# What does 'normal conditions' mean in plant capacity?

**What does it mean when an oxygen plant's capacity is stated in Nm3/h at normal conditions?**

A capacity in Nm³/h is stated at a reference condition of temperature and pressure, and WHO (2023) notes that the unit determines the actual gas volume; EU Implementing Regulation 2018/2066 on greenhouse gas monitoring, for example, defines the standard conditions behind normal cubic metres as 273.15 K and 101,325 Pa.

WHO, Foundations of medical oxygen systems (2023), explains that a plant capacity in Nm³/h refers to a reference condition of temperature and pressure, that some capacities are expressed in Sm³/h, based on standard reference conditions, and that the units used determine the actual gas volume produced.

In EU law, Commission Implementing Regulation (EU) 2018/2066 on the monitoring of greenhouse gas emissions defines standard conditions as a temperature of 273.15 K and a pressure of 101,325 Pa, the conditions defining normal cubic metres (Nm³).

Compressor datasheets can use another basis. The ULTRASCREW datasheet measures air flow in accordance with ISO 1217:2009, Annex C, with values referring to 1 bar absolute, 0% relative humidity and 20 °C inlet air, and the ULTRACECO SC datasheet gives reference conditions of 1 bar absolute inlet pressure and 20 °C suction temperature.

## Capacity and purity

The Global Fund briefing note on PSA plant oxygen purity (3 April 2024) gives an example: a 20 Nm³/h PSA plant set to 93 ± 3% could produce 20 Nm³/h, while the same equipment set to 95 ± 1% would produce 18 Nm³/h. The note adds that the purity decreases as more oxygen is demanded from the plant.

The ULTRACON concentrator datasheet gives each model's oxygen flow at 93% and at 95%, in Nm³/h, Nl/min and SCFH, together with the compressed air it consumes in Nm³/h; the 14D, the largest model in the table, is listed at 111.4 Nm³/h at 93% and 90.4 Nm³/h at 95%. The ULTRAOX SC and ST model tables give each plant's oxygen capacity in l/min and m³/h.

### Is Nm³/h the same as Sm³/h?

Not necessarily. WHO (2023) notes that some capacities are given in Sm³/h, based on standard reference conditions, and that the units used determine the actual gas volume produced.

### Does a higher purity setting change the rated flow?

Yes. In the Global Fund (2024) example, the same PSA equipment gives 20 Nm³/h at 93 ± 3% and 18 Nm³/h at 95 ± 1%.

## Sources

- [Commission Implementing Regulation (EU) 2018/2066, Article 3(52), consolidated text of 1 January 2025](http://publications.europa.eu/resource/celex/02018R2066-20250101)
- [WHO, Foundations of medical oxygen systems (17 February 2023), section 2.1](https://iris.who.int/server/api/core/bitstreams/4ad6f5fd-089e-4142-8439-b64e98d816d8/content)
- [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)
- [ULTRACON datasheet, p. 3](https://www.ultracontrolo.com/products/ultracon)
- [ULTRASCREW datasheet, p. 3](https://www.ultracontrolo.com/products/ultrascrew)
- [ULTRACECO SC datasheet, p. 5](https://www.ultracontrolo.com/products/ultraceco-sc)
- [ULTRAOX SC datasheet, p. 5](https://www.ultracontrolo.com/products/ultraox)
- [ULTRAOX ST datasheet, p. 5](https://www.ultracontrolo.com/products/ultraox-st)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/what-does-rated-capacity-at-normal-conditions-mean/