TLDR;
- Oxygen fraction (FO2) stays constant at ~20.9%, but partial pressure of oxygen (ppO2) — what sensors actually measure — changes with barometric pressure and humidity. This is normal physics, not a device fault.
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All Divesoft analyzers provide a builtin compensation for ambient pressure, to avoid manual adjustment when travelling to different altitude.
Explanation
Air is roughly 20.9% oxygen — this is the fraction of oxygen (FO2), and it stays essentially constant regardless of pressure or altitude.
What an oxygen sensor actually measures is not FO2 directly, but the partial pressure of oxygen (ppO2) — the share of total pressure contributed by oxygen molecules. It's calculated as:
ppO2 = FO2 × total pressure
Since FO2 stays constant but total pressure doesn't, ppO2 changes whenever barometric pressure changes — with weather, altitude, or indoor conditions. Lower pressure means fewer oxygen molecules in a given volume of air, even though the percentage (FO2) is unchanged.
Humidity adds a small additional effect: water vapor takes up part of the total pressure, slightly reducing the pressure available to oxygen and other dry gases. So humid air has a marginally lower ppO2 than dry air at the same barometric pressure - see table at the end of this article.
Divesoft analyzers compensate for barometric pressure automatically. Small variations in readings with pressure or humidity reflect real physical changes in the air, not a device fault.
What to check
- Ensure the analyzer has stabilized to ambient conditions before measuring.
- Avoid measuring immediately after a sudden change in environment (e.g. using a hairdryer to heat up frozen analyzer forgotten overnight in a car).
- Calibrate under stable conditions where possible.
When to contact service
If readings deviate significantly beyond what pressure and humidity can reasonably explain, please contact an authorized Divesoft Service Point.
You can find your nearest authorized service location on our Service Centers page.
Oxygen content in ambient air based on temperature and relative humidity
