I am working on a problem trying to find out why a 0.010” orifice can be used to open a pilot actuated, spring return pneumatic 5/2 directional control (spool) valve, but a 0.0045” orifice only seems to work intermittently. The air system that I am working with is 100 psig. The air enters the control valve, is routed thru Ø0.063” passage thru the metal valve body, thru a flame arrestor, thru the orifice, thru a Ø0.068” x 6” length tube, and into a pilot. One the pilot is opened the air flows thru a Ø0.068” x 5” length tube, into a Ø0.068” orifice, thru another flame arrestor, and into a different port of the valve body, with the same Ø0.063”, and into a chamber where the air is trying to force the piston to open. The piston is being held closed by a spring. The spring rate is 4.59 lb/in (or 804 N/m). While assembling the valve in the assembly, the spring is compressed by 0.183” from free length. When the piston is moved to the open position, it compressors the spring an additional 0.209” The diameter of the piston is 0.407”. I do not have a device to measure flow rate of the air going thru the orifice, but I did some online research that say that thru the Ø0.0045” orifice the air flow at 100 psig should be 0.0215 CFM and the air flow thru the Ø0.010” orifice at 100 psig should be 0.1412 CFM. If I do simple calculations for the forces, setting the spring force equal to the force (thrust) from the air, I get that the valve should open at ~ 34-35 psig. But, I know flow rate HAS to have something to do with it , but can’t decipher what that is. How do I figure out what is keeping the valve from piston from actuating with the Ø0.0045” orifice, but working with the 0.010” orifice?