Title basically. Why is a tire with no air at 0 psi if atmospheric pressure is at 14.7. Donee just not count that and the psi gauges compensate for it? Or is something else going on?
In: Mathematics
14.7 psi is the ambient pressure all around us, right? psi gauge inserted into a tire is measuring the *difference in pressure* between in the tire and outside the tire. I think you were sort of guessing that yourself in your question. It seemed like you were thinking along the right lines.
If the tire was so empty that it was less pressure than outside, you could imagine a negative reading on the gauge even though there’s some pressure exerted by the air lingering in the tire.
What you are observing is called gauge pressure. Its common in things like tires or other pressure systems, where the gauge measures how far above or below atmospheric pressure something is.
It does that because the way gauges measure pressure is by having the gas in the container push against the surroundings of the gauge (usually on a spring).
So it just tells you what the pressure is vs the surroundings, where that is air, water, air on top of a mountain or whatever.
For the same reason you zero a scale when you put a container on it…so you only get the weight of the contents, not the contents and the container.
You want to know what the pressure is minus the atmosphere. So the gauges come with the atmosphere zero’d out already.
Gauges are showing relative pressure. They are showing you the differential between the outside pressure vs. the pressure that is measured by whatever is providing resistance in the gauge. Since we are always assumed to be at 1atm, pressure measurements for most purposes are given relative to that value.
That’s a relative pressure. 0 psi just means there’s 0 pressure difference inside vs outside of the ball.
If we were to put the ball in a vacuum chamber and remove all the air, we would measure 14.7 psi inside the ball and it would appear to inflate
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