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Author Topic: Why does immersing a pressure cooker in cold water cause the pressure to vanish?  (Read 4033 times)

neilep

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Dearest Luvvlies (Happy Christmas by the way !!)

I cooked a lovely dinner tonight in the pressure cooker (You really must get one George)

There are three ways to release the pressure.

I can leave it and it will eventually fizzle out after a long time, I can open up a valve for hot steamy pressure cooked food sauna fun, or, I can immerse the bottom half of the cooker into a sink filled with cold water !...

It's the third option that I am enquiring about. The release of pressure is almost immediate !!..but there's no steam or anything like that so please...can you tell me:

1: How does immersing a pressure cooker in cold water kill the pressure inside a sealed pressure cooker ?

2: Where on earth does the pressure go ?

THANKING you in advance

By the way...dinner was delish !!
« Last Edit: 24/12/2006 09:13:29 by chris »

Soul Surfer

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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.

Early steam engines used this process to get power.  first the steam was let into the cylinder to push the piston up.  Then some cold water was let into the cylinder to make it condense into water and the atmospheric pressure pushed the piston back down
« Last Edit: 06/11/2006 23:24:20 by Soul Surfer »

neilep

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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.

THANK YOU IAN...thank you so much !

Now that you have explained it, it does seem obvious and I yet again find myself humbled by you.

Thank you for your continued presence here.

eric l

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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 100C, at 0.8 atm it is 93C and at 1.2 atm it is 104C.  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 96C.

Here we lowered the boiling temperature by decreasing the pressure, with your pressure cooker you lowered the temperature in order to decrease the pressure.

elegantlywasted

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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.

Karen W.

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 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?


Heliotrope

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Merry Christmas ?
It's November.

Humbug.
;-)

neilep

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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 100C, at 0.8 atm it is 93C and at 1.2 atm it is 104C.  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 96C.

Here we lowered the boiling temperature by decreasing the pressure, with your pressure cooker you lowered the temperature in order to decrease the pressure.

Wonderful experiment Eric ..THANK YOU

neilep

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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.

WOOO !!...this sounds just as fun....presumably this should be done at arms length eh ?

neilep

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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?



...and a Happy New Year too Karen Mam  ;)

neilep

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Merry Christmas ?
It's November.

Humbug.
;-)

Happy Christmas to you too Dave !!...that's the spirit !!  ;)


Heliotrope

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Karen W.

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HAPPY NEW YEAR TO YOU TOO NEIL! YOU TOO HELIOTROPE ( IS IT DAVE ) ?  My favorite time of year.... 41 shopping days left YIKES!! I AM LATE AGAIN!!

 

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