1.What Temperature Instruments Do We Use? by-AK BARLA ANS- Temperature is most commonly measured by Resistance Temperature Devices (RTD) and Thermocouples, and to a lesser degree Infrared (IR). Temperature is a very common process variable and temperature control is critical to maintaining product quality and ensuring safe and reliable operation of many processes. Thermocouples Thermocouples are fabricated from two electrical conductors made of two different metal alloys. At one end of the cable the two conductors are electrically shorted together by crimping, welding, etc. This end of the thermocouple, the hot or sensing junction, is thermally attached to the object to be measured. The other end, the cold or reference junction is connected to a measurement system. Thermocouples generate an open-circuit voltage, called the Seebeck voltage that is proportional to the temperature difference between the sensing (hot) and reference (cold) junctions: VS = V (THot − TRef ) Where : VS = Seebeck Voltage V = Proportionality Constant THot = Temperature of the Sensing Junction TRef = Temperature of the Reference Junction The relationship between the temperature difference and the voltage produced has been documented for several commonly used combinations of metals. Junctions Since thermocouple voltage is a function of the temperature difference between junctions, it is necessary to know both voltage and reference junction temperature in order to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to maintain it at a fixed, known temperature. Most controllers today measure the temperature of the reference junction.
Posted on: Sat, 29 Mar 2014 01:55:18 +0000
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