Short answer
bar, MPa and psi are pressure units; the terms gauge and absolute indicate the reference of the pressure. Unit conversion does not change the reference. Especially in vacuum, gas and steam assessments, it must be clear whether the value is given relative to the atmosphere or to absolute zero.
Technical explanation
Gauge pressure shows the difference relative to ambient pressure. In absolute pressure the reference is absolute zero; at the same point, absolute pressure is the sum of gauge pressure and local atmospheric pressure. Assuming a fixed value for atmospheric pressure in every case can introduce an error depending on the measurement approach. For unit conversion, the equality 1 bar = 0.1 MPa = 100,000 Pa is used. This equality only changes the unit scale; it does not convert a gauge value into an absolute value.
Topic-specific checkpoints
| Item | Description |
|---|---|
| Unit | bar, MPa or psi; defines the scale of the number. |
| Reference | Gauge/differential or absolute; defines the zero point of the number. |
| Conversion | The unit factor only changes the scale. |
| Record | Stored together with the measurement point, sensor and date. |
How is it assessed in practice?
In data entry, use three separate fields for the numerical value, unit and reference. When a user writes 'minus 0.8 bar', do not automatically move it to the absolute pressure field; ask for the reference or verify it from documentation. In a comparison table, do not compare two values before their references are aligned. If a pressure-temperature relationship is to be read from a steam table, also record the reference used by the table. A calculation output should not be a precise-looking number that hides the uncertainty in the input.
Original editorial recommendation; not a clause of a standard.
Frequently asked questions
Does 0 bar always mean there is no pressure?
No. 0 bar gauge indicates approximately ambient pressure, while 0 bar absolute is a different physical reference. Read the measurement reference along with the unit on the display.
Source coverage
-
S023
NIST Guide to the SI, Appendix B.8 NIST · accessed 2026-10-19 Alphabetical entries: bar, inch, pound-force per square inch
-
S024
Hose and Flexible Tubing, MS-01-180 Swagelok · accessed 2026-10-19 PDF pages 4–5 / printed 253–254: Nomenclature; pages 8–9: selection, installation and use