Vacuum gauge

01

Match the gauge type to the pressure range

Leybold explains that pressures in vacuum technology span about 2000 mbar to 10⁻¹² mbar, more than 15 orders of magnitude, so no single gauge covers them; each type has "a characteristic measuring range that usually extends over several orders of magnitude", and measurement uncertainty rises very rapidly, by up to 100% in some cases, near the range limits, so the catalogue range differs from the range for precise measurement. [1]

For rough vacuum it describes direct, gas-independent gauges: a Bourdon gauge accurate only to about 10 mbar unless atmospheric pressure changes are corrected, capsule gauges accurate to about 10 mbar, and a diaphragm gauge readable to about 0.3 mbar between 1 and 20 mbar. Capacitance diaphragm gauges are used from atmospheric pressure to 1·10⁻⁴ mbar, each sensor covering about three decades (for example 1000 to 1 Torr, or 1 to 10⁻³ Torr). [2] Edwards notes that commercially available direct gauges have a practical lower limit of 10⁻⁴ mbar, so indirect gauges are needed at lower pressures. [4]

Constant-resistance Pirani (thermal conductivity) gauges measure from 10⁻⁴ to 1013 mbar with a time constant of a few milliseconds, suiting control and monitoring; between 10⁻³ and 10 mbar, their range of highest sensitivity, uncertainty is around 15% of reading and significantly greater outside it. Ionisation gauges are the main instruments for high and ultra-high vacuum. Leybold describes transmitter versions (THERMOVAC, Penning, IONIVAC) that need, for example, +24 V and give 4–20 mA or 0–10 V. [3]

Editorial calculation: the field table uses pascals; 1 mbar = 100 Pa, so the Pirani range of 10⁻⁴ to 1013 mbar is 10⁻² Pa to about 101,300 Pa.

Give the supplier the pressure range to read and the range where accuracy matters, gases and vapours present (including process gases and venting gas), the process connection (KF, CF, thread), whether the reading drives interlocks or setpoints, output signal and supply, cable length, ambient and bake-out temperature, and contamination or corrosion risks.

02

Correct gas-dependent readings

Leybold distinguishes direct gauges, which measure force on an area and read independently of gas type, from indirect gauges (thermal conductivity, ionisation), whose readings depend on the gas because they sense properties that vary with molar mass. Indirect gauge scales are based on air or nitrogen; for other gases the true pressure equals the indicated pressure multiplied by a correction factor, either a constant or, for Pirani gauges, read from a pressure-dependent calibration curve. [1] [3]

Leybold gives two warnings: an ionisation gauge reading "a gas essentially consisting of pump fluid molecules" reads about ten times too high, and a Pirani gauge on a chamber vented with argon shows only about 40 mbar when the argon has reached atmospheric pressure, so the cover could open; for such cases it calls for pressure switches or gas-independent gauges. [1] [3]

Edwards states that "Indirect gauges are often calibrated for nitrogen", that true pressure equals indicated pressure divided by the gas's relative sensitivity S, and lists ion-gauge sensitivities including helium 0.14–0.25, hydrogen 0.36–0.60, air 0.97–1.00 and krypton 1.72–1.94; it adds that with Pirani gauges "many users forget or do not know that they are gas dependent", giving large errors at the extremes of the quoted range. [4] Editorial calculation: an ion gauge showing 1 × 10⁻⁶ mbar in helium, with S between 0.14 and 0.25, corresponds to a true pressure of about 4 × 10⁻⁶ to 7 × 10⁻⁶ mbar.

03

Compare what suppliers offer

Suppliers linked to this page list vacuum gauges; each card links to the page reviewed.

Listings include Pirani vacuum gauges with KF connections from a vacuum equipment maker, digital vacuum gauges with flanges from a pressure-gauge maker, and digital vacuum gauges sold as refrigeration and HVAC service tools. Ask each for gauge principle, range, accuracy by range and gas calibration.

04

Trace the gauge components

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your system

A vacuum gauge quotation needs values for your system. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the range, gas and interface data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Range: [full range; range needing accuracy]
Gases: [process gases; vapours; venting gas]
Use: [reading; setpoint; interlock]
Connection: [KF/CF/thread; size]
Signal: [output; supply; cable]
Environment: [temperature; bake-out; contamination]

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