Oxygen Concentrators: Continuous Flow vs. Pulse-Dose, Explained
The single most common spec confusion in respiratory sourcing comes down to one question: how the machine delivers oxygen. Get the delivery mechanism right against the prescription, and stationary-vs-portable, LPM ratings, and lifespan all fall into place.
Respiratory Care
Continuous flow vs. pulse-dose: what the machine is actually doing
Continuous flow delivers a steady, uninterrupted stream of oxygen measured in liters per minute (LPM) — the patient receives the set rate whether they are inhaling, exhaling, or holding their breath. It is the only delivery method compatible with bilevel and CPAP bleed-in, high-flow needs, pediatric use, and patients who breathe through their mouth or have an irregular respiratory pattern.
Pulse-dose (also called on-demand or intermittent flow) uses a sensor to detect the start of each inhalation and releases a measured bolus of oxygen only at that moment. Because it wastes nothing on the exhale, a pulse unit delivers far more therapy per battery-hour and per pound — which is why nearly every truly portable concentrator is pulse-dose. The trade-off: a pulse unit must reliably trigger off the patient's breath. Shallow breathers, sleeping patients, and anyone needing flow during apnea can fail to trigger, leaving them under-oxygenated.
The procurement rule is simple: a continuous-flow prescription cannot be filled with a pulse-only device. Confirm the order's delivery mode before you confirm anything else.
Stationary vs. portable — and why purity drops when you shrink the box
Stationary (home) concentrators are the workhorses: typically 5 to 10 LPM continuous flow, AC-powered, 30 to 50 lbs, and built to run continuously for years. They hold oxygen purity at roughly 90–96% across their full flow range, which is what makes them suitable for higher liter flows and 24/7 use.
Portable oxygen concentrators (POCs) trade flow capacity for mobility. Most weigh 3 to 10 lbs, run on swappable batteries plus AC and DC adapters, and are overwhelmingly pulse-dose. The physics of a smaller sieve bed mean purity can sag at the highest pulse settings — a unit rated 90% ±3% at setting 2 may fall toward the bottom of that band at setting 5. That is the hidden catch: a POC's top setting number does not always carry the same oxygen concentration as its lower settings.
Match the form factor to the patient's life, not the brochure: a homebound patient on 4 LPM continuous needs a stationary unit; an active patient on a pulse prescription who travels needs a POC plus enough batteries to cover their longest outing.
Reading a prescription against the spec sheet
An oxygen order has three parts that map directly to the equipment rating. Work them in order before you select a unit.
1. Confirm the delivery mode
Does the order say continuous flow or pulse/on-demand? This is the gate. Continuous orders rule out pulse-only POCs entirely. Nocturnal and CPAP/BiPAP-supplemented orders are almost always continuous — never substitute a pulse unit for sleep use.
2. Match the LPM or pulse setting to capacity
For continuous flow, the prescribed LPM must sit comfortably inside the unit's rated range — a 5 LPM order on a 5 LPM-max machine leaves no headroom. For pulse-dose, confirm the prescribed setting number AND the bolus volume (mL per breath), since the same 'setting 3' differs between models.
3. Check oxygen purity at the prescribed rate, not the peak
Read the purity spec (e.g. 90–96%) at the actual flow the patient needs. On stationary units purity is stable; on POCs and at a unit's maximum continuous output it can dip. The spec line you care about is purity at the patient's setting — not the headline number.
4. Size batteries and power to the use pattern
For POCs, total battery runtime is a function of the pulse setting — runtime printed for setting 2 may halve at setting 5. Spec battery count against the patient's longest stretch away from AC power, and confirm DC/airline adapter availability if travel is in scope.
Maintenance and lifespan — what drives replacement
Concentrator longevity is mostly about the sieve bed and filtration. Build these intervals into a service or par-level plan.
Gross particle (cabinet) filter
Washable foam filter on the intake. Rinse and dry weekly to biweekly. A clogged intake filter is the most common cause of low-output and overheat faults — and the cheapest to prevent.
Outlet / bacteria filter and cannula
Replace per the manufacturer interval (commonly 6–12 months for the outlet filter; cannulas more frequently for hygiene). Patient-contact consumables drive most of the recurring spend, so stock them as a kit.
Sieve bed (molecular sieve)
The zeolite sieve separates oxygen from nitrogen and is the heart of the machine. It degrades over years of run-hours; falling purity that filter changes don't fix points here. Some units use replaceable columns; many stationary units need the bed serviced or the unit retired.
Expected service life
A quality stationary concentrator runs roughly 4–7 years (15,000–20,000+ hours) with maintenance; POCs trend shorter (3–5 years) given duty cycle and battery wear. Plan replacement on accumulated run-hours and a downward purity trend, not on calendar age alone.
Frequently asked questions
Can a pulse-dose portable concentrator fill a continuous-flow prescription?
No. A continuous-flow order requires steady, uninterrupted oxygen regardless of the breathing cycle. Pulse units release oxygen only on detected inhalation, so they cannot deliver flow during exhale, apnea, or to a mouth-breather. Substituting one for the other is a clinical safety error, not a cost optimization.
Why does oxygen purity drop at higher settings on a portable unit?
A POC's compact sieve bed has a finite capacity to separate oxygen from nitrogen. As you demand more oxygen per minute at higher settings, the same bed has less time to purify each volume, so concentration can drift toward the bottom of the rated band. Always read the purity spec at the patient's prescribed setting, not the headline figure.
Is LPM on a stationary unit the same as a pulse setting on a portable?
No, and conflating them is the classic sourcing mistake. LPM is a continuous volume per minute; a pulse 'setting' corresponds to a bolus volume delivered per breath. There is no fixed 1-to-1 conversion — a pulse setting of 3 is not '3 LPM.' Match the order's stated mode to the unit's matching rating.
What's the single most overlooked spec when ordering a POC?
Battery runtime tied to the prescribed setting. Marketing runtime is usually quoted at a low setting; at the patient's actual setting it can be far shorter. Size the number of batteries against the patient's longest time away from a power source, and confirm AC and DC charging are both included.
How do I know when a concentrator should be retired?
Track run-hours and oxygen purity. When measured purity falls below the unit's spec and a filter and sieve service doesn't restore it, the sieve bed is exhausted and the unit should be repaired or replaced. Persistent overheat or low-flow alarms after filter maintenance are the same signal.
Spec the right delivery mode the first time
Bring us the prescription's delivery mode, flow rate, and the patient's use pattern, and we'll walk through the continuous-vs-pulse and stationary-vs-portable decision against real ratings — mechanism and selection logic, no pitch.