What about relative vacuum and absolute vacuum?

- Jan 11, 2019-

What about "relative vacuum" and "absolute vacuum"?

 IKS PVD Design the best deposition system for your needs,contact with us now,


"The customer requires 0.08MPa,

The vacuum pump has an absolute vacuum of 160mbar."


I believe that many users heart calculator dada dada dada for a long time still do not know how to calculate.


So let’s introduce the Vacuum concept at first.

What is Vacuum?


Vacuum: True space theoretically means that there is nothing in the volume. (there is no real vacuum.)

A vacuum is a condition in which the pressure inside a container is less than the normal atmospheric pressure (101,325 Pa).


Vacuum Degree: Represents the thinness of a gas in a vacuum, usually expressed as a pressure value.In practical application, vacuum degree usually has absolute vacuum and relative vacuum.


Absolute Vacuum

Also called "absolute pressure",or "extreme vacuum".


Namely, "theoretical vacuum" is taken as the zero position, indicating how much higher the pressure is than "theoretical vacuum". The Numbers are all positive, and the smaller the number, the closer it is to absolute vacuum, which means higher vacuum. Absolute vacuum is usually suffixed with "abs."


Vacuum degree from high to low:

2 mbar abs.> 100 mbar abs.> atm. (standard atmosphere)


Relative Vacuum

Also called "relative pressure", "gauge pressure", "negative pressure".

That is, atmospheric pressure as a zero, that is, how much pressure lower than the atmosphere. Denoted by a negative value, the greater the absolute value of this negative value, the higher the vacuum degree. A relative vacuum usually USES the suffix "g".


Vacuum degree from high to low:

-980 mbar g> -100 mbar g>atm. (standard atmosphere)



Absolute vacuum & Relative vacuum


The international vacuum industry universal, is also the most scientific is to use the "absolute vacuum" logo; However, because the method of measuring "relative vacuum" is simple and the measuring instruments are common (for example, all vacuum meters are relative vacuum meters), "relative vacuum" is commonly used to mark in China.



General vacuum meters are calibrated relative vacuum values

When measuring a vacuum explicitly, the "-" in front of the "relative vacuum" is usually hidden for convenience.

For example, the vacuum scale of the device is 0.098MPa, which is actually -0.098MPa


Finally, the conversion of pressure units.

The commonly used pressure expression units in the laboratory are Pa, KPa, MPa, mbar, bar, mmH2O, Psi, etc.

The conversion method between them is shown in the table below:


How do you convert relative vacuum to absolute vacuum?


reduction formula

Relative vacuum = absolute vacuum (absolute pressure) - atmospheric pressure

For example, for a diaphragm vacuum pump, the ultimate vacuum is 8 mbar abs. Its relative vacuum is -1005 mbar g.

Algorithm: 8-1013 (standard atmospheric pressure) = -1005


Conclusion: the requirement mentioned at the beginning of the article is 0.08mpa, which is usually the dial pressure, i.e., the relative pressure -0.08mpa g= -800mbar g. The absolute vacuum is 213mbar. The vacuum pump can achieve an absolute vacuum of 160mbar, higher than 213mbar, so it can meet the requirements.