01
Why Cv is not a universal flow rate
Cv originated as a coefficient rather than a direct volumetric-flow rating. For compressible air, calculated flow changes with pressure ratio and reference conditions. A catalog's nominal flow test may use another standard or another upstream/downstream condition, so identical-looking numbers are not automatically comparable.
02
Compare coefficients on the same basis
Cv and metric Kv are related but not numerically equal. ISO 6358 commonly reports sonic conductance C with critical pressure ratio b. Use an explicit, documented conversion only when the source definitions, units, and reference states are known; otherwise compare manufacturer curves at the intended operating point.
03
Select for the complete pneumatic function
Flow capacity is only one valve requirement. Confirm valve function, normal state, actuation method, minimum pilot pressure, voltage, port thread, media, pressure range, leakage, response time, and environmental approvals before treating a Cv match as a replacement match.
Selection checklist
What to verify before selection
- 01
Coefficient convention and units
- 02
Upstream absolute pressure
- 03
Downstream pressure or pressure ratio
- 04
Reference temperature and air condition
- 05
Required actuator fill or exhaust time
- 06
Valve function and pilot limits
Application caution
Common mistakes
- Reading Cv as a direct SCFM value
- Treating Cv and Kv as numerically identical
- Selecting a replacement valve from coefficient and port size only
FAQ