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DEPARTMENT OF ELECTRONICS AND INSTRUMENTATION

ENGINEERING

EI2251 INDUSTRIAL INSTRUMENTATION-1

SEMESTER :IV

BRANCH : EIE

Lesson Planning Sheet

Sub

Code

Name of the

Subject

Sem Department of

Electronics &

Instrumentation

No. of

Student

s

Time

L T P

EI2251 INDUSTRIAL

INSTRUMENTATION-1 IV Name of Faculty:

M.THIRUMAGAL

40 3 0 0

L-Lecture, T-Tutorial, P-Practical

Sl.

No

Lesson / Topic

Covered

(each lecture session

wise)

Methodology

Pract

ical /

Gue

st

Lect

ure

Ind

ustr

ial Visit

Theory Coverage Tutorial Support

Black

-

Board

OHP Power

Point

Exercis

e

Assignment

s


1. 1. MEASUREMENT OF

FORCE, TORQUE AND

VELOCITY

INTRODUCTION

2. Electric balance –

Different types of load

cells

3. Hydraulic, pneumatic

strain gauge

4. Magneto elastic and Piezo

electric load cell

5. Different methods of

torquemeasurements:

introduction

6. strain gauge-Relative angular

twist-

7. Speed measurement:- -

Stroboscope.

8. Capacitive tacho

9. Dragcup type tacho-D.C

Ac tacho

Unit 2

1 MEASUREMENT OF

ACCELERATION,

VIBRATION AND

DENSITY- introduction

2 Accelerometers:- LVDT,

Piezo-electric,

typeaccelerometer

3 Strain gauge and Variable

reluctance type


4 Mechanical type vibration

instruments –

5 Seismic instruments as

anaccelerometer –

6 Vibrometers : Calibration

of vibration pickups –

7 Units of density and

specific gravity – Baume

scale, and API scale

8 Pressure head type

densitometers- Floattype

densitometers

9 – Ultrasonic

densitometer- Bridge

type gas densitometer.

Unit 3

1 PRESSURE

MEASUREMENT

introduction

2 Units of pressure-

Manometers-

introduction- -

3 Different types –Elastic

type pressure gauges:

Bourdontube, bellows and

diaphragms-

4 Elastic elements with

LVDT and straingauges –

Capacitive type pressure

gauge –Piezo-resistive

pressure sensor-


5 Resonator pressure

sensor

6 Measurement of vacuum:-

McLeod gauge-Thermal

conductivity gauges

7 - Ionization gauges:– Cold

cathode type and hot

cathode

8 Testing and calibration of

pressure gauges

9 -

Dead weight tester.

10 Electrical methods

Unit4

1 Temperature

measurement-intro

2 Definitions and

standards-Primary and

secondary fixed points

3 Calibration

ofthermometers

4 Different types of filled

in system thermometer-

5 Sources of errors in

filledin systems and their

compensation

6 – Electrical methods of

temperature

measurement

7 Signal conditioning of

industrial Bimetallic

thermometers


8 3 lead and 4 lead RTDs

9 Thermistors.

10 Bimetallic thermometers

and theircharacteristics-

Unit 5

1 THERMOCOUPLES AND

RADIATION

PYROMETERS- intro

2 Thermocouples-Laws of

thermocouple Response of

thermocouple –––-–

3 Fabrication of industrial

thermocouples –

Signalconditioning of

thermocouple output-

4 –Isothermal block

reference junctions –

Commercial circuits for

cold junction

compensation-

5 Special techniquesfor

measuring high

temperature using

thermocouples

6 Radiation fundamentals

Radiation methods of

temperature

measurement

7 Total radiation

pyrometers-Optical

pyrometers-


8 Fiber optic temperature

measurement

9 Two colour radiation

pyrometers

UNIT 1

MEASUREMENT OF FORCE, TORQUE AND VELOCITY

AIM:

Discussion of load cells, torque meter and various velocity pick-ups.

KEY WORDS:

Load cell- strain gauge- torque measurement- torque meters- speed measurement-

Tachogenerators-stroboscope


UNIT-1

MEASUREMENT OF FORCE , TORQUE AND VELOCITY

Force may be defined as a cause that produces resistance or obstruction to any moving body, or

changes the motion of a body, or tends to produce these effects. Force is given as F=MA.

Force measurement is also done by electric means in which the force is first converted into

displacement at an elastic element and the displacement is measured

A vector quantity has both magnitude and direction

Units of force

S.I unit= Newton (N)


ONE NEWTON:

The force capable of giving a mass of one Kg an acceleration of one meter per second

Types of forces

Frictional force:

Friction is a surface force that opposes relative motion

Tensional force:

Tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar

object on another object. Measured in newtons (or sometimes pounds-force)

Compression force:

Opposite of tension

Elastic force:

Elastic force is the physical property of a material that returns to its original shape after

the stress

Various methods of measuring force:

1. Balancing on standard mass, either directly or through levers

2. Measuring acceleration of the body if its mass is known on which the unknown force is

applied

3. Balancing against a magnetic force of a current-carrying coil and a magnet

4. Transducing the force to fluid pressure and then measuring the pressure.

5. Force to elastic member and measuring the resulting deflection

6. Measuring the change in precession of a gyroscope caused by an applied torque due to

applied force

7. Measuring the change in natural frequency of a wire tensioned by the force.

Measurement methods

1. Direct method

2. Indirect method

Measuring devices: Different types of electrical type force transducers are given below

Force gauge-Load cell etc.,

http://en.wikipedia.org/wiki/Physical_property

http://en.wikipedia.org/wiki/Stress_(mechanics)


Spring scale

Strain gauge

Load cell

A load cell is a transducer that is used to convert a force into electrical signal

This conversion is indirect and happens in two stages

Stage-1: The force being sensed deforms a strain gauge.

Stage-2:The strain gauge converts the deformation (strain) to electrical signals

Types of Load cell:

1. Hydrostatic load cell

2. Pneumatic load cell

3. Magneto elastic load cell

4. Piezo electric load cell

Hydrostatic Load cell:

The force is made to exerted on the load platform which is connected to the diaphragm

The diaphragm seals the chamber filled with fluid connected to bourdon gauge.

During the measurement process the applied force pressurizes the oil which in turn

activate the burdon gauge and the needle connected to it indicates the magnitude of the pressure

exerted

Full load deflection:;0.05 mm

Measurement range:0-20 Tonnes

Tare compensation of 0.2MPa is done

Used in static measurement

Also called as plunger


Pneumatic load cell:

The force is applied to the platform connected to the sealing diaphragm of air chamber

The applied force is measured by means of flapper nozzle principle described as follows

The chamber is equipped with constant air supply

The platform acts like a flapper and creates the backpressure on through the nozzle in the

chamber according to the force applied on it

This counter balance the platform and equilibrium is attained the value of pressure inside

the chamber indicated on meter gives the value of the force applied

If the

Applied mass = W

Output pressure=p

Diaphragm stiffness=ks

Flapper-nozzle gain=kf

Area of diaphragm=α

p= W / (ks/kf + α)

The force is made to exert on the load platform which in turn compresses the fluid closed

by the diaphragm resulting in deflection of meter pointer

Used in static measurement

Magneto elastic load cell:


Also called as pressductor load cell

Principle:

Stress on ferromagnetic material alters the magnetic moments resulting in the

change in permeability of the material.

The change in permeability is directly proportional to the applied force/stress.

Working:

Primary and secondary windings are wounded at right angles on diagonally drilled

hole pairs on the transducer body enclosed with laminated sheets of ferromagnetic material

Secondary windings remain undisturbed under no load condition. On load condition the

angle between the primary and secondary changes the resulting flux linkage is given by

Ф=αB cosθ

α = C.S.A of material

Ф = Total flux linkage

B= Magnetic flux density

Cosθ = change in angle

Piezo Electric Load Cell:

and induces a voltage proportional to the force applied

es= -n dФ /dt

n = turn ratio (n2/n1)

Unlike

strain gages that can measure

static forces, piezoelectric

force sensors are mostly used

for dynamic- force

measurements

such as oscillation,

impact, or high speed

compression or tension. Any

force applied to the

piezoelectric sensing element

produces a separation of

charges within the atomic


structure of the material, generating an electrostatic output voltage. The polarity of the voltage

generated depends on the atomic structure of the material and the direction in which the force is

applied.

However, any leakage path lets electrons redistribute across the material, dropping the

voltage output back to zero. Internal leakage paths are formed by impurities within the crystal

while external paths are created by the electronics used to measure the voltage generated. All

leakages must be considered to determine the discharge time constant (DTC). The DTC typically

follows an exponential curve similar to an RC time constant and is used to determine the sensor’s

lowest frequency response.

In a typical quartz-based force sensor, a charge-collection electrode is sandwiched

between two quartz-crystal elements. The quartz elements are oriented to supply the same

polarity voltage to the electrode when compressed, while the opposite polarity is applied to the

sensor housing. This assembly resides between two mounting disks held together by an elastic,

beryllium-copper stud and then weld-sealed within the enclosure to prevent contamination. The

stud preloads the quartz elements to assure all parts are in intimate contact and to provide good

linearity and tensile-force measurements.

When a force is applied to the impact cap, the quartz elements generate an output voltage

which can be routed directly to a charge amplifier or converted to a low-impedance signal within

the sensor. The use of the direct sensor output demands that any connector, cable, and charge

amplifier input must maintain a high insulation resistance on the order of >10≠″ Ω.

Low-impedance quartz sensors have an internal MOSFET amplifier. Its output is a low-

impedance voltage signal that uses standard cabling. However, force sensors with internal

amplifiers do require external power to operate the amp.

TORQUE MEASUREMENT

The force which tends to change the linear motion or rotation of a body.

It is also defined as the turning or twisting moment of a force about an axisT= FX D

T=Torque

F=Force

D=perpendicular distance from the axis of rotation of the line of action of the force

Methods of measurement:

1. Inline rotating sensor based torque measurement

2. Inline stationary sensor based torque measurement

In line rotating sensor based torque measurement:


Strain gauge based measurement:

A strain gage can be installed directly on a shaft. Because the shaft is rotating, the torque sensor

can be connected to its power source and signal conditioning electronics via a slip ring. The strain

gage also can be connected via a transformer, eliminating the need for high maintenance slip

rings. The excitation voltage for the strain gage is inductively coupled, and the strain gage output

is converted to a modulated pulse frequency (Figure 6-5). Maximum speed of such an

arrangement is 15,000 rpm.

Strain gages also can be mounted on stationary support members or on the housing itself. These

"reaction" sensors measure the torque that is transferred by the shaft to the restraining elements.

The resultant reading is not completely accurate, as it disregards the inertia of the motor.

