1. Sep 3, 2004

ntdmods

When operating a pressure cooker (autoclave) at high altitudes, why is it necessary to adjust pressure or cooking times to maintain the target temperature ? The weight of the ball that holds in pressure is still the same, therefore temperature and pressure should be the same. At 15psi the temperature should be 121°C at sea level or at 5000 feet. Since the weight that is keeping in pressure is always constant, the pressure inside the vessel should also remain constant. But, altitude adjustments are usually recommended. Is the reduction of air pressure at higher altitudes enough to affect the weight?

Edit: Let me see if I can explain the problem a bit better...
Air pressure at sea level is roughly 14.7psi. Air pressure at 5000ft is roughly 12.2psi. Would this difference in pressure have any significant effect on the internal pressure of the vessel by acting upon the weight used to regulate the pressure? Basically there is roughly 2.5psi difference between the altitudes. Would this correspond to a 2.5psi difference in the internal pressure of the vessel? I.E.- it would only reach a maximum of 12.5psi at 5000ft instead of 15psi at sea level?

It makes sense to me that if I were to take a pressure cooker at sea level pressure (cooking at 15psi) and reduce the external pressure to that at 5000ft, the weight regulating internal pressure would be more easily displaced. Thus, steam would vent violently from the sudden change in external pressure.

I don't know which way to go on this. Since the weight is constant, I would think that the pressure would also remain constant. But then again, if the outside pressure is lower, it seems as though there would be less force acting upon the weight, therefore allowing it to vent internal pressure at a lower external pressure.

If so, would this loss in external pressure acting upon the weight be enough to alter internal pressure significantly? Is there any way I can calculate the difference? Is the difference in direct relation to the external pressure (I.E. - a 2psi difference in external pressure would result in a 2psi loss of internal pressure)?

Last edited: Sep 3, 2004
2. Sep 3, 2004

HallsofIvy

Staff Emeritus
Believe it or not there exist websites with info on this! I googled on "Pressure Cooker" and "altitude" and got this:
http://missvickie.com/library/altitude.html
(I wonder if that is Tiny Tim's "miss vickie"?)

Basically, the point is that the pressure cooker controls ITS pressure in reference to the outside pressure. If you are using a pressure cooker at high altitude, the pressure cooker sets its internal pressure PROPORTIONAL TO the outside air pressure which is slightly lower than at sea level.

Last edited by a moderator: Apr 21, 2017
3. Sep 3, 2004

ntdmods

Ok...

First, I have googled previously on this subject, and indeed found the link you provided. I have not found a website that explains why this happens though, which is why I posted here.

I think I understand the concept now after sleeping on it. For example:
A pressure cooker operating at sea level with a 15psi internal pressure will actually have a 29.7psi absolute pressure. At 5000ft altitude, the outside air pressure is only 12.2psi, therefore the internal absolute pressure would only be 27.7psi. The weight still maintains 15psi internal pressure, but since the outside pressure has been reduced, the absolute pressure is also reduced. Thus the need for increased cooking times or increased pressures (just if anyone is curious, this is not for cooking but sterilization of media).

Am I correct?

4. Sep 3, 2004

GOD__AM

The lower pressure of the air lets the valve blow off steam more frequently and as a result heat is released more frequently.

5. Sep 4, 2004

LURCH

...Because the amount of pressure pushing on the weight is calculated by the difference between internal and external pressures?