Reference — Clinical Skills
Medical Gas Cylinders & Regulators
Compressed medical gases are stored in color-coded, high-pressure cylinders and delivered through reducing valves and flowmeters — this reference covers the color codes, the cylinder-factor duration math, the safety connection systems, and the pressure-reduction hardware every respiratory therapist has to read at the bedside.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Compressed medical gases are stored in color-coded, high-pressure cylinders and delivered to patients through reducing valves and flowmeters. Respiratory therapists must read cylinder labels, estimate how long a cylinder will last, connect the correct gas safely, and understand the pressure-reduction hardware that makes that gas usable at the bedside. Standardized color codes and indexed safety connections exist specifically to prevent the wrong gas from being delivered.
Cylinder Color Codes
U.S. cylinder colors, per CGA/NFPA convention:
| Gas | U.S. color | Note |
|---|---|---|
| Oxygen (O₂) | Green | International/ISO color is white |
| Air | Yellow | |
| Carbon dioxide (CO₂) | Gray | |
| Nitrous oxide (N₂O) | Blue | |
| Nitrogen (N₂) | Black | |
| Helium (He) | Brown | |
| Helium/oxygen (heliox) | Brown and green | |
| CO₂/oxygen | Gray and green |
U.S. color codes are a convenience, not the legal identifier — always read the printed cylinder label to confirm contents.
Cylinder Sizes, Pressures & Duration
| Cylinder size | Typical use | O₂ capacity (L, full) | Cylinder factor (L/psig) |
|---|---|---|---|
| D | Portable | ~400 L | 0.16 |
| E | Portable (wheelchair, transport) | ~625 L | 0.28 |
| G | Stationary | ~5,300 L | 2.41 |
| H / K | Large stationary / bulk | ~6,900 L | 3.14 |
A full oxygen cylinder is pressurized to a service (working) pressure of about 2200 psig at room temperature (70°F). The cylinder factor (L/psig) converts remaining gauge pressure into remaining volume for that cylinder size.
Duration formula
Duration (min) = (gauge pressure in psig × cylinder factor) ÷ flow (L/min)
Worked example: An E cylinder reading 2000 psig running at 2 L/min lasts (2000 × 0.28) ÷ 2 = 280 minutes ≈ 4.7 hours.
Change the cylinder before it empties — replace at roughly 200 psig of residual so the cylinder never runs fully empty. This protects against moisture/contaminant ingress and leaves a safety reserve.
Safety Connection Systems
- PISS (Pin Index Safety System):for small cylinders (sizes A–E) that use a yoke/post-valve connection. A unique pin-and-hole pattern for each gas physically prevents attaching a regulator meant for a different gas. Oxygen uses pin positions 2 and 5.
- DISS (Diameter Index Safety System):for low-pressure (≤200 psig) threaded connections — the outlets of flowmeters/regulators and wall/station outlets. Each gas has a unique thread/shoulder diameter so connectors are not interchangeable.
- ASSS (American Standard Safety System):gas-specific threaded valve outlets for large cylinders (e.g., H/K).
One-line summary: small cylinders → PISS (pin/yoke); large cylinders → ASSS threads; low-pressure threaded outlets → DISS.
Reducing Valves & Regulators
A reducing valve lowers the high cylinder pressure (~2200 psig) to a usable working pressure of about 50 psig— the standard pressure that powers hospital gas outlets, ventilators, and blenders. A regulator is a reducing valve combined with a flowmeter, so it controls both pressure and flow.
- Single-stage reducing valve:reduces pressure in one step (~2200 → ~50 psig). Adequate and standard for most clinical use.
- Multi-stage (two- or three-stage) reducing valve: reduces pressure in successive steps, giving more precise, stable flow as the cylinder empties — but is heavier and costlier. Used where precision matters (e.g., research, lab).
Clinical Notes
- Always confirm gas identity by reading the label, not color alone.
- “Crack” the cylinder valve (briefly open, then close, pointed away from people) before attaching a regulator to blow out dust from the outlet.
- Open cylinder valves slowly and fully, standing to the side of the regulator.
- Never use oil, grease, or petroleum lubricant on oxygen fittings — it can ignite in high-pressure oxygen.
- Board pearl: memorize E = 0.28 and H/K = 3.14 L/psig; duration = (psig × factor) ÷ flow.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
- Cairo JM. Mosby's Respiratory Care Equipment. 11th ed. Elsevier; 2022.