Strain gages used for torque measurements include foil, diffused semiconductor, and thin film

types. These can be attached directly to the shaft by soldering or adhesives. If the centrifugal

forces are not large--and an out-of-balance load can be tolerated--the associated electronics,

including battery, amplifier, and radio frequency transmitter all can be strapped to the shaft.

Torque measurement by relative angular twist method by proximity probe type

Proximity and displacement sensors also can detect torque by measuring the angular

displacement between a shaft's two ends. By fixing two identical toothed wheels to the shaft at


some distance apart, the angular displacement caused by the torque can be measured. Proximity

sensors or photocells located at each toothed wheel produce output voltages whose phase

difference increases as the torque twists the shaft.

Torque measurement by relative angular twist method by optical type

Another approach is to aim a single photocell through both sets of toothed wheels. As torque

rises and causes one wheel to overlap the other, the amount of light reaching the photocell is

reduced.

SPEED MEASUREMENT:

Speed is defined as rate of change of position of an object with respect to time.

Units of speed

Meters per second (symbol m s−1

or m/s), the SI derived unit;

Kilometers per hour (symbol km/h);

Miles per hour (symbol mph);

Knots (nautical miles per hour, symbol kn or kt);

Feet per second (symbol fps or ft/s);

Mach number, speed divided by the speed of sound;

The speed of light in vacuum (symbol c) is one of the natural units:

Revolution per minute (rpm)

Measuring methods:

Generally speed is calculated using tachometers which calculates the angular speed in

revolution per minute(rpm) of the object and converted into the form required

Types of Tachometer:

1. Mechanical tachometer: Associated only with mechanical units to measure speed

2. Electrical tachometer: Associated with transducer for converting rotational speed to

electrical quantity.

Capacitive tacho:

http://en.wikipedia.org/wiki/Meters_per_second

http://en.wikipedia.org/wiki/SI_derived_unit

http://en.wikipedia.org/wiki/Kilometers_per_hour

http://en.wikipedia.org/wiki/Miles_per_hour

http://en.wikipedia.org/wiki/Knot_(nautical)

http://en.wikipedia.org/wiki/Nautical_mile

http://en.wikipedia.org/wiki/Foot_per_second

http://en.wikipedia.org/wiki/Mach_number

http://en.wikipedia.org/wiki/Speed_of_sound

http://en.wikipedia.org/wiki/Speed_of_light

http://en.wikipedia.org/wiki/Natural_units


It uses the principle of charging the capapcitor and discharging through a meter alternately . If

the charging and discharging is controlled by the speed of the equipment, the average discharge

current would be ppl to the speed, if (ω)omega is the speed of rotation, I=C R (ω)

Drag cup tachometer:

This type is very common in rotational speed measurement. The source angular speed

rotates a permanent magnet. An aluminum disc or cup is held close to the rotating magnet

restrained by a control spring. When the magnet rotates eddy current is set up in the drag cup or

disc and a torque is produced which tries to oppose the field produced by the eddy current. The

cup is thus dragged or rotated in the direction of the rotating magnet. Due to the restraining action

of the spring and angular rotation is indicated by the pointer which is proportional to speed.

D.C Tachogenerators

The transducer that converts speed of rotation directly into electrical signal is an

induction pickup such a tachometer is more commonly used for speed cpntrol rotating equipments


Here the dc generator with the output voltage from the commutator is directly proportional to the

speed measured

A.C Tacho generators:

Here the operation is similar to the dc tachometer but the magnet rotates in the

stationary coil proportional to the speed to be measured in a stationary coil and generates a a.c

voltage which is signal conditioned and displayed in the units of speed

Stroboscopic method

Stroboscopes are used to measure the speed of rotation or frequency of vibration of a

mechanical part or system. They have the advantage over other instruments of not loading or

disturbing the equipment under test. Mechanical equipment may be observed under actual

operating conditions with the aid of stroboscopes. Parasitic oscillations, flaws, and unwanted

distortion at high speeds are readily detected. The flashing-light stroboscopes employ gas

discharge tubes to provide a brilliant light source of very short duration.

http://www.answers.com/topic/vibration

http://www.answers.com/topic/unwanted


UNIT 2

MEASUREMENT OF ACCELERATION, VIBRATION AND DENSITY

AIM:

. Exposure to various accelerometer pick-ups, vibrometers, density andviscosity pick-ups

KEY WORDS:

Accelerometers:- LVDT - Vibrometers - density and specific gravity – densitometers- Ultrasonic densitometer-

Bridge type gas densitometer


UNIT-II

MEASUREMENT OF ACCELERATION, VIBRATION AND DENSITY

MEASUREMENT OF ACCELERATION

Accelerometers:

An accelerometer is a device that measures the vibration, or acceleration of motion of a structure. The

force caused by vibration or a change in motion (acceleration) causes the mass to "squeeze" the

piezoelectric material which produces an electrical charge that is proportional to the force exerted upon

it. Since the charge is proportional to the force, and the mass is a constant, then the charge is also

proportional to the acceleration.

There are two types of piezoelectric accelerometers (vibration sensors). The first type is a "high

impedance" charge output accelerometer. In this type of accelerometer the piezoelectric crystal produces

an electrical charge which is connected directly to the measurement instruments. The charge output

requires special accommodations and instrumentation most commonly found in research facilities. This

type of accelerometer is also used in high temperature applications (>120C) where low impedance

models cannot be used.

The second type of accelerometer is a low impedance output accelerometer. A low impedance

accelerometer has a charge accelerometer as its front end but has a tiny built-in micro-circuit and FET

transistor that converts that charge into a low impedance voltage that can easily interface with standard

instrumentation. This type of accelerometer is commonly used in industry. An accelerometer power

supply like the ACC-PS1, provides the proper power to the microcircuit 18 to 24 V @ 2 mA constant

current and removes the DC bias level, they typically produces a zero based output signal up to +/- 5V

depending upon the mV/g rating of the accelerometer. All OMEGA(R) accelerometers are this low

impedance type.

LVDT accelerometer


A second type of accelerometer takes advantage of the natural linear displacement

measurement of the LVDT to measure mass displacement. In these instruments, the LVDT core

itself is the seismic mass. Displacements of the core are converted directly into a linearly

proportional ac voltage. These accelerometers generally have a natural frequency less than 80 Hz

and are commonly used for steady-state and low-frequency vibration. Figure shows the basic

structure of such an accelerometer.

Piezoelectric Accelerometer:

The piezoelectric accelerometer is based on a property exhibited by certain crystals where

a voltage is generated across the crystal when stressed. This property is also the basis for such

familiar sensors as crystal phonograph cartridges and crystal microphones. For accelerometers,

the principle is shown in Figure 5.28. Here, a piezoelectric crystal is spring-loaded with a test

mass in contact with the crystal. When exposed to an acceleration, the test mass stresses the

crystal by a force (F = ma), resulting in a voltage generated across the crystal. A measure of this

voltage is then a measure of the acceleration. The crystal per se is a very high-impedance source,

and thus requires a high-input impedance, low-noise detector. Output levels are typically in the

millivolt range. The natural frequency of these devices may exceed 5 kHz, so that they can be

used for vibration and shock measurements.

Variable Reluctance


This accelerometer type falls in the same general category as the LVDT in that an

inductive principle is employed. Here, the test mass is usually a permanent magnet. The

measurement is made from the voltage induced in a surrounding coil as the magnetic mass moves

under the influence of acceleration. This accelerometer is used in vibration and shock studies

only, because it has an output only when the mass is in motion. Its natural frequency is typically

less than 100 Hz. This type of accelerometer often is used in oil exploration to pick up vibrations

reflected from underground rock strata. In this form, it is commonly referred to as a geophone.

Seismic instruments as accelerometer

The mass is connected through the parallel spring and damper arrangement to the housing

frame. This frame is then connected to the vibration source whose characteristics are to be

measured. The mass tends to remain fixed in its spatial position, so that the vibration motion is

registered as a relative displacement between the mass and the housing frame. The displacement

is then sensed and indicated by an appropriate transducer. The seismic instrument may be used

for either displacement or acceleration measurement by proper selection of mass, spring and

damper combinations.

Vibration instruments

Calibration of vibration pick ups:

Constant Acceleration method: Constant acceleration methods, which are suitably

only for calibrating accelerometers include the tilting- support method and the centrifuge.

The tilting-support method utilises the accelerometer’s inherent sensitivity to gravity..

Static acceleration over the range +-1g may be accurately applied by fastening the

accelerometer to a tilting support whose tilt support whose tilt angle from vertical is

accurately measured. This method requires that the accelerometer respond to static

accelerations; therefore piezoelectric devices cannot be calibrated in this way.

This method consists of a modified electro dynamic vibration shaker which has

been carefully designed to provide uniaxial pure sinusoidal motion which is equipped with

an accurately calibrated moving coil velocity pick up to measure its table motion. If a

motion is purely sinusoidal, knowledge of its velocity pickup to measure its table motion.

If Motion is known to be purely sinusoidal, knowledge of its velocity and frequency

enables accurate calculation of the displacement and displacement. The motion frequency

is easily obtained with high accuracy by electronic counters. This technique is thus useful

for displacement , velocity or acceleration pickups.

LASER DOPPLER VIBROMETERS:


During the last years the growing importance of the correct determination of the state of

conservation of artworks has been stated by all personalities in care of Cultural Heritage. There

exist many analytical methodologies and techniques to individuate the physical and chemical

characteristics of artworks, but at present their structural diagnostics mainly rely on the expertise

of the restorer/technician and the typical diagnostic process is accomplished mainly through

manual and visual inspection of the structure. For this reason, many innovative optical techniques

have been tried and applied to this issue and in these pages we will show some examples

regarding the use of the laser Doppler vibrometer (LDV); The basic idea behind the employment

of LDV is to substitute human senses and contact sensors with measurement systems capable of

remote acquisition and, if necessary, of remote structural excitation: surfaces are very slightly

vibrated by mechanical and acoustical actuators, while a laser Doppler vibrometer means the

objects measuring surface velocity and producing 2D or 3D maps. Think, for example, of a fresco

with delaminated areas: where these defects occur, velocity is higher than neighbouring areas so

defects can be easily spotted by a LDV. Laser vibrometers also identify structural resonance

frequencies thus leading to a complete characterization of these defects, and this holds true also

for massive structures, like towers, buildings, churches.

Laser Doppler Vibrometers, or better Scanning Laser Doppler Vibrometers (SLDV), have been

applied to different types of movable or decorative artworks, like frescoes, icons, mosaics,

ceramics, inlaid wood and easel painting, with different degrees of success, but always showing

an impressive list of important advantages:

— no remarkable intrusivity,

— remote measurements,

— ample frequency response,

— high sensibility,

— portability.

