Where did all these pressure units come from?

Several months ago, we posted about the vacuum unit conversion tool that we have added to our website.  This calculator lets you convert between 8 different pressure units that are commonly used to measure vacuum.

But why are there so many units used to measure vacuum?  Some of it is caused by dueling measurement systems – metric vs. imperial, some of it is due to gauge vs. absolute pressures, some is driven by historical reasons, and some is for convenience.

Metric and imperial units.  Pressure is force divided by area.  Using standard metric units, the basic measure of force is equivalent to 1 N/m2.  This standard unit of pressure has been defined as the Pascal, where 1 Pa = 1 N/m2.  In the imperial system, using units of lbsf and in2, you get pressure measurements in pounds per square inch or PSI.

Gauge vs. absolute pressure.  Depending upon what is being measured and why, using either gauge or absolute pressure measurements will probably make more sense, or be more convenient.  For example, if you are working in the vacuum range, using absolute pressure measurements often makes the most sense.  But if you’re work is driven by a pressure differential relative to atmosphere, gauge units may be the better choice.  Either way, it’s important to be clear whether you are using gauge or absolute units.  As a consequence of this choice between gauge and absolute pressures, several common pressure units – like inches of Mercury (in. Hg), PSI, and Pa – are all used as both absolute and gauge units.

Historical units.  Some commonly used units are derived from historical methods used to measure pressure.   For example, work done in the 1600’s to measure atmospheric pressure is the basis of the vacuum unit torr, where 760 torr is equal to 1 atmosphere.

Convenience.  Because of the wide range of pressures that are measured in the physical world, different units come into play for the sake of convenience.  For example, 1 Pa is a relatively small amount of pressure; 1 atmosphere is defined as 101,325 Pa (or 101.325 kPa).   The Pascal provides good resolution in the fine vacuum range, but can belie measurement precision greater than is achievable in this range.  Instead, the unit micron was created.  Also called a millitorr, the micron is the equivalent of 1/1000th of 1 torr.  At the other end of the spectrum the unit kilopound per square inch (ksi) was introduced for use in materials science, for example to measure the Young’s modulus for materials.

Contact us to request a Vacuum Unit Conversion slide rule.