Overview of Temperature Measurement

NEIL VIJAY PUNIA

Temperature Measurement MethodMechanical or Non-Electric method Liquid-in-glass thermometer-Change in pressure Constant-volume gas thermometer Bimetallic ThermometerElectric method Resistance-Temperature Detectors Thermistors ThermocouplesRadiation Method Total radiation Pyrometer Selective Radiation Pyrometer Infrared(IR) Pyrometer

What Is ThermometerA device to measure temperature.A thermometer has two important elements:

• The temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature

• Some means of converting this physical change into a value.

Glass ThermometerMercury filled in a glass

tube and a glass bulb at the bottom.

As the temperature increases, the mercury rises in the glass tube.

The glass tube is calibrated in Celsius, Fahrenheit or both.

RTD

How it works:Utilizes the fact that

resistance of a metal changes with temperature.

Make up:Traditionally made up

of platinum, nickel, iron or copper wound around an insulator.

Temperature range:From about -196°C to

660°C.

Thin Film RTD

RTD geometry

Sheathing: stainless steel or iconel, glass, alumina, quartz.

Metal sheath can cause contamination at high temperatures and are best below 250ºC.

At very high temperatures, quartz and high-purity alumina are best to prevent contamination.

Resistance MeasurementSeveral different bridge circuits are used to

determine the resistance. Bridge circuits help improve the accuracy of the measurements significantly. Bridge output voltage is a function of the RTD resistance.

Types of RTD

Film thermometers Wire-wound thermometers

Coil elements

RTD Advantages and DisadvantagesAdvantages:

StableVery accurateChange in

resistance is linearWide temperature

operating range(-196°C to 660OC)

Good stability at high temperature

Disadvantages:ExpensiveCurrent source

requiredSmall change in

resistanceSelf heatingLess rugged than

thermocouples.Affected by shock

and vibration

ApplicationsApplications of RTD's includeAir conditioning and refrigeration servicing Food Processing Stoves and grills Textile production Plastics processing Petrochemical processing Micro electronics Air, gas and liquid temperature measurement Exhaust gas temperature measurement

Thermistor

How it works:Like the RTD a

thermistor uses the fact that resistance of a metal changes with temperature.

Make up:Generally made up of

semiconductor materials

Temperature Range:About -60°C to 200°C

Thermistor

Thermistor Non-Linearity

Thermistor Advantages and Disadvantages

Advantages:Very sensitive (has

the largest output change from input temperature)

Quick responseMore accurate than

RTD and Thermocouples

No 4-wire bridge is required as with an RTD.

Disadvantages:Output is a non-

linear functionLimited

temperature range.Require a current

sourceSelf heatingFragile

Thermocouple

How it works:Made up of two different

metals joined at one end to produce a small voltage at a given temperature.

Make up:Made of up two different

metals.Temperature Range

0°C to 750°C

A few Thermocouples

Circuit Diagram

49K

1K

1K

50K

1K

1K

50K

50K

-Vin+

+-

+-

+-

+Vout-

+

-Thermocouple

4.7μF

7417

1 2

5V 15V

Fan

Relay

Thermocouple Advantages and Disadvantages

Advantages:Self Powered (does

not require a current or voltage source)

RuggedInexpensiveSimple

Disadvantages:Extremely Low

Voltage output (mV)Not very stableNeeds a reference

point

Choice Between RTDs, Thermocouples, Thermistors Cost – thermocouples are cheapest by far, followed by RTDs Accuracy – RTDs or thermistor Sensitivity – thermistor Speed - thermistor Stability at high temperatures – not thermistor Size – thermocouples and thermistor can be made quite

small Temperature range – thermocouples have the highest

range, followed by RTDs Ruggedness – thermocouples are best if your system will be

taking a lot of abuse

PyrometerPyrometer derived from the Greek root pyro,

meaning fire.A Pyrometer, is a non-contact instrument

that detects an object's surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object.

Idea : Every object whose temperature is above absolute zero emits radiation.

Disappearing-filament pyrometer

Advantages of Pyrometers High temperatures measuremens Fast response No adverse effects on temperatures and

materials Measuring moving objects Measuring objects which are difficult to

access

Infrared ThermometerThey work by focusing infrared heat onto a

sensor that can convert infrared energy into temperature units

Detect IR radiation(λ=0.7 –1000 µm)Non-contact Wide range (30-4500 °C) Accuracy: 1% of reading

Infrared ThermometryInfrared thermometers measure the amount

of radiation emitted by an object.Peak magnitude is often in the infrared

region.Surface emissivity must be known. This can

add a lot of error.Reflection from other objects can introduce

error as well.Surface whose temp you’re measuring must

fill the field of view of your camera.

Benefits of Infrared ThermometerCan be used for

Moving objectsNon-contact

applications where sensors would affect results or be difficult to insert or conditions are hazardous

Large distancesVery high

temperatures