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First, I understand that the boiling point of water is increased as the pressure is increased. At 15 PSI, the boiling point is about 250F.

My question, which seems like nobody can answer with certainty, is this: Is there a continuous boil within the pressure cooker once it reaches temp/pressure?

I understand that there needs to be a boil at some point to create the steam. But once it reaches that pressure, then what? An electric PC will turn the heating element off until the pressure drops. A user will turn the heat down on an analog PC. Does the liquid just boil constantly while the heat is off? It probably maintains temp & pressure for a lot longer in a sealed environment. In my mind, if there were a continuous boil, that steam can only build so much until it exceeds the pressure that the PC is capable of maintaining and it begins to vent. So, sure you could have a constant boil, but that means it would also be constantly venting, right?

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  • $\begingroup$ No, the boiling stops when the pressure is reached. It is the boiling that increases the pressure. $\endgroup$
    – Digiproc
    Nov 13 '19 at 14:51
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    $\begingroup$ At the boiling point, in a closed container, the vapor and liquid are in equilibrium. Vapor will condense to liquid, and liquid will turn to vapor, but in equal quantities. It is not a static equilibrium, but dynamic on the molecular level. $\endgroup$
    – Jon Custer
    Nov 13 '19 at 15:10
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    $\begingroup$ @JonCuster Why not post as an answer? $\endgroup$
    – Bob D
    Nov 13 '19 at 15:11
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    $\begingroup$ @Digiproc Your comment is not correct. You can maintain boiling as long as you continue to supply heat. Once you stop supplying heat, boiling will stop. But, if you turn on the heat again, boiling will start again. $\endgroup$ Nov 13 '19 at 15:30
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    $\begingroup$ @ChetMiller Right, I just tried to extend it to the general case where there's no pressure relief. I feel inclined to point it out, because heating liquids in an unvented closed space can lead to some really bad situations. $\endgroup$
    – JMac
    Nov 13 '19 at 17:54
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It seems likely that the lid of a pressure cooker would be at a slightly lower temperature than the heated bottom. This could allow the steam from a very slow boil to condense without venting.

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  • $\begingroup$ From what I've seen of pressure cookers, this does seem to be the case. I didn't even really think about that. There's pretty much always a layer of dripping condensation on the lid (or at least with glass lids). $\endgroup$
    – JMac
    Nov 13 '19 at 18:33
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The way my older pressure cooker works is that, once it reaches operating temperature, it boils all the time. It has a pressure relief valve that has a weight that is placed over a conical tube sticking up in the lid. The weight has different diameter holes drilled partway through it, so that as larger a hole is placed over the conical vent tube, it will go down farther to have a larger sealing surface, so less steam pressure will lift it to release steam. This changes the relief pressure so it can be set to boil at different temperature / pressures. It is made to be used on a cook stove. If a lot of steam is coming out of the vent (rapid boil), you can reduce heat. If no steam is coming out of the vent you can increase heat, so that ideally, a little steam comes out so you know it is boiling. If it is not boiling, it may be below desired temperature.

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