# If I hard boil eggs in a pressure cooker, will I get the same results regardless of altitude?

This seems like it should be obvious but for some reason (my chemistry is very rusty) I'm having a hard time articulating why the hardboiled egg recipe in an instant pot should yield the same results regardless of altitude.

Questions:

1. Would I get the same results at altitude using the pressure cooker?
2. Why? or why not?

Example recipe using pressure (via damndelicious.net)

INGREDIENTS:
1 cup water
6-12 large eggs

DIRECTIONS:
1. Add water to a 6-qt Instant Pot®. Place metal trivet into the pot. Gently add eggs on top of the trivet.
2. Select manual setting; adjust *pressure to high*, and set time for 3-7 minutes.* When finished cooking, quick-release pressure according to manufacturer’s directions.
3. Cool eggs in a bowl of ice water for 1 minute. Drain well and peel.

NOTES:
3 minutes: Soft, just set whites and runny yolks
4-5 minutes: Medium, firmer whites and jammy yolks
6 -7 minutes: Hard, firm whites and fully set yolks

• The question is what exactly sets the pressure? Having an Instant Pot myself, I believe the pressure is set by the temperature of the heating element, so the pressure inside the pot should be independent of the pressure outside the vessel. Oct 3, 2020 at 20:17
• A traditional pressure cooker uses a weight that blocks the opening. In that type of setup, the pressure of the pressure cooker is relative to ambient, not an absolute setting. Oct 3, 2020 at 20:52
• @Andrew - the Instant Pot does not use a weight. High vs low pressure is just a setting on the control panel. And the safety valve does not set the pressure. Just different equipment. Oct 3, 2020 at 21:31
• Instant Pot uses a mechanical technique to control the pressure, described here: instantpot.com/portfolio-item/… Oct 4, 2020 at 5:52

According to their website, the manufacturers say "The Ultra and Max models of Instant Pot come with the ability to adjust pressure settings to account for altitude."

So, yes, the timings will be affected by altitude, even in a pressure cooker. This is probably because the pressure inside the cooker is controlled by some method that uses the difference in pressure between the inside and the outside. Decrease the external pressure and that will decrease the internal pressure.

There is a USDA factsheet on high altitude cooking: https://www.fsis.usda.gov/shared/PDF/High_Altitude_Cooking_and_Food_Safety.pdf

The size of the eggs will probably have an even bigger effect on the cooking time though, with larger szed eggs taking longer to cook. Since you're aiming for hard-boiled eggs though, it shouldn't be too difficult. Your "recipe" suggests 6 - 7 minutes and even making a very generous allowance for altitude and egg size, 10 minutes should be fine. If you're aiming for hard-boiled eggs, it would be difficult to over cook them.

Soft-boiled eggs would require much more precise timing and that can be best achieved by, and I have to say this, eggsperiment.

• There is language elsewhere on their site that says they explicitly increase cooking times for altitude, indicating that, as you said, the pot creates a pressure difference in pressure-cooker mode, hence the internal pressure, and thus the cooking times, will be affected by ambient pressure: "there is no indication the altitude has been adjusted. However, you may note the Cook time will be adjusted once the altitude has been updated. For example, depending on your altitude, a 00:30 minute Cook cycle may be automatically adjusted to 00:34 minutes when it begins cooking." Oct 4, 2020 at 6:27
• eggspiriment!!!! Oct 5, 2020 at 21:17
• I actually prefer soft boiled. I'm at sea level now. I guess the thing to do is perfect my recipe here and then take my pressure cooker to altitude and try it there. ...or zoom with my friend who has the same cooker and lives at altitude. Oct 5, 2020 at 21:20

An ideal pressure cooker is an isolated vessel in which what goes on inside is independent of what happens outside. It allows the temperature of what you put in the cooker to be raised until the vapor pressure reaches a set point (a pressure higher than the pressure at the standard boiling point of water). Since cooking time depends only on the temperature of the water it should remain the same at different altitudes provided the inside of the cooker is pressurized to the same level.

However, pressure cookers are usually fitted with safety valves (or some system that regulates the pressure to prevent accidents). A safety valve prevents the pressure within the vessel from climbing to a dangerously high level which might result in a failure. The safety valve is actuated whenever the pressure difference between interior and exterior exceeds a safety threshold, reducing the internal pressure back to a safe level. The pressure inside such a cooker does depend on the external pressure (it cannot be higher than the difference deemed safe).

As an interesting aside, the Wikipedia writes that

The pressure cooker was invented in the seventeenth century by the physicist Denis Papin, and works by expelling air from the vessel, and trapping the steam produced from the boiling liquid inside. This raises the internal pressures and permits high cooking temperatures.

In fact, what Papin was first to do was to routinely incorporate a safety valve into his pressure cooker designs. The modern pressure cooker would not be safe without such mechanisms.

When we cook something inside a pressure cooker, we are able to manipulate the pressure inside of the pressure cooker. This is useful, because we know that changes in pressure can cause changes in boiling point. Let me explain.

## Boiling Point In Terms of Pressure

A chemists definition of boiling point is the point at which the vapor pressure of the liquid overcomes the atmospheric pressure. The vapor pressure increases as we increase the temperature of a liquid.

Let's say that we were to boil a pot of water at sea level, where the pressure is very high. It would require a lot of heat to get the vapor pressure high enough to overcome the atmospheric pressure. Now let's say that we boil a pot of water at the top of Mt. Everest. Since the air pressure is so low, it would require less heat to overcome the atmospheric pressure and therefore boil the water. Thus, a pot of boiling water at 0 ft above sea level is hotter than one at 100000 feet above sea level.

Now that you understand the basic concept of boiling, lets investigate how a pressure cooker works. By manually increasing the pressure inside the container, we are able to cause the water to boil at a higher temperature. The outside atmospheric pressure may be different, but that doesn't matter, because the pressure cooker is sealed and the pressure is kept constant inside.

Hope this helps!

When water evaporates the hotter molecules leave and they take some heat with them. Which cools down what's left in the pot. You keep heating your pot and more molecules evaporate, which in turn cools down the pot more. When you just started to heat water - the amount of heat absorbed by the water is greater than the cooling effect of evaporation and the temperature keeps rising. But once you get to boiling - these processes balance each other. You keep heating, but evaporation keeps cooling down the pot at the same rate. Hence no net increase of temperature.

But if you were to trap the evaporated molecules in the pot? They wouldn't leave and thus they wouldn't cool down the pot. So you'd be adding more heat and there won't be evaporation to cool it down. The temperature will keep rising and rising.

When you cook in normal conditions the air particles keep hitting water (they exert pressure) which controls how many water particles are able to evaporate. At high altitudes the pressure goes down and so the water evaporates more readily (and thus cools the pot down faster). The goal of pressure cooker is to trap some of the evaporated water and thus keep the pressure high.

PS: Physics SE is a better place for such questions.