Differences among Gauge Pressure, Absolute Pressure and Vacuum Degree

Pressure is one of the most commonly used terms nowadays, yet different types of pressure should not be confused. For industrial production processes, 

chemical operators and instrument maintenance personnel need to understand various pressure concepts.

Concepts of Absolute Pressure, Gauge Pressure and Vacuum Degree
Absolute pressure, also known as true pressure, refers to the pressure calculated starting from absolute zero pressure (perfect vacuum), abbreviated as absolute pressure.

Gauge pressure is the pressure calculated based on the local atmospheric pressure at the time of measurement. When the pressure of the measured system equals the local atmospheric pressure, 

the pointer of the pressure gauge points to zero, which means the gauge pressure is zero.

Vacuum degree: When the absolute pressure of the measured system is lower than the local atmospheric pressure, 

the difference between the local atmospheric pressure and the absolute pressure of the system is defined as vacuum degree. 

The measuring instrument used under this condition is called a vacuum gauge.

The relationship among absolute pressure, gauge pressure and vacuum degree is shown in the figure:
Figure: Relationship between Absolute Pressure, Gauge Pressure and Vacuum Degree
A – Pressure at measuring point is lower than local atmospheric pressure
B – Pressure at measuring point is higher than local atmospheric pressure
Pressure Gauge
Differences between Gauge Pressure, Absolute Pressure and Differential Pressure
Absolute pressure is the absolute pressure (engineering term; the physical term is absolute pressure intensity).
Differential pressure means pressure difference.
Formula:
Absolute Pressure − Atmospheric Pressure = Gauge Pressure

In the vacuum industry, "gauge pressure" specifically refers to the relative gas pressure value measured by an ordinary vacuum gauge (relative pressure gauge), 

expressed as a negative number. It represents the difference between the measured gas pressure and atmospheric pressure, also known as negative pressure.

Vacuum Gauge
The term "vacuum" describes a gas state where the pressure within a given space is lower than 101325 Pascals (one standard atmospheric pressure, approximately 101 kPa).
There are generally two ways to indicate vacuum degree:
Indicated by absolute pressure / absolute vacuum degree (how much higher the pressure is than theoretical perfect vacuum).

In practical applications, the absolute pressure value of a vacuum pump ranges from 0 to 101.325 kPa. Absolute pressure must be measured with an absolute pressure instrument. 

At 20°C and an altitude of 0 m, the initial reading of an instrument for vacuum measurement (absolute vacuum gauge) is 101.325 kPa (one standard atmospheric pressure).

Indicated by relative pressure / relative vacuum degree (how much lower the pressure is than atmospheric pressure).

Relative vacuum degree is the pressure difference between the measured object and the atmospheric pressure at the measurement location, measured with an ordinary vacuum gauge. 

Under non-vacuum conditions (normal atmospheric pressure), the initial reading of the gauge is zero. When measuring vacuum, the reading ranges from 0 to −101.325 kPa (usually expressed as a negative value).

For example, if the reading of micro vacuum pump model PH2506B is −75 kPa, it means the pump can achieve a vacuum state 75 kPa lower than the atmospheric pressure at the measurement location.