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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