1. Introduction
In industrial pressure measurement, you often encounter terms like absolute pressure, gauge pressure, and vacuum pressure. Understanding their differences and relationships is essential for accurate pressure monitoring and selection of pressure instruments.
This article explains the definitions, formulas, and practical implications of these three types of pressure.
2. Absolute Pressure (P<sub>abs</sub>)
Absolute pressure is the total pressure exerted by a fluid (gas or liquid), measured relative to a perfect vacuum (absolute zero pressure). It includes both the atmospheric pressure and the additional pressure applied by the system.
Key Features:
Reference point: absolute vacuum (0 Pa)
Value is always ≥ 0
Commonly used in scientific and thermodynamic calculations
Example:
If the local atmospheric pressure is 100 kPa and the pressure gauge reads 250 kPa (gauge pressure), then:
3. Gauge Pressure (P<sub>gauge</sub>)
Gauge pressure is the pressure measured relative to the ambient atmospheric pressure. It indicates how much higher or lower the system pressure is compared to the atmosphere.
Key Features:
Reference point: local atmospheric pressure
Can be positive (above atmospheric) or negative (below atmospheric, treated as vacuum)
Commonly used in industrial instruments and pipelines
Formula:
Pgauge=Pabs−Patm
Example:
A water pipe shows 0.5 MPa on the gauge. Assuming atmospheric pressure is 0.1 MPa, the absolute pressure inside the pipe is:
Pabs=0.5+0.1=0.6 MPa
4. Vacuum Pressure (P<sub>vac</sub>)
When the measured pressure is lower than atmospheric pressure, it is considered vacuum. Vacuum pressure is defined as the difference between atmospheric pressure and absolute pressure.
Formula:
Pvac=Patm−Pabs
Key Features:
Used when gauge pressure is negative
Indicates the degree of vacuum (how close the pressure is to a perfect vacuum)
The higher the vacuum value, the lower the actual gas density inside
Example:
If the atmospheric pressure is 100 kPa and the absolute pressure inside a sealed chamber is 20 kPa:
Pvac=100−20=80 kPa
5. Relationship Diagram (Suggested Illustration)
Below is a conceptual diagram that illustrates the relationship among absolute pressure, atmospheric pressure, gauge pressure, and vacuum:
6. Practical Notes
In most industrial applications, pressure instruments display gauge pressure by default.
When high-precision measurement or vacuum systems are involved, absolute pressure or vacuum degree becomes critical.
Unless otherwise specified, pressure values mentioned in engineering documentation generally refer to gauge pressure.
7. Summary Table
| Type | Reference Point | Value Range | Common Usage |
|---|---|---|---|
| Absolute Pressure | Absolute Vacuum (0 Pa) | ≥ 0 Pa | Scientific calculations, vacuum systems |
| Gauge Pressure | Atmospheric Pressure | Positive/Negative | Most industrial instruments |
| Vacuum Pressure | Atmospheric Pressure | 0 to Patm | Vacuum pumps, sealed containers |