How to apply thermocouples
Although many sensors can measure temperature, the three most common are resistance temperature detectors (RTD), thermocouples, and thermistors. The thermocouple (TC) is the most common of those, and is made up of two dissimilar metals joined together at one end and open at the other (output end), producing a voltage for a given temperature.
Although many sensors can measure temperature, the three most common are resistance temperature detectors (RTD), thermocouples, and thermistors. The thermocouple (TC) is the most common of those, and is made up of two dissimilar metals joined together at one end and open at the other (output end), producing a voltage for a given temperature. A thermocouple's voltage signal increases as temperature rises.
TCs are ubiquitous for several reasons, principally their low cost. TCs are also extremely rugged. Generally they are simply a spot weld of two alloys at one end. However metal-sheathed TCs are available for harsh or corrosive environments.
TCs also come in a wide variety. Different alloys allow different ranges and sensitivity of measurement, including: J, K, T, E, R, S, B, and N. The most common types are J, K, and T.
Generally, TCs can cover a wide temperature range. For example, Type K spans -260oC to +1,370oC. The ranges of all types of thermocouples are listed in NIST's (National Institute of Standards and Technology) reference tables ( www.nist.gov ).
A notable characteristic of thermocouples is their non-linear voltage compared to temperature. Consequently, linearization is required.
The open-end signal is a function of not only closed-end temperature—the point of measurement—but also the temperature at the open end. Only by holding T2 at a standard temperature can the measured signal be considered a direct function of the change in T1. The industry standard for T2 is 0oC; most tables and charts accept this assumption.
In industrial instrumentation, the difference between the actual temperature at T2 and 0oC is usually corrected electronically within the instrumentation—the adjustment is known as cold junction compensation or ice point reference. See the graphic.
Advantage: no power
Thermocouples have many advantages over other types of temperature sensors. For one, they are self-powered, requiring no external source. They're also extremely rugged and can withstand harsh environments.
Thermocouples are also inexpensive—compared to RTDs and thermistors—and come in a wide variety of types and have a wide temperature range. For instance, Type C thermocouples have a rating to 2,340oC (4,208oF), while Type N thermocouples are rated to -270oC (-450oF).
Thermocouples' nonlinearity requires cold-junction compensation (CJC) for linearization. Additionally, voltage signals are low—normally, in the tens to hundreds of microvolts. This requires careful techniques to eliminate noise and drift in a low voltage measurement.
Another drawback is thermocouples' lower accuracy compared to RTDs or thermistors. They are also the least sensitive temperature sensor. Depending on the thermocouple type, a one degree change could mean a few microvolts change in signal.
Avoiding some common mistakes when setting up and using thermocouples will yield better measurements. Frequently one problem is that the CJC is not configured or compensated properly, or at all. This leads to inaccurate or nonlinear temperature measurements.
Another mistake is to use copper wire from the thermocouple connection to the measurement device. Doing so essentially introduces another thermocouple into the measurement, because any junction of dissimilar metals forms a thermocouple.
On the measurement device side, the voltmeter being used may not be sensitive enough for thermocouple measurements. To avoid this problem, make sure the voltmeter is sensitive and accurate enough for the low voltage (micro- to milli-volt) measurements.
For more Temperature Measurement Basics, see “ Resistance Temperature Detectors ”
Dale Cigoy, senior applications engineer, Keithley Instruments Inc.
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