Bayard-Alpert ion gauge

The gauge is a triode with the heated filament serving as a cathode. The electrons emitted from the heated cathode are attracted to a helical grid by a DC potential. Most of the electrons pass through the grid and collide with gas molecules. The positive ions formed in this way are attracted to the central ion collector wire by a negative voltage applied to it. When the temperature is constant, the gauge reading is proportional to pressure. The gauge is sometimes called a hot-filament gauge or Bayard–Alpert gauge and it is the widely used for the region from 10⁻³ to 10⁻¹⁰ Torr.

The exhibit is a Bayard-Alpert gauge tube from Saturn 2000R GC/MS (Varian Inc.). It used thoria-coated iridium (ThO₂-Ir) filament, which is burnout resistant. Therefore, it exhibited a high tolerance to air and water in the vacuum manifold. There was a time delay associated with heating the filament; from 15 to 20 seconds was usually required after the filament was turned on to obtain a stable reading. The ion gauge measured pressures between 10^-6 and 10^-2 Torr. A logarithmic amplifier amplified the collector current, and the data system interpreted this current as measured vacuum. Fixed pressure readings with nominally identical gauges exhibited variations of ±15 %. The ion trap mass spectrometer with internal ionization Saturn 2000R was manufactured in 2000.

Wikipedia: Hot filament ionization gauge
Wikipedia: Pressure measurement