Oxygen and PSA
How do you convert Nm³/h to litres per minute?
How do you convert oxygen plant capacity between Nm3/h, L/min and kg per day?
Short answer
Oxygen plant capacity converts with fixed factors: multiply L/min by 60 and divide by 1,000 to get m³/h, as WHO's 2020 interim COVID-19 guidance does, and reverse those steps to go back; for mass, Open Critical Care gives 1.428 g of oxygen per litre of gas at 0 °C and 1 atm.
From L/min to m³/h and back
In its interim guidance on oxygen sources for COVID-19 treatment centres (4 April 2020), WHO notes that PSA plants are sized by output capacity in m³/h and converts flows in two steps: litres per minute times 60 gives litres per hour, and litres per hour divided by 1,000 gives cubic metres per hour. Its worked example is 75 severe COVID-19 patients at 10 L/min: 75 × 10 × 60 = 45,000 L/h = 45 m³/h.
Published tables give pairs of values in both units:
| Source | Capacity (as published) | Flow |
|---|---|---|
| WHO, Foundations of medical oxygen systems (2023), Table 2.1 | 3 Nm³/h | 50 L/min |
| WHO (2023), Table 2.1 | 16 Nm³/h | 266 L/min |
| WHO (2023), Table 2.1 | 32 Nm³/h | 533 L/min |
| ULTRACON datasheet, model 1, at 93% | 0.32 Nm³/h | 5.3 Nl/min |
| ULTRACON datasheet, model 14D, at 93% | 111.4 Nm³/h | 1,856 Nl/min |
| ULTRAOX ST datasheet, model 1640 | 164 m³/h | 2,734 L/min |
Check the reference conditions
WHO (2023) explains that Nm³/h refers to a reference condition of temperature and pressure, that capacity is sometimes expressed instead in Sm³/h, based on standard reference conditions, and that the unit used determines the actual gas volume produced. When comparing datasheets, check which unit each one uses. The ULTRACON table also gives standard cubic feet per hour, for example 11.2 SCFH for model 1 at 93%.
From volume to kilograms per day
Open Critical Care works out the mass from the ideal gas law: one mole of O2 weighs 32 g and occupies 22.4 L at standard temperature and pressure, which it takes as 0 °C and 1 atm, giving 1.428 g per litre. At 22 °C a mole occupies 24.2 L, giving 1.322 g per litre, so that 1,000 L of gaseous oxygen weighs 1.32 kg at that temperature.
To express a plant's output in kilograms per day, take the volume per day, which Open Critical Care obtains by multiplying the hourly capacity by 24 (a 16 m³/h plant gives 384 m³ per day in its example), and multiply the litres by the grams per litre at the matching temperature. Open Critical Care's figures are for oxygen (O2); Oxygen 93 from a PSA plant contains 90.0% to 96.0% oxygen, the rest mainly argon and nitrogen, according to the International Pharmacopoeia (13th edition, 2025).
More questions
Is a normal cubic metre the same as a cubic metre?
Not necessarily. WHO's 2023 guidance notes that Nm³/h and Sm³/h use different reference conditions, and that the unit used determines the actual gas volume produced.
How many litres are in a cubic metre?
1,000 litres, the factor WHO uses in its 2020 interim guidance to convert litres per hour to cubic metres per hour.
Sources
- WHO, Oxygen sources and distribution for COVID-19 treatment centres, interim guidance (4 April 2020), Table 1
- WHO, Foundations of medical oxygen systems (17 February 2023), sections 2.1 and 2.5, Table 2.1
- Open Critical Care, Oxygen FAQ: How much does gaseous oxygen (O2) weigh?
- Open Critical Care, Oxygen FAQ: How do you calculate the conversion between a PSA plant's rate of oxygen production and cylinder volumes that can be filled?
- The International Pharmacopoeia, 13th edition (2025), Medicinal oxygen
- ULTRACON datasheet, p. 3 (also ULTRAOX datasheet 2022, p. 15)
- ULTRAOX ST datasheet, p. 5 (model table)
Ultra Controlo figures come from its technical datasheets. Third-party facts were read at the source on the date shown.