Categories of exhibits

Thermocouple gauge

Exhibit no. 532

Thermocouple gauge is a device that uses thermal conductivity for the measurement of the pressures in vacuum systems. In this type of gauge, a wire filament is heated by running current through it. As molecules interact with the filament, heat is transferred to them, which changes the filament temperature. The temperature is measured by a thermocouple attached to the filament. As the pressure drops, the thermocouple gets hotter and its output increases. Readout is accomplished on a millivolt meter calibrated to indicate pressure. Thermocouple gauges are typically used to measure pressures in the range 10−4 mbar up to 1 mbar.

The exhibit is Varian 531 thermocouple tube sensor from ion trap GC/MS Saturn 2000R (Varian Inc.) manufactured in 2000. It contains two short resistance wires of different composition, which are crossed and connected together to serve as a heater and a thermocouple. Such design gives shock resistance and provides the minimum mass required for fast response. The gauge’s main purpose was to enable the diffusion pump controller to detect gross leaks and foreline pump failure. The gauge also indicated that the vacuum system has been pumped down to the diffusion pump’s required operational pressure. It measured vacuum pressures in the 2 Torr to 1 x 10-3 Torr range.

Wikipedia: Thermocouple as vacuum gauge
Wikipedia: Pressure measurement