Industrial Temperature Measurement

Temperature

• Controlling temperature is one of the most common processes in industrial electronics and manufacturing.

• Manufacturing processes that are affected by temperature are referred to as thermal systems.

Temperature

• Temperature is a measure of the average kinetic energy of the particles that make up a body.-The greater the kinetic energy of the particles is, the higher the temperature of the body will be

• Temperature is the ability of one body to transfer thermal energy to another body- If two bodies are in thermal equilibrium and no thermal energy is exchanged, the bodies are at the same temperature

Temperature

• Molecular motion creates heat known as thermal energy

• Thermal movement from hot to cold is called thermodynamics

• Absolute zero (no molecular motion) means no heat is produced

Temperature scales & Units

There are 4 scales that can be used to measure temperature.

• Celsius/Fahrenheit units are used in the common everyday scales

• Kelvin/Rankine are used when working with the Absolute Temperature Scale ( these are typically used in engineering and research calculations)

Temperature Measurement

Like other process measurements, temperature measuring devices are divided into 3 general categories:indicatorssensors/transducers, switchestransmitters

The design and construction of these devices is based on how different materials react when subjected to heat and what type of measurement is required i.e. indication only, point measurement, analog output, etc.

Temperature devices are specified according to their construction and are classified as follows:

• Liquid Expansion devices• Bimetallic Elements• Electrical Sensors (thermistors, RTD,

thermocouples, semiconductor)• Optical pyrometers• Change-of-state Devices

Liquid in Glass Thermometer

Advantages

• Easy portability• Independence of auxiliary equipment• Low cost• Compatibility with most environments• Moderate ruggedness• Wide range (it has been use to measure

temperatures as low as 70K and as high as 1000°C, but it most frequent use is within -40°C to 250 °C)

Disadvantages

• a large sensing element• Impossibility for continuous automatic

readout,• Long time constant• Awkward dimensions, and hysteresis (except

for special types),• breakage(mercury contamination)

Filled Thermal SystemFilled thermometers, Gas Filled Thermometers

• Closed system, contains a gas or a volatile liquid and relies on pressure measurements to provide temperature indications.

• Various types are used but all have similar components and share the same principle of operation.

Closed System- no external energy required

• When temperature changes , fluid either expands or contracts, which caused Bourdon tube to move, thereby moving the position of the needle on the scale

Bimetallic Thermometers

• Based on the principle 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 temperature element is found in many mechanical temperature switches as well as indicators.

• Strips of metals with different thermal expansion coefficients are bonded together at the same temperature

• When the temperature increase, the assembly bends.

• When this happens, the metal strip together with the larger temperature coefficient of expansion expands more than the other strip.

• The angular position-vs-temperature relation is established by calibration so you can use the device as a thermometer

Bimetallic Thermometer

• The bimetallic strip can be wound as a helix and will twist when heated. This twisting action can be used to drive a pointer over a calibrated temperature scale. These temperature sensors are low cost and have accuracy ranges of between 2-5% and are mostly used for local readings. They are not suitable for providing a continuous output measurement

Change-of-State Temperature Measurement Devices

• Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers or liquid crystals whose appearance changes once a certain temperature is reached. They are used, for instance, with steam traps – when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black

• Response time typically takes minutes, so these devices often do not respond to transient temperature changes.

• Accuracy is lower than with other types of sensors.

• Change in state is irreversible, except in the case of liquid-crystal displays.

Radiation Temperature SensorsOptical Pyrometers

• In general terms these devices measure the amount of radiation emitted by a surface. Electromagnetic energy radiates from all matter regardless of its temperature. In many process situations, the energy is in the infrared region. The intensity of an object’s emitted IR energy increases in proportion to its temperature and measured as the target’s emissivity, that indicates an object’s temperature

Non Contact Measurement

Used in High Temperature Applications

• Radiation Temperature sensors can be used as hand-held local temperature devices or can be installed to provide a continuous signal. They are used in many industries where extremely high temperature and/or non-contact measurements are required. (Extrusion presses, rolling mills, strip annealing, Tank refractory’s, mold temperature, bottle machines, Kiln shell, and many more)

Electrical Temperature Sensors

• Resistance-Thermistors- Resistance Temperature Detectors (RTDs)

• Thermo-electric-Thermocouples

Thermistors

• Thermally sensitive resistors that change resistance with changes in temperature (in a predictable manner)

• They are highly sensitive and have very reproducible resistance vs temperature properties

• Typically used over a small temperature range, (compared to other temperature sensors) because of their non-linear characterstics

• Manufactured from oxides of nickel, manganese, iron, cobalt, magnesium, titanium and other metals

• They are epoxy or glass encapsulated, or bare bead, many of the standardized types are color coded.