Moreover all existing systems are completely PC controlled and this allows digital data storage

and easy data transfer to other applications like software packages for structural and modal

analysis, and to spreadsheets applications like Excel or Matlab.

The application to historical buildings is more recent [2] and still limited but looks promising and

will be the subject of much research in the immediate future. Of course there still exist a lot of

difficulties, mainly related to the non-optically collaborative surfaces of tested structures and the

necessity of working at great distances to get data that can be considered representative of the

examined object. These two factors work one against the other, and this makes the application of

SLDV mainly a ―prototype‖ application yet, but already exist situations where this is not the case

anymore [3].

Also we must not forget other problems, like instrument isolation from ground vibrations and the

realization of special excitation techniques but it has been already demonstrated the capability of

the LDV to acquire non-intrusively vibrational data on not-treated surfaces up to 10-15 meters, a

real asset when dealing with large structures. Regular monitoring of important parameters related

to the state of conservation of these huge objects, like frequencies of resonance, is thus possible

with no external intervention on the structure and may be performed quickly and with a high

degree of accuracy.

Optical Sensors for Vibration Measurements

When we say ―optical sensors‖, we mean an immense variety of instruments, devices and

systems. Just think of such different instruments like infra red thermal cameras or a Bragg grating

strain sensor. Even to measure vibrations we may have such different solutions like laser based,


LED based, fibre based sensors; not to mention the physical scale of such sensors, going from the

micro scale, e.g. optical MEMS accelerometers, to relatively ―immense‖ laser Doppler

vibrometers. If we confine ourselves to laser based instrumentation, we may mention full field

techniques (holography, shearography, ESPI, for example) or focused beam ones (laser Doppler

vibrometers).

The advantages of optical sensors are outstanding and their use is spreading more and more, day

after day. Think for example to optical fibres sensors: their solid state design is resistant to

vibration, unaffected by electromagnetic interference (nor do they create additional EMI), and,

because the light source can be located far away from explosive materials, do not run the risk of

sparking an explosion. They also offer superior multiplexing capabilities, thanks to the possibility

of having multiple sensors in a single fibre line.

Also, fibre optic sensors fall into a variety of sensor types: chemical, temperature, strain,

biomedical, electrical and magnetic, rotation, vibration, displacement, pressure, and flow. Many

of these categories were developed by military organizations during the nineties. These sensors

are extremely effective at creating "smarter" structures, widely used nowadays for chemical

sensing (especially in the petrochemical industry), transportation, building and structural

monitoring, and biomedical.

However, fibre sensors must be placed in contact or closed to the object to be measured, and so

they maybe not used in many occasions, where the objects cannot be reached or are impossible to

modify, e.g. a fresco in a church.

For cases like these, instrumentation based on a laser beam used as a probe is much more suited,

and we will deal exclusively with these devices in the following of this publication. Major

advantages of such instruments rely not only in this absence of invasivity, but also in their high

sensibility and in their capacity of acquiring detailed data in the terms of space, time, and

frequency. Many of these systems are still quite expensive, but their contribution to solve design,

production process, or quality control problems is invaluable.

More specifically we will deal with focused laser beam instruments, laser Doppler vibrometers.

We will avoid detailed mathematical description of involved theory, preferring a more intuitive

approach to make this matter more palatable to a wider range of learners.

The scanning version of the LDV may automatically and accurately measure point-by-point

surface velocities using interferometric techniques and a couple of galvanometric driven mirrors

steering the laser beam. In this way it is possible to scan a grid of acquisition points acquiring

response spectra and time histories of the velocity of each point; these data are then processed and

presented as 2D or 3D colour maps. Modern SLDVs may scan 100 points/second for a total

number of more than 100.000 points working with a maximum frequency in the range of some

tens of MHz, and with a lower limit of less than a Hertz. Full-scale highest range is typically 10

m/s with lower ranges in the order of 1 mm/s, corresponding to a displacement of some tens of

nanometres.

These features make the SLDV an ideal instrument in applications where it is impossible or very

difficult to use standard vibration measuring devices, such as accelerometers. Accelerometers will

load the examined structures and may even damage the delicate surface of precious objects.

Moreover, to perform an accurate vibrational analysis it would require to employ many

transducers or to move one all around the tested piece and in both cases time and cost would rise

considerably

MEASUREMENT OF DENSITY:

Density:

http://alpha1.infim.ro/cost/pagini/handbook/chapters/ldv.htm

http://alpha1.infim.ro/cost/pagini/handbook/chapters/shea.htm


Density is defined as an objects mass per unit volume. Mass is a property.

Mass and Weight - the Difference! - What is weight and what is mass? An explanation of

the difference between weight and mass.

The density can be expressed as

ρ = m / V = 1 / vg (1)

where

ρ = density (kg/m3)

m = mass (kg)

V = volume (m3)

vg = specific volume (m3/kg)

The SI units for density are kg/m3. The imperial (BG) units are lb/ft

3 (slugs/ft

3). While people

often use pounds per cubic foot as a measure of density in the U.S., pounds are really a measure

of force, not mass. Slugs are the correct measure of mass. You can multiply slugs by 32.2 for a

rough value in pounds.

Unit converter for other units

The higher the density, the tighter the particles are packed inside the substance. Density is a

physical

property constant at a given temperature and density can help to identify a substance.

Densities and material properties for common materials

Relative Density (Specific Gravity)

Relative density of a substance is the ratio of the substance to the density of water, i.e.

Specific Weight

Specific Weight is defined as weight per unit volume. Weight is a force.

Mass and Weight - the difference! - What is weight and what is mass? An explanation of

the difference between weight and mass.

Specific Weight can be expressed as

γ = ρ g (2)

where

γ = specific weight (N/m3)

ρ = density (kg/m3)

g = acceleration of gravity (m/s2)

The SI-units of specific weight are N/m3. The imperial units are lb/ft

3. The local acceleration g is

under normal conditions 9.807 m/s2 in SI-units and 32.174 ft/s

2 in imperial units.

http://www.engineeringtoolbox.com/mass-weight-d_589.html

http://www.engineeringtoolbox.com/unit-converter-d_185.html

http://www.engineeringtoolbox.com/material-properties-t_24.html

http://www.engineeringtoolbox.com/mass-weight-d_589.html

http://www.engineeringtoolbox.com/accelaration-gravity-d_340.html


Ultrasonic densitometer

Twin folks are inserted into liqu9d or gas media whose density needs to be

measured since the natural frequency of the forks is a function of density of the media, small

changes in the natural frequency must be monitored accurately

Pressure head type densitometers:

The pressure at the bottom of the tank of the constant liquid column is proportional to

density and the weight of the given volume of the fluid is proportional to density.

It compares hydrostatic pressures due to the height of the liquids in two tanks. one is the

reference tank, consisting of a liquid of constant height and density. The other tank maintains the

height constant by overflow, so that the manometer can be directly in terms of density

measurement.

Float type densitometer:

The plumet is located entirely under the liquid surface the effective weight of the chain on

the plumet varies depending on the position of the plumet which in turn the function of density of

the liquid

Bridge type gas densitometer

It consist of four arm of pipe connections like wheat stone bridge for a balanced flow the

detector elements are equally cooled and when they are connected in the wheat stone bridge and

they indicates null balance. The detector bridge unbalance will therefore will beameasure of gas

density


UNIT 3

PRESSURE MEASUREMENT

AIM:

To have an adequate knowledge about pressure transducers

KEY WORDS:

Manometers- pressure gauges- Elastic elements- bellows- diaphragms- LVDT and straingauges- Piezo-resistive

pressure sensor- Ionization gauges- Dead weight tester.


UNIT-III

PRESSURE MEASUREMENT

Pressure :

Pressure is defined as force per unit area. It is usually more convenient to use pressure

rather than force to describe the influences upon fluid behavior. The standard unit for pressure is

the Pascal, which is a Newton per square meter.

For an object sitting on a surface, the force pressing on the surface is the weight of the

object, but in different orientations it might have a different area in contact with the surface and

therefore exert a different pressure

Units of pressure

http://hyperphysics.phy-astr.gsu.edu/hbase/mass.html#wgt


Pressure measurements:

Manometer methods:

1. U-tube manometer:

When there is a pressure difference between two ends of the tube the liquid

goes down on one side of the tube and up on other side the difference in liquid levels from one

side to other indicates the difference in pressure

Well type manometer:

The well type manometer is

widely used because the

reading a single leg is required in it consist of very large diameter vessel connected on one side to

a very small size tube thus the zero level moves very little when pressure is applied

Inclined manometer:

Inclined manometer is used to measure very small pressure differences the manometer is tipped

so that the liquid moves a longer distance through the tube as it rises

Elastic type pressure gauges:


Bourdon-tube designs

Since the invention of the Bourdon-tube gauge more than a century ago, pressure gauge

manufacturers have been developing different types of gauges to meet specific needs without ever

changing the basic principle of the Bourdon tube's operation. Bourdon-tube gauges, Figure 1, are

now commonly available to measure a wide range of gauge, absolute, sealed, and differential

pressures, plus vacuum.

They are manufactured to an accuracy as high as 0.1% of span and in dial diameters from 1-1/2 to

16 in. A variety of accessories can extend their performance and usefulness. For example,

snubbers and gauge isolators can be installed to protect the sensitive internal workings of the

gauge from pressure spikes. The availability of Bourdon-tube pressure gauges to meet specific

needs, coupled with their inherent ruggedness, simplicity, and low cost has resulted in their wide

use in many applications.

Gauges using C-shaped Bourdon tubes as the

elastic chamber - the type shown in Figure 1 -

are by far the most common. Pressurized fluid

enters the stem at the bottom (which is

sometimes center-back-mounted instead) and

passes into the Bourdon tube. The tube has a

flattened cross section and is sealed at its tip.

Any pressure in the tube in excess of the

external pressure (usually atmospheric) causes

the Bourdon tube to elastically change its shape

to a more circular cross section.

This change in shape of the cross section tends to straighten the C-shape of the Bourdon tube.

With the bottom stem end fixed, the straightening causes the tip at the opposite end to move a

short distance - 1/16 to 1/2 in., depending on the size of the tube. A mechanical movement then

transmits this tip motion to a gear train that rotates an indicating pointer over a graduated scale to

display the applied pressure. Often, a movement is incorporated to provide mechanical advantage

to multiply the relatively short movement of the tube tip.

Bellows and diaphragms:

Low-pressure applications do not generate

enough force in the Bourdon tube to operate the

multiplying mechanism; therefore, Bourdon-

tube gauges are not generally used for pressure

spans under 12 psi. For these ranges, some other

form of elastic chamber must be used, a metallic

bellows, Figure 4, for example. These bellows

generally are made by forming thin-wall tubing.

