I built a pressure chamber to test small parts for leaks at 1000 psi. I can measure the pressure with an accuracy of ± 1 psi. I found that small changes in ambient temperature ± 2 °F can result in a slow but profound change in the pressure in the system, say 25 psi. Insulating the chamber helped a little, but I really need to control for the change in temperature associated with changing pressure. As a side note, with the ambient temperature constant, running the system from 0 to 1000 psi over 2 minutes and then monitoring the pressure always results in a slow and relatively consistent pressure drop that slowly tapers off. If the pressure is then dropped over 2 minutes to 200 psi and then monitored, the pressure slowly rises consistently over time again slowly tapering off. These results lead me to conclude that pressure induced temperature change is the cause.