If rice is cooked in a pressure cooker on the Siachen glacier, at sea beach, and on Deccan plain, which of the following is correct about the time taken for cooking rice?

  • one of the locations cooks the fastest (which?)
  • all three locations cook at the same rate

Pressure is inversely proportional to height.

When you are closing the lid before cooking on 3 different altitudes, the air that gets trapped inside the cooker in the beach is having the highest pressure. So wont the cooking of food be faster in beach since you reach a particular temperature must faster as there is already some pressure inside the cooker? Like if cooking of food has to take place when you reach a particular pressure (inside cooker) isn't it much faster in the low altitude (beach)

  • $\begingroup$ Although its cooking pressure has nothing to do with the external pressure when first sealed, it will still cook fastest at sea level as the pressure release valve is not an absolute but rather a pressure difference between internal and external pressure. $\endgroup$ – Adrian Howard Jan 26 at 19:48

This is a common misunderstanding.

I'll start answering by telling how a normal utensil cooks food. At lower altitudes we know that the pressure is high. And, at higher pressure water boils competitively slower. Therefore, cooking at a higher pressure will obviously be simple. Now, when you go up higher and higher, you'll find that the pressure decreases, and therefore cooking food becomes hard at higher altitudes(since boiling point of water reduces).

Now, in the case of pressure cookers, they work by making a high pressure environment inside them. To be more clear, when you keep a pressure cooker on the stove putting the whistle on it, what happens is that the water inside turns to steam initially. But when more and more water gets converted to steam, the pressure inside the cooker increases a hence the boiling point of water increases. That means, it will take more heat for water to convert to steam and or food inside will get cooked with this heat.

Now again, as I've said before, a pressure cooker works by raising its internal pressure. And it's this internal pressure that helps in cooking the food inside. But infact, the pressure inside the pressure cooker does depend upon the outside pressure. To be more clear, the pressure inside a pressure cooker is the sum of the pressure mentioned on its whistle and the atmospheric pressure. The reason is that the atmosphere exerts pressure on all things on the surface of Earth. As a result, the pressure inside the cooker will vary with the altitude(i.e., it will decrease with increase in height). Obviously, the temperature at which the water boils inside a pressure cooker which is kept at Siachen glacier will be much lower than the temperature at which the water boils inside a pressure cooker at the beach. Therefore, the food will take time to get cooked at the Siachen glacier even though your using a pressure cooker.

You can find more details in this question asked at chemistry.stackexchange and in this faq on pressure cookers by hippressurecooker.
P.S. Sorry for that misleading mistake that I'd made in this answer. And thanks for those corrections😁

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    $\begingroup$ The internal pressure does change with external pressure as the vent is affected by outer pressure $\endgroup$ – Adrian Howard Jan 26 at 19:11
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    $\begingroup$ lee, you'll find that the heat of vaporization of water decreases as temperature increases, so it takes less heat to vaporize a pound of water inside a pressure cooker than it does to vaporize that same pound of water outside the pressure cooker. Also, the pressure cooker maintains a constant pressure difference between the ambient atmosphere and the inside of the cooker, so at lower ambient pressures (aka higher altitudes), the pressure and temperature inside the cooker will be lower than it is at sea level. $\endgroup$ – David White Jan 26 at 19:23
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    $\begingroup$ Given that my Instant Pot instruction manual tells you how you change the cooking time (in the pressure cook mode) with altitude, this answer is not correct for all equipment. $\endgroup$ – Jon Custer Jan 26 at 23:50
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    $\begingroup$ Re, "...at higher pressure water boils...slower..." I think what you are trying to say is that at higher pressure, it takes longer for a given rate of heating to boil the water. The reason why, as you already know, is that at higher pressure, you have to heat the water to a higher temperature before it will begin to boil. It takes longer because it takes more heat to start it boiling. $\endgroup$ – Solomon Slow Jan 27 at 1:32
  • $\begingroup$ Sorry guys, edited my answer and corrected those mistakes. Thanks for the corrections. $\endgroup$ – lee Jan 27 at 6:15

Pressure cookers usually regulate their internal pressure with a weight on the relief valve. If so, the internal pressure is that stated on the weight plus the external atmospheric pressure. As the temperature at which the food cooks is the boiling point of the internal water, which goes up with the internal pressure, the cooking time will depend on the external atmopheric pressure.

At sea level (15 psi or approximately 1 bar) water boils at 212F. With an extra 1 bar (the usual regulator setting) in the cooker the total pressure will be 2 bar and the water will boil at 250F. If you are at 10,000 ft the exterior air pressure is about 10psi or approximately .6 bar. The total pressure in the cooker will then be 1.6 bar. The water then boils at 237F, and this is the temperature at which the food will cook.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – rob Jan 26 at 18:51

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