1.5 Key NFPA 99 Definitions & Terminology
Key Takeaways
A station outlet is the point on a positive-pressure gas system where users connect and disconnect; a station inlet is the matching point on a vacuum system.
In this code, the patient or use side of a pressure gas system is downstream, toward the outlets, and the source side is upstream, toward the central supply.
A zone valve controls gas or vacuum to one area, and a source valve isolates an entire central supply system, including its dryers and final line regulators.
Medical support gas means nitrogen or instrument air used to power equipment; it is never breathed as part of treatment.
WAGD is the capture and removal of gases vented from the patient breathing circuit during normal operation of anesthesia or analgesia equipment.
Why Definitions Get Their Own Domain
NITC weights Definitions at 8% of the written exam, and most other questions assume you read code terms precisely. Many wrong choices on medical gas exams swap one similar term for another: a riser valve for a service valve, a station outlet for a station inlet, or medical air for instrument air. The definitions below are summarized in plain language from NFPA 99's terminology. On the job, the exact wording in the adopted edition controls.
System Hardware
| Term | Plain-language meaning |
|---|---|
| Central supply system | The complete source of a medical gas or vacuum, such as a cylinder manifold, cryogenic system, compressor or pump package, or concentrator system |
| Header | A pipe assembly that connects cylinders or containers to a central supply, with its leads, filter, check valves, and shutoff valve |
| Manifold | Equipment that joins cylinders or containers through headers and switches automatically between the primary and secondary headers |
| Primary / secondary / reserve | Supply in use / supply that takes over when the primary is depleted / emergency supply that serves when the others cannot |
| Average day's supply | The facility's normal daily consumption, used to size headers and to trigger the low-contents alarms |
| Source valve | A shutoff valve at the immediate connection of a central supply system that isolates the whole source |
| Main line valve | A shutoff valve in the main supply line inside the building served |
| Riser valve | A shutoff valve in a riser, next to the main line |
| Service valve | A shutoff valve in branch (lateral) piping off a riser, ahead of the zone valve boxes on that branch |
| Zone valve | A valve that controls the gas or vacuum to a particular area |
| In-line valve | An optional valve that isolates part of a branch, such as individual rooms, for service |
| Auxiliary connection | A tee, valve, and removable cap or plug for connecting a temporary supply |
| EOSC | Emergency oxygen supply connection: an exterior inlet for a temporary oxygen source |
Terminal Devices
| Term | Plain-language meaning |
|---|---|
| Station outlet | The point on a positive-pressure gas pipeline where the user makes connections and disconnections |
| Station inlet | The point on a vacuum pipeline where the user makes connections and disconnections |
| Primary / secondary valve | In an outlet, the secondary valve closes automatically when the primary valve assembly is removed |
| DISS | Diameter Index Safety System; noninterchangeable threaded connections covered by CGA V-5 |
| Quick coupler | A gas-specific, noninterchangeable connector that is not threaded |
| Demand check | A self-closing fitting that lets a device be disconnected without releasing gas |
| Manufactured assembly | A factory-built device such as a headwall, column, boom, or medical gas rail with outlets already piped |
Directions and Places
- Source side vs. patient (use) side: for pressure gases, the source side is upstream and the patient side is downstream, toward the outlets. "Install the pressure indicator on the patient side of the zone valve" means downstream of it.
- Anesthetizing location: an area the governing body designates for administering inhalation anesthetic agents.
- Category 1-4 spaces: patient care spaces classified by the governing body's risk assessment.
- Breached system: a system opened by pipeline intrusion, physical separation, or removing, replacing, or adding a component. It must be inspected and tested again.
Gases and Purity Terms
- Patient medical gas: a piped gas such as oxygen, nitrous oxide, helium, carbon dioxide, or medical air, used for human respiration or for calibrating respiratory devices.
- Medical support gas: nitrogen or instrument air used for medical support purposes such as running surgical tools, booms, and pendants. It is not breathed as part of treatment.
- Nitrogen NF / oil-free, dry nitrogen: nitrogen meeting the National Formulary standard, free of moisture and hydrocarbons. It is the required brazing purge gas and test gas.
- Oxygen USP vs. Oxygen 93 USP: cylinder or cryogenic oxygen tested to at least 99% purity, versus concentrator oxygen tested to 90-96%.
- Medical air: air from cylinders, bulk containers, or medical air compressors, or reconstituted from oxygen USP and nitrogen NF, that meets the USP medical air monograph.
- Medical-surgical vacuum: a method that provides drainage, aspiration, and suction to remove body fluids from patients.
- WAGD (waste anesthetic gas disposal): capturing and carrying away gases vented from the patient breathing circuit during normal operation of anesthesia or analgesia equipment.
People
- Authority having jurisdiction (AHJ): the organization, office, or individual that enforces the adopted code.
- Governing body: the health care organization's leadership, which sets risk categories and designates spaces.
- Responsible facility authority: the person the facility names to operate and manage its medical gas systems.
Testing Vocabulary
| Term | Meaning |
|---|---|
| Blowdown | Clearing newly installed piping with nitrogen NF before outlets and components are installed |
| Initial pressure test | Holding piping at 1.5 times operating pressure (at least 150 psi) while every joint is checked for leaks |
| Standing pressure test | A long hold (24 hours for installers, 10 minutes for verifiers) to detect leakage |
| Cross-connection test | Confirming that each outlet delivers only its own system's test gas |
| Piping purge test | Intermittent high-volume flow at each outlet until a white cloth shows no discoloration |
| Particulate test | Flowing a measured gas volume through a fine filter to check cleanliness |
| Purity test | Comparing hydrocarbon and moisture levels at a remote outlet with the source gas |
| Final tie-in test | Leak testing and purging after new piping is joined to a live system |
| Operational flow pressure drop test | Confirming outlets and inlets deliver the required flow without excessive pressure or vacuum loss |
What is the difference between a station outlet and a station inlet in NFPA 99 terminology?
An outlet is mounted in a wall and an inlet is mounted in a ceiling column or boom
An outlet is a DISS fitting and an inlet is a quick coupler
An outlet has no secondary valve, while an inlet always has both primary and secondary valves
An outlet serves a pressure gas system; an inlet serves a vacuum system
Nitrogen piped at 175 psi to power surgical drills and articulating booms is classified as which of the following?
A patient medical gas
A medical support gas
Medical air
A nonmedical laboratory gas
A drawing note says to install a pressure gauge on the patient side of the zone valve for an oxygen line. Where does the gauge go?
Downstream of the zone valve, between it and the outlets
Upstream of the zone valve, between the valve and the riser
At the source valve in the supply room
On the master alarm panel
Sections you finish are checked off in the contents.