Respiratory Therapy · Equipment and Quality Control
Medical Gas Sources, Delivery Systems, and Oxygen Devices
On this page 7 sections
In 30 seconds
Medical gases travel from a bulk or cylinder source, through a Regulator Drops high pressure to a safe constant Full entry → that drops pressure to a safe working level, and through a flowmeter that measures flow, before reaching a delivery device. Low-flow devices (Nasal cannula Low-flow prongs at the nares Full entry →, Simple mask Low-flow face mask with side holes Full entry →, Partial rebreather Simple mask plus a reservoir bag Full entry →, Non-rebreather Mask with reservoir and one-way valves Full entry →) deliver a fixed flow that mixes with variable room air, so the delivered oxygen concentration changes with breathing. High-flow devices (Venturi mask Entrains room air at a fixed ratio Full entry →, HFNC Heated, humidified high-flow cannula Full entry →, LVN/aerosol masks, hoods/tents) meet or exceed inspiratory demand, giving a stable concentration.
Why this matters
Gas handling is a high-consequence safety domain: a mislabeled cylinder, tilted Thorpe tube Upright variable-orifice flowmeter Full entry →, drifting blender, or empty transport cylinder can silently cause under- or over-delivery. The technologist verifies equipment and recognizes problems — not independently selecting flow or concentration. Oxygen targets and device choices follow provider orders and are verified against current NBRC/AARC references and manufacturer IFU, with safety concerns escalated immediately.
The college version
1. Gas Sources: Cylinders and Bulk Supply
Oxygen, air, and other gases are stored as bulk liquid systems feeding a hospital's central piping, or in compressed-gas cylinders for portability and backup. Cylinders are color-coded (in the U.S., oxygen is green; air is yellow), though the label — never color alone — is authoritative. Size is designated by letter: an E cylinder is the small portable tank for transport and backup; an H cylinder is a large cylinder for extended stationary supply. A Duration-of-flow calculation Gas remaining from pressure, factor, and flow Full entry → estimates how long a cylinder lasts: duration = (gauge pressure × cylinder factor) ÷ flow rate, where the cylinder factor is a constant for each size. This is a planning estimate, verified against the gauge and current references.
2. Regulators and Flowmeters
A regulator reduces high, variable cylinder pressure to a constant, safe working pressure (about 50 psi), protecting devices and stabilizing flow as the cylinder empties. A flowmeter then measures and displays flow. The Thorpe tube is a gravity-dependent, variable-orifice flowmeter with a float in a tapered tube, so it must be upright and read at the correct point on the float. The Bourdon gauge Position-independent pressure flowmeter Full entry → is a position-independent, pressure-based flowmeter usable under downstream restriction. A Restrictor Fixed-orifice, preset-flow device Full entry → (fixed-orifice) delivers only a preset flow, found on some transport setups.
3. Air Compressors, Oxygen Concentrators, and Gas Blenders
An Air compressor Makes medical air from ambient air Full entry → makes medical-grade air from ambient air, powering air-driven devices and blenders. An oxygen concentrator does not store oxygen but extracts it continuously from room air (typically by molecular-sieve adsorption of nitrogen), for long-term or home use. A gas blender proportionally mixes air and oxygen to a set FiO₂; it needs routine calibration against an oxygen analyzer and troubleshooting when concentration drifts or supply pressures unbalance. The technologist verifies these systems; applying a specific concentration follows a provider order.
How it works
- Gas is stored or generated at a source (bulk, cylinder, compressor, or concentrator).
- A regulator reduces pressure to a constant working level.
- A flowmeter measures and displays flow.
- Optionally, a blender proportions air and oxygen to a target concentration.
- A delivery device presents gas as a fixed low flow (with room-air entrainment) or a high flow meeting inspiratory demand.
- The technologist verifies setup against orders and guidelines and watches for alarms.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Low-flow device | High-flow device | Low flow = variable FiO₂; high flow = stable FiO₂ |
| Thorpe tube | Bourdon gauge | Thorpe is gravity-dependent; Bourdon is pressure-based |
| Oxygen concentrator | Oxygen cylinder | Concentrator makes oxygen; cylinder stores it |
| Regulator | Flowmeter | Regulator controls pressure; flowmeter measures flow |
| Partial rebreather | Non-rebreather | Non-rebreather has one-way valves limiting rebreathing |
| Blender | Flowmeter | Blender sets concentration; flowmeter sets flow |
Memory aids
"GREEN" — Gauge the cylinder (duration), Regulator reduces pressure, Entrainment defines low- vs high-flow, Examine the label, Never titrate without an order.
Quick review
Topic Recap
Medical gases move from a high-pressure source through a regulator and flowmeter to a delivery device. Low-flow devices deliver a variable FiO₂; high-flow devices deliver a stable concentration. Cylinder sizing, color codes, duration-of-flow, and blender calibration are equipment-and-safety concepts to verify against current references.
