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If I inflate my bike tire to say, 60psi, which is about 4atm. Does this mean the pressure on the inside of the tire is 4atm + 1atm? or does it mean it's actually 4atm?

It seems only reasonable that the pressure inside is 4atm + 1atm, because my bike pump (for example) doesn't start at 15psi (1atm). Does this mean that if I took my bike pump to space it would then read -15psi (-1atm)?

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  • $\begingroup$ It really depends on what your instrument is meant to measure, absolute pressure or differential pressure w.r.t. atmospheric pressure. Roughly speaking, you can easily realize if the manometer you're using is absolute of differential looking at the measurement when the tire of your bike is flat: if you read 0 -> differential; if you read the atmospheric p -> absolute manometer. If you read something else -> wrong offset of the instrument $\endgroup$
    – basics
    Commented Sep 28, 2023 at 18:59
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    $\begingroup$ $\uparrow$ also worth mentioning that differential is also known as gauge pressure if you've seen that term before. But this should really just have made this an answer rather than a comment. $\endgroup$
    – Triatticus
    Commented Sep 28, 2023 at 19:25
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    $\begingroup$ Does this answer your question? Gauge pressure clarification $\endgroup$
    – Jon Custer
    Commented Sep 28, 2023 at 20:36

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The quoted pressures are what are called “gauge pressures” ie the pressure above atmospheric pressure which is around 15 psi.

So a gauge pressure of 60 psi corresponds to an absolute pressure of 60 + 15 = 75 psi.

Does this mean that if I took my bike pump to space it would then read -15psi (-1atm)?

Yes, if the pressure gauge allowed a negative pressure reading.

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