While reading through my Twitter feed the other day, I stumbled across this conversation:
This caused me to wonder a bit what was going on. After all, the whole point behind a pressure cooker is that it creates its own zone of pressure inside the cooker that is completely separate from whatever is going on outside of the pressure cooker. The only thing that made sense is that the device that regulates the pressure must be affected somewhat by the outside pressure as well.
I recently purchased an electronic pressure cooker, and when I'm not doing food-related things I am working with electronic sensors and the like. So when I was trying to imagine how a pressure cooker worked, I had thoughts of an electronic sensor that fed information back to a microcontroller that would open a valve blah blah blah. Crazy talk. I mean, I'm sure there are pressure cookers out there that do that. They are likely Japanese, and contain the phrase "fuzzy logic" on the marketing materials. They were likely made in the 1990's. But that's way overthinking the problem.
Most pressure cookers regulate pressure with a valve. A valve is a simple device that lets fluid substances such as liquid or gas flow through it under some circumstances, but not in others. Some keep water from flowing through a pipe one direction and not another, some keep gases from flowing through a pipe when it's closed, but let the gases flow when it's open, and so on. Valves have been around for ages. They've been around far longer than microcontrollers and fuzzy logic.
A simple mechanism for a pressure cooker is to have a piece of metal, shaped kind of like a hat, with a hole in it. The metal is of a specific weight. Most of the time, it sits at the bottom of the shaft it rests in, and it blocks liquids and gasses from escaping. However, when pressure builds inside the cooker past a certain point, the hat is lifted, the hole in the hat meets a hole to the outside workld, and the gasses escape until the pressure inside the pot drops enough to let the valve fall back down and keep gasses inside.
This hat mechanism can follow a number of forms, from a weighted ball bearing to a spring and so on. The general idea is the same, though: it provides resistance until the pressure underneath lifts the mechanism up to release gasses. Which is where we hit our snag.
Most of the work with these pressure valves is done by gravity and/or spring forces. These would be relatively independent of air pressure. But some of the work holding down these valves is also done by the atmosphere, which pushes down on everything. Not as much as gravity, of course, but it still pushes. And if the atmospheric pressure is lower in higher altitudes, then it will take less pressure to open the valve, thus lowering the cooking pressure inside the system.
Some pressure cookers come with valves made for high-altitude cooking, so you can replace them and not adjust your cooking time. However, if they don't, you'll just have to add a few minutes to the cooking time. Check your manual or call your manufacturer to find how what adjustments to make. Or you can just follow Jill's advice: