Absolute pressure uses a perfect vacuum as its zero reference. Gauge pressure uses the surrounding atmospheric pressure as its reference. A negative gauge reading therefore means pressure below the local atmosphere, not negative absolute pressure. Always give both the unit and the reference when specifying a vacuum requirement.
For a gauge referenced to the current local atmosphere, the relationship is:
Absolute pressure = gauge pressure + local atmospheric pressure.
At atmospheric pressure, such a gauge reads zero. Below atmospheric pressure, it reads a negative value. The same gauge reading can correspond to different absolute pressures at different locations or times because atmospheric pressure changes with altitude and weather. These definitions and the relationship are explained by WIKA's guide to pressure references.
Assume local atmospheric pressure is 101.325 kPa, equal to one standard atmosphere, and the instrument reads −80.000 kPa gauge. Then:
Absolute pressure = 101.325 + (−80.000) = 21.325 kPa absolute.
This equals 21,325 Pa = 213.25 mbar = 0.21325 bar, or approximately 159.95 Torr. The extra digits illustrate the calculation, not an instrument accuracy claim. Standard atmosphere is a defined reference; it is not a measurement of conditions at your site.
If local atmospheric pressure were instead 95.000 kPa, the same −80.000 kPa gauge reading would correspond to 15.000 kPa absolute. Use the actual local reference pressure for a site conversion.
Use pascals as a common basis when comparing figures. The values below follow NIST's pressure conversion table and NIST SP 811, Appendix B.9.
| Unit | Value in pascals | How to use it |
|---|---|---|
| 1 Pa | 1 Pa | SI pressure unit; 1 Pa = 1 N/m². |
| 1 kPa | 1,000 Pa | Use when a kilopascal value is easier to read. |
| 1 mbar | 100 Pa | 1 mbar = 0.1 kPa. |
| 1 bar | 100,000 Pa | 1 bar = 100 kPa = 1,000 mbar. |
| 1 Torr | ≈133.3224 Pa | Retain the stated unit when comparing vacuum data. |
| 1 mmHg, conventional | ≈133.3224 Pa | The rounded factor matches Torr here; do not treat this as an assertion of exact identity. |
The factors for kPa, mbar and bar above are exact; the Torr and conventional mmHg factors are rounded. A real mercury-column instrument also requires its measurement conditions to be considered. For precision work, use the applicable calibration and unit definition rather than extending the rounded table beyond its stated precision.
Changing kPa to mbar does not convert gauge pressure into absolute pressure. That requires the reference-pressure calculation as well.
Ultimate pressure describes the lowest pressure attainable under the stated conditions. It is not a promise that a working installation will maintain that pressure while processing gas. Leybold explains that attainable ultimate pressure can depend on pump fluids, gas released from vessel surfaces and leak tightness. See Leybold's vacuum pressure definitions.
For selection, state the pressure needed during the actual process and ask for performance data at that operating point. Include the gas load and installation details. If quoting an ultimate-pressure value from a datasheet, attach the stated test conditions and clarify whether it is a total or partial pressure value. Do not use an isolated end-pressure figure to infer cycle time or the performance of a complete system.
These wording examples and the conversion examples above illustrate notation and calculation only. They are not product performance claims or pump-selection recommendations.
State your required process pressure separately from the pump's catalogue value. A useful pressure specification identifies:
For example, a message can state: “Required absolute pressure: [value] [unit], measured at [location], during [operating stage]. Required flow and reference conditions: [details]. Gauge-to-absolute conversion, if used: [gauge reading] plus [measured local atmospheric pressure].” Replace the placeholders with your process requirements, not values copied from a model name.
Use the full vacuum pump inquiry checklist to add duty cycle, gas details, installation and electrical information. Send us your target pressure and operating requirements.
