What should a technician check first when PSA purity falls?

The USAID procedure refers to the operation and maintenance manual for purging, to the manufacturer's recommendation for calibrating the PLC, and to the manufacturer or supplier for anything its tips do not solve. The sections below group what each source says about the analyser, the demand, the feed air, the valves and the sieve beds.

Confirm the reading

• The USAID troubleshooting table lists a faulty sensor and a faulty PLC reading among its tips for low purity, the PLC to be calibrated as the manufacturer recommends; its annual inspections include calibrating the oxygen sensor.
• Global Fund's daily maintenance checklist records purity both on the plant's PLC and with a handheld oxygen analyser. Its guide to expected values (5 June 2025) gives 90 to 96%, treats above 90% as medical grade and says a reading below 85% should lead to an investigation.
• AIGA 113/20, adopted from EIGA Doc 195, asks for analysers to be calibrated periodically with a certified calibration gas, and includes the 90% low oxygen alarm and its automatic shutdown in the safety checks, a check that could be made with a mixture of 10% nitrogen in oxygen.

Compare demand with production

The USAID procedure warns against drawing more oxygen than the plant's capacity, because purity drops, and its tips for low tank pressure include checking consumption against production. See why PSA purity drops when demand rises.

The feed air: water, oil and the dryer

• Water: the USAID procedure says never to run the plant if the automatic water drains are faulty or not draining, and, if water appears at the drains at start-up, to check the dryer against its manual. Its daily inspection includes the dryer dew point.
• Carbon dioxide trend: NHS England's HTM 02-01 (2026) notes that increasing or fluctuating carbon dioxide readings in PSA-generated oxygen can be an early indication of dryer failure or poor compressor maintenance.
• Moisture and the sieve: AIGA 113/20 notes that an increase in moisture would lead to accelerated damage of the oxygen 93 adsorbers, and PATH (November 2023) says high humidity can accelerate the degradation of zeolite sieve beds.
• Oil: PATH notes that missed oil-separating (coalescing) filter changes can lead to downstream oil contamination of the sieve beds, which then need full disassembly, cleaning and zeolite replacement.

Valves and leaks

• Valves: besides the open, closed and clean checks in the answer above, PATH lists leaky valves, resulting in low purity output, fire or noise, among the common issues of plants in disrepair; the USAID monthly inspection checks solenoid valves for corrosion.
• Sounds and dust: at start-up, the USAID procedure asks the operator to watch for abnormal sounds in the generator towers, exhaust blowdown and dust or powder at the silencers, and to follow the manufacturer's manual if they appear.
• Leaks: its leak test leaves the plant isolated and pressurised, for example at 5 bar, for one hour; any pressure drop is then traced with soapy water.

The sieve beds

PATH lists contamination of the sieve beds, resulting in low purity output, among the common failures. PATH and CHAI (July 2021) say sieve beds must be recharged periodically if the plant's oxygen generating capacity or purity falls, and HTM 02-01 Part B (2026) asks for molecular-sieve charges to be replaced at the supplier's intervals or if the material has been proven ineffective. See how often PSA zeolite needs replacing.

On ULTRAOX SC plants, purity is measured by one or two zirconium oxide analysers, and the ULTRAOX touch screen shows oxygen purity, alarm information and a record of all contingencies, according to the ULTRAOX SC and ULTRAOX 2022 datasheets. See how purity is measured in a PSA plant.