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Did you know… relative humidity has an influence on the calibration of your DO sensor?

Did you know… relative humidity has an influence on the calibration of your DO sensor?

Yes it does!

When you measure dissolved oxygen in liquids, you normally calibrate your sensor in air. That’s OK. Air is a well-known substance and practically everywhere available. The oxygen content in air depends on it partial pressure. In dry air this partial pressure is 209.5 mbar for an atmospheric pressure of 1 bar. As we all know, the air we breathe is mostly not dry. It contains water, as vapour, causing humidity. If there is water vapour in the air the partial pressure of oxygen reduces. That is why you need to compensate for relative humidity while calibrating the oxygen sensor in air.

Example: If you calibrate in air at 1013 mbar and 30°C with a relative humidity of 80%, you have 205 mbar partial pressure (or 20.24% O2) instead of 209.5 mbar (20.68%) in dry air. This means a difference of more than 2% of measured value. If you don’t take this into account while calibrating, it results in a calibration error.

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