What is the difference between absolute vacuum and gauge vacuum in vacuum systems? How to convert?

Absolute vacuum and gauge vacuum are two different ways of describing the degree of vacuum.

absolute vacuum

Absolute Vacuum:

Absolute vacuum refers to the degree of vacuum relative to absolute vacuum. In the measurement coordinate system of absolute vacuum, the physical absolute vacuum is set to point 0, that is, a pressure of 0 Pa represents absolute vacuum.

Absolute vacuum degree is a value obtained by comparing the tested vacuum pressure with the absolute vacuum. This value is a positive value greater than 0, because in any real system, the pressure can never be lower than the absolute vacuum. In order to avoid misunderstanding with the gauge pressure, there is often a special nominal value after the unit of positive pressure value. abs, which means that the value is absolute pressure vacuum degree.

The smaller the value of the absolute pressure vacuum degree, the higher the vacuum degree. The higher the vacuum degree, that is, the deeper the vacuum degree. For example, a vacuum system with a nominal pressure of 1kPa.abs has a higher vacuum degree than a vacuum system with a nominal pressure of 10kPa.abs. Generally speaking, people stipulate that the atmospheric pressure at normal temperature and pressure (20C sea level) is 101.325kPa, which also represents absolute pressure.

Gauge Vacuum:

Gauge vacuum is the degree of vacuum relative to the external ambient pressure of the system being measured. It is a value obtained by comparing the vacuum pressure with the current ambient pressure of the test (usually the current atmospheric pressure) through a vacuum gauge. Gauge vacuum can also be considered relative vacuum because it is the degree of vacuum relative to the environment.

The gauge pressure vacuum will be tested with the local external atmospheric pressure as the 0 point. The test vacuum system is lower than the external atmospheric pressure, so its reading is a negative value, such as -30kPa or -80kPa, the latter (-80kPa) represents A lower degree of vacuum than the former (-30kPa), or the latter is closer to absolute vacuum than the former.

Because the gauge pressure will be measured as the 0 point of the local atmospheric pressure at the time, the gauge pressure will be affected by the climate, atmospheric pressure at the time, and local altitude. The same vacuum system will often measure different values ​​at different times and locations due to different climate conditions. Different gauge pressures; in extreme situations, such as on plateaus or extreme weather conditions, this effect is very significant.

Therefore, in order to evaluate the vacuum degree of a vacuum system under a unified standard, people will specially agree on a certain gauge. The pressure is measured at normal temperature and pressure. Once it leaves the agreed test 0 point (20C sea level altitude 101.325kPa) of normal temperature and pressure, the gauge pressure will also change accordingly and it will no longer be accurate.

 The advantages and disadvantages of absolute pressure and gauge pressure:

Absolute pressure has a constant measurement base point because absolute vacuum is the 0 point. Therefore, when absolute pressure is used to identify the system parameters of a vacuum system, comparisons can be made more accurately. However, absolute pressure testing instruments are often more expensive and the testing is more troublesome.

Gauge pressure can be easily tested with a negative pressure meter, which has the advantage of fast, simple and low-cost testing. However, since its measurement point 0 is affected by the environment and location, the values ​​measured for the same system at different times and locations will change and will not change. They have objective invariance and must be measured under uniform normal temperature and pressure before they can be compared with each other.

 Conversion method between the two:

The relationship between absolute pressure and gauge pressure can be converted by the following formula:

PAbsolute = PAtmosphere + PGauge

For example, a vacuum pump nominal at: standard atmospheric pressure, gauge pressure =-88kPa, its absolute pressure is:

PAbsolute = P101.325kPa + P-88kPa = P13.325kPa  its absolute pressure 13.325kPa。

Similarly, this vacuum pump nominal at standard atmospheric pressure, gauge pressure =-88kPa, when the local atmospheric pressure is lower than the standard atmospheric pressure, only 95kPa pressure, if you want to test its gauge pressure, then we can calculate through the formula:

PGauge = PAbsolute – PAtmosphere = P13.325kPa – P95kPa = P-81.675kPa   That is, when the atmospheric pressure is 95kPa, the vacuum pump with a nominal gauge pressure of -88kPa at normal temperature and pressure is actually measured at -81.675kPa

In particular, we know that if the local real-time atmospheric pressure is known, the above formula can be used to convert absolute pressure to gauge pressure, or gauge pressure to absolute pressure.

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