[FrankJ]

Cv is the conductance of flow. (inverse of resistance.) It is a handy way of knowing how much a particular valve or fitting flows. The easiest example is with liquid. A valve with a Cv of 1 will flow 1 Gal/Min with a pressure drop of 1 PSI. Note the unit dependance. Cv with pneumatics is more difficult because the flow is referenced to standard conditions. (Cubic feet per min at 14.47 PSI at 32 F, or 60F or 68F or 70F). Lets not get started with metric definitions. Anyway Cvs determined with consistent standards with let you know the relative size without worrying about all the illogic & math behind it. Which is what standards are all about.

Any way no matter how free flowing a valve is, the restrictions in the 1/4 OD tube restrict the flow to something acceptable to the GDC. I think that was what Al was saying. You would have to know the length of tube to calculate the exact Cv for it.