Thermocouple Gauge

About thermocouple gauge

The thermocouple gauge is one of the group of gauges working on the thermal conductivity principle. The Pirani and thermistor gauges also belong to this group. At low pressure, the kinematics theory of gases predicts a linear relationship between pressure and thermal conductivity. Thus measurement of thermal conductivity gives an indication of pressure.

Operation of the gauge depends on the thermal conduction of heat between a thin, hot metal strip in the center and the cold outer surface of a glass tube (which is normally at room temperature). The metal strip is heated by passing a current through it and its temperature is measured by a thermocouple. The temperature measured depends on the thermal conductivity of the gas in the tube and hence on its pressure. A source of error in this instrument is the fact that heat is also transferred by radiation as well as conduction. This is of a constant magnitude, independent of pressure, and so could be measured and corrected for. However, it is usually more convenient to design for low radiation loss by choosing a heated element with low emissivity. Thermocouple gauges are used to measure pressures typically in the range 10-4 mbar up to 1 mbar.

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