Temperature vs Resistance Characteristics

• Most thermistors exhibit a negative temperature coefficient (NTC)

• Non-linear T vs R

Positive Temperature Coefficients

• Thermistors can also be made to have a PTC response

• Used more for current overload than temperature measurement

General Specifications (NTCs)

• usually specified by their resistance at room temperature-For example an NTC Thermistor T25 could have a resistance of 3.0kΩ, 5.0kΩ, 10.0kΩ at 25°C

• Accuracy is very good to average• Response time is fast to moderate• Typically used over small temperature ranges

Resistance Temperature Detector RTDs

• Change resistance in a linear relationship to the applied heat

• Very accurate temperature vs resistance characteristics and reproducible

• Excellent interchangeability and stability• Can be used ass a temperature standard

RTD Materials

• Platinum• Nickel• Copper

RTD Construction

• Wire wound• Coil• Hollow Annulus• Film

Wire wound

• The wire wound sensing element is built by winding a small diameter platinum sensing wire around a non-conducting mandrel

Classical construction, excellent interchangeability

Coil

• The coiled element sensor, made by inserting the helical sensing wires into a packed powder-filled insulating mandrel, provides a stain-free sensing element

Good Stability, rugged

Hollow Annulus

• The hollow annulus-type element is made by winding platinum sensing wire around a hollow corrosion-resistant metal mandrel. The entire unit is coated with an insulating material

Fastest response, most espensive

Film

• Film type sensing element is made by depositing a thin layer of platinum in a resistance pattern on a ceramic substrate. A layer of glass applied for protection

Newer design, easier to make, interchangeability is not the best

Temperature- Resistance Characteristics

• RTD’s are specified by their resistance at zero deg C and the material they are made of

Platinum (Pt)-Pt100 (100Ω@0°C) 0.4ohms/C-Pt1000 (1000Ω@0°C) 4ohms/C

-Nickel Ni120 (120Ω@0°C)-Copper Cu10 (10Ω@0°C)

RTD’s Temperature vs Resistance Charactersitics

Temperature Coefficient

• Temperature coefficient or alpha α is used by the manufacturers to standardize the RTD’s slope of TR curve

• The alpha describes the average resistance change per unit temperature from the ice point to the boiling point of water

• A Pt100 and Pt1000 have an α=0.00385

Wiring configuration

• RTDs are typically use with a bridge circuit

Wiring Configuration

• These bridge circuits are built into the transmitter, PLC, DCS, PID Controller etc

Effects of lead wire resistance

Assume the RTD is measuring 100°CR(PT100)= 138.5Ω RLEAD=20Ω

Total resistance measured will be 158.5Ω which is a temperature of 153°C

3- wire RTDs

RTD Specifications

• Platinum RTD’s can measure temperatures from -200°C to 650°C. (IEC says -200°C to 850°C)

• A “bare wire” RTD has a fast response time however the protective “sheaths” and “wells” slow down the response.

Example of Manufacturers Specs

• Time Constant of 2.2 sec or less• Temperature Range: -200 to +600°C• Pressure Range: Vacuum to +50,000 psia• Accuracy: ± 1ohm@0°C(±0.1% for 100ohm)• Reproducibility: ±0.1% of resistance for

100ohm element

Spring loaded RTD Thermowell

RTD color coding

RTDs Pros and Cons

Advantages• Linear• Stable output over a long period of time• Ease of recalibration• Accurate readings over narrow temperature spansDisadvantages (when compared to thermocouples)• Smaller overall temperature range (-330 to 930°F)• Higher initial cost• Fragile in rugged industrial environments

Thermocouples (TCs)

• A thermocouple consists of two pieces of dissimilar metals with their ends joined together (by twisting, soldering or welding)

• Based on Seebeck Effect which simply states that an electromotive force (emf) is created at the junction of 2 dissimilar metals when heated.

Seebeck Effect produces a mV

• When heat is applied to the junction of 2 dissimilar metals, a voltage , in the range of millivolts (mV), is generated at the open leads.

The mV per degree will depend on the combinations of metals used. Manufacturers produce a variety of combinations specified as “types” examples – Type T, J, K, E

Thermocouple Types

• Manufacturers have perfected a variety of metal combinations and specify them as “types”

Example -Type T, J, K, E• Each produces its own specific mV per degree, these values

are published in the TC cables• Each type has a different temperature range (Type T can only

measure up to 400°C, Type K 1300°C)• The types are color coded to make it easy to identify them in

the field.

Thermocouple Table

Reference Junction is 0°C

Measuring Temperatures- ExampleAccording to the type J table the meter should read 5.269mV when the TC is measuring a temperature of 100°C

But it reads 4.25mV instead

Because the meter leads form another junction which produces another emf equal to room temperature

The mV at the reference junction temperature must be added to the meter

Measuring & Reference JunctionType J(Iron/Constantan)

Reference Junction Compensation

• Ice Bath• Electronic Ice Point• Thermocouple Transmitters and Controllers

are internally compensated• Make sure to match TC types with the

equipment ( some instruments will allow several types but must be configured.

Ice Bath Compensation

Thermocouples Pros and ConsAdvantages• A wide temperature from -300 to 2300°F• Fast response time (under a second in some cases)• Low initial cost and durability• Thermocouples are able to withstand rugged applicationsDisadvantages• wide accuracy range, especially at elevated temperature• Difficult to recalibrate seeing though they are dependant upon

the environment, and• Installation can be very expensive if long lengths of

thermocouple wire are needed

Limitations of electrical thermometers

• Sensor cable’s resistance and its temperature dependency

• Junction resistances• Thermal voltages• Thermal noise in resistors• Measurement current• Non-linear temperature dependencies• Electrical perturbations• Inaccuracy at least ± 0.1 °C