After opening a valve and releasing some gas, it makes sense that pressure in the chamber decreases, but I don't understand why the pressure starts increasing again after a while instead of remaining constant. Hope someone can provide some insight for me :)
Without more details of your specific application and the volumes and pressured observed its difficult to comment with authority, but the effect you have observed may be due to one of the following.
1
A faulty (passing) inlet valve, allowing gas at higher pressure enter the chamber.
2
When you part depressurise the chamber (assuming it is a fixed volume) you may be getting a decrease in temperature (according to the combined gas law) and/or a decrease in temperature due to the JT (Joules Thompson) effect. Then after you stop depressurising and isolate, the chamber slowly increases back to its normal temperature. Since the chamber is a fixed volume, as you increase temperature of gas inside, a small increase in pressure is observed.
Gas remaining in the chamber as it is depressurised has undergone a near isentropic Joule-expansion (NB not isenthalpic Joule Thompson). This is because the gas being left in the chamber is expanding and doing work in accelerating the gas which leaves through the nozzle or valve. The central core can be very cold and the average gas temperature will be generally below ambient. As the gas and the container wall temperatures equalise and as heat leaks in from ambient, the average temperature of the contents will rise, so assuming the valve on the container is closed, the pressure of the residual contents will rise in accordance with the Ideal gas law P=R.T/V, so dP = (R/V).dT
The expansion causes cooling, which makes the gas contract. As it warms again, the gas tries to expand but with the vessel closed, it increases in pressure slightly.
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