It describes the relationship between the pressure drop across an orifice valve or other assembly and the corresponding flow rate.
Mathematically the flow coefficient Cv (or flow-capacity rating of valve) can be expressed as
For gas flow in a pneumatic system the Cv for the same assembly can be used with a more complex equation.
With Cv = 1.0 and 200 psia inlet pressure, the flow is 100 standard cubic feet per minute (scfm).
The flow is proportional to the absolute inlet pressure, so the flow in scfm would equal the Cv flow coefficient if the inlet pressure were reduced to 2 psia and the outlet were connected to a vacuum with less than 1 psi absolute pressure (1.0 scfm when Cv = 1.0, 2 psia input).
The complete definition also says that the flow medium must have a density of 1000 kg/m3 and a kinematic viscosity of 10−6 m2/s, e.g.