01
b: definition and engineering context
Critical pressure ratio. ISO 6358 ratio marking transition between choked and subsonic flow behavior.
The term belongs to the flow & valves context. Its exact limits can depend on the cited standard, catalog convention, unit system, reference state, and component configuration.
02
Why b affects pneumatic selection
Needed with sonic conductance for standardized pneumatic flow calculations.
- Choked flow: Increasing component conductance or upstream pressure may be needed for more flow.
- SCFM: Always confirm the reference temperature, pressure, and humidity convention.
- NL/min: Normal conditions vary by convention; record the supplier basis.
- ANR: Keep ANR values tied to the catalog's stated reference definition.
03
How to evaluate b in a catalog or replacement project
Read the value together with its unit, test basis, operating pressure, temperature, circuit state, and manufacturer definition.
For replacement work, compare the complete datasheet and drawing. A matching abbreviation or nominal size does not by itself prove functional or dimensional interchangeability.
- Sonic flow: Condition where mass flow no longer increases by lowering downstream pressure at fixed upstream conditions.
- Standard cubic feet per minute: Volumetric flow referenced to stated standard conditions.
- Normal litres per minute: Volumetric flow normalized to a stated reference condition.
- Atmosphère Normale de Référence: Reference-air condition used by some pneumatic catalogs for normalized flow.
Selection checklist
What to verify before selection
- 01
Needed with sonic conductance for standardized pneumatic flow calculations.
- 02
Increasing component conductance or upstream pressure may be needed for more flow.
- 03
Always confirm the reference temperature, pressure, and humidity convention.
- 04
Normal conditions vary by convention; record the supplier basis.
Application caution
Common mistakes
- Treating b as a universal definition without checking the manufacturer or standard context
- Mixing gauge and absolute values, incompatible units, or different reference conditions in one comparison
- Selecting a component from the term or nominal value alone without checking the complete operating point and drawing
FAQ