The most common type of pressure range for pressure measurement instrumentation is one which has a gauge reference. So if you want to measure vacuum pressure, it would be convenient to use a gauge pressure device which is the most widely available pressure range format. So is it possible to measure vacuum with a gauge pressure range instrument, and if so how do you select the correct pressure range?
There are different levels of vacuum quality, from low partial, to very high, but they we all reside within the confines of a pressure range between local atmospheric air pressure and full vacuum where no pressure exists at all.
A vacuum, regardless of quality, which is generated in vessel or chamber, will always be measured as a negative pressure relative to the ambient environment air pressure surrounding the vessel or chamber.
A gauge reference pressure measuring device provides pressure readings relative to ambient air pressure. A positive gauge pressure range would not be suitable for measuring a vacuum, since it will only measure pressures that are greater than atmospheric pressure. Therefore only gauge pressure ranges that include a negative pressure range will be suitable for measuring a vacuum pressure.
For example a 0 to -1 bar or -15 to 0 psi gauge range will cover the entire range of vacuums, and is a popular range offered by manufacturers. It can also be combined with positive pressure ranges which are called compound pressure ranges, such as -1 to +10 bar g or -760 mmHg to + 100 psi g.
If you only interested in measuring a partial or low vacuum you will need a smaller gauge range such as 0 to -1 psi or 0 to -250 mbar g, which provide a better accuracy and resolution than the larger ranges, for measuring the full vacuum range.
A gauge pressure range is not suitable for measuring high vacuum pressures, since you can only use a full vacuum range device, which compromises the accuracy and resolution so much that the errors are greater than the difference between full vacuum and the high vacuum reading.
Gauge pressure measurement devices which are suitable for measuring vacuum
will often be described as having a negative gauge range, or a compound range for measuring both vacuum and positive pressure.
