Because argon comes through with it, and the sieve cannot tell the two apart.

Air is about 78 per cent nitrogen, 21 per cent oxygen and just under 1 per cent argon. A zeolite molecular sieve adsorbs nitrogen strongly and lets oxygen pass - that is how the separation works. But argon's adsorption behaviour on zeolite is very close to oxygen's, so the sieve passes argon just as readily.

Remove essentially all the nitrogen and what is left is oxygen plus all of the original argon. Because the argon has been concentrated along with the oxygen, it ends up at around five per cent of the product - which caps purity in the low-to-mid nineties.

Getting past that requires separating argon from oxygen, which needs cryogenic distillation - an entirely different scale and type of plant.

So if a process genuinely requires oxygen above that ceiling, on-site PSA is not the technology, and delivered gas is the answer.

Those wanting oxygen enrichment rather than pure oxygen - which covers most industrial uses.

Aquaculture is a major one: dissolving oxygen into water allows far higher stocking densities and improves growth and survival rates, and the purity ceiling is irrelevant.

Wastewater treatment uses oxygen instead of air to intensify aerobic digestion, achieving the same treatment in a much smaller footprint. Ozone generation for water disinfection is markedly more efficient and produces higher ozone concentrations when fed oxygen rather than air.

Industrially it is used for furnace and kiln enrichment, glassmaking, metal cutting and heating, and in some fermentation and biotechnology processes.

What it is not automatically suitable for is medical use. Medical oxygen is a regulated pharmaceutical product with its own standards and regulatory approvals, and supplying it from an on-site generator is a substantially higher bar that a standard industrial machine does not meet.

Oxygen-compatible materials, oxygen-clean components, and no hydrocarbons anywhere in contact with the gas.

Oxygen does not burn, but it makes other things burn far more easily and at temperatures where they would be perfectly safe in air. Materials that are non-flammable in air can ignite and sustain combustion in an oxygen-enriched stream.

The specific and serious hazard is hydrocarbons. Oil or grease in contact with pressurised oxygen can ignite spontaneously, without any spark or flame, releasing enough energy to ignite the surrounding metal. That is why oxygen pipework, valves, fittings and seals must be cleaned for oxygen service to a defined standard and kept clean.

Practical requirements: no ordinary lubricants, oxygen-compatible seal and valve materials, cleaned-for-oxygen-service components, and installation practices that avoid introducing contamination. Ventilation matters too, since leaked oxygen enriches the surrounding atmosphere and makes ordinary materials and clothing readily flammable.

Favourably for continuous industrial duty, on the same whole-cost basis as nitrogen.

Delivered oxygen carries gas cost, cylinder or tank charges, deliveries, handling and storage - and liquid oxygen additionally loses product continuously to boil-off, so a site pays for gas that evaporates while waiting to be used.

Against that, a generator costs capital plus the electricity to make and treat the compressed air it consumes, plus maintenance.

For an aquaculture site, a treatment works or an ozone system running continuously, the calculation is usually clearly in the generator's favour, and the elimination of delivery logistics has value beyond the arithmetic - particularly at remote sites where deliveries are difficult or unreliable.

For intermittent or low-volume use, or where high purity is genuinely required, delivered oxygen remains the sensible answer. As always, include the compressed air consumption in the running cost.

Same PSA principle, different adsorbent - and a different product stream, which changes everything downstream.

A nitrogen PSA machine uses carbon molecular sieve, which captures oxygen and passes nitrogen. An oxygen PSA machine uses zeolite, which captures nitrogen and passes oxygen. Both cycle between two vessels, pressurising one while venting the other.

Mechanically the machines look similar and share the same dependence on clean, dry, oil-free feed air.

The differences that matter are on the product side. The oxygen machine's output is an oxidising, fire-intensifying gas, so all downstream materials, seals, valves and lubricants must be oxygen-compatible and cleaned for oxygen service. The nitrogen machine's output is an asphyxiant, so the concern is oxygen depletion monitoring in enclosed spaces instead.

The two hazards are opposite and the precautions are not interchangeable - which is worth stating plainly on a site that runs both.

Clean, dry, oil-free compressed air at a specified quality, plus power for the controls - and the air is the dominant utility.

The zeolite is damaged by water and by oil, both of which occupy adsorption sites and are not released during the depressurisation cycle. Water is particularly damaging because zeolites are strongly hygroscopic. So the manufacturer specifies an air quality standard, normally met with coalescing filtration, drying to a stated dewpoint, and carbon filtration for oil vapour.

The volume of air consumed per unit of oxygen produced is significant and should be included in the economic assessment, along with the electricity to compress it.

The generator itself needs relatively little electrical power - it is the compressor upstream that dominates consumption.

Also confirm the ambient conditions. PSA performance falls as feed air temperature rises, so a machine in a hot plant room or an unventilated container will not meet its rated output.

Continuous oxygen purity measurement with alarm, plus atmospheric monitoring in the plant room - and the second is often forgotten.

Product purity should be measured continuously rather than spot-checked, with an alarm on low purity and, where the process warrants it, an automatic vent that dumps off-specification gas rather than delivering it. Purity drifts downward as sieve degrades or a valve leaks, so a trend is far more informative than a periodic reading.

Atmospheric monitoring inside the plant room addresses the other risk. A leak enriches the surrounding air, and oxygen enrichment is not detectable by smell or sensation while making clothing and ordinary materials dangerously flammable. Oxygen enrichment monitors with alarms are standard practice in enclosed generator rooms.

Ventilation should be checked as part of the installation rather than assumed. A generator installed in a container or a small room needs designed ventilation for exactly this reason.