Pirani pressure switch

Exhibit no. 754

Vacuum switches monitor vacuum status and open or close a connection when there is a change in the vacuum level. 

The exhibit is one of two Pirani switches with a long heated wire, which were used in the MS902 double-focusing mass spectrometer manufactured by AEI Scientific Apparatus Ltd. in 1969. They were located on the rotary pump inlet pipelines. When the pressure dropped to approximately 0.22 torr, the switches completed the circuit to a relay, which energized heaters of the diffusion pumps and started the ionization gauges.

Wikipedia: Pressure measurement

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