How is oxygen purity measured in a PSA plant?

Continuous measurement at the plant

The International Pharmacopoeia (13th edition, 2025) states that the oxygen content of Oxygen 93% is continuously monitored during production. HTM 02-01, the NHS England guidance (2026), asks for a duplex calibrated paramagnetic oxygen monitoring system made of two oxygen analysers, two concentration indicators, two oxygen flow monitors and a concentration and flow recorder, with connections for calibration cylinders. If the concentration falls below 94%, the monitoring system should isolate the PSA from the pipeline so that the emergency or reserve supply operates, and an independent monitoring system should also isolate the plant below 94%. The 2020 WHO interim specification for PSA plants (superseded by WHO's technical specifications of 31 December 2024) asked for an oxygen concentration monitor with ±1% accuracy, a continuous display of concentration and pressure, and an alarm below 90%.

AIGA 113/20, adopted from EIGA Doc 195, also asks for two oxygen analysers to monitor and record the output continuously. They connect to the control and alarm systems so that the main shut-off valve closes automatically if the concentration falls outside the pharmacopoeia specification, and carbon monoxide, carbon dioxide and moisture analysers can do the same if impurity levels exceed it. A sampling outlet allows oil, nitric oxide, nitrogen dioxide and sulfur dioxide to be tested, continuously or periodically depending on the site's risk management plan. In Portugal, the ACSS specification ET 03/2006 (revised 2024) recommends permanent control with oxygen analysers, with the analyser and the shut-off valve upstream of the oxygen 93 reservoir.

The analyser specified for identity tests

HTM 02-01 names a paramagnetic analyser as the specified instrument for the identity of oxygen when pipelines are tested, and its acceptance criterion for oxygen from a PSA plant is at least 94%.

Checks by hand and in the laboratory

• Global Fund's daily maintenance checklist records the purity shown on the plant's PLC and the purity measured with a handheld analyser, and its sustainability guidance (2024) asks for purity to be recorded daily with at least two different analysers.
• HTM 02-01 Part B (2026) asks for the quality of gas from PSA plants and medical air compressors to be tested at least quarterly.
• The USAID RISE operating procedure, written for hospitals in India, recommends having oxygen samples tested by an accredited laboratory, preferably every six months, to validate the plant and the analyser fitted in it, and lists calibration of the oxygen sensor among the annual inspections.
• HTM 02-01 notes that increasing or fluctuating carbon dioxide readings in PSA-generated oxygen can be an early indication of dryer failure or poor compressor maintenance.

Ultra Controlo analysers

The ULTRAOX SC datasheet lists one or two oxygen analysers with a zirconium oxide probe.

Ultra Controlo also supplies a portable oxygen purity analyser for on-site checks of oxygen concentration in pipelines and cylinders. QuPHARM, launched in May 2026, measures oxygen, carbon monoxide and carbon dioxide at the same time, with an optional moisture transmitter, and has an output per channel for integration with the hospital's building management system.