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That's easy. The pressure in the cooker is steam produced by boiling the water in the cooker and when you cool it down under the cold tap the moment the temperature drops below 100 degrees c the steam all condenses back into water and leaves a partial vacuum. Some sorts of pressure cooker with a lid that is held shut by the pressure pop inwards to let the air in when this happens.
This reminds me of a rather spectacular experiment we did in physics class.Fill a flat bottomed flask halfway with water, and heat it on a plate till it starts boiling.Switch off the current, boiling goes on. Put on a rubber stopper and press it on firmly. Boiling stops (almost immediately). (If the stopper is not pressed on firmly enough, it will pop off and boiling will start again). With the stopper still on, put the flask bottoms up under a running cold water tap. Bailing starts again.Explanation : The boiling temperature of water - or any other liquid - is related to the pressure above the water. At 1 atm, this temperature is 100°C, at 0.8 atm it is 93°C and at 1.2 atm it is 104°C. When you put on the stopper, steam is entrapped and pressure will rise so that the water does not boil any more. Under the cold water tap, steam condenses, and pressure drops so that the water can start boiling again, even if the temperature drops to 98 or even 96°C.Here we lowered the boiling temperature by decreasing the pressure, with your pressure cooker you lowered the temperature in order to decrease the pressure.
you could also hold a pop can in boiling water for a bit, then immediatly immerse it in an ice bath, the pressure change causes the can to implode upon itsself.
Merry Christmas Neil! I have used a pressure cooker for many many years, I have never known about releaseing the pressure that way! I have always layed a fork on the rocker and lid to allow the pressure to escape. I will have to try that! BTW what loveliness did you make for dinner Kind Sir?
Merry Christmas ?It's November.Humbug.;-)Happy Christmas to you too Dave !!...that's the spirit !!