Considerations for measuring vacuum

How you measure vacuum pressure to best effect is highly dependent upon the type of application. This article written by the experts at SensorsOne looks the different vacuum pressure measurement strategies, and when one should be used over the other

There are two reference points for measuring a vacuum pressure: you can either measure how much the pressure is below local atmospheric pressure or how much it is above absolute zero vacuum. Both methods are measuring the same pressure point but each will lead to differing results over time because the measuring reference point for one is fixed (absolute zero vacuum) and the other is variable (atmospheric air pressure).

If for example you are looking to ensure that there is adequate suction pressure being drawn by a vacuum pump or you are trying to maintain a slightly lower pressure in a laboratory than the local barometric pressure to ensure no laboratory air escapes, you would be interested in measuring a negative gauge pressure. Therefore, if the barometric pressure changes you will always be able to maintain suction pressure or containment of the laboratory environment because the pressure you are controlling will track with the barometric pressure.

Absolute pressure

However, if you are looking to simulate altitude in an environmental chamber or determine whether an adequate vacuum seal has been achieved for preserving food you would measure the absolute pressure. As you need to apply a vacuum that is a fixed value independent of the ambient barometric pressure reading you can be sure that changes in atmospheric air pressure will not influence the pressure measurement.

The negative gauge pressure range for measuring the full range of vacuum is -1 to 0bar gauge, but pressure ranges such as -1 to 2bar gauge which are called compound pressure ranges are more common because the processes will often require measurement of vacuum and positive pressures at the same point. The absolute pressure range for measuring the full range of vacuum is 0 to 1bar absolute, but for improved resolution at higher vacuums a lower pressure range such as 0-100mbar absolute is used.

Because negative and absolute pressure ranges are so similar it is sometimes assumed they are the same, so you may want to watch out for this when sourcing a pressure transducer or pressure transmitter for measuring vacuums in the future.

01 November 2006

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