Gauge vs absolute pressure (psig vs psia)
Understand gauge vs absolute pressure (psig vs psia), why thermodynamic properties use absolute pressure, and how to convert safely (Patm varies with altitude and weather).
Many pressure readings in HVAC and automotive A/C are reported as gauge pressure (psig, barg, kPa(g)), but most thermodynamic property correlations (including CoolProp) require absolute pressure (psia, bar(a), kPa(a)). Mixing them up is one of the most common reasons PT charts and saturation calculations look "wrong".
Definitions
Gauge pressure is measured relative to the local atmosphere. Absolute pressure is measured relative to a perfect vacuum.
Pabs = Pgauge + Patm
Pgauge = Pabs - Patm
Common unit labels
- psia = pounds per square inch absolute; psig = gauge.
- bar(a) or bara = absolute; bar(g) or barg = gauge.
- kPa(a) = absolute; kPa(g) = gauge.
Why properties use absolute pressure
- Absolute pressure has a physical zero (vacuum). Gauge pressure can be negative, but absolute pressure cannot.
- Most equations of state and saturation correlations are written on an absolute basis.
- For PT charts, Psat(T) is an absolute pressure by definition.
A quick mental check
At standard sea-level conditions, atmospheric pressure is about 101.3 kPa (~ 1.013 bar, ~ 14.7 psi). That means:
- 0 psig is not zero pressure -- it's roughly 14.7 psia.
- 0 barg is roughly 1 bar(a) (but not exactly unless Patm is exactly 1 bar).
Because Patm varies with altitude and weather, you should convert using a local barometric value whenever precision matters.
Using FluidTool correctly
FluidTool uses absolute pressure for calculations (Pa, kPa, bar, MPa, psi). If your instrument shows gauge pressure, convert it to absolute first.
For PT-chart style lookups, use a Two-phase input pair such as T + Q or P + Q. For supercritical/transcritical cases (e.g., CO2 high side), use P + T instead of assuming saturation.
What this page is NOT
This page explains pressure definitions only. It does not provide "normal operating pressure" targets, charging advice, retrofit guidance, or service procedures.
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