# Why does PSA purity drop when demand rises?

**Why does the oxygen purity of a PSA plant fall when hospital demand exceeds its rated flow?**

PSA purity drops with demand because a PSA plant trades flow against purity: Global Fund guidance states that as more oxygen is demanded from a PSA plant, its purity will decrease. Above the rated flow, a USAID operating procedure for Indian hospitals warns that purity and pressure fall and that valves, sensors and gauges can fail.

## Flow and purity in the datasheets

Manufacturers' tables show the trade-off directly. The ULTRACON concentrator table gives, for every size, a lower output at 95% oxygen than at 93%:

| ULTRACON model | Output at 93% (Nm³/h) | Output at 95% (Nm³/h) |
|---|---|---|
| 1 | 0.32 | 0.3 |
| 6 | 6.5 | 5.3 |
| 10 | 18.2 | 14.8 |
| 12D | 73 | 59.4 |
| 14D | 111.4 | 90.4 |

The Global Fund purity note (2024) gives the same kind of example: equipment that produces 20 Nm³/h at 93% (±3%) produces 18 Nm³/h when set to 95% (±1%).

## Why it happens

In a PSA plant the zeolite preferentially adsorbs nitrogen while letting oxygen pass, and once it is saturated the vessel pressure is lowered to release the nitrogen, as PATH describes. HTM 02-01 (2026) notes that regeneration itself uses a proportion of the product gas, which should be taken into account when sizing the plant. When the hospital draws more oxygen than the plant is rated for, purity falls, and the USAID procedure adds that oxygen pressure drops too.

## What to do about it

- Test at full load: the Global Fund commissioning guideline holds the flow at the plant's rated maximum for 24 hours, with purity above 90% at all times.
- Compare production with use: the USAID procedure's troubleshooting for low tank pressure is to check consumption against production, and Global Fund guidance says an overused plant should be supplemented with secondary or tertiary sources.
- Use oxygen rationally: Global Fund guidance recommends training health workers on the rationale of oxygen therapy.
- Keep the alarm and changeover working: see purity alarms.
- Add capacity: ULTRAOX SC plants are modular and increase flow capacity progressively, and ULTRAOX ST plants can be expanded without replacing the concentrator.

### Does a plant set to 95% give less oxygen than one set to 93%?

Yes, for the same equipment: the Global Fund purity note gives 20 Nm³/h at 93% (±3%) against 18 Nm³/h at 95% (±1%), and the ULTRACON table shows lower flows at 95% for every size.

### How can a hospital tell that the plant is overdrawn?

Falling purity and falling pressure are the effects of overdrawing that the USAID procedure lists; it asks operators to compare production with consumption.

## Sources

- [Global Fund / Build Health International, Briefing Note: PSA Plant Oxygen Purity (3 April 2024)](https://resources.theglobalfund.org/media/lenpqaoa/cr_c19rm-psa-oxygen-purity_technical-briefing-note_en.pdf)
- [USAID RISE, SOP: PSA Plant Operations and Preventive Maintenance (March 2022)](https://media.path.org/documents/SOP_on_PSA_Plant_USAID_RISE_04032022_F7fUBr6.pdf)
- [PATH and CHAI, Oxygen Generation and Storage: Pressure/Vacuum Swing Adsorption Plant (July 2021)](https://media.path.org/documents/O2_generation_and_storage_PSA_VSA_v1.pdf)
- [NHS England, HTM 02-01 Part A (2026), paragraph 6.110](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)
- [Global Fund / Build Health International, Overview: PSA Oxygen Plant Commissioning Guidelines (28 August 2024)](https://resources.theglobalfund.org/media/qp3h1vxo/cr_c19rm-psa-oxygen-plant-commissioning_overview_en.pdf)
- [Global Fund / Build Health International, Overview: Sustainability Guidance for PSA Oxygen Plants (22 August 2024)](https://resources.theglobalfund.org/media/bwrp43cm/cr_c19rm-psa-oxygen-plant-sustainability_overview_en.pdf)
- [ULTRACON datasheet, p. 3 (model table); ULTRAOX datasheet 2022, p. 15](https://www.ultracontrolo.com/products/ultracon)
- [ULTRAOX SC datasheet, p. 2](https://www.ultracontrolo.com/ultraox)
- [ULTRAOX ST datasheet, p. 4](https://www.ultracontrolo.com/products/ultraox-st)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/why-does-psa-purity-drop-when-demand-rises/