# Kv — Metric flow coefficient

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

## Definition

Metric flow coefficient: Metric coefficient convention commonly used for valve flow capacity. Cv and Kv are related conventions but are not numerically identical. Read the applicable product data and technical standard before treating a catalog value as interchangeable.

## Key facts

- **Term:** Kv
- **Full form:** Metric flow coefficient
- **Selection effect:** Cv and Kv are related conventions but are not numerically identical.

## Kv: definition and engineering context

Metric flow coefficient. Metric coefficient convention commonly used for valve flow capacity.

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 Kv affects pneumatic selection

Cv and Kv are related conventions but are not numerically identical.

- C: Use with critical pressure ratio and absolute pressure conditions.
- b: 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.

## How to evaluate Kv 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 conductance: ISO 6358 flow characteristic representing conductance in choked flow.
- Critical pressure ratio: ISO 6358 ratio marking transition between choked and subsonic flow behavior.
- 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.

## What to verify before selection

- Cv and Kv are related conventions but are not numerically identical.
- Use with critical pressure ratio and absolute pressure conditions.
- Needed with sonic conductance for standardized pneumatic flow calculations.
- Increasing component conductance or upstream pressure may be needed for more flow.

## Common mistakes

- Treating Kv 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 Kv mean in pneumatics?

Metric flow coefficient. Metric coefficient convention commonly used for valve flow capacity.

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

Cv and Kv are related conventions but are not numerically identical.

### Can Kv 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.