Capacitance Manometer

Capacitance manometers, Pirani gauges, cold-cathode gauges, and ionization gauges each measure vacuum pressure through a different physical principle, because no single gauge technology covers the entire pressure range a fab needs to measure, from near-atmospheric down to ultra-high vacuum.

A capacitance manometer's gas-species independence is a real practical advantage, since it measures pressure directly as mechanical force rather than through a gas-property-dependent effect that varies between different process gases. Thermal and ionization gauges both measure pressure indirectly, through an effect — heat loss, or ionization rate — that depends on which specific gas is present, meaning their readings need gas-specific calibration; a capacitance manometer measures pressure as the actual physical deflection of a diaphragm under gas pressure, giving a reading that's accurate regardless of what gas mixture happens to be in the chamber, which matters for processes running varying or mixed gas chemistries.

Ionization and cold-cathode gauges reach the lowest pressures precisely because they measure something that only becomes detectable once gas density drops low enough that individual molecules, and the ionization events they produce, become countable. At the deepest vacuum levels, there simply aren't enough gas molecules left for a mechanical or thermal effect to register meaningfully; ionization-based gauges instead ionize whatever residual gas remains and measure the resulting ion current, a signal that stays measurable even at pressures where other gauge types have essentially nothing left to detect.

Capacitance manometers, Pirani gauges, and ionization/cold-cathode gauges together span the entire pressure range a fab needs to monitor, and most vacuum systems run two or more of these gauge types simultaneously specifically because no single technology covers the full journey from rough vacuum to ultra-high vacuum accurately on its own.