01
What the choked-flow region means
When the downstream-to-upstream absolute pressure ratio is sufficiently low, the controlling passage reaches sonic conditions. Further lowering downstream pressure does not materially increase mass flow at the same upstream condition. In this region, sonic conductance provides a compact description of component capacity.
02
Why critical pressure ratio b is also needed
The b value identifies the transition between choked and subsonic behavior under the ISO model. Once the operating pressure ratio moves out of the choked region, the predicted flow depends on both C and b rather than C alone.
03
Use catalog C values carefully
Confirm the units, reference atmosphere, test direction, component configuration, and whether the rating covers a fixed or variable flow path. Regulators and components with strong hysteresis or internal feedback may require their own flow curves rather than a simple steady-state coefficient comparison.
Selection checklist
What to verify before selection
- 01
C units and reference state
- 02
Critical pressure ratio b
- 03
Upstream absolute pressure
- 04
Downstream absolute pressure
- 05
Test direction and component state
- 06
Required flow at the real operating point
Application caution
Common mistakes
- Entering gauge pressure where absolute pressure is required
- Using C without b in the subsonic region
- Assuming C, Cv, Kv, and nominal flow are interchangeable labels
FAQ