Lecture 3 In The Name of Allah - Ali...

Lecture 3

Dr. Ali Karimpour Mar 2015

Instrumentation

In The Name of Allah

Dr. Ali Karimpour

Associate Professor

Ferdowsi University of Mashhad

Lecture 3


2

Temperature measurement

Topics to be covered include:

v Resistive Thermometers

v Thermistor Thermometers

v Thermocouple Thermometers

v Semiconductor Junction Thermometers

v Bimaterials Thermometers (Bimetal)

v Pyroelectric(Infrared) Thermometers

v Fiber-Optic Thermometers

v …….

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3

Resistive Thermometers

v PTC

Positive Temperature Coefficient.

v NTC

Negative Temperature Coefficient.

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4


Sensitivity is defined as the amount of resistance change of the sensor

per degree of temperature change.

Nickel is very good for sensitivity.

However, have a limited temperature

range,

Copper has a very linear resistance-to-

temperature relationship. However,

copper oxidizes at moderate

temperatures and cannot be used above

150°C.

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5


Platinum is the best metal for RTD

elements.

• It follows a very linear resistance-

to-temperature relationship.

• It follows its resistance-to-

temperature relationship in a highly

repeatable manner over its

temperature range.

• It has the widest temperature range

among the metals used to make RTDs.

• Platinum is not the most sensitive metal; however, it is the metal

that offers the best long-term stability.

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6


Wire-wound RTD.

Thin film RTD.

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7


Industrial Platinum Resistance Thermometers (IPRTs) are designed to

withstand industrial environments and are almost as durable as

thermocouples.

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8

Linearity (least square method)

bauyii T

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iii

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ˆ

b

aui

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1

Calibration

b

a

u

u

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YA YAAA TT

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9


Calibration

...)()()(1 3

03

2

02010 TTTTTTRR

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10


Calibration

Exercise 3-1: Resistance of an RTD at 60°C is 106Ω. The resistance

Variation with time is:

a) Derive a first order model for system and find FSO%(Suppose

113.6 is full scale).

b) Derive a second order model for system and find FSO %(Suppose

113.6 is full scale).

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PTC100.

• It is often from platinum.

• Its resistance at 0°C is 100 Ω.

• Its sensitivity is 0.00385

Example 3-1: What is the resistive

of PTC100 at 300°C ?

215.5 Ω.

Platinum RTD usually used in -200°C800°C

Nickel RTD usually used in -100°C260°C

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12

Thermistor Thermometers

A thermistor is a thermally sensitive resistor whose primary function is to

exhibit a change in electric resistance with a change in body temperature.

Unlike a wire-wound or metal film resistance temperature detector

(RTD), a thermistor is a ceramic semiconductor.

Thermistors have a large temperature coefficient compare to RTD but

it is nonlinear.

Thermistors usually used in -90°C130°C

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13


Calibration

)(1010

TTRR

RTD

Thermistor

)11

(

0

0TT

eRR

10R

dT

dR

1

0

dTR

dRIts sensitivity is 0.00385

RTdT

dR2

0

2

0

TRdT

dR Its sensitivity is -0.045

Calibration

T: Degrees Kelvin

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Calibration

Calibration curve of Thermistor by

Steinhart-Hart equation:

3)(lnln1

RCRBAT

T: Degrees Kelvin

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Calibration

Exercise 3-2: TR91 Series 10KΩ@25°C Thermistor calibration table

is:

a) Derive β for TR91 and find maximum error in ohms.

LSM. use and ofln derive :Hint)

11(

0

0TT

eRR

b) Derive A, B and C for Steinhart-Hart equation of TR91 and find

maximum error in ohms.

LSM. use :Hint

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16

RTD and Thermistor Thermometers

Suitable circuits to convert R V

Voltage divider. Voltage divider with follower.

DD

SEN

OUTV

RR

RV

1

1

Advantages:

• Simplicity

• Detection of open sensor (failure)

Disadvantages:

• Poor noise rejection

• Voltage is non-linear function of resistance

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17



Voltage divider by current source.

SENSENOUTIRV

Advantages:

• Linearity of resistance to voltage conversion.

• Robust in noise.

Disadvantages:

• Cost.

Two

wireFour

wire

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18



Wheatstone bridge

Two

wire

Three

wire

Two

wire

s

RTD

OU

RR

R

RR

RU

3

3

21

2

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19


RTDs and Thermistors self heating

The ability of the sensor to dissipate heat is defined by its dissipation

factor, δ, which has units of mW/°C.

The definition for δ is the amount of power that it takes to the

raise the body temperature of the sensing element 1°C.

