Vacuum Expressions and Units

In everyday speech there are many different expressions and units for pressure below the atmospheric pressure. It is therefore important to relate to the same vocabulary in discussions.

ExpressionsUnits
Under pressure-kPabar
Absolute pressureinHgmm H2O
% vacuum (% of vacuum)mmHgtorr
Negative pressurehPambar

Different terms for pressure in relation to “absolute vacuum”

This diagram shows the relation between absolute negative and positive pressures. It also illustrates the problem that may occur if the pressure is not clearly specified. 30 kPa can imply three different values.

Physically there is only one kind of ”pressure” and that is the one that starts from ”0” or absolute vacuum. All above ”0” is pressure and correctly named absolute pressure.

Normal atmospheric pressure (101.3 kPa is used as a reference, which is why the terms “positive pressure” or “negative pressure” are used.

In earlier posts, the term “% vacuum” was used, where 0% was atmospheric pressure and 100% absolute vacuum. Consequently, in industry -kPa is the unit used most often since it nearly corresponds to “% vacuum”. In the chemical branch of industry, and in deep vacuum, mbar is generally used.

Thus, it is very important to be clear about which unit and reference point is meant. In this site, -kPa is generally used (as in industry), and for laboratory pumps, mbar absolute is specifically used.