Oxygen Partial Pressure vs Oxygen Percentage: Why 32–40 kPa Does Not Mean 32–40%
The percentage tells you how much of the gas mixture is oxygen. Partial pressure tells you the pressure contributed by that oxygen.
In a pressurised chamber those numbers are not interchangeable. This guide explains the calculation, the physiology and why accurate units matter for both science and safety.
At a total pressure of 150 kPa, an oxygen partial pressure of 32–40 kPa corresponds to an oxygen fraction of roughly 21.3–26.7%, assuming an approximately uniform dry gas mixture. That is why writing “32–40% oxygen” is not equivalent to writing “32–40 kPa oxygen partial pressure”.
The calculation is simple — but the units must be right
Dalton’s law says that the total pressure of a gas mixture is the sum of the partial pressures contributed by its component gases. For oxygen:
Rearranging the same relationship gives the oxygen fraction: FiO₂ = pO₂ ÷ total pressure.
21% oxygen does not always create the same oxygen partial pressure
Approximately the oxygen partial pressure of dry atmospheric air near sea level.
The oxygen fraction can remain near ordinary air while pressure raises oxygen partial pressure substantially.
What does Ekonstal’s 31.5–40 kPa oxygen specification imply?
Ekonstal currently states that its chamber operates at approximately 1500 hPa (150 kPa) total pressure and an oxygen partial pressure of approximately 31.5–40 kPa.
So the manufacturer’s specification describes an approximate oxygen fraction range of 21–26.7% at 150 kPa, subject to actual measured chamber conditions and the usual assumptions used in this simple gas-law conversion.
Why does partial pressure matter to breathing physiology?
Gas exchange is driven by pressure gradients. Oxygen moves from inspired gas into the lungs and then into blood according to partial-pressure gradients, not simply because a display shows a particular percentage.
This is why altitude affects oxygen availability even though the oxygen fraction of air remains close to 21%: total atmospheric pressure falls, so oxygen partial pressure also falls. A pressurised environment does the opposite.
Partial pressure matters physiologically; oxygen concentration matters strongly for fire safety
The distinction works both ways. For respiratory physiology, partial pressure is fundamental. For fire risk, the concentration of oxygen in the surrounding atmosphere is also a critical engineering parameter.
HSE guidance warns that oxygen enrichment makes materials easier to ignite and fires burn more intensely. Separate HSE compressed-gas guidance notes that room atmospheres above about 25% oxygen by volume can be dangerous, particularly in confined spaces.
The original chamber paper had the gas-pressure units wrong
In 2024, the authors issued a formal correction to the 2020 chamber study because specific information about chamber gas pressures had been incorrectly reported.
The corrected paper now states ten two-hour sessions at a total pressure of 1500 hPa with 32–40 kPa partial pressure of oxygen, 0.7–2 kPa carbon dioxide and 0.4–0.5 kPa hydrogen.
This is why new Oxygen Clinic technical content should use the corrected units rather than copying legacy percentages from older descriptions.
Same idea, different combinations of pressure and oxygen fraction
| Example | Total pressure | O₂ fraction | Approx. O₂ partial pressure |
|---|---|---|---|
| Dry air near sea level | 101.3 kPa | 20.95% | 21.2 kPa |
| 150 kPa chamber at 21% O₂ | 150 kPa | 21% | 31.5 kPa |
| 150 kPa chamber at 26.7% O₂ | 150 kPa | 26.7% | ≈ 40 kPa |
| Illustrative 200 kPa, 100% O₂ | 200 kPa | 100% | 200 kPa |
The final row is a physics illustration only; it is not a description of Oxygen Clinic or a treatment recommendation.
What should Oxygen Clinic publish?
The technically safest public wording is to use the manufacturer’s stated units directly:
Total pressure: approx. 1500 hPa / 150 kPa
Oxygen partial pressure: approx. 31.5–40 kPa
Carbon dioxide partial pressure: approx. 0.7–2 kPa
Hydrogen partial pressure: approx. 0.4–0.5 kPa
Converted percentages can be shown as explanatory approximations only after the actual chamber-control specification is confirmed with the manufacturer or operational logs.
Oxygen partial pressure: quick answers
What is oxygen partial pressure?
It is the share of total gas pressure contributed by oxygen. In a simple gas mixture, oxygen partial pressure is oxygen fraction multiplied by total pressure.
Does 32 kPa oxygen mean 32% oxygen?
Not unless total pressure is 100 kPa. At 150 kPa total pressure, 32 kPa oxygen corresponds to approximately 21.3% of the gas mixture.
What oxygen percentage corresponds to 40 kPa at 150 kPa?
Approximately 26.7%, because 40 ÷ 150 ≈ 0.267.
Why does altitude matter if air is still about 21% oxygen?
At altitude, total atmospheric pressure falls. Because partial pressure equals gas fraction multiplied by total pressure, oxygen partial pressure also falls even when oxygen remains about 21% of the mixture.
Why does oxygen percentage matter for fire safety?
Oxygen-enriched atmospheres make many materials easier to ignite and can make fires burn more intensely. That is why chamber engineering, materials, ignition control and actual oxygen concentration matter.
What specification should I use for Oxygen Clinic?
Until direct chamber logs or manufacturer confirmation support another format, use the manufacturer’s published total pressure and gas partial-pressure values rather than converting the kPa figures directly into percentage labels.
Sources used for this guide
Now put oxygen, CO₂, hydrogen and pressure together
The chamber atmosphere only makes sense when all of its gas parameters are interpreted using the same units and total pressure.