Example 3-2: An RTD with R=500Ω at 20°C and sensitivity

coefficient 0.005 1/°C with dissipation factor δ=30 mw /°C let

R1=R2=500 Ω. Derive R3 for balance of bridge in 0°C. Us=10V

R3=450 Ω. In the case of no dissipation.

R3=454 Ω. By considering of dissipation.

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20

Thermocouple Thermometers

The Seebeck effect is the conversion of temperature differences

directly into electricity

(Thomas Johann Seebeck 1821)

For example, the Seebeck coefficients

of Cu is 1.5 μV/K, and of Al −1.5 μV/K.

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The Peltier effect is the presence of heating or cooling at an electrified

junction of two different conductors(Jean Charles Athanase Peltier

1834.)

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The Seebeck effect:

How to use?

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Seebeck Coefficients for Some Metals

and Alloys, Compared to Platinum.

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Properties of several different thermocouple types

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Two additional thermocouple:

External ice-point reference junction:

Thermocouple refrigeration method:

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Using two ovens to simulate ice-point reference:

Using a thermometer to sense the temperature:

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Scanning many thermocouples.

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Electronic ice-point compensation.

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The sensitivity of RTD is very small.

The thermistor’s output is quite nonlinear.

Resistive, thermistor and thermocouple thermometer

The RTD and thermistor’s

output is …...

The thermocouple output is a very

small signal that is nonlinear

with temperature.

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31

Semiconductor Junction Thermometers

A silicon temperature sensor 730 mV at –40°C and 300 mV at 150°C

The narrowly specified Vbe ranges between 580 mV -620 mV at 25°C.

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Linearity error (in mV) vs. temperature for a silicon temperature

sensor

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LM335 provides 10 mV/°K (A zener diode)

LM34 provides 10 mV/°F

AD592 provides 0.001 mA /°K

Or LM335 provides 2.73+10 mV/°C .

Output:

+10 mV/°C .

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LM34 provides 10 mV/°F

5 °F 50 mV

300 °F 3.00 V

-5 °F ??

-5 °F -50mV

300 °F 3.00 V

Sensors are not near to source??

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Sensors are not near to source??

We use two wires not three!

We transmit current instead of voltage!

IQ is 70 μA

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Most available thermometers

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Bimaterials Thermometers (Bimetal)

Bimetallic thermometers make use

of change of volume with temperature.

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Pyroelectric(Infrared) Thermometers

An infrared (IR) thermometer is a noncontact radiant energy detector.

IR thermometers capture the invisible infrared energy that is naturally

emitted from all objects warmer than absolute zero (0 K).

IR thermometers are ideal for moving targets such as glass, plastic, and

steel. They do not interfere with the process.

The TC or RTD will not respond to these temperature changes

quickly enough to permit close control.

The IR thermometer will instantly measure the actual product

temperature, not the environment surrounding the product.

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Distance does not affect the measurement. Models are available that

can measure from 0.3 to 91 m.

However, the farther away from the target the sensor is, the larger the

spot.

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The infrared thermometer must be carefully aimed to avoid focusing on

reflected solar radiation.

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Fiber-Optic Thermometers

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Photon Thermometers

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45

Some more thermometers

Temperature sensing by expansion

Temperature sensing by displacement

Temperature sensing

by pressure.

Lecture 3


System Design

46

Two-point control: (also called on–off control)

is the simplest type of closed-loop control strategy.

Inverting Amplifier

PVSPSPPVerror VVVR

RV

R

RV )(

2

3

2

3

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System Design

Hysteresis circuit

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System Design

Exercise 3-3: Consider following TR91 Series 10KΩ@25°C Thermistor

calibration table.

Design a circuit that turn on the fan of a car at 95°C and turn it off at

85°C.

Lecture 3


References

v Industrial Control Electronics J.M. Jacob, Prentice-Hall,

1989

v مبانی اندازه گیری در سیستمهای ابزار دقیق، حمید رضا تقی راد و سید

1392علی سالمتی، انتشارات دانشگاه صنعتی خواجه نصیرالدین طوسی،

v Principles and Practice of Automatic Process Control,

Third edition, C. A. Smith, A. Corripio, Wiley 2006

v Instrument Control Engineers' Handbook Fourth

Edition, Béla G. Lipták, editor-in-chief. CRC Press 2003

49

Lecture 3


Some Useful websites for the course

50

v http://saba.kntu.ac.ir/eecd/ecourses/instrumentation.htm

v http://profsite.um.ac.ir/~shoraka/Instrumentation.htm

v http://karimpor.profcms.um.ac.ir/index.php/courses/10328

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