Choosing the right vacuum gauge
No single gauge covers the whole vacuum range, so the question is always which sensor for which decade. Capacitance manometers measure pressure directly and are gas-independent, from atmosphere down to roughly 10⁻⁴ mbar. Pirani and thermocouple gauges infer pressure from heat loss and cover atmosphere to about 10⁻⁴ mbar, but their reading depends on the gas. Hot- and cold-cathode ionisation gauges take over from about 10⁻³ mbar down into UHV, and are also gas-dependent.
Pick the sensor for the decade you actually control, not for the widest range on the datasheet. Then check the gas-correction factors for your process gas, the flange, whether you need a bakeable or corrosion-resistant version, the output and protocol your control system expects, and the calibration interval. Combination gauges — Pirani plus cold cathode in one body — are the practical choice where one port has to cover atmosphere to high vacuum.
Typical duties: process control on furnaces and coaters, load-lock and interlock switching, leak-test benches, and laboratory rigs. Sensors drift and filaments fail; keeping a spare of the gauge you depend on is cheaper than the downtime.
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FAQ
Which gauge type covers my pressure range?
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.
Why does my gauge read differently from another gauge on the same system?
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.
How often do gauges need calibration?
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.
Can a gauge be cleaned instead of replaced?
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.