However, to obtain a reasonable fatigue life and

motion that is more linear with pressure, a coil

spring supplements the inherent spring rate of

the bellows. These spring-loaded bellows

gauges generally are used in pressure ranges having spans to 100 psi and to 1 in. Hg.

Simplified view of spiral Bourdon-tube

pressure gage and movement.

Cross-sectional view of spring-loaded

bellows pressure gauge.


Metallic diaphragms also are used as the elastic chamber in low-pressure gauges. A diaphragm

plate is formed from thin sheet metal into a shallow cup having concentric corrugations. To make

an element with a low spring rate that generates substantial deflection from a small change in

pressure, two plates can be soft soldered, brazed, or welded at their periphery to form a capsule,

and additional capsules can be joined at their centers to form a stack, Figure 5.

Generally, the measured pressure is applied to the interior of the element and no supplemental

coil springs are used. A 2-in. diameter capsule (two plates) will provide about 0.060 in. of motion

without exceeding the elastic limit of the material. This is usually enough to operate a high-ratio

multiplying movement because diaphragm deflection can transmit high force.

Diaphragm elements often are used in gauges to indicate absolute pressure. In this form, the

diaphragm element is evacuated. sealed, and mounted within a closed chamber. The pressure to

be measured is admitted to the closed chamber and surrounds the diaphragm element. Changes in

the measured pressure cause the element to deflect, but because atmospheric pressure is excluded

and has no effect on the indication, the gauge may be calibrated in terms of absolute pressure. If

the applied pressure is atmospheric pressure, the gauge is known as a barometer.

Diaphragm elements also may be used in an opposing arrangement. By evacuating one side of the

assembly, the gauge can indicate absolute pressure. If a pressure is applied to one side of the

assembly, and a second pressure is applied to the other side, then the differential pressure will be

indicated. The differential pressure is limited with respect to the static pressure that can be

applied. That is, the gauge may be suitable to indicate between 10 psi and 12 psi, but not be

suitable to indicate between 100 psi and 102 psi. Also, the consequence of inadvertently applying

full pressure to one side of the element and no pressure to the other side of the element must be

considered.

Elastic element with LVDT based pressure measurement

Any change in pressure will given to bellows which in turn actuate the core of the

LVDT and produces output in the secondary the value of output is directly proportional to the

pressure input to the bellows

Capacitive type pressure transducer:


Any change in pressure causes the change in distance between diaphragm and fixed

plate which the unbalance the bridge the bridge is proportional to pressure applied.

Piezoresistive pressure sensor:

Piezoresistive materials are materials that change resistance to the flow of current when they are

compressed or strained. Metal is piezoresistive to some degree, but most pressure sensors use the

semiconductor silicon. When force is put on the silicon, it becomes more resistant to a current

pushing through. This resistance is usually very linear--twice as much pressure results in twice as

large a change in resistance.

A Piezoresistive Pressure Sensor contains several thin wafers of silicon embedded between

protective surfaces. The surface is usually connected to a Wheatstone bridge, a device for

detecting small differences in resistance. The Wheatstone bridge runs a small amount of current

through the sensor. When the resistance changes, less current passes through the pressure sensor.

The Wheatstone bridge detects this change and reports a change in pressure.

Resonant Wire pressure sensor:


The resonant-wire pressure transducer was introduced in the late 1970s. In this design , a wire is

gripped by a static member at one end, and by the sensing diaphragm at the other. An oscillator

circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes

the wire tension, which in turn changes the resonant frequency of the wire. A digital counter

circuit detects the shift. Because this change in frequency can be detected quite precisely, this

type of transducer can be used for low differential pressure applications as well as to detect

absolute and gauge pressures.

The most significant advantage of the resonant wire pressure transducer is that it generates an

inherently digital signal, and therefore can be sent directly to a stable crystal clock in a

microprocessor. Limitations include sensitivity to temperature variation, a nonlinear output

signal, and some sensitivity to shock and vibration. These limitations typically are minimized by

using a microprocessor to compensate for nonlinearities as well as ambient and process

temperature variations.

Resonant wire transducers can detect absolute pressures from 10 mm Hg, differential pressures

up to 750 in. water, and gauge pressures up to 6,000 psig (42 MPa). Typical accuracy is 0.1% of

calibrated span, with six-month drift of 0.1% and a temperature effect of 0.2% per 1000¡ F.

Measurement of vacuum:

McLeod gauge:

A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer

until the pressure is a fewmmHg. The gas must be well-behaved during its compression (it must

not condense, for example). The technique is slow and unsuited to continual monitoring, but is

capable of good accuracy.

Useful range: above 10-4

torr [3]

(roughly 10-2

Pa) as high as 10−6

Torr (0.1 mPa),

mPa is the lowest direct measurement of pressure that is possible with current technology. Other

vacuum gauges can measure lower pressures, but only indirectly by measurement of other

pressure-controlled properties.

The McLeod gauge measures the pressure of gases by compressing a known volume with a fixed

pressure. The new volume is then a measure of the initial absolute pressure.

-- The McLeod gauge has been used until recently for calibrating other gauges.

http://en.wikipedia.org/wiki/McLeod_gauge

http://en.wikipedia.org/wiki/Torr

http://en.wikipedia.org/wiki/Pressure_measurement#cite_note-THV_V-2


- It covers the vacuum range between 1 and 10-6

torr.

Thermal Designs:

The thermal conductivity of a gas changes with its pressure in the vacuum range. If an element

heated by a constant power source is placed in a gas, the resulting surface temperature of the

element will be a function of the surrounding vacuum. Because the sensor is an electrically heated

wire, thermal vacuum sensors are often called hot wire gauges. Typically, hot wire gauges can be

used to measure down to 10-3 mm Hg.

Pirani guage:

In this design, a sensor wire is heated electrically and the pressure of the gas is

determined by measuring the current needed to keep the wire at a constant temperature

Ionisation gauges:

Hot cathode vaccum guage

The operating principles of this gauge are similar to the Penning gauge except that the electrons

are produced by a hot filament and accelerated to a grid. The pressure range covered is either 1 to

10-5

torr or 10-2

to 10-7

torr, depending on the electrode structure. Electrons emitted from the

filament ionize residual gas molecules in the container being evacuated; the ion current arriving at

the collector plates is directly proportional to the pressure and the ionization probability of the

residual gas. This is a clean, accurate gauge that can be used down to about 10-6

torr; below this

pressure its accuracy is reduced due to the soft X-rays produced by electrons striking the grid.

These X-rays generate a current in the collector circuit independent of pressure.


Bayard-Alpert hot-filament ionization gauge. In this ionization gauge, the cross section of the

collector is reduced to minimum to reduce the X-ray effect. This is achieved by inverting the

gauge—that is, the collector (a fine wire) is surrounded by the grid. The pressure range covered is

10-3

to 10-9

torr or down to 10-11

torr if a modulated instrument is used. Operating principles are

the same as for the other ionization gauges

Cold cathode vacuum guage:

This gauge makes use of the fact that the rate of ion production by a stream of electrons in a

vacuum system is dependent on pressure and the ionization probability of the residual gas. Also

called the Penning gauge, it consists of two cathodes opposite one another with an anode centrally

spaced between them inside a metal or glass envelope. Outside the envelope a permanent magnet

provides a magnetic field to lengthen the path travelled by the electron in going from cathode to

anode, thus increasing the amount of ionization occurring within the gauge. Normally the anode

is operated at about 2 kV, giving rise to a direct current caused by the positive ions arriving at the

cathode. The pressure is indicated directly by the magnitude of the direct current produced. The

pressure range covered by this gauge is from as low as 10-7

torr. It is widely used in industrial

systems because it is rugged and simple to use.

Testing and calibration of pressure gauges-Dead weight tester.

Dead weight tester

Deadweights are usually used for pressure gauge calibration as they come with high accuarcy, So

they can be used as primery standard (as mentioned before).there are many types of them

depending on the application and they are operated with oil (hydrulic) or with air (penumatic).

Deadweight testers are the basic primary standard for accurate measurement of pressure.

Deadweight testers are used to measure the pressure exerted by gas or liquid and can also

generate a test pressure for the calibration of numerous pressure instruments.

Hope this helps!

Description

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Known weights are placed on a rotating plate on top of a calibrated piston, connected by tubing to

the pressure sensor being tested. This puts a known force (weights) on a known surface area

(piston). The rotation eliminates any static friction that would affect the reading.

Dead Weight Testers.

1 - Handpump

2 - Testing Pump

3 - Pressure Gauge to be calibrated

4 - Calibration Weight

5 - Weight Support

6 - Piston

7 - Cylinder

8 - Filling Connection

Dead weight testers are a piston-cylinder type measuring device. As primary standards, they are

the most accurate instruments for the calibration of electronic or mechanical pressure measuring

instruments.

They work in accordance with the basic principle that P= F/A, where the pressure (P) acts on a

known area of a sealed piston (A), generating a force (F). The force of this piston is then

compared with the force applied by calibrated weights. The use of high quality materials result in

small uncertainties of measurement and excellent long term stability.

Dead weight testers can measure pressures of up to 10,000 bar, attaining accuracies of between

0.005% and 0.1% although most applications lie within 1 - 2500 bar. The pistons are partly made

of tungsten carbide (used for its small temperature coefficient), and the cylinders must fit together

with a clearance of no more than a couple of micrometers in order to create a minimum friction

thus limiting the measuring error. The piston is then rotated during measurements to further

minimise friction.

The testing pump (2) is connected to the instrument to be tested(3), to the actual measuring

component and to the filling socket. A special hydraulic oil or gas such as compressed air or

nitrogen is used as the pressure transfer medium. The measuring piston is then loaded with

calibrated weights (4). The pressure is applied via an integrated pump (1) or, if an external

pressure supply is available, via control valves in order to generate a pressure until the loaded

measuring piston (6) rises and 'floats' on the fluid. This is the point where there is a balance

between pressure and the mass load. The piston is rotated to reduce friction as far as possible.


Since the piston is spinning, it exerts a pressure that can be calculated by application of a

derivative of the formula P = F/A.

The accuracy of a pressure balance is characterised by the deviation span, which is the sum of the

systematic error and the uncertainties of measurement.

Today's dead weight testers are highly accurate and complex and can make sophisticated physical

compensations. They can also come accompanied by an intelligent calibrator unit which can

register all critical ambient parameters and automatically correct them in real time making

readings even more accurate.

