A simple relationship with an important consequence
Absolute pressure = gauge pressure + local atmospheric pressure. Atmospheric pressure is not always exactly 1 bar; it changes with altitude and weather. An open vessel has zero gauge pressure but does not contain a vacuum. The letter “e” in bar(e) also generally denotes pressure relative to the atmosphere.
Pressure differences and pressure ratios
Subtracting two gauge pressures referenced to the same atmosphere gives the same difference as subtracting absolute pressures. This does not hold for ratios. It is the ratio of absolute pressures that is used in volume conversion and the physics of compression. Check each field description when entering calculator values.
What to check in practice
- State whether every pressure value is gauge, absolute or differential.
- Use local atmospheric pressure where altitude matters.
- Check the sensor's measurement type, not just its range and unit.
Example scenario
At an atmospheric pressure of 1.00 bar(a), 7 bar(g) means an absolute pressure of 8 bar(a). A drop from 8 to 7 bar(g) is a difference of 1 bar, but the absolute pressure ratio is 9/8, not 8/7.
What to watch out for
- Substituting gauge pressure into pV = mRT.
- Adding atmospheric pressure to a pressure drop.
- Treating 0 bar(g) as an absence of air.
Is bar(a) a different unit from bar(g)?
One bar has the same magnitude in both cases. The measurement zero point differs. Converting bar to kPa therefore does not by itself change gauge pressure to absolute pressure.