Contrary to popular belief, water never reaches its boiling point inside the pressure cooker, which accelerates cooking by the simple increase in water temperature.
Pressure and temperature conditions inside the cooker (1→2) prevent the liquid from boiling, except if the water steam is cooled quickly provoking a fast decrease in pressure (2→3).
If we watch the diagram, the boiling point will be achieved as long as the called “state change line” is crossed, this line separates the liquid (L) and gas (G) areas in the thermodynamic diagram. However, when the lid of the pot is closed most of the gas inside it will be air, not water steam, so at every point the pressure inside will be the addition of water steam, whose amount is increased due to evaporation as temperature rises, and due to air, whose partial pressure is responsible of, as the pressure cooker heats in the inside, it goes farther and farther from saturation, which prevents water from boiling inside the pot, i.e. the state change line never crosses the 1→2 line that represents the evolution in pressure and temperature conditions inside the pot.
At the same time, once maximum pressure is achieved that the valve determines (by its weight or a spring) in the inside of the pot, it cannot be changed, and kept in high heat does not accelerate cooking, it simply increases water evaporation and steam loss through the valve.
Exceptionally, boiling can be produced if a quick cooling of the air and water steam mix is produced; for example, if we wish to open the pot quickly and we place the pot under running water. The water cools the container’s walls, which produces water steam condensation and quick pressure decrease in the inside of the pot so it reaches the state change line (2→3), and sudden water boiling is produced with such virulence that it can even cause liquid escaping through the sealing gasket or the valve itself. It can also happen if the pot is opened when still pressurized, with the risk of suffering burns by liquid splashing or the steam itself.
The pot can pressure by itself, if after cooling and opened the steam inside escapes. If it is closed again, as it continues to cool, pressure decrease and vapo
rization is produces at less temperature, with new steam production and movement of the escape valve.
Mountaineers also use pressure cookers, since otherwise cooking can be difficult. At sea level atmospheric pressure is 1 atmosphere and boiling temperature in an open sauce pan is 100° C. However, at higher altitude atmosphere pressure is less and, therefore, water boiling temperature decreases, as Charles Darwin described in The Beagle Journey:
«In the place where we slept water boiled at a lower temperature that in lower altitude due to a reduction in atmospheric pressure, being the opposite case in the Papin pot. So, after several hours cooking in the water potatoes were almost the same as in the beginning. The potatoes were even left in the fire during the whole night and boiled again in the next morning and they were still not cooked. I remembered that while I listened my mates discussed the reason for that; they had reached the conclusion that the bloody potatoes were not cooking material.
Oddly enough, the higher the altitude the faster a pressure cooker Works, since if the starting point (1) is lower, the 1→2 line would reach a higher temperature in the pot’s working temperature.