Which temperature sensor do I need? An RTD or a thermocouple?

Temperature is probably the most measured parameter across applications and industries we work with on a daily basis. But there’s still confusion as to what temperature sensor to use.

I very often hear the Lesman inside sales team talking a customer through the selection process, so I thought I’d provide a quick tutorial on temperature sensor selection.

What’s an RTD?

A resistance temperature detector works by tracking the linear change in electrical resistance of a pure metal — like nickel, copper, or platinum — that’s caused by a change in temperature. An RTD element consists of a length of fine coiled wire wrapped around a ceramic or glass core. It is fragile, and usually protected by a sheathed probe. Since platinum has a wide temperature range, and produces highly accurate and stable measurements, it’s the most common RTD material.

Pros: High accuracy, stability, repeatability, and long life

Cons: Cost, slow response time, narrow temperature range (not good in very low or very high temperatures), and more expensive to purchase

What’s a Thermocouple?

This was one of the first lessons I taught our marketing specialist, when she was building her first Lesman catalog. “Okay, so break it down. Thermo = something to do with temperature. Couple = two of something. Ready for the next part?”

A thermocouple consist of two different electrically conductive metal alloys. Together, they produce a voltage that’s proportional to the temperature difference between either end of the pair of conductors. Combinations of specific alloys produce a predictable difference between temperature and voltage (e.g., Type J thermocouples, made of copper and constantan, have a range of 32° to 559°F with a standard error range of ±4°).

Pros: Inexpensive, cover a wide range of temperatures (-328° to 3092°F), have a fast response time, and are durable in applications where there’s high vibration

Cons: Not as accurate as RTDs, signal stability degrades over time, and can require frequent replacement, especially in high temperatures. Subject to “cold junction error”, a degradation of the millivolt signal that varies based on the ambient temperature where the measuring device is installed.

Which One Do I Need?

So, with a very basic understanding of how these sensors work, we turn our attention to your application and your specific needs.

Decision Factor #1: Process Temperature

Less than 400°F
Either an RTD or thermocouple will work. Go to Decision Factor #2.
Between 400° and 932°F
You’ll most likely want a thermocouple. Go to Decision Factor #2 to verify.
Greater than 932°F
For high temperature applications, you’ll need a thermocouple. RTDs don’t function at these high temperatures.

Decision Factor #2: Accuracy

±0.01% or ±0.1%
Only an RTD will let you achieve this level of accuracy
±1% or wider
If accuracy isn’t critical, or you need a more rugged, durable sensor for high vibration applications, a thermocouple is the way to go.

Which One Do I Have Now?

So, you’re replacing a temperature sensor, and it’s not clear which one you have. Here’s how to tell.

Though they do come in two-wire and four-wire varieties, most RTDs are three-wired sensors, where two wires are one color and the third is another. The problem is that no two RTD manufacturers follow the same standard.

Thermocouples are two-wire devices, and follow an industry standard for wire colors. The chart below can help you identify the thermocouple type you have.

The chart comes from page 489 of the Lesman catalog’s reference section, along with standard wiring diagrams for thermocouples and transmitters, thermocouple material tolerances, and more.

If you need more information, a while back, I explained the differences between accuracy, repeatability, and stability. Check it out in “I’m not getting the readings I expect to see.”

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This entry was posted on February 9, 2012, 10:54 am and is filed under Burner Control, Combustion, Cost Issues, Flame Safety, Installation, Measurement, RTDs, Temperature, Thermocouples. You can follow any responses to this entry through RSS 2.0. You can leave a response, or trackback from your own site.

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