Thermocouple Sensor

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Selecting the Right Temperature Sensor for the Industries

A thermocouple is a widely used sort of sensor that is used to measure temperature. Thermocouples are popular in industrial control applications because of their relatively inexpensive and wide measurement ranges. Particularly, thermocouples do well at measuring high temperatures where other common sensor types cannot function. Try operating an incorporated circuit (LM35, AD 590, etc.) at 800C.

Thermocouples are fabricated from two electrical conductors made of two different metal alloys. The conductors are typically built into a cable developing a heat-resistant sheath, often with an integral shield conductor. At one end of your cable, both conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple--the recent junction--is thermally coupled to the object to be measured. The other end--the cold junction, sometimes called reference junction--is connected to a measurement system. The goal, of course, is to look for the temperature near the hot junction.

If low temperatures are being measured, it should be noted that the "hot" junction, which is somewhat of a misnomer, may in fact be at a temperature lower than that of the reference junction.

In order to determine the temperature at the hot junction, since thermocouple voltage is a function of the temperature difference between junctions, it is necessary to know both voltage and reference junction temperature. Consequently, a thermocouple measurement system must either look at the reference junction temperature or control it to keep up it in a fixed, known temperature.

Most industrial thermocouple measurement systems decide to measure, rather than control, the reference junction temperature. This is due to the fact that it is more often than not less costly to easily add a reference junction sensor to an existing measurement system rather than add-on a full-blown temperature controller.

Several methods are commonly used to linearize thermocouples. At the low-cost end of the solution spectrum, anybody can restrict thermocouple operating range such that the thermocouple is almost linear to within the measurement resolution. At the opposite end in the spectrum, special thermocouple interface components (integrated circuits or modules) are available to perform both linearization and reference junction compensation inside the analog domain. In general, neither of such methods is well-designed for cost-effective, multipoint data acquisition systems.

As well as linearizing thermocouples from the analog domain, it is actually possible to perform such linearizations from the digital domain. This is accomplished through either piecewise linear approximations (using look-up tables) or arithmetic approximations, or sometimes a hybrid of these two methods. For more information please visit Thermocouple Sensors