# Critical pressure ratio

> Source: https://rodlesspneumatic.com/resource-center/pneumatic-glossary/b/
> Language: en-US
> Category: Flow & valves

## Definition

Critical pressure ratio: ISO 6358 ratio marking transition between choked and subsonic flow behavior. Needed with sonic conductance for standardized pneumatic flow calculations. Read the applicable product data and technical standard before treating a catalog value as interchangeable.

## Key facts

- **Term:** b
- **Full form:** Critical pressure ratio
- **Selection effect:** Needed with sonic conductance for standardized pneumatic flow calculations.

## 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.

## 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.

## 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.

## What to verify before selection

- Needed with sonic conductance for standardized pneumatic flow calculations.
- Increasing component conductance or upstream pressure may be needed for more flow.
- Always confirm the reference temperature, pressure, and humidity convention.
- Normal conditions vary by convention; record the supplier basis.

## 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

## Frequently asked questions

### What does b mean in pneumatics?

Critical pressure ratio. ISO 6358 ratio marking transition between choked and subsonic flow behavior.

### Why does b matter when selecting a component?

Needed with sonic conductance for standardized pneumatic flow calculations.

### Can b values be compared directly between catalogs?

Only after the unit, reference condition, test method, operating point, and manufacturer definition have been reconciled.

## Technical references

- [ISO 5598:2020](https://www.iso.org/standard/67985.html): Provides the international fluid-power vocabulary used to interpret pneumatic terminology.
- [ISO 6358-1:2013](https://www.iso.org/standard/56612.html): Defines steady-state flow-rate testing and compressible-flow characteristics for pneumatic components.