# How does PSI work?

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I know that the average PSI at sea level is 14.7 PSI, I just don’t understand why gauges and other PSI measuring equipment dont account for outside atmospheric conditions.

For example, if you were filling a scuba tank at sea level, the gauge would say 0 PSI instead of 14.7. And when you do fill the tank up by 100 PSI (I don’t know how much a scuba tank holds), it would say 100, not 114.7.

It’s the same with vacuum chamber gauges. Instead of going from 14.7-0, they go from 0 to around -30 PSI. Where is the extra 15.3 PSI coming from? I assume that it is just standard to start at 0 PSI for all gauges, but it’s a bit confusing. Because if you were were on top of a mountain then the gauge wouldn’t be accurate.

In: Physics

Those gauges can only measure the difference in pressure between the two sides.

If both are at 14.7 psi then there’s 0 difference. This is known as “gauge pressure” or “differential pressure” (those are slightly different but that doesn’t matter for ELI5 imo)

There’s a *ton* of different ways to measure pressure but a lot of them still follow this same principle of only measuring a difference in pressure. A really simple one is a tire pressure gauge with that little stick. That works by having air exiting the tire push against the piston. The higher the pressure the farther it pushes the stick. But there’s the thing. The surrounding air is also pusing on the stick from the other end. So if you tire was at the exact same pressure as the surrounding air it won’t be able to overcome that and the stick wont move.

There ARE ways to measure the absolute pressure, which would show that 14.7. But that just isn’t needed for most things, and it’s harder to measure so we don’t bother.

Why would the gauge read 14.7 instead of 0, or 114.7 instead of 100? The gauge is telling you the pressure inside the rigid tank. The extra 14.7 you’re referencing is outside the tank.

They do account for it in more technical fields you will often see 0psi(a) or 0psi(g). (a) means absolute in which case you would be discussing the pressure compared to atmospheric. (g) refers to gauge, as in this is the pressure of the system discounting the pressure of air, as the gauge has already been calibrated to account for it.

This is done because it is easier to keep track of, and it just makes more logical sense. For instance if I held my finger an inch from your skin and asked do you feel pressure you wouldn’t answer “Yes the 14.7psi from the air”.

The gauges are measuring against the outside air, since that’s generally what you care about. You don’t really care how much air is in your tire, you care how hard that air is pressing out, and that varies depending on how much air is outside the tire. A balloon with the same amount of air might be regular size at your house, very tiny at the bottom of your pool, and pop at the top of a mountain.

The gauges are essentially using a balloon to measure relative pressure, which will tell you how much more the air on one side is pressing than the air on the other side.