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)?