I've lived at a high altitude (between 6000' /1830 m and 7200'/2200m) for several years now, and I'm still trying to find consistency in high-altitude baking and the adjustments needed for different types of bakes. I've been experimenting with baking in my pressure cooker (instant pot). Since low air pressure and higher rate of evaporation are two big reasons for adjusting recipes at a higher elevation, this made me wonder—does using a pressure pot negate the need for high-altitude adjustments, or is there another set of adjustments needed that would be different from a standard oven? Does the pressure inside a pressure cooker remain constant despite the elevation at which it's being used?
You still will need to adjust for altitude with an instant pot. The cooker is adding 12-15 psi above the ambient pressure, which is lower at altitude. That means the total pressure inside the pot is lower than most recipes might expect, leading to longer cook times.
A bit of the science is explained here:
Practically speaking, what all that science amounts to is this: In a sealed pressure cooker, the boiling point of water goes up as pressure increases.
At standard atmospheric pressure, the boiling point of water is 212°F. But in a standard American pressure cooker, the pressure reaches 1 atm or 15 psi (pounds per square inch) above standard atmospheric pressure*, or 2 atm, which is typically the maximum pressure limit on most cookers. At 30 psi, the boiling point of water is about 250°F.
Using a Pressure Cooker at High Elevation
What about pressure cooking above sea level? You might be aware that general cooking times and temperatures for certain recipes differ in places like Denver, CO, or high up in the Andes. At high altitudes, the atmospheric pressure is lower**. For example, in Denver, the ambient pressure is around 12.2 psi.
** Pressure is lower at higher elevations because most of the air molecules in the atmosphere are held close to the earth’s surface by gravity, which means there are fewer air molecules above a higher altitude surface than there are above a surface at a lower altitude.
In general, a pressure cooker adds pressure above the given atmospheric pressure. That means the force that closes the valve as pressure builds in the chamber includes the force of atmospheric pressure. For example, if the atmospheric pressure in Denver is 12.2 psi, then the absolute pressure of the chamber at full pressure is 27.2 (12.2 psi + 15 psi)—nearly 3 psi less than at sea level. Looking at our trusty ideal gas equation, we know that lowering pressure will lower the temperature in a system. In this case, the boiling point of water in a sealed chamber cooking at high pressure will be 244.8°F, almost 6 degrees lower than the same system at sea level.
Of course, a lower boiling point means slower cooking. What does that mean for you? It means you have to increase cooking time to accommodate lower pressure and lower cooking temperature in order to get the same results. A good rule of thumb is to increase cooking time by about five percent for every 1000 feet above 2000 feet elevation.