When you boil potatoes on the stove top, it can take half an hour to cook them 'til tender. However, when you boil them in a pressure cooker, presto, they're ready in about five minutes. What's the difference? What makes pressure cookers cook so quickly?
Let's start with the potatoes boiling on the stove top. The temperature of the boiling water won't go above 212 degrees Fahrenheit, no matter how high you turn up the burner or how long you leave the pot on the stove. That's because, at normal air pressure, water boils at 212 degrees and escapes as steam.
In a pressure cooker, on the other hand, the steam can't make such an easy getaway. The sealed lid keeps the steam inside, raising the pressure inside the cooker to about twice normal air pressure.
Now here's the secret of speedy pressure cooking. As pressure rises, so does the boiling point of water. At the higher pressure inside the cooker, the boiling point of water goes up to about 252 degrees. As a result, food cooks at a much higher temperature in a pressure cooker, and thus, cooks more quickly.
Pressure cookers control cooking temperature by maintaining a certain pressure. A vent on the lid releases steam when the right pressure, and therefore the right cooking temperature is reached. To prevent disasters like boiled potatoes exploding onto the ceiling, modern pressure cookers also have a safety valve to release steam, just in case the vent gets stuck.