UNIT 4

TEMPERATURE MEASUREMENT

AIM:

To have an idea about the temperature standards, calibration and signalconditioning used in RTD’s

KEY WORDS:

Thermometers- Filled in thermometers- Bimetallic thermometers- RTDS- thermistors


UNIT-IV

TEMPERATURE MEASUREMENT

Temperature measurement:

Measurement of the hotness of a body relative to a standard scale. The fundamental scale of

temperature is the thermodynamic scale, which can be derived from any equation expressing the

second law of thermodynamics. Efforts to approximate the thermodynamic scale as closely as

possible depend on relating measurements of temperature-dependent physical properties of

systems to thermodynamic relations expressed by statistical thermodynamic equations, thus in

general linking temperature to the average kinetic energy of the measured system. Temperature-

measuring devices, thermometers, are systems with properties that change with temperature in a

simple, predictable, reproducible

In the establishment of a useful standard scale, assigned temperature values of thermodynamic

equilibrium fixed points are agreed upon by an international body (General Conference of

Weights and Measures), which updates the scale about once every 20 years. Thermometers for

interpolating between fixed points and methods for realizing the fixed points are prescribed,

providing a scheme for calibrating thermometers used in science and industry.

The scale now in use is the International Temperature Scale of 1990 (ITS-90). Its unit is the

kelvin, K, arbitrarily defined as 1/273.16 of the thermodynamic temperature T of the triple point

of water (where liquid, solid, and vapor coexist). For temperatures above 273.15 K, it is common

to use International Celsius Temperatures, t90 (rather than International Kelvin

Temperatures, T90), having the unit degree Celsius, with symbol °C. The degree Celsius has the

same magnitude as the kelvin. Temperatures, t90, are defined as t90/°C = T90/K - 273.15, that is, as

differences from the ice-point temperature at 273.15 K. The ice point is the state in which the

liquid and solid phases of water coexist at a pressure of 1 atm (101,325 pascals). [The Fahrenheit

scale, with symbol °F, still in common use in the United States, is given by tF/°F = (t90/°C × 1.8)

+ 32, ortF/°F = (T90/K × 1.8) - 459.67.] The ITS-90 is defined by 17 fixed points.

Primary thermometers are devices which relate the thermodynamic temperature to statistical

mechanical formulation. The fixed points of ITS-90 are all based on one or more types of gas

thermometry or on spectral radiation pyrometry referenced to gas thermometry. Secondary

thermometers are used as reference standards in the laboratory because primary thermometers are

often too cumbersome. It is necessary to establish standard secondary thermometers referenced to

one or more fixed points for interpolation between fixed points. Lower-order thermometers are


used for most practical purposes and, when high accuracy is required, can usually be calibrated

against reference standards maintained at laboratories, such as the U.S. National Institute of

Standards and Technology, or against portable reference devices (sealed boiling or melting point

cells).

Primary and secondary thermometers

Thermometers can be divided into two separate groups according to the level of knowledge about

the physical basis of the underlying thermodynamic laws and quantities. For primary

thermometers the measured property of matter is known so well that temperature can be

calculated without any unknown quantities. Examples of these are thermometers based on the

equation of state of a gas, on the velocity of sound in a gas, on the thermal noise (seeJohnson–

Nyquist noise) voltage or current of an electrical resistor, and on the angular anisotropy of gamma

ray emission of certain radioactive nuclei in amagnetic field. Primary thermometers are relatively

complex.

Secondary thermometers are most widely used because of their convenience. Also, they are

often much more sensitive than primary ones. For secondary thermometers knowledge of the

measured property is not sufficient to allow direct calculation of temperature. They have to be

calibrated against a primary thermometer at least at one temperature or at a number of fixed

temperatures. Such fixed points, for example, triple points and superconducting transitions, occur

reproducibly at the same temperat

C alibration of thermometers:

Thermometers can be calibrated either by comparing them with other calibrated thermometers or

by checking them against known fixed points on the temperature scale. The best known of these

fixed points are the melting and boiling points of pure water. (Note that the boiling point of water

varies with pressure, so this must be controlled.)

The traditional method of putting a scale on a liquid-in-glass or liquid-in-metal thermometer was

in three stages:

1. Immerse the sensing portion in a stirred mixture of pure ice and water at 1 Standard

atmosphere (101.325 kPa ; 760.0 mmHg) and mark the point indicated when it had come

to thermal equilibrium.

2. Immerse the sensing portion in a steam bath at 1 Standard atmosphere (101.325 kPa ;

760.0 mmHg) and again mark the point indicated.

3. Divide the distance between these marks into equal portions according to the temperature

scale being used.

Other fixed points were used in the past are the body temperature (of a healthy adult male) which

was originally used by Fahrenheit as his upper fixed point (96 °F (36 °C) to be a number divisible

by 12) and the lowest temperature given by a mixture of salt and ice, which was originally the

definition of 0 °F (−18 °C).[14]

(This is an example of a Frigorific mixture). As body temperature

varies, the Fahrenheit scale was later changed to use an upper fixed point of boiling water at

212 °F (100 °C).[15]

These have now been replaced by the defining points in the International Temperature Scale of

1990, though in practice the melting point of water is more commonly used than its triple point,

the latter being more difficult to manage and thus restricted to critical standard measurement.

Nowadays manufacturers will often use a thermostatbath or solid block where the temperature is

held constant relative to a calibrated thermometer. Other thermometers to be calibrated are put

into the same bath or block and allowed to come to equilibrium, then the scale marked, or any

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deviation from the instrument scale recorded.[16]

For many modern devices calibration will be

stating some value to be used in processing an electronic signal to convert it to a temperature.

Precision, accuracy, and reproducibility:

The "Boyce MotoMeter" radiator cap on a 1913 Car-Nation automobile, used to measure

temperature of vapor in 1910s and 1920s cars.

The precision or resolution of a thermometer is simply to what fraction of a degree it is possible

to make a reading. For high temperature work it may only be possible to measure to the nearest

10°C or more. Clinical thermometers and many electronic thermometers are usually readable to

0.1°C. Special instruments can give readings to one thousandth of a degree. However, this

precision does not mean the reading is true or accurate.

Thermometers which are calibrated to known fixed points (e.g. 0 and 100°C) will

be accurate (i.e. will give a true reading) at those points. Most thermometers are originally

calibrated to a constant-volume gas thermometer.[citation needed]

In between a process

of interpolation is used, generally a linear one.[16]

This may give significant differences between

different types of thermometer at points far away from the fixed points. For example the

expansion of mercury in a glass thermometer is slightly different from the change in resistance of

a platinum resistance of the thermometer, so these will disagree slightly at around 50°C.[17]

There

may be other causes due to imperfections in the instrument, e.g. in a liquid-in-glass thermometer

if the capillary varies in diameter.[17]

For many purposes reproducibility is important. That is, does the same thermometer give the

same reading for the same temperature (or do replacement or multiple thermometers give the

same reading)? Reproducible temperature measurement means that comparisons are valid in

scientific experiments and industrial processes are consistent. Thus if the same type of

thermometer is calibrated in the same way its readings will be valid even if it is slightly

inaccurate compared to the absolute scale.

An example of a reference thermometer used to check others to industrial standards would be a

platinum resistance thermometer with a digital display to 0.1°C (its precision) which has been

calibrated at 5 points against national standards (-18, 0, 40, 70, 100°C) and which is certified to

an accuracy of ±0.2°C.[18]

According to a British Standard, correctly calibrated, used and maintained liquid-in-glass

thermometers can achieve a measurement uncertainty of ±0.01°C in the range 0 to 100°C, and a

larger uncertainty outside this range: ±0.05°C up to 200 or down to -40°C, ±0.2°C up to 450 or

down to -80°C.[19]

Temperature Measurement: Filled-System Thermometers:

Many physical properties change with temperature, such as the volume ofa liquid, the length of a

metal rod, the electrical resistance of a wire, thepressure of a gas kept at constant volume, and the

volume of a gas kept atconstant pressure. Filled-system thermometers use the phenomenon of

thermal expansion of matter to measure temperature change.

The filled thermal device consists of a primary element that takes the formof a reservoir or bulb, a

flexible capillary tube, and a hollow Bourdon tubethat actuates a signal-transmitting device and/or

a local indicating temperaturedial. A typical filled-system thermometer is shown in Figure .In this

system, the filling fluid, either liquid or gas, expands as temperatureincreases. This causes the

Bourdon tube to uncoil and indicate thetemperature on a calibrated dial.


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The filling or transmitting medium is a vapor, a gas, mercury, or another liquid. The liquid-filled

system is the most common because it requires a bulb with the smallest volume or permits a

smaller instrument to be used.The gas-filled system uses the perfect gas law, which states the

following for an ideal gas:

T = kPV

where:

T = temperature

k = constant

P = pressure

V = volume

If the volume of gas in the measuring instrument is kept constant, then the ratio of the gas

pressure and temperature is constant, so that

The only restrictions on Equation are that the temperature must be expressed in degrees Kelvin

and the pressure must be in absolute units.

Different types of Filled in thermometers:

1.Gas filled thermometers

2.Liquid filled thermometers

3.Mercury filled thermometers

4.Vapour pressure Thermometers

Sources of errors in filled system:

1.Ambient temperature effect:


The change of temperature causes volume changes in the capillary tube and the bourdon tube

therby causing error in measurement.

2.Head or elevation effect:

If the thermometer bulb is placed at a different height with respect to the bourdon tube, elevation

errors are produced

3.Barometric effect:

The effect due to change in the atmospheric pressure is known as the barometric effect.

4.Immersion effect:

If the bulb is not properly immersed or fully immersed and the head of the bulb is lost due to

conduction through the not properly insulated heat from the bulb is lost due to conduction

through the extension neck and thermal well.This causes what is known as immersion error.

5.Radiation effect:

radiation error occurs due to temperature difference between the bulb and other solid bodies

around.

Bimetallic Strip Thermometers

Bulb thermometers are good for measuring temperature accurately, but they are harder to use

when the goal is to control the temperature. The bimetallic strip thermometer, because it is made

of metal, is good at controlling things.

The principle behind a bimetallic strip thermometer relies on the fact that different metals

expand at different rates as they warm up. By bonding two different metals together, you can

make a simple electric controller that can withstand fairly high temperatures. This sort of

controller is often found in ovens. Here is the general layout:

Two metals make up the bimetallic strip (hence the name). In this diagram, the green metal would

be chosen to expand faster than the blue metal if the device were being used in an oven. In a

refrigerator, you would use the opposite setup, so that as the temperature rises the blue metal

expands faster than the green metal. This causes the strip to bend upward, making contact so that

current can flow. By adjusting the size of the gap between the strip and the contact, you control

the temperature.

You will often find long bimetallic strips coiled into spirals. This is the typical layout of a

backyard dial thermometer. By coiling a very long strip it becomes much more sensitive to small

http://home.howstuffworks.com/refrigerator.htm


temperature changes. In a furnace thermostat, the same technique is used and a mercury switch

is attached to the coil. The switch turns the furnace on and off. `

Electrical methods Of Temperature Measurement:

Thermistors.:

A thermistor is a type of resistor whose resistance varies significantly(more than in standard

resistors) with temperature. The word is a portmanteau of thermal and resistor . Thermistors are

widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors,

and self-regulating heating elements.

