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# What is the relation between Absolute pressure , Gage pressure, Vacuum pressure, and Atmospheric Pressures?

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What is the relation between Absolute pressure , Gage pressure, Vacuum pressure, and Atmospheric Pressures?

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Atmospheric pressure is the pressure applied by the air on the ground as a result of the fluid volume of air directly above.....it is roughly 14.7psi or 1 bar. Atmospheric pressure is constantly changing asa result of the eweathter...stormy is generally low pressure (about .96bar) whereas fine weather can be a result of high pressure (1.012 bar) as a result this is standardised as atmospheric pressure at 1 bar....a more useful unit is mBar, but can also be psi, mmHg etc.

The variation above or below 1 bar of pressure as measured is generally known as "gauge" pressure as it does not take into account the surrounding atmospheric pressure (which is hydrostatic btw) therefore only measuring the difference between the "zero" which is in effect atmospheric and the vessel being measured.

Similarly vacuum pressure is a measure of pressure below that of surrounding ambient....generally in Bar or Torr. Vacuum pressure is ALWAYS that below the surrounding medium (which is generally air although occasionally within the confines of a high pressure environ. ).

Absolute pressure is a corrected pressure which takes into account the surrounding air pressure to read the true pressure. It is not always the case, but it can generally be read as pressure + 1bar as it completely negates the effects of the surrounding medium...comparing directly to a total vacuum.

Absulute Pressure = atmospheric Pressure + Gauge Pressure, and when it is zero then it is Vacuum!

Gauge pressure is the pressure measured by an instrument. Atmospheric pressure is the ambient air pressure. Absolute pressure=gauge pressure+atmospheric pressure

For gauge pressure, P(absolute)= P(atmospheric) + P(gauge)

For vacuum pressure, P(absolute)= P(atmospheric) - P(vacuum)

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