01
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.
02
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.
03
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.
Selection checklist
What to verify before selection
- 01
Use when the required fail-state needs flow.
- 02
Check minimum pilot or differential pressure.
- 03
Useful where zero differential operation matters, usually with lower maximum flow.
- 04
Leakage, overlap, and contamination tolerance affect application behavior.
Application caution
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
FAQ