Pressure is one of the most commonly used words in today's society, but this pressure is not the same pressure. For industrial processes, chemical operators and instrumentation maintenance personnel need to understand the concept of different pressures.
The concepts of absolute pressure, gauge pressure and vacuum
Absolute pressure, or true pressure, is the pressure calculated with absolute zero pressure as a starting point. or vacuum as the starting point for the calculation of the pressure, absolute pressure, or absolute pressure for short.
The gauge pressure, or gauge pressure for short, is the pressure calculated using the local atmospheric pressure at the time as a starting point. When the pressure of the system being measured is equal to the local atmospheric pressure at the time, the pointer of the pressure gauge points to zero. That is, the gauge pressure is zero.
Vacuum, when the absolute pressure of the measured system is less than the local atmospheric pressure, the difference between the local atmospheric pressure and the absolute pressure of the system is called the vacuum degree. The pressure measuring instrument used at this time is called a vacuum gauge.
The relationship between absolute pressure, gauge pressure and vacuum is shown in the diagram
The relationship between absolute pressure, gauge pressure and vacuum
A- Pressure at the pressure measurement point is less than the prevailing atmospheric pressure
B- B- pressure at the pressure measurement point is greater than the prevailing atmospheric pressure
Pressure gauge
Difference between gauge pressure, absolute pressure and differential pressure
absolute pressure is the absolute pressure (the engineering term, the physical term is absolute pressure).
differential pressure is the pressure difference (or pressure difference)
i.e.: absolute pressure - atmospheric pressure = gauge pressure.
Gauge pressure" in the vacuum industry means in particular: the relative pressure of a gas measured with an ordinary vacuum gauge (relative pressure gauge), expressed as a negative number, is the difference between the measured gas pressure and the atmospheric pressure. Also called negative pressure.
Vacuum gauges
A "vacuum" is a state of gas in a given space where the pressure is below 101325 Pascals (i.e. a standard atmospheric pressure of about 101KPa).
There are usually two ways of marking a vacuum.
One is the marking by "absolute pressure", "absolute vacuum" (i.e. how much higher than the "theoretical vacuum").
In practice, the absolute pressure of a vacuum pump is between 0 and 101.325 KPa. The absolute pressure value is measured with an absolute pressure gauge, the initial value of which is 101.325 KPa (i.e. one standard atmosphere) at 20°C and altitude = 0.
Negative pressure gauge
The second is marked by "relative pressure" and "relative vacuum" (i.e. how much lower than "atmospheric pressure").
The "relative vacuum" is the difference between the pressure of the object to be measured and the atmospheric pressure at the place of measurement. It is measured with an ordinary vacuum gauge. In the absence of vacuum (i.e. at atmospheric pressure), the initial value of the gauge is 0. When a vacuum is measured, it is between 0 and -101.325 KPa (usually expressed as a negative number).
For example, one miniature vacuum pump PH2506B has a measured value of -75KPa, which means that the pump can pump to a vacuum state 75KPa lower than the atmospheric pressure at the place of measurement.
Source: Reprint
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