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During the calibration of oxygen sensors, there are two possibilities
to take the humidity into account:
(1)
The relative humidity and the temperature of the
ambient air must be determined during the calibration.
(2)
The calibration standard is prepared in a closed vessel
either filled with water or partly filled with e.g. wet
cotton wool or a wet sponge. This ensures a constant
humidity of 100% RH and there is no need to measure
the humidity.
Option (1) is utilized in the calibration standard "Ambient Air",
whereby option (2) is utilized for the calibration standards "Water".
Another parameter even more important for the air calibration
standard is the atmospheric pressure. The principle parameter
measured by oxygen sensors is not the partial volume (i.e. "% O2"),
but the partial oxygen pressure (i.e. "mbar") (see also appendix
11.3). So an oxygen level of e.g. 20.7% O2 (determined as
described above by a given humidity and temperature) is
converted internally by the software into a partial pressure of
oxygen essentially by multiplying the relative oxygen level with the
atmospheric pressure of e.g. 990 mbar:
0.207 x 990 mbar = 205 mbar
giving a partial oxygen pressure of e.g. 205 mbar. This is the
essential calibration value used internally by the software. The
atmospheric pressure can be influenced 1) by weather changes
(e.g. varying between ca. 990 and 1030 at sea level) and 2) by the
elevation above sea level (e.g. at 1000 m elevation the typical
atmospheric pressure is about 900 mbar compared to 1013 mbar at
sea level).
Thus in summary, there are three important parameters to be
known for the air calibration standard:
