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Measuring vacuum pressure with negative gauge or absolute pressure ranges
5th May 2007
There are two reference points for measuring a vacuum pressure: either you can 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 as the barometric pressure changes you will always be able to maintain suction pressure and containment of the laboratory air because the pressure you are controlling will track with changes in barometric 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. Since you need to apply a vacuum that is a fixed value independent of the ambient baro pressure reading you can be sure that changes in atmospheric air pressure will not influence the pressure measurement.
The typical negative gauge pressure range for measuring vacuum is 0 to -1 bar gauge but if the barometric pressure is below 1 bar absolute -1 bar will never be achieved and if the barometric pressure is above 1 bar absolute then full vacuum cannot be measured. Negative gauge pressures can be combined with positive pressures to create a compound pressure range such as -1 to 2 bar gauge for processes that involve vacuum purging and pump filling.
The typical absolute pressure range for measuring vacuum pressures is 0 to 1 bar absolute.
Since negative gauge and absolute reference vacuum ranges are measuring the same pressure it is often assumed that they are the same measurement carried out in different directions. However as explained above this is not the case, so it is important to understand which type of reference is required before selecting a pressure instrument for measuring over the vacuum range.
Pressure Measurement Resource Links Convert a pressure unit Convert an altitude in feet or meters to a pressure unit Convert temperature units to and from degrees celsius, fahrenheit or kelvin Explain an IP rating for ingress protection Explanations for pressure measurement terms Create a pressure sensor specification
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