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.
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