# NO — Normally open

> Source: https://rodlesspneumatic.com/resource-center/pneumatic-glossary/no/
> Language: en-US
> Category: Valves & circuits

## Definition

Normally open: Flow path is open in the defined normal or de-energized condition. Use when the required fail-state needs flow. Read the applicable product data and technical standard before treating a catalog value as interchangeable.

## Key facts

- **Term:** NO
- **Full form:** Normally open
- **Selection effect:** Use when the required fail-state needs flow.

## NO: definition and engineering context

Normally open. Flow path is open in the defined normal or de-energized condition.

The term belongs to the valves & circuits context. Its exact limits can depend on the cited standard, catalog convention, unit system, reference state, and component configuration.

## Why NO affects pneumatic selection

Use when the required fail-state needs flow.

- Pilot operated: Check minimum pilot or differential pressure.
- Direct acting: Useful where zero differential operation matters, usually with lower maximum flow.
- Spool: Leakage, overlap, and contamination tolerance affect application behavior.
- Poppet: Often offers short travel and tight shutoff; verify required actuation force.

## How to evaluate NO 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.

- Pilot-actuated valve: Main element is shifted using pilot pressure.
- Direct-actuated valve: Solenoid or mechanical input acts directly on the sealing element.
- Sliding valve element: Internal element routes air between valve ports by moving along a bore.
- Seat-type valve element: Sealing element opens away from a seat.

## What to verify before selection

- Use when the required fail-state needs flow.
- Check minimum pilot or differential pressure.
- Useful where zero differential operation matters, usually with lower maximum flow.
- Leakage, overlap, and contamination tolerance affect application behavior.

## Common mistakes

- Treating NO 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 NO mean in pneumatics?

Normally open. Flow path is open in the defined normal or de-energized condition.

### Why does NO matter when selecting a component?

Use when the required fail-state needs flow.

### Can NO 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 1219-1:2012](https://www.iso.org/standard/60184.html): Defines graphical symbols and circuit-documentation principles for fluid-power systems.