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I have a new inflatable stand-up paddle board, which is supposed to be inflated to 15 psi. I had to pump a long time and it became very rigid—and when I opened the valve it let out a lot of air. I wondered what the pressure was in atmospheric pressures, and was surprised to see that 15 psi is very close to 1 atm (1.02069 atm).

I'm at a complete loss to understand how something that is evidently under high pressure, is actually at atmospheric pressure—since I am just above sea level. What is the explanation? (Wasn't the air inside the board also at 1 atm before I inflated it, when it was just lying on the ground? What's keeping the rubber taut when it's inflated?)

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    $\begingroup$ 15psi gauge or absolute? Likely gauge, which means it was inflated to 30psi absolute, or 15psi above ambient at sea level. Most inflation gauges read relative to ambient. $\endgroup$
    – Jon Custer
    May 13 at 13:39
  • $\begingroup$ That would make sense then, thank you. What a relief, I thought I must be really off in my understanding of pressure somehow! $\endgroup$
    – adam.baker
    May 13 at 14:57
  • $\begingroup$ @JonCuster Please post answers as answers, not comments. $\endgroup$
    – noah
    May 13 at 16:37

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for the purposes of inflating tires, rafts, basketballs, etc. the ambient (atmospheric) pressure is taken to be zero: a tire pressure gauge will read zero pressure when it is actually sensing ambient pressure, 15 PSI. The measured pressure in pounds per square inch is then called PSIG, where the "G" stands for "gauge"

For physics purposes, a vacuum is considered zero pressure and ambient is therefore 15 PSI. So a physics gauge will read 15 PSI when it is not connected to anything. This measurement is called PSIA ; the "A" stands for absolute.

Therefore 0 PSIG = 15 PSIA, and 15 PSIG = 30 PSIA.

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