# PSA, cylinders or liquid oxygen: what is the difference?

Under AIGA 113/20, oxygen 93 produced by PSA can be manufactured on site within the healthcare facility, while medicinal oxygen supplied in cylinders or as a cryogenic liquid is required to be manufactured by an approved supplier with a manufacturer’s licence.

## Three ways to supply hospital oxygen

|  | On-site PSA plant | Cylinders from a gas supplier | Liquid oxygen (bulk tank) |
|---|---|---|---|
| Where it is made | On site, from ambient air (WHO interim guidance, April 2020; International Pharmacopoeia, 2025) | Manufactured by an approved supplier with a manufacturer’s licence, which AIGA 113/20 says is required | Always off site, by cryogenic distillation of ambient air (WHO interim guidance, April 2020; International Pharmacopoeia, 2025) |
| Who answers for its quality | Qualified personnel of the healthcare facility, normally the head pharmacist (AIGA 113/20) | The medical gas supplier (AIGA 113/20) | The medical gas supplier (AIGA 113/20) |
| Grade | Oxygen 93: 90.0% to 96.0% oxygen, the rest mainly argon and nitrogen (International Pharmacopoeia, 2025) | Above 99% oxygen (HTM 02-01, 2026) | At least 99.5% oxygen (Ph. Eur. 0417; International Pharmacopoeia, Oxygen 99.5%) |
| Form at the hospital | Gas, which can be piped directly to bedside terminal units (WHO interim guidance, April 2020) | Gas at high pressure: the rated working pressure typically ranges from 137 to 150 bar, depending on the manufacturer (WHO, Foundations of medical oxygen systems, 2023) | Liquid stored at down to −183 °C in a VIE (vacuum-insulated evaporator) and vaporised to gas for the pipeline (HTM 02-01, 2026) |
| Electricity at the hospital | Needed: HTM 02-01 (2026) says all components of the PSA supply system should be connected to the essential electrical supply | Not needed: cylinders require no electricity to be functional (PATH and CHAI, 2021) | Not needed: the tank feeds the pipeline by self-vaporisation (WHO interim guidance, April 2020) |
| Checks and monitoring | Daily checklist, with oxygen purity recorded on the PLC and with a handheld analyser (Global Fund / BHI checklist, 2025) | Periodic inspection and pressure testing at intervals set by national or international transport rules (see [how often cylinders are retested](id:how-often-must-medical-oxygen-cylinders-be-retested)) | Telemetry should be installed so that the hospital and the gas supplier can monitor vessel levels and pressures continuously (HTM 02-01, 2026) |

## Sources

- [European Pharmacopoeia (EDQM)](https://www.edqm.eu/en/newslist/-/asset_publisher/JaJ4OiL8a8pm/content/id/93758)
- [WHO, Oxygen sources and distribution for COVID-19 treatment centres, interim guidance (4 April 2020)](https://iris.who.int/server/api/core/bitstreams/17eea7fd-59d9-4523-86eb-acec74fcfd27/content)
- [WHO, Foundations of medical oxygen systems (17 February 2023)](https://iris.who.int/server/api/core/bitstreams/4ad6f5fd-089e-4142-8439-b64e98d816d8/content)
- [The International Pharmacopoeia, 13th edition (2025), Medicinal oxygen](https://digicollections.net/phint/202502/index.html#d/b.6.1.226)
- [AIGA 113/20 (adopted from EIGA Doc 195), On site generation of oxygen 93% for medicinal use, section 1](https://asiaiga.org/uploaded_docs/en_AIGA_113_20_Generation_of_Oxygen_93_for_Medicinal_Use.pdf)
- [NHS England, HTM 02-01 Part A (2026), paragraphs 6.35 to 6.37, 6.48, 6.112 and 6.126](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [PATH and CHAI, Oxygen Generation and Storage: Oxygen Cylinders (July 2021)](https://media.path.org/documents/O2_generation_and_storage_oxygen_cylinders_v1.pdf)
- [Global Fund / Build Health International, PSA plant Daily Maintenance Checklist (2025)](https://resources.theglobalfund.org/media/xbanfgrc/cr_c19rm-psa-plant-daily-maintenance_checklist_en.pdf)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/psa-vs-cylinders-vs-liquid-oxygen/