Temperature Sensors Survey
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Common Temperature Measurement Sensors
This page surveys the most common temperature measurement sensor types.
Thermocouples | RTDs | Thermistors | Infrared | Solid State | Bimetallic
Thermocouple
These sensors depend on differences in charge mobility in two dissimilar metals to produce a potential difference. The developed potentials are small, but measurable.
Thermocouples are rugged, and tolerate a wide range of temperatures, but they have many drawbacks, including low signal levels, long-term stability, and noise. Their response time tends to be very slow, but this often results from the packaging more than the device physics.
The measurement is actually temperature difference, not an absolute temperature level, so a supplementary measurement is required to establish the reference temperature. The resulting temperature correction is called cold junction compensation.
Despite their limitations, thermocouples are used with success in a variety of applications. You can obtain moderate accuracy using standard device curves with no calibration. Careful calibration can improve accuracy to within a degree C or so.
RTD
RTDs, short for "resistive thermal devices," are built basically the same way as wire-wound or thin-film resistors, but using materials with relatively high levels of resistivity variation as a function of temperature. They are generally between thermocouples and thermistors in terms of speed, ruggedness, signal level and temperature range.
If not severely stressed, they have good long term stability.
They cost more, but offer superior linearity and good accuracy using standard curves without calibration. With individual device calibration, the accuracy is even better.
Though most common metals exhibit RTD effects, platinum alloys have the best range and performance, and are by far the most popular.
Thermistor
Negative temperature coefficient thermistors, the ones commonly used for temperature measurements, change resistance dramatically in response to temperature changes, but they have some limitations.
They are vulnerable to physical damage and chemical contamination — for example, water can cause problems. They require careful protective packaging, and this often limits how they can be used. Response is moderately fast, but typically limited because of the packaging. The thermal characteristics are highly nonlinear, so you must apply corrections that differ for every device type. For full accuracy you must calibrate. They have a limited range compared to other thermal sensors.
For the temperatures where liquid water can exist, thermistors usually perform very well.
Infrared
These sensors include special optical and electrical components to detect infrared blackbody radiation from a very specific location, without direct physical contact.
These sensors are very useful for measuring extreme temperatures through a viewing port, under conditions that would rapidly destroy other sensor types.
The accuracy, stability, and repeatability are not very good, so they need a lot of attention.
The receiver/converter electronics scale the signal to convenient levels for acquisition and recording.
Solid state
These devices use the thermal properties of semiconductor junction voltages to detect temperature.
These devices are appealing because sensor, power-gain amplifiers, and "linearization" can be fabricated on the same chip.
The drawbacks are that the operating range of the electronics limits the sensing range, and any calibration other than offset adjustment is impractical.
These are extremely handy for measuring ambient temperatures around circuit boards, and popular for "cold junction compensation" in combination with thermocouples. Accuracy is moderate.
Bimetallic
We mention this one only in passing. A sensor bonded to a bimetallic strip senses strain in proportion to temperature changes. This is too bulky, slow, and vulnerable to mechanical and electrical interference for most applications, but compatible.