Basics of Temperature Measurement

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Major Terms used in Temperature measurement

Thermocouple

Measure temperature by correlating the the voltage differential between the junction of two different metal alloys and a reference voltage to temperature

RTD

Measures temperature by correlating the resistance of a resistive element with the temperature

Temperature Transmitter

Converts the low-level signal generated by one of these temperature sensor types and converts it into a higher power level (4-20 ma) signal that can be transmitted over long distance

What is thermocouple ?

Most popular type of temperature sensor. Can measure wide range of temperatures Are interchangeable and have standard connectors Two thin metal wires welded together to form a junction. Almost any type of metal can be used.There are preferred metal used for their predictable output voltages and ability to withstand large temperature gradient.

How Thermocouple works ?

The junction between two metals generates a voltage that is a function of temperature.This is know as thermoelectric or seebeck effect.

The picture shows two different metal alloy leads welded together to form a thermocouple juction.

The differential is measured and compared with the known voltage/temperature relationship to determine the temperature of the environment being measured.

It is not possible to simple measure the voltage differential directly and the metal leads of a voltmeter would create an addition thermocouple junction.

A thermocouple output also nonlinear complex polynomial equation.

Types of Thermocouple.

Thermocouple are made from different metal alloy pairs which all have different characteristics such as temperature range and accuracy.

Thermcouple comes as either:

Bare wire bead which is low cost Built in to probes such as needle,insulated,catheter,capsule,direct immersion,surface mount, and micro thermocouple.

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Different types of Thermocouples are

Type K Type E Type N Type J Type B Type R Type S

Type K Thermocouple

General purpose TC Low cost and highly available in wide array of probes Available in 200 deg C to 1200 deg C Sensitivity is 41μV/°C

Type E Thermocouple

Has a higher output (68μV/°C) which makes it well suited for low temperature (cryogenic) use

Type N Thermocouple

Suitable for high temperature measurement at lower cost than platinum TC(type B R S)

High stability and resistant to high temperature oxidation

Can withstand temperature above 1200 deg C

Type J Thermocouple

Less popular than type K

Used with older equipment that cannot accept modern thermocouples

Limited range of -40 deg C to 750 deg C

Sensitivity is 52μV/°C

Type B

Platinum TCs suitable for high temperature measurement only>1600 deg C

Type R

High cost and low sensitivity make them not suitable for general purpose use

Type S

Type B TCs due to shape of their temp / V curve give the same output at 0 deg C and 42 deg C making them useless below 50 deg C

Consideration of Using thermocouple

1.Connection problem

Measurement errors can be caused by unintentional thermocouple junctions. If longer length thermocouple leads are needed,use correct type of thermocouple extension wire

2.Lead resistance

TC wires are very thin and have high resistance and therefor can experience signal noise or errors due to the input impedance of measuring device.TC leads should therefor be kept short.If long leads are needed,use proper thermocouple extension wire,which is thicker.

3.Noise

The millivolt output of a thermocouple is very prone to signal noise. Interference can be minimized by twisting both wires together to ensure they both pickup the same noise.

4.De-calibration

Thermocouple wire makeup can alter over time at the extreme of operating temperature.Make sure probe insulation is sufficient for operating environment.Element oxidation and corrosion can also effect calibration and function

5.Thermal shunting

The mass of thermocouple mounting location nad self heating/cooling may affect the ability to read an accurate temperature.Consider high mass probed when range rate is critical.Be aware of heat dissipation issues when full-immersion is not possible

## Thermocouple transmitter calibration

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Advantages and Disadvantages of Thermocouple

Advantages -

Popular in most temperature measurement applications Low cost Robust and resistant to shock and vibration Wide temperature range Simple to manufacture Require no excitation power No self heating Can be made very small

Disadvantages of thermocouple

Produce relatively low,non-linear output signal Requires a sensitive and stable measuring device Low signal level,so very noise susceptible