Re: Measurement - Pressure Drop vs. Leak Rate
If I have a closed system with 10 psi in it, and I measure the pressure again after 5 minutes and I have 8 psi in it, what is my average leak rate? I think it is 2 psi/5 minutes which is 0.4 psi/min.
When I connect an Air Data Test Set up to an aircraft and perform a leak check on the pitot-static system, I'm going to get pressure per unit time (or altitude or airspeed/time), and I'm not going to care about the volume of the system--Piper Cub or 787. The test set doesn't care.
What is of concern are the variables--most important being adiabatics. Unless the system is temperature stabilized, gas temperature changes will cause pressure changes--for a given volume--that will mimic a leak. Leak checks performed immediately after evacuation or pressurization will run that risk.
So, as a baseline, I would pressurize the system, wait 60 seconds, and measure/record the pressure every minute for 5 minutes. I would plot the values and see how curvy it was. Too curvy at the top means wait longer before starting to record. If it's not too curvy, then establish a 1st order slope (regression line). You could then predict a pressure leak anywhere along the line for a given time.
In other words, for a 2 minutes leak check x, after waiting to stabilize, you should get a pressure value y ?some error.