Temperature tutorial: Thermocouple vs. RTD vs. thermistor

One of the potentially challenging choices you may have to face is selecting a temperature measuring device. Like many things in the process industries, you have a range of options and there are tradeoffs that you need to balance. The three main technologies you have available from which to choose are thermocouples , RTDs , and thermistors . Each has its particular advantages, drawbacks, and application niches. But if you choose carefully from current offerings, you may not need to pay any attention to the specific technology.

Temperature sensors have many specifications, but the basics are:

Measuring range;

Stability;

Accuracy;

Sensitivity; and

Response time.

If you know what these need to be for your application, they can be the basis for your decision and you won’t have to worry about measurement technology. When you find a device that does what you want and has a transmitter to convert the measurement into 4-20 mA or digital output, there is probably little reason for you to concern yourself with whether it’s a thermocouple or RTD.

However, if you’re a stick shift, change your own oil type and you like to hardwire sensors, understanding the fine points of temperature devices can give you a challenge.

Thermocouples are available in countless configurations. They indicate temperature by providing a very small voltage signal generated by a junction of dissimilar metals. There are at least eight calibration groups, expressed as letters (B, E, J, K, R, S, and T) that have their own characteristics and ranges. For example, B type thermocouples typically have the highest limits at 3000+ °F.

Thermocouples have a number of key strengths:

Wide variety of measuring ranges, including very high limits;

Many physical sizes and configurations;

Fast response times;

Tiny measuring point;

Moderate price; and

Very simple configuration (You can even make your own!)

However there are drawbacks:

Medium accuracy and sensitivity;

Linearity is only fair;

Specific types have to have matching cable (e.g., type K thermocouple has to have type K cable.); and

Signal strength is very low and prone to EMI problems

The last two drawbacks can be solved by using an appropriate transmitter that can convert the signal to something more robust (e.g. 4-20 mA) at the sensor. Moreover, sophisticated electronics can mitigate some of the linearity issues. But without these, you could have a temperamental device that sends a temperature spike to the I/O device if a worker turns on his walkie-talkie too close to the sensor.

RTDs (resistance temperature detector) use the fact that some metals (usually platinum) increase their electrical resistance as they get hotter. To measure the change, the sensor output is fed into a Wheatstone bridge with a reference voltage.

RTDs have characteristics that compare well against thermocouples:

More stable;

More accurate;

Greater repeatability;

Better sensitivity and linearity; and

More robust signal less prone to EMI problems (although can still benefit from a transmitter).

Other RTD attributes don’t compare as well against thermocouples:

Narrower measuring range, particularly at the high end;

More expensive;

Require an external power source;

Slower response time; and

At some temperatures, the reference voltage can actually heat the sensor and throw it off.

Thermistors act in much the same way as RTDs, but use a semiconductor device to change resistance rather than a metallic element. They have their own characteristics in the group:

Narrowest measuring range, by far;

Lowest stability and linearity;

Accuracy and response time comparable to thermocouples;

Highest sensitivity;

Least expensive; and

Robust signal.

If the application isn’t too critical and within the temperature limitations, thermistors can be an appropriate choice.

For most practical applications below 1000 °F where accuracy matters, an RTD is usually the best approach. When the sensor is combined with a transmitter at the device, any problematic characteristics (other than cost) can be mitigated if not eliminated entirely.

Virtually all instrumentation manufacturers make some type of temperature device using one of the technologies mentioned. You can search on the controleng.com site using the Control Engineering Buyer’s Guide.

— Edited by Peter Welander , process industries editor Control Engineering Instrumentation & Sensors eNewsletter( Register here and scroll down to select your choice of eNewsletters free.)

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