Vacuum Gauges

Pirani (thermal conductivity) covers roughly atmosphere down to 10⁻¹ Pa; cold cathode or hot cathode ionisation gauges take over from about 10⁻¹ Pa down to 10⁻⁸ Pa; capacitance and piezoresistive gauges are for the rough range where gas-type independence matters. Wide-range combination gauges pair a Pirani with a cold cathode in one head.

Thermal and ionisation gauges are gas-species dependent — they are calibrated for nitrogen, and helium, argon or solvent vapour will read high or low by a factor. Position matters too: a gauge on a long, narrow port reads the port, not the chamber. Compare only gauges of the same type, in the same gas, mounted properly.

Annually is the common quality-system requirement; more often if the process contaminates the sensor. Pirani filaments drift as they get coated, and cold cathode heads drift as the anode gets sputter-coated. If readings and pump-down times stop agreeing, suspect the gauge before the pump.

Cold cathode heads are designed to be opened and cleaned, and doing so restores most of the accuracy. Pirani sensors with a coated filament are generally replaced. Either way, recalibrate after cleaning — cleaning changes the response.