Thermal Conductivity Limit in Molecular Flow Pirani gauges measure pressure by sensing the heat loss from a heated filament (typically platinum) to the surrounding gas. At higher pressures, in the viscous-flow regime, thermal conductivity increases linearly with gas density, providing excellent sensitivity. As pressure drops into the transition region (roughly 10 Torr to 10⁻² Torr),

Plasma Discharge Behavior at High Pressure Cold cathode gauges rely on the Penning (magnetron) discharge to ionize gas molecules. A high negative voltage (–2000 V operating, –2500 V for startup) combined with a ~100 gauss magnetic field traps electrons in long spiral paths, creating an avalanche of ions whose current is proportional to pressure. This

Structural Differences Hot cathode and cold cathode ionization gauges both measure vacuum by ionizing gas molecules and collecting the resulting ion current, but their fundamental designs differ dramatically in how electrons are generated and how the discharge is sustained. A hot cathode gauge, often based on the Bayard-Alpert or triode geometry, relies on a thermionic