Thermistors differ from resistance temperature detectors (RTD) in that the material used in a

thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature

response is also different; RTDs are useful over larger temperature ranges, while thermistors

typically achieve a higher precision within a limited temperature range [usually −90 °C to 130

°C].

Thermistor symbol

Assuming, as a first-order approximation, that the relationship between resistance and

temperature is linear, then:

where

= change in resistance

= change in temperature

= first-order temperature coefficient of resistance

Thermistors can be classified into two types, depending on the sign of . If is positive,

the resistance increases with increasing temperature, and the device is called a positive

temperature coefficient (PTC) thermistor, or posistor. If is negative, the resistance

decreases with increasing temperature, and the device is called a negative temperature

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coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a as

close to zero as possible(smallest possible k), so that their resistance remains nearly constant

over a wide temperature range.

Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance

(alpha) or is used. It is defined as [1]

For example, for the common PT100 sensor, or 0.385 %/°C. This

coefficient should not be confused with the parameter below.

1. Steinhart-Hart equation

2. B parameter equation

3. Conduction model

4. Self-heating effects

5. Applications

1. Steinhart-Hart equation

In practice, the linear approximation (above) works only over a small temperature range.

For accurate temperature measurements, the resistance/temperature curve of the device

must be described in more detail. The Steinhart-Hart equation is a widely used third-order

approximation:

where a, b and c are called the Steinhart-Hart parameters, and must be specified for each

device. T is the temperature in kelvins and R is the resistance in ohms. To give resistance

as a function of temperature, the above can be rearranged into:

where

and

The error in the Steinhart-Hart equation is generally less than 0.02 °C in the

measurement of temperature[citation needed]

. As an example, typical values for a thermistor

with a resistance of 3000 Ω at room temperature (25 °C = 298.15 K) are:

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2. B parameter equation

NTC thermistors can also be characterised with the B parameter equation, which is

essentially the Steinhart Hart equation with ,

and ,

where the temperatures are in kelvins and R0 is the resistance at temperature T0

(usually 25 °C = 298.15 K). Solving for R yields:

or, alternatively,

where . This can be solved for the temperature:

The B-parameter equation can also be written as . This can be used to

convert the function of resistance vs. temperature of a thermistor into a linear function of

vs. . The average slope of this function will then yield an estimate of the value of the

B parameter.

3. Conduction model

Many NTC thermistors are made from a pressed disc or cast chip of a semiconductor such as a

sintered metal oxide. They work because raising the temperature of a semiconductor increases the

number of electrons able to move about and carry charge - it promotes them into the conduction

band. The more charge carriers that are available, the more current a material can conduct. This is

described in the formula:

= electric current (amperes) = density of charge carriers (count/m³)

= cross-sectional area of the material (m²)

= velocity of charge carriers (m/s)

= charge of an electron ( coulomb)


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The current is measured using an ammeter. Over large changes in temperature, calibration is

necessary. Over small changes in temperature, if the right semiconductor is used, the resistance of

the material is linearly proportional to the temperature. There are many different semiconducting

thermistors with a range from about 0.01 kelvin to 2,000 kelvins (−273.14 °C to 1,700 °C).

Most PTC thermistors are of the "switching" type, which means that their resistance rises

suddenly at a certain critical temperature. The devices are made of a doped polycrystalline

ceramic containing barium titanate (BaTiO3) and other compounds. The dielectric constant of this

ferroelectric material varies with temperature. Below the Curie point temperature, the high

dielectric constant prevents the formation of potential barriers between the crystal grains, leading

to a low resistance. In this region the device has a small negative temperature coefficient. At the

Curie point temperature, the dielectric constant drops sufficiently to allow the formation of

potential barriers at the grain boundaries, and the resistance increases sharply. At even higher

temperatures, the material reverts to NTC behaviour. The equations used for modeling this

behaviour were derived by W. Heywang and G. H. Jonker in the 1960s.

Another type of PTC thermistor is the polymer PTC, which is sold under brand names such as

"Polyswitch" "Semifuse", and "Multifuse". This consists of a slice of plastic with carbon grains

embedded in it. When the plastic is cool, the carbon grains are all in contact with each other,

forming a conductive path through the device. When the plastic heats up, it expands, forcing the

carbon grains apart, and causing the resistance of the device to rise rapidly. Like the BaTiO3

thermistor, this device has a highly nonlinear resistance/temperature response and is used for

switching, not for proportional temperature measurement.

Yet another type of thermistor is a silistor, a thermally sensitive silicon resistor. Silistors are

similarly constructed and operate on the same principles as other thermistors, but employ silicon

as the semiconductive component material.

4. Self-heating effects

When a current flows through a thermistor, it will generate heat which will raise the temperature

of the thermistor above that of its environment. If the thermistor is being used to measure the

temperature of the environment, this electrical heating may introduce a significant error if a

correction is not made. Alternatively, this effect itself can be exploited. It can, for example, make

a sensitive air-flow device employed in a sailplane rate-of-climb instrument, the electronic

variometer, or serve as a timer for a relay as was formerly done in telephone exchanges.

The electrical power input to the thermistor is just:

where I is current and V is the voltage drop across the thermistor. This power is converted to heat,

and this heat energy is transferred to the surrounding environment. The rate of transfer is well

described by Newton's law of cooling:

where T(R) is the temperature of the thermistor as a function of its resistance R, is the

temperature of the surroundings, and K is the dissipation constant, usually expressed in units of

milliwatts per degree Celsius. At equilibrium, the two rates must be equal.

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The current and voltage across the thermistor will depend on the particular circuit configuration.

As a simple example, if the voltage across the thermistor is held fixed, then by Ohm's Law we

have and the equilibrium equation can be solved for the ambient temperature as a

function of the measured resistance of the thermistor:

The dissipation constant is a measure of the thermal connection of the thermistor to its

surroundings. It is generally given for the thermistor in still air, and in well-stirred oil. Typical

values for a small glass bead thermistor are 1.5 mW/°C in still air and 6.0 mW/°C in stirred oil. If

the temperature of the environment is known beforehand, then a thermistor may be used to

measure the value of the dissipation constant. For example, the thermistor may be used as a flow

rate sensor, since the dissipation constant increases with the rate of flow of a fluid past the

thermistor.

5. Applications

PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for

fuses. Current through the device causes a small amount of resistive heating. If the current is large

enough to generate more heat than the device can lose to its surroundings, the device heats up,

causing its resistance to increase, and therefore causing even more heating. This creates a self-

reinforcing effect that drives the resistance upwards, reducing the current and voltage available to

the device.

PTC thermistors are used as timers in the degaussing coil circuit of CRT displays and televisions.

When the unit is initially switched on, current flows through the thermistor and degauss coil. The

coil and thermistor are intentionally sized so that the current flow will heat the thermistor to the

point that the degauss coil shuts off in under a second.

NTC thermistors are used as resistance thermometers in low-temperature measurements of the

order of 10 K.

NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They

present a higher resistance initially which prevents large currents from flowing at turn-on, and

then heat up and become much lower resistance to allow higher current flow during normal

operation. These thermistors are usually much larger than measuring type thermistors, and are

purposely designed for this application.

NTC thermistors are regularly used in automotive applications. For example, they monitor things

like coolant temperature and/or oil temperature inside the engine and provide data to the ECU

and, indirectly, to the dashboard. They can be also used to monitor temperature of an incubator.

Thermistors are also commonly used in modern digital thermostats and to monitor the

temperature of battery packs while charging.

Resistive Temperature Detectors (RTD).

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Another type of electrical resistance temperature sensor is the Resistance Temperature Detector

or RTD. RTD's are precision temperature sensors made from high-purity conducting metals such

as platinum, copper or nickel wound into a coil and whose electrical resistance changes as a

function of temperature, similar to that of the thermistor. Also available are thin-film RTD's.

These devices have a thin film of platinum paste is deposited onto a white ceramic substrate.

RTD

Resistive temperature detectors have positive temperature coefficients (PTC) but unlike the

thermistor their output is extremely linear producing very accurate measurements of temperature.

However, they have poor sensitivity, that is a change in temperature only produces a very small

output change for example, 1Ω/oC. The more common types of RTD's are made from platinum

and are called Platinum Resistance Thermometer or PRT's with the most commonly available

of them all the Pt100 sensor, which has a standard resistance value of 100Ω at 0oC. However,

Platinum is expensive and one of the main disadvantages of this type of device is its cost.

Like the thermistor, RTD's are passive resistive devices and by passing a constant current through

the temperature sensor it is possible to obtain an output voltage that increases linearly with

temperature. A typical RTD has a base resistance of about 100Ω at 0oC, increasing to about 140Ω

at 100oC with an operating temperature range of between -200 to +600

oC.

Because the RTD is a resistive device, we need to pass a current through them and monitor the

resulting voltage. However, any variation in resistance due to self heat of the resistive wires as the

current flows through it, I2R, (Ohms Law) causes an error in the readings. To avoid this, the RTD

is usually connected into a Whetstone Bridge network which has additional connecting wires for

lead-compensation and/or connection to a constant current source.

Signal conditioning of RTDs:

Several types of signal conditioning should be considered when using RTDs and thermistors, as

described below.

Current Excitation

Because RTDs and thermistors are restive devices, your data acquisition (DAQ) system must

provide a current excitation source to measure a voltage across the device. This current source

must be constant and precise.

2, 3, and 4-Wire Configurations (RTDs only)

RTDs come in 2, 3, and 4-wire configurations. Therefore, your system must handle support

whatever type of RTD you choose. (Thermistors are typically 2-wire devices because they have

higher resistance characteristics, thus eliminating lead resistance considerations.)

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Linearization

Both the RTD and themistor output voltage is not linear with temperature. Therefore, your system

must perform linearization either in hardware or software.

An Overview of RTD Lead Wire Compensation

Lead Wire Resistance

RTDs are resistive devices, so lead wire resistance directly affects its accuracy.The error can be

quite large, depending on the lead wire resistance (measured in ohms / foot).For example, an

uncompensated 2-wire circuit using 30 gauge wires can have an error as high as 1.2°F per

foot!Fortunately, there is a method to compensate for the lead wire resistance.

2-wire:One lead wire is connected to each end of the element.This arrangement is suitable for

uses where the lead wire resistance may be considered as a constant in the circuit, or where

changes in the lead wire resistance due to ambient temperature changes can be ignored.

3-wire:This is the most common of RTD configurations.One lead wire is connected to one end of

the element and two lead wires are connected to the other end.The purpose of the third lead is to

compensate for the lead wire resistance, thereby increasing accuracy.An instrument capable of

utilizing a 3- wire RTD must be used to benefit from this configuration.

