How to Convert Absolute Pressure to Gauge Pressure Read more: How to Convert Absolut

Most modern pressure gauges take into account the use of atmospheric pressure on the systems being read. Appliances and machines such as well pumps, air compressors and tire gauges will read by gauge pressure. This of course does not take into account the total system or absolute pressure. By using a basic formula, you can convert absolute pressure to gauge pressure or visa versa.[LEFT][COLOR=#000000]
Understand that absolute pressure or total system pressure is generally defined as the pressure measured from an absolute vacuum or zero pounds per square inch (PSI). Gauge pressure is that which is generally read on a gauge and takes into consideration the atmospheric pressure we are placed under every day. Atmospheric pressure changes with temperature and altitude above sea level. On average this would be 14.6 pounds per square inch (PSI) above an absolute vacuum reading of 0 PSI. In other words, the reading of 0 on a gauge would really have 14.6 PSI already into account.

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Realize that the basic formula for finding gauge pressure is Pg = Ps -- Pa. Where Pg is equal the gauge pressure reading, Ps is the total system pressure reading or absolute pressure, and Pa is the atmospheric pressure that on average will be 14.6 PSI.
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Covert the absolute pressure or system pressure of 100 PSI into a gauge pressure reading. Plug the numbers into the formula and the equation will read Pg = 100 - 14.6.; the gauge pressure is equal to 85.4 PSI.
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Find the absolute pressure or total system pressure on a tire gauge reading when testing a tire that reads 36 PSI. Translating the formula will read as follows, Ps = Pg + Pa. The answer will be 36 + 14.6 = 50.6 PSI.
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[CENTER][SIZE=5][B]Negative Pressure[/B]
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The term “negative pressure” is used in physics and engineering to refer to a situation in which an enclosed area has lower pressure than the area around it. Any compromise in the divide between the area of negative pressure and the more highly pressurized area around it would cause substances to flow into the area of negative pressure. Negative pressure is useful for a number of applications, including the prevention of oil spills, quarantine of highly contagious patients, and in the household vacuum cleaner.

Pipelines commonly have areas of negative pressure. Usually this is an intentional choice. For example, undersea pipelines used for oil and other materials are kept in a state of negative pressure so that if they rupture, seawater will flood the pipe. If the pipes were positively pressurized, their contents would explode into the ocean, potentially creating a hazardous spill. Negative pressure can also be dangerous, as is the case when municipal waterlines lose pressure, potentially sucking contaminated groundwater up into the water supply. In pipes, pressure is carefully monitored with the use of gauges, and is controlled with valves.

In quarantine situations, a room with negative pressure will suck air into it when doors or windows are opened. This prevents contagions from escaping through opened doors and windows, and makes it safer for medical personnel to care for the patient. Most research labs have rooms with negative pressure for studying dangerous diseases, preceded by a series of checkpoints to ensure that only authorized individuals enter the room. Negative pressure pipelines and vent hoods are also used in laboratory situations, to vent dangerous substances away from scientists.

Many homeowners interact with negative pressure on a fairly frequent basis, when they use a vacuum cleaner. When a vacuum is switched on, an area of negative pressure is formed in the bag or canister of the vacuum, which sucks air in as it tries to even out its internal pressure. Along with the air, the vacuum picks up particulate matter, leaving the floors cleaner. The basic vacuum principle is also used in a great deal of electronics and industrial applications.
The understanding and control of negative pressure have greatly contributed to many scientific and technological advances. The opposite principal, positive pressure, or an area of higher pressure than the surrounding regions, is also used to help control environments. Many manufacturing facilities, for example, use positively pressurized “clean rooms” for handling delicate substances such as computer chips.

[B]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.[/B]


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.

Thanks Mohamed. Good information.