The thermocouple gauge uses the thermal conductivity property of gases, by incorporating a wire filament which is heated by a constant source of power. Attached to this filament is a thermocouple, which measures the temperature of the wire. At high pressures, the large number of gas molecules striking the heated wire carries energy away and cools the wire. At low pressures, the smaller number of gas molecules striking the wire causes less cooling, and thus a higher temperature. This is the figure of the principle operation of the theory;
The thermocouple output voltage responds to these temperature changes to give an indication of pressure: low gas pressure gives high filament temperature which gives high thermocouple output voltage; high gas pressure gives low filament temperature which gives low thermocouple output voltage. The meter measuring the thermocouple voltage is calibrated in pressure units to give a direct indication of pressure. At pressures below about 10-3 torr, the heat loss from the filament is primarily through radiation since the density of gas molecules is so low. Since the heat loss due to radiation is constant, the resulting temperature corresponds to the “zero” reading on the meter. The thermocouple gauge is a simple, rugged device which is very useful at rough vacuum pressures. The meter covers the pressure range of 1 to 2000 millitorr. This is the schematic diagram of the thermocouple circuit.
[Schematic source: Duniway Stockroom Corp.]