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8
Calibration Standards
8.1
The Air Calibration Standard
The Air Calibration standard can be
ambient air
water-vapor saturated air
air saturated water (100% air saturation)
When inserting fragile needle-type oxygen sensors into the
calibration standards, ensure that the sensor tips are not hitting
against e.g. the bottom of the flask or any hard object. Always use
a proper lab stand for mounting the oxygen sensor!
All air calibration standards described in the following rely on the
virtually constant oxygen content in the earth’s atmosphere of
about 20.95%O2 in dry air. Slight deviations might be given in
closed rooms occupied by many people (or e.g. candles,
combustion engines) consuming the oxygen. So if in doubt, ensure
a good ventilation of the room with fresh air e.g. by opening a
window for some minutes.
Furthermore, the relative humidity of the air causes deviations
from the ideal value of 20.95%O2. Simply speaking, the water
vapor in humid air replaces a fraction of the oxygen, resulting in a
diminished oxygen level of e.g. 20.7%O2. For temperatures around
and below 20°C, this effect causes fortunately only a maximum
deviation of about 0.5%O2. However, for higher temperatures at
30°C or even 40-50°C, the humidity of the air gets a significant
influence on the actual oxygen level. For example, ambient air at
body temperature (37°C) with 100% relative humidity contains only
19.6%O2 compared to dry air with 20.95%O2.
