Compact Pirani Gauge

In this gauge, a filament (usually gold plated tungsten or platinum) is heated up to around 100 to 150 °C by the passage of a current. As gas molecules collide with the sensing filament, it loses heat. The filament forms one arm of a Wheatstone bridge (an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit). In gauges with variable resistance, the sensing filament is heated with a constant current. As gas pressure increases, the temperature of the filament decreases so that the bridge becomes out of balance. The bridge current serves as a measure for the gas pressure. In the gauges built today, the heating voltage applied to this bridge is regulated so that the resistance and therefore the temperature of the filament remain constant, regardless of the heat loss. This means that the bridge is always balanced and the voltage applied to the bridge is a measure of the pressure. Pirani gauges have a measuring range from 10^-4 to 1000 Torr.

The exhibit is a Balzers TPR 250 Compact Pirani Gauge mounted to a sideported zero length flange. It was unmounted from a Biflex II MALDI-TOF mass spectrometer (Bruker Daltonics, 1990s).

Wikipedia: Pirani Gauge
Wikipedia: Pressure measurement