Lecture Temp Sensors

8/6/2019 Lecture Temp Sensors

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Instrumentation:Types OfTemperature sensors

Instrumentation and Process Control

(Credit hr 4)


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Basic Types OF Temperature Measuring

Devices 1) Thermocouples

2) RTDs

3) Thermistors

4) Infrared Thermometry

How to Choose

Standards, cost, accuracy, stability, sensitivity, size, contact/non-

contact, temperature range, fluid type


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1) Thermocouples

Based on Seebeck effect(In 1821 existance of

thermoelectric circuit due to electromagnetic effects

of metal studies)

If two wires of dissimilar metals are joined at both ends andone end is heated, current will flow. The joined end is called

junction. Measuring junction in process and reference

junction to compensate the difference in voltage.

If the circuit is broken, there will be an open circuit voltage

across the wires.

Voltage is a function of temperature and metal types.

For small (Ts, the relationship with temperature is linear

For larger(Ts, non-linearities may occur

V TE!


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Measuring the Thermocouple Voltage If you attach the thermocouple directly to a voltmeter, you will

have problems.

You have just created another junction! Your displayed voltagewill be proportional to the difference between J

1and J2 (and

hence T1 and T2). Note that this is Type T thermocouple.


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External Reference Junction

A solution is to put J2 in an ice-bath; then you know

T2, and your output voltage will be proportional to

T1-T2.


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Software Compensation

How can we find the temperature of the reference

junction? Use a thermister or RTD.

Once the temperature is known, the voltage

associated with that temperature can be subtractedoff.

Then why use thermocouples at all(Advantages)

Thermocouples are cheaper, smaller, more flexible and

rugged, and operate over a wider temperature range. Most data acquisition systems have software

compensation built in(controllers). To use Labview.


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Hardware Compensation

With hardware compensation, the temperature of the

isothermal block again is measured, and then a

battery is used to cancel out the voltage of thereference junction.

This is also called an electronic ice point reference.

With this reference, you can use a normal voltmeter

instead of a thermocouple reader. You need a

separate ice-point reference for every type ofthermocouple.


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Thermocouple Types

If you do your own

calibration, you can

usually improve on the

listed uncertainties.


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Thermocouple Types, cont.

Type B very poor below 50C; reference junction temperaturenot important since voltage output is about the same from 0 to42 C

Type E good for low temperatures since dV/dT (E) is high forlow temperatures

Type J cheap because one wire is iron; high sensitivity butalso high uncertainty (iron impurities cause inaccuracy)

Type T good accuracy but low max temperature (400 C); onelead is copper, making connections easier; watch for heat being

conducted along the copper wire, changing your surface temp Type K popular type since it has decent accuracy and a wide

temperature range; some instability (drift) over time

Type N most stable over time when exposed to elevatedtemperatures for long periods


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2) RTDs (Resistance Temperature

Detectors)(eg PT-100) : Resistivity of metals is a function of temperature. When the

temperature of a wire changes the resistance to flow of currentalso changes.(early work of Faraday)

Platinum often used since it can be used for a wide temperature

range. It has lower temp coefficient than Ni but stable and highresistance for corrosion and oxidation (-190 to 660 oC).Nickel ornickel alloys are cheap, but they arent as accurate.(eg PT-100gives 100 oms resistance at O0C).

Rt=Ro(1+at).

Where Rt=Resistance at any temp toC

,Ro=Resistance at O0C, a=Temp coefficient

Thin Film RTDs are new. To make these, a platinum or metal-glass slurry film is deposited onto a ceramic substrate. Thesubstrate is then etched with a laser. These RTDs are verysmall but arent as stable (and hence accurate).


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RTD geometry

From Nicholas & White, Traceable Temperatures.

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

Metal sheath can cause contamination at high temperatures andare best below 250C.

At very high temperatures, quartz and high-purity alumina are

best to prevent contamination.


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Resistance/Temperature Conversion

For very accurate results, do your own calibration.

Several electronic calibrators are available.

The most accurate calibration that you can do easily yourselfis to use a constant temperature bath and NIST-traceable

thermometers. You then can make your own calibration

curve correlating temperature and voltage.


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Accuracy and Response Time

Response time is longer than thermocouples; for a

sheath, response time can easily be 10 s.


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Potential Problems Related to RTDs and

Thermocouple RTDs are more fragile than thermocouples.

An external current must be supplied to the RTD. This current

can heat the RTD, altering the results. For situations with high

heat transfer coefficients, this error is small since the heat isdissipated to air. For small diameter thermocouples and still air

this error is the largest. Use the largest RTD possible and

smallest external current possible to minimize this error.

Be careful about the way you set up your measurement device.

Attaching it can change the voltage.


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4) Thermistors

Thermistors also measure the change in resistance with

temperature.

Thermistors are very sensitive (up to 100 times more than RTDs

and 1000 times more than thermocouples) and can detect very

small changes in temperature. They are also very fast. Due to their speed, they are used for precision temperature

control and any time very small temperature differences must be

detected.

They are made of ceramic semiconductor material (metal

oxides). The change in thermistor resistance with temperature is very

non-linear. Mostly used as safety device eg motor protection.


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Thermistor Non-Linearity


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Infrared Thermometry(Pyrometers) Infrared thermometers measure the amount of

radiation emitted by an object. It is based on Planks

Law which predicts very accurately the radiant power

emitted by a Black Body per unit area per unit wave

length or complete radiation. Peak magnitude is often in the infrared region.

Reflection from other objects can introduce error as

well. Surface whose temp youre measuring must fill the

field of view of your camera.


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Benefits of Infrared Thermometry

Can be used for

Moving objects

Non-contact applications

where sensors wouldaffect results or be

difficult to insert or

conditions are hazardous

Large distances

Very high temperatures

Eg blast furnace

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Lecture Temp Sensors