Knowledge Check
- Which device delivers the least stable oxygen concentration as breathing changes?
- Which flowmeter works correctly when tilted and under downstream resistance?
- Why does a non-rebreather deliver more oxygen than a simple mask?
- What two properties determine how long an E cylinder lasts at a given flow?
- Which device entrains room air at a fixed ratio for a stable concentration?
Answers and Rationales
- A nasal cannula (or any low-flow device). Low-flow devices deliver less than peak inspiratory flow, so varying room-air entrainment changes FiO₂ breath to breath.
- A Bourdon gauge. It measures pressure rather than relying on gravity, so it is position-independent.
- Its reservoir bag and one-way valves reduce room-air entrainment and rebreathing, keeping more of each breath from the oxygen source.
- Gauge pressure and cylinder factor (duration = pressure × factor ÷ flow).
- A Venturi (air-entrainment) mask, whose fixed entrainment ratio makes concentration largely independent of breathing.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of oxygen supply like a house's water system. The big tank (bulk supply) holds a lot under high pressure; a valve (regulator) drops it to a usable level; the faucet (flowmeter) sets how much comes out; the cup (delivery device) decides how it is used.
A low-flow device is like sipping from a slow-running faucet while also breathing in air around the stream — how much "pure" water you swallow depends on how fast you drink. A high-flow device is like holding a full glass directly under the stream so it always fills whatever you can swallow.
This stops being exact because oxygen is also about concentration (fraction of inspired oxygen, FiO₂), not just volume. Low-flow devices deliver a fixed flow that dilutes with variable room air, so the concentration breathed changes breath to breath; high-flow devices control both, keeping the percentage steady.
Simple Example
A person breathing quietly has a low inspiratory flow, so a nasal cannula's small flow mixes with less room air and yields a higher oxygen percentage. The same person panting hard entrains far more room air, so that same cannula flow delivers a much lower percentage. Delivered oxygen depends on the patient's own breathing — the core difference between low-flow and high-flow systems.
Worked example
- Match the source to the need. Transport or code scenarios favor a portable E cylinder; ICU or wall outlets depend on bulk supply.
- Trace the pressure chain. High-pressure source → regulator → flowmeter → device; a break anywhere changes what reaches the patient.
- Decide low-flow versus high-flow by asking whether the device meets total inspiratory demand.
- Reason about concentration, not a single number. Explain why each device's oxygen percentage is variable (low-flow) or stable (high-flow), and confirm any applied flow against provider orders, AARC guidelines, and manufacturer IFU.
- Escalate. A failed blender, empty or leaking cylinder, or pressure alarm warrants escalation to qualified clinicians or biomedical engineering per local protocol.
Key takeaways
- High yield: Low-flow devices deliver a fixed flow but a variable FiO₂; high-flow devices deliver a stable FiO₂ by meeting inspiratory demand.
- High yield: A Venturi mask entrains room air at a fixed ratio for a precise concentration largely independent of breathing.
- High yield: HFNC provides heated, humidified high flow and a mild distending (PEEP-like) pressure.
- High yield: A Thorpe tube must be vertical; a Bourdon gauge is position-independent.
- High yield: Oxygen cylinders are green, air is yellow — verify by label, never color alone.
- High yield: Duration-of-flow depends on gauge pressure, cylinder factor, and flow; smaller cylinders empty faster at higher flows.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Compare medical gas sources, including compressed-gas cylinders and bulk supply, and explain color codes and cylinder sizing.
- Describe how regulators, flowmeters, air compressors, oxygen concentrators, and gas blenders work and how each is calibrated or troubleshot.
- Distinguish low-flow from high-flow oxygen devices and explain the mechanism of each device.
- Explain the conceptual basis for duration-of-flow calculations and gas-handling safety.
Key vocabulary
- Cylinder color codes
- Tank colors hinting at contents
- E / H cylinder
- Small portable vs. large stationary tank
- Duration-of-flow calculation
- Gas remaining from pressure, factor, and flow
- Regulator
- Drops high pressure to a safe constant
- Thorpe tube
- Upright variable-orifice flowmeter
- Bourdon gauge
- Position-independent pressure flowmeter
- Restrictor
- Fixed-orifice, preset-flow device
- Air compressor
- Makes medical air from ambient air
- Oxygen concentrator
- Extracts oxygen from room air
- Gas blender
- Mixes air and oxygen to a set FiO₂
- Nasal cannula
- Low-flow prongs at the nares
- Simple mask
- Low-flow face mask with side holes
- Partial rebreather
- Simple mask plus a reservoir bag
- Non-rebreather
- Mask with reservoir and one-way valves
- Venturi mask
- Entrains room air at a fixed ratio
- HFNC
- Heated, humidified high-flow cannula
- LVN / aerosol mask
- Large-volume nebulizer with mask
- Oxygen hood / tent
- Enclosure around a patient
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