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The Difference between Absolute and Gauge Pressure

Two completely different ideas of pressure are more closely related than you think

When you deal with calculating pressures, whether in class or on the job, there are two types of pressure that are important to distinguish between — absolute and gauge pressure. They are different in terms of when to measure them, but conceptually they are very similar.

How are they related?

Well, simply they are related by the equation:

Pabs= Patm + Pgauge

What this says is that that absolute pressure is equal to the sum of the atmospheric pressure and gauge pressure. We’ve already covered it before but for review, we know that the atmospheric pressure is the pressure exerted by the air on an object. Gauge pressure is therefore the pressure that’s exerted on an object after the effects of atmospheric pressure have been removed.

One way to represent this is by imagining the pressure exerted on a ball submerged deep under water, say in the ocean. The ball has two forms of pressure acting on it—the pressure exerted from the fluid onto the ball, which we know is hydrostatic pressure, and the other is atmospheric pressure, which is the pressure added from the atmosphere. Using the above equation, the gauge pressure (Pgauge) in this case is the just the pressure exerted by the ocean.

In the case of absolute pressure for the ball, it is the pressure felt by both the atmosphere and the fluid. Most of the time, the pressure encountered from the atmosphere is ignored either because it is neutralized from the pressure acting on both sides (say in a pipe), or because the pressure is very small in comparison to the total pressure we are looking at (like in the example of a ball being deep underwater). However, it is important to realize that the atmosphere always exerts some sort of pressure on everything, including you and me. This is why when astronauts go into outer space (where there is no pressure), they have to wear suits so that their body does not expand in strange ways.

Where could you encounter it?

Besides in the AP Physics 2 exam, if you do research or are purchasing equipment that is pressure dependent, such as a compressed gas tank, you may see the pressure presented to you in terms of absolute or gauge pressure, and you would have to understand how to decipher it. So, again, gauge pressure is generally without atmospheric pressure taken into account whereas absolute pressure is the sum of both the atmospheric pressure and also gauge pressure. Most of the time, you'll be dealing with gauge pressure, but it's important to understand the difference because many exams, especially the AP exam, may try to ask you questions about both.

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