I'm having trouble calculating the required pressure to get the desired volume flow rate (at outlet conditions). I'm no expert at compressible fluid systems, but I ended up being the one to design it... The system must feed helium vertically through 3/8" ID, 112 ft long tubing (ID can vary, I just chose 3/8" for the availability of very lightweight tubing). The desired flow rate is 100 ft3/min or greater (Outlet Pressure = 0.1083 psig). It would be nice to maximize this and hit flow speeds of Mach 1 at the outlet. So far, I have used Fanno Flow equations and found an inlet pressure of about 1.9 psig. The flow rate here came out to about 100 ft3/min. Does this pressure make sense? It seems a bit low to me... especially to hit Mach 1. Am I headed in the right direction at least? If anyone has experience with this, any help would be appreciated, Thanks.