4-wire:The most accurate of the RTD configurations, this element uses two wires for each end of

the element. Building on the 3-wire concept, compensation is made for the resistance in each lead

wire, creating a highly-accurate temperature-measurement device for critical applications.An

instrument capable of utilizing a 4-wire RTD must be used to benefit from this configuration.

To use an RTD, a small voltage is passed through the element and then measured. The resistance of the element reduces the voltage and this voltage drop can be converted into

a temperature measurement. With most RTD’s, the higher the temperature, the higher the

resistance. The following diagram represents a simple 2-wire RTD circuit. An instrument

is hooked to one red wire and sends a voltage thru that red wire, through the element and

back thru the other red wire. This reading is then converted to a temperature by the

instrument. The only problem with this simple 2-wire circuit is that you read the

resistance of the lead wire along with the resistance of the element. There is no way to

separate the three resistances.

Resistance of circuit = 5 + 100 + 5 = 110 ohms


The 3-wire circuit does allow for compensation of lead wire resistance, which is

normally

done by the measuring instrument. The instrument measures the resistance between the

red and the white leads and then subtracts the resistance between the two reds.

5 (w) + 100 + 5 (r) = 110 – (5 (w) + 5 (r)) = 100

The problem with the 3-wire circuit is that the formula assumes that all three wires are

the same resistance. This is not a problem on short lead wire lengths but can become a

problem as the length of the extension lead wires increases. The 4-wire circuit is a true 4-

wire bridge circuit that eliminates any differences in lead resistances.

The 4-wire bridge circuit eliminates lead wires resistance electrically instead of

mathematically.


UNIT 5

THERMOCOUPLES AND RADIATION PYROMETERS

AIM:

To have a sound knowledge about thermocouples and pyrometrytechniques

KEY WORDS:

Thermocouple- cold junction compensation- Radiation methods- pyrometers- fibre optic method

of temperature measurement-


UNIT-5

THERMOCOUPLES AND RADIATION PYROMETERS

One of the most common industrial thermometer is the thermocouple. It was discovered by

Thomas Seebeck's in 1822. He noted that a voltage difference appeared when the wire was heated

at one end. Regardless of temperature, if both ends were at the same temperature there was no

voltage difference. If the circuit were made with wire of the same material there was no current

flow.

A thermocouple consists of two dissimilar metals, joined together at one end, and produce a small

unique voltage at a given temperature. This voltage is measured and interpreted by a

thermocouple thermometer.

The thermoelectric voltage resulting from the temperature difference from one end of the wire to

the other is actually the sum of all the voltage differences along the wire from end to end

Thermocouples can be made from a variety of metals and cover a temperature range 200 o

C to

2,600 oC. Comparing thermocouples to other types of sensors should be made in terms of the

tolerance given in ASTM E 230.

Base metal thermocouples

Thermocouple

Maximum Temperature (oC)

Continuous Spot

Copper-Constantan 400 500

Iron-Constantan 850 1,100

Chromel-Constantan 700 1,000

Chromel-Alumel 1,100 1,300

Nicrosil-Nisil 1,250 -

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Tungsten-Molybdenum* 2,600 2,650

* Not used below 1250 oC.

Advantages with thermocouples

Capable of being used to directly measure temperatures up to 2600 oC.

The thermocouple junction may be grounded and brought into direct contact with the material

being measured.

Disadvantages with thermocouples

Temperature measurement with a thermocouple requires two temperatures be measured, the

junction at the work end (the hot junction) and the junction where wires meet the instrumentation

copper wires (cold junction). To avoid error the cold junction temperature is in general

compensated in the electronic instruments by measuring the temperature at the terminal block

using with a semiconductor, thermistor, or RTD.

Thermocouples operation are relatively complex with potential sources of error. The materials of

which thermocouple wires are made are not inert and the thermoelectric voltage developed along

the length of the thermocouple wire may be influenced by corrosion etc.

The relationship between the process temperature and the thermocouple signal (millivolt) is not

linear.

The calibration of the thermocouple should be carried out while it is in use by comparing it to a

nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration

bath, the output integrated over the length is not reproduced exactly.

Thermocouple Types

Thermocouples are available in different combinations of metals or calibrations. The four most

common calibrations are J, K, T and E. Each calibration has a different temperature range and

environment, although the maximum temperature varies with the diameter of the wire used in the

thermocouple.

Some of the thermocouple types have standardized with calibration tables, color codes and

assigned letter-designations. The ASTM Standard E230 provides all the specifications for most of

the common industrial grades, including letter designation, color codes (USA only), suggested

use limits and the complete voltage versus temperature tables for cold junctions maintained at 32

oF and 0

oC.

There are four "classes" of thermocouples:

The home body class (called base metal),

the upper crust class (called rare metal or precious metal),

the rarified class (refractory metals) and,

the exotic class (standards and developmental devices).

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The home bodies are the Types E, J, K, N and T. The upper crust are types B, S, and R, platinum

all to varying percentages. The exotic class includes several tungsten alloy thermocouples usually

designated as Type W (something).

Instrument

Temperature

Range

Accuracy

Recommended

(oF)

Maximum

(oF)

Type J probes 32 to 1336 -310 to 1832

1.8 to 7.9oF or 0.4% of

reading above 32oF,

whichever is greater

Type K probes 32 to 2300 -418 to 2507

1.8 to 7.9oF or 0.4% of

reading above 32oF,


Type T probes -299 to 700 -418 to752

0.9 to 3.6oF or 0.4% of

reading above 32oF,


Type E probes 32 to 1600 32 to 1650

1.8 to 7.9oF or 0.4% of

reading above 32oF,


Type R probes 32 to 2700 32 to 3210

2.5oF or 0.25% of

reading, whichever is

greater

Type S probes 32 to 2700 32 to 3210

2.5oF or 0.25% of

reading, whichever is

greater

Temperature Conversions

oF = (1.8 x

oC) + 32

oC = (

oF - 32) x 0.555

Kelvin = oC + 273.2


oRankin =

oF + 459.67

A thermocouple is a junction between two different metals that produces a voltage related to a

temperature difference. Thermocouples are a widely used type of temperature sensor for

measurement and control [1]

and can also be used to convert heat into electric power. They are

inexpensive [2]

and interchangeable, are supplied fitted with standard connectors, and can measure

a wide range of temperatures. The main limitation is accuracy: system errors of less than one

degree Celsius (C) can be difficult to achieve. [3]

Any junction of dissimilar metals will produce an electric potential related to temperature.

Thermocouples for practical measurement of temperature are junctions of specific alloys which

have a predictable and repeatable relationship between temperature and voltage. Different alloys

are used for different temperature ranges. Properties such as resistance to corrosion may also be

important when choosing a type of thermocouple. Where the measurement point is far from the

measuring instrument, the intermediate connection can be made by extension wires which are less

costly than the materials used to make the sensor. Thermocouples are usually standardized against

a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of

cold-junction compensation to adjust for varying temperature at the instrument terminals.

Electronic instruments can also compensate for the varying characteristics of the thermocouple,

and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature

measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Laws for thermocouple

Law of homogeneous material

A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the

application of heat alone, regardless of how it might vary in cross section. In other words,

temperature changes in the wiring between the input and output do not affect the output voltage,

provided all wires are made of the same materials as the thermocouple.

Law of intermediate materials

The algebraic sum of the thermoelectric emfs in a circuit composed of any number of dissimilar

materials is zero if all of the junctions are at a uniform temperature. So If a third metal is inserted

in either wire and if the two new junctions are at the same temperature, there will be no net

voltage generated by the new metal.

. Law of successive or intermediate temperatures

If two dissimilar homogeneous materials produce thermal emf1 when the junctions are at T1 and

T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the

junctions are at T1 and T3 will be emf1 + emf2

SIGNAL CONDITIONING OF THERMOCOUPLE OUTPUT:

Difficulties Measuring with Thermocouples

It is not easy to transform the voltage generated by a thermocouple into an accurate temperature

reading for many reasons: the voltage signal is small, the temperature-voltage relationship is

nonlinear, reference junction compensation is required, and thermocouples may pose grounding

problems. Let's consider these issues one by one.

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Voltage signal is small: The most common thermocouple types are J, K, and T. At room

temperature, their voltage varies at 52 µV/°C, 41 µV/°C, and 41 µV/°C, respectively. Other less-

common types have an even smaller voltage change with temperature. This small signal requires

a high gain stage before analog-to-digital conversion. Table 1 compares sensitivities of various

thermocouple types.

Voltage Change vs. Temperature Rise

(Seebeck Coefficient) for Various Thermocouple Types at 25°C.

Thermocoup

le Type

Seebeck Coefficient

(µV/°C)

E 61

J 52

K 41

N 27

R 9

S 6

T 41

Because the voltage signal is small, the signal-conditioning circuitry typically requires gains of

about 100 or so—fairly straightforward signal conditioning. What can be more difficult is

distinguishing the actual signal from the noise picked up on the thermocouple leads.

Thermocouple leads are long and often run through electrically noisy environments. The noise

picked up on the leads can easily overwhelm the tiny thermocouple signal.

Two approaches are commonly combined to extract the signal from the noise. The first is to use a

differential-input amplifier, such as an instrumentation amplifier, to amplify the signal. Because

much of the noise appears on both wires (common-mode), measuring differentially eliminates it.

The second is low-pass filtering, which removes out-of-band noise. The low-pass filter should

remove both radio-frequency interference (above 1 MHz) that may cause rectification in the

amplifier and 50 Hz/60 Hz (power-supply) hum. It is important to place the filter for radio

frequency interference ahead of the amplifier (or use an amplifier with filtered inputs). The

location of the 50-Hz/60-Hz filter is often not critical—it can be combined with the RFI filter,

placed between the amplifier and ADC, incorporated as part of a sigma-delta ADC, or it can be

programmed in software as an averaging filter.

Reference junction compensation: The temperature of the thermocouple's reference junction

must be known to get an accurate absolute-temperature reading. When thermocouples were first

used, this was done by keeping the reference junction in an ice bath. Figure 2 depicts a

thermocouple circuit with one end at an unknown temperature and the other end in an ice bath


(0°C). This method was used to exhaustively characterize the various thermocouple types, thus

almost all thermocouple tables use 0°C as the reference temperature.

Basic iron-constantan thermocouple circuit.

But keeping the reference junction of the thermocouple in an ice bath is not practical for most

measurement systems. Instead most systems use a technique called reference-junction

compensation, (also known as cold-junction compensation). The reference junction temperature is

measured with another temperature-sensitive device—typically an IC, thermistor, diode, or RTD

(resistance temperature-detector). The thermocouple voltage reading is then compensated to

reflect the reference junction temperature. It is important that the reference junction be read as

accurately as possible—with an accurate temperature sensor kept at the same temperature as the

reference junction. Any error in reading the reference junction temperature will show up directly

in the final thermocouple reading.

