# Sonic Conductance (C)

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

## Definition

Sonic conductance, symbol C, is an ISO 6358 flow characteristic that describes a pneumatic component's conductance in the choked-flow region. It is used with upstream absolute pressure, reference conditions, and the critical pressure ratio b to model mass or normalized volumetric flow. C is not a direct synonym for Cv or a catalog NL/min rating.

## Key facts

- **Standard basis:** ISO 6358 steady-state flow characterization
- **Paired value:** Critical pressure ratio b
- **Pressure rule:** Use absolute pressure in compressible-flow equations

## 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.

## 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.

## 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.

## What to verify before selection

- C units and reference state
- Critical pressure ratio b
- Upstream absolute pressure
- Downstream absolute pressure
- Test direction and component state
- Required flow at the real operating point

## 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

## Frequently asked questions

### Does choked flow mean the air stops flowing?

No. It means mass flow has reached a limit for the current upstream condition and controlling restriction.

### Can sonic conductance be converted to Cv with one universal factor?

Only with clearly defined units, reference conditions, and modeling assumptions. A bare number is not enough.

## Technical references

- [ISO 6358-1:2013 — Pneumatic flow-rate characteristics](https://www.iso.org/standard/56612.html): Steady-state test methods and reporting conventions for compressible-fluid components.
- [ISO 5598:2020 — Fluid power vocabulary](https://www.iso.org/standard/67985.html): International vocabulary reference for fluid power systems and components.