While cooking food in my pressure cooker today, I pulled the whistle using my spatula to let the steam off before I opened it to check on how well cooked the food was. Before, I opened the pressure cooker, I let the whole steam drain out. I opened the cooker, saw the food wasn't cooked enough and closed the lid again. Note that I turned off the stove while doing this.

For some reason, I tried pulling the valve again after closing the lid, and interstingly, gas came out of it. How is this possible?

  • $\begingroup$ By mistake I posted it two times. Not sure what happend. $\endgroup$ Feb 7 at 13:13

1 Answer 1


The pressure cooker (and possibly also the stove under it) was still hot. When it was opened, some colder air entered, it was then heated when the cooker was closed, and the pressure inside became higher than the pressure in the surrounding room.

A somewhat similar thing happens when you open a sparkling water/Coca-cola/Champaign bottle and then close it again - although the pressure builds up for a different reason.

  • $\begingroup$ "it was heated when the cooker was closed"..? I a m positive that the stove was off during the process I described $\endgroup$ Feb 7 at 13:14
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    $\begingroup$ @TrystwithFreedom, It didn't instantly assume room temperature when you turned it off. It still was hot. It was hotter than the room air that entered when you opened the lid. You trapped cooler air inside the hot vessel when you re-closed the lid. Heat lingering in the vessel warmed the trapped, cool air; and that's what caused the pressure increase. Pressure is inversely proportional to absolute temperature for a trapped volume of gas. $\endgroup$ Feb 7 at 13:40
  • $\begingroup$ It wasn't only air that got heated after entering. A bigger effect was more water vaporizing within the cooker. $\endgroup$ Feb 7 at 13:53
  • $\begingroup$ @ChetMiller good point. I agree. $\endgroup$
    – Roger V.
    Feb 7 at 14:25
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    $\begingroup$ To add to this, even if the pressure cooker is turned off or removed from the heat source, the latent energy inside may still be enough to continue to boil water to add steam pressure other than the cooler room-temperature air being heated. $\endgroup$
    – David S
    Feb 7 at 20:44

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