Choosing the right Temperature Sensor for Your Industries

A thermocouple is really a frequently used kind of sensor which is used to measure temperature. Thermocouples are popular in industrial control applications due to their relatively inexpensive and wide measurement ranges. Specifically, thermocouples do well at measuring high temperatures where other common sensor types cannot function. Try operating a built-in circuit (LM35, AD 590, etc.) at 800C.

Thermocouples are fabricated from two electrical conductors made from two different metal alloys. The conductors are normally built into a cable possessing a heat-resistant sheath, often having an integral shield conductor. At one end from the cable, both the conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple--the new junction--is thermally coupled to the object to get measured. One other end--the cold junction, sometimes called reference junction--is connected to a measurement system. The objective, naturally, is to ascertain the temperature nearby 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 it at a fixed, known temperature.

Most industrial thermocouple measurement systems decide to measure, instead of control, the reference junction temperature. This really is simply because that it is typically cheaper to merely add a reference junction sensor with an existing measurement system instead of add on a full-blown temperature controller.

Several methods are typically employed to linearize thermocouples. On the low-cost end of the solution spectrum, one could restrict thermocouple operating range in a way that the thermocouple is almost linear to in the measurement resolution. At the opposite end in the spectrum, special thermocouple interface components (integrated circuits or modules) are for sale to perform both linearization and reference junction compensation inside the analog domain. On the whole, neither of such methods is well-suited for cost-effective, multipoint data acquisition systems.

Along with linearizing thermocouples in the analog domain, it is easy to perform such linearizations in the digital domain. This is accomplished by way of either piecewise linear approximations (using look-up tables) or arithmetic approximations, or occasionally a hybrid of such two methods. For more information please visit Thermocouple Temperature Sensor

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