# Sonic flow

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

## Definition

Sonic flow: Condition where mass flow no longer increases by lowering downstream pressure at fixed upstream conditions. Increasing component conductance or upstream pressure may be needed for more flow. Read the applicable product data and technical standard before treating a catalog value as interchangeable.

## Key facts

- **Term:** Choked flow
- **Full form:** Sonic flow
- **Selection effect:** Increasing component conductance or upstream pressure may be needed for more flow.

## Choked flow: definition and engineering context

Sonic flow. Condition where mass flow no longer increases by lowering downstream pressure at fixed upstream conditions.

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 Choked flow affects pneumatic selection

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.
- Qn: Compare only when pressure conditions and measurement conventions match.

## How to evaluate Choked flow 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.

- 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.
- Nominal or normalized flow rate: Catalog flow value defined under a stated test condition.

## What to verify before selection

- 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.
- Keep ANR values tied to the catalog's stated reference definition.

## Common mistakes

- Treating Choked flow 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 Choked flow mean in pneumatics?

Sonic flow. Condition where mass flow no longer increases by lowering downstream pressure at fixed upstream conditions.

### Why does Choked flow matter when selecting a component?

Increasing component conductance or upstream pressure may be needed for more flow.

### Can Choked flow 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.