Here is a brief discussion on 3 technologies available for temperature mesaurements in real time and intended for closed loop applications.
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| Fig. 01 Reference images of thermocouples, RTD's and thermsistors, respectively. |
It is possible that if you use a handheld temperature instrument you can not for sure say what sensor it is in use: a thermocouple, an RTD or a thermsistor. This does not mean you do not need to know which option choose among them. Thererfore, this post is intended for providing a aid for comparison among these technologies.
What is best for closed control loops?
For this case, thermocouples and RTD's are best rated. These two options have almost the same advantages and a few disadvantages. However, from the industrial point of view both are very good choices. For very precise measurements as in a lab or some scientific experiment in which high precision is required RTD's could be a better option because of its linearity.
In other words, in industry you will usually not need high precision of the order of decimals and the measurement range is not too large so that thermocouples or RTD's can virtually do the same job.
When would you use a thermsistor?
On the other hand, thermsistors are more commonly used for thermal switches. For example, these can be found installed in small and large electric motors for protection against undesireble increments of temperature.
However, there are some hand-held temperature instruments using thermsistors such as immersion thermometers which may represent another option against thermocouples and RTD's.
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| Fig. 02 An immersion thermometer based on a thermsistor |
The models, as the one shown in Fig. 02, are not too expensive and precision may not be capital. The common practice is that thermsistors are used for soldering on electronic boards rather than as probe for a closed control loop.
One further comment: it should be rare to find a PID controller for industrial application accepting a thermsistor as an input.
Thermocouples vs RTD's
Here is a list of non repeated advantages of each sensor that could be helpful in a complicated application.
Thermocuples
- are commonly used for very high temperatures. For example, above 400 C and more,
- are cheaper than RTD's. A thermocouple sensor could be three or twice cheaper than an RTD,
- in demanding applications, thermocouples provide faster response than RTD's (about two or three times). This faster response is noticeable at machine level.
RTD's
- are commonly used for low temperatures. For example, below 400 C,
- may have cheaper installation since extension wires may be of copper rather than a special alloy,
- are more precise than thermocouples. RTD's precision is usually about 0.1 C while for thermocouples it is about 1 C. Several factors may affect this,
- have linear relationship between electrical resistance and temperature. This feature makes RTD's a better candidate for large ranges of measurements. Thermocouples have non linear voltage-temperature relationship so that these are better recommended for short ranges of measurements,
- have better response to oxidation. Thermocouple wires may be affected by oxidation of the alloys.
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| Fig. 03 Plots of linearity of RTD's and thermocouples response. It can be seen that for RTD's it is lnear while for thermocouples it is non linear. |
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