A variety of sensors are available for measuring the reference temperature:

1. Thermistors: They have fast response and a small package; but they require linearization

and have limited accuracy, especially over a wide temperature range. They also require

current for excitation, which can produce self-heating, leading to drift. Overall system

accuracy, when combined with signal conditioning, can be poor.

2. Resistance temperature-detectors (RTDs): RTDs are accurate, stable, and reasonably

linear, however, package size and cost restrict their use to process-control applications.

3. Remote thermal diodes: A diode is used to sense the temperature near the thermocouple

connector. A conditioning chip converts the diode voltage, which is proportional to

temperature, to an analog or digital output. Its accuracy is limited to about ±1°C.

4. Integrated temperature sensor: An integrated temperature sensor, a standalone IC that

senses the temperature locally, should be carefully mounted close to the reference

junction, and can combine reference junction compensation and signal conditioning.

Accuracies to within small fractions of 1°C can be achieved.

Compensation,signal conditioning, and respose of thermocouple

Voltage signal is nonlinear: The slope of a thermocouple response curve changes over

temperature. For example, at 0°C a T-type thermocouple output changes at 39 µV/°C, but at

100°C, the slope increases to 47 µV/°C.

There are three common ways to compensate for the nonlinearity of the thermocouple.

Choose a portion of the curve that is relatively flat and approximate the slope as linear in this

region—an approach that works especially well for measurements over a limited temperature

range. No complicated computations are needed. One of the reasons the K- and J-type

thermocouples are popular is that they both have large stretches of temperature for which the

incremental slope of the sensitivity (Seebeck coefficient) remains fairly constant.


Variation of thermocouple sensitivity with temperature. Note that K-type's Seebeck

coefficient is roughly constant at about 41 µV/°C from 0°C to 1000°C.

Another approach is to store in memory a lookup table that matches each of a set of thermocouple

voltages to its respective temperature. Then use linear interpolation between the two closest

points in the table to get other temperature values.

A third approach is to use higher order equations that model the behavior of the thermocouple.

While this method is the most accurate, it is also the most computationally intensive. There are

two sets of equations for each thermocouple. One set converts temperature to thermocouple

voltage (useful for reference junction compensation). The other set converts thermocouple

voltage to temperature. Thermocouple tables and the higher order thermocouple equations can be

found at http://srdata.nist.gov/its90/main/. The tables and equations are all based on a reference

junction temperature of 0°C. Reference-junction compensation must be used if the reference-

junction is at any other temperature.

Grounding requirements: Thermocouple manufacturers make thermocouples with both

insulated and grounded tips for the measurement junction (Figure 4).

Thermocouple measurement junction types.

The thermocouple signal conditioning should be designed so as to avoid ground loops when

measuring a grounded thermocouple, yet also have a path for the amplifier input bias currents

when measuring an insulated thermocouple. In addition, if the thermocouple tip is grounded, the

amplifier input range should be designed to handle any differences in ground potential between

the thermocouple tip and the measurement system ground (Figure 5).

http://srdata.nist.gov/its90/main/


Grounding options when using different tip types.

For nonisolated systems, a dual-supply signal-conditioning system will typically be more robust

for grounded tip and exposed tip types. Because of its wide common-mode input range, a dual-

supply amplifier can handle a large voltage differential between the PCB (printed-circuit board)

ground and the ground at the thermocouple tip. Single-supply systems can work satisfactorily in

all three tip cases if the amplifier's common-mode range has some ability to measure below

ground in the single-supply configuration. To deal with the common-mode limitation in some

single-supply systems, biasing the thermocouple to a midscale voltage is useful. This works well

for insulated thermocouple tips, or if the overall measurement system is isolated. However, it is

not recommended for nonisolated systems that are designed to measure grounded or exposed

thermocouples.

Practical thermocouple solutions: Thermocouple signal conditioning is more complex than that

of other temperature measurement systems. The time required for the design and debugging of the

signal conditioning can increase a product's time to market. Errors in the signal conditioning,

especially in the reference junction compensation section, can lead to lower accuracy. The

following two solutions address these concerns.

The first details a simple analog integrated hardware solution combining direct thermocouple

measurement with reference junction compensation using a single IC. The second solution details

a software-based reference-junction compensation scheme providing improved accuracy for the

thermocouple measurement and the flexibility to use many types of thermocouples.

Measurement Solution 1: Optimized for Simplicity

Figure 6 shows a schematic for measuring a K-type thermocouple. It is based on using the

AD8495 thermocouple amplifier, which is designed specifically to measure K-type

thermocouples. This analog solution is optimized for minimum design time: It has a

straightforward signal chain and requires no software coding.

http://www.analog.com/en/sensors/analog-temperature-sensors/ad8495/products/product.html


Measurement solution 1: optimized for simplicity.

How does this simple signal chain address the signal conditioning requirements for K-type

thermocouples?

Gain and output scale factor: The small thermocouple signal is amplified by the AD8495's gain

of 122, resulting in a 5-mV/°C output signal sensitivity (200°C/V).

Noise reduction: High-frequency common-mode and differential noise are removed by the

external RFI filter. Low frequency common-mode noise is rejected by the AD8495's

instrumentation amplifier. Any remaining noise is addressed by the external post filter.

Reference junction compensation: The AD8495, which includes a temperature sensor to

compensate for changes in ambient temperature, must be placed near the reference junction to

maintain both at the same temperature for accurate reference-junction compensation.

Nonlinearity correction: The AD8495 is calibrated to give a 5 mV/°C output on the linear

portion of the K-type thermocouple curve, with less than 2°C of linearity error in the –25°C to

+400°C temperature range. If temperatures beyond this range are needed, Analog Devices

Application Note AN-1087 describes how a lookup table or equation could be used in a

microprocessor to extend the temperature range.

Handling insulated, grounded, and exposed thermocouples: Figure 5 shows a 1-MΩ resistor

connected to ground, which allows for all thermocouple tip types. The AD8495 was specifically

designed to be able to measure a few hundred millivolts below ground when used with a single

supply as shown. If a larger ground differential is expected, the AD8495 can also be operated

with dual supplies.

More about the AD8495: Figure 7 shows a block diagram of the AD8495 thermocouple

amplifier. Amplifiers A1, A2, and A3—and the resistors shown—form an instrumentation

amplifier that amplifies the K-type thermocouple's output with a gain appropriate to produce an

output voltage of 5 mV/°C. Inside the box labeled "Ref junction compensation" is an ambient

temperature sensor. With the measurement junction temperature held constant, the differential

voltage from the thermocouple will decrease if the reference junction temperature rises for any

reason. If the tiny (3.2 mm × 3.2 mm × 1.2 mm) AD8495 is in close thermal proximity to the

reference junction, the reference-junction compensation circuitry injects additional voltage into

the amplifier, so that the output voltage stays constant, thus compensating for the reference

temperature change.

http://www.analog.com/static/imported-files/application_notes/AN-1087.PDF


AD8495 functional block diagram.

summarizes the performance of the integrated hardware solution using the AD8495:

Solution 1 (Figure 6) Performance Summary

Thermocou

ple Type

Measurement

Junction

Temperature

Range

Reference

Junction

Temperature

Range

Accuracy

at 25°C

Power

Consump

tion

K –25°C to +400°C 0°C to 50°C

±3°C (A

grade)

±1°C (C

grade)

1.25 mW

Measurement Solution 2: Optimized for Accuracy and Flexibility

Figure 8 shows a schematic for measuring a J-, K-, or T-type thermocouple with a high degree of

accuracy. This circuit includes a high-precision ADC to measure the small-signal thermocouple

voltage and a high-accuracy temperature sensor to measure the reference junction temperature.

Both devices are controlled using an SPI interface from an external microcontroller.

Measurement solution 2: Optimized for accuracy and flexibility.

How does this configuration address the signal conditioning requirements mentioned earlier?

Remove noise and amplify voltage: The AD7793, shown in detail in Figure 9—a high-

precision, low-power analog front end—is used to measure the thermocouple voltage. The

http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7793/products/product.html


thermocouple output is filtered externally and connects to a set of differential inputs, AIN1(+)

and AIN1(–). The signal is then routed through a multiplexer, a buffer, and an instrumentation

amplifier—which amplifies the small thermocouple signal—and to an ADC, which converts the

signal to digital.

AD7793 functional block diagram.

Compensate for reference junction temperature: The ADT7320 (detailed in Figure 10), if

placed close enough to the reference junction, can measure the reference-junction temperature

accurately, to ±0.2°C, from –10°C to +85°C. An on-chip temperature sensor generates a voltage

proportional to absolute temperature, which is compared to an internal voltage reference and

applied to a precision digital modulator. The digitized result from the modulator updates a 16-bit

temperature value register. The temperature value register can then be read back from a

microcontroller, using an SPI interface, and combined with the temperature reading from the

ADC to effect the compensation.

ADT7320 functional block diagram.

Correct nonlinearity: The ADT7320 provides excellent linearity over its entire rated

temperature range (–40°C to +125°C), requiring no correction or calibration by the user. Its

digital output can thus be considered an accurate representation of the reference-junction state.

To determine the actual thermocouple temperature, this reference temperature measurement must

be converted into an equivalent thermoelectric voltage using equations provided by the National

Institute of Standards and Technology (NIST). This voltage then gets added to the thermocouple

voltage measured by the AD7793; and the summation is then translated back into a thermocouple

temperature, again using NIST equations.

http://www.analog.com/en/temperature-sensing-and-thermal-management/digital-temperature-sensors/adt7320/products/product.html


Handle insulated and grounded thermocouples: Figure 8 shows a thermocouple with an

exposed tip. This provides the best response time, but the same configuration could also be used

with an insulated-tip thermocouple.

Table 3 summarizes the performance of the software-based reference-junction measurement

solution, using NIST data:

Solution 2 Performance Summary

Thermocou

ple Type

Measurement

Junction

Temperature

Range

Reference

Junction

Temperature

Range

Accuracy

Power

Consump

tion

J, K, T Full Range –10°C to +85°C

–20°C to +105°C

±0.2°C

±0.25°C

3 mW

3 mW

Conclusion

Thermocouples offer robust temperature measurement over a quite wide temperature range, but

they are often not a first choice for temperature measurement because of the required trade-offs

between design time and accuracy. This article proposes cost-effective ways of resolving these

concerns.

The first solution concentrates on reducing the complexity of the measurement by means of a

hardware-based analog reference junction compensation technique. It results in a straightforward

signal chain with no software programming required, relying on the integration provided by the

AD8495 thermocouple amplifier, which produces a 5-mV/°C output signal that can be fed into

the analog input of a wide variety of microcontrollers.

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