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?)