We have developed a PDF called
"Understanding Oxygen in Air"
that discusses the following topics,
some of which are summarized below:
Absolute and Relative
Gas Concentration
Effect of Barometric Pressure
on the Output of the Apogee
Oxygen Sensor
Example of Pressure Correction
Effect of Temperature
on the Output of the Apogee
Oxygen
Sensor
Example of Temperature
Correction in Soil
EFFECT OF BAROMETRIC PRESSURE:
The sensor is not pressure compensated so changes
in barometric pressure change the mV output of the
sensor. Barometric pressure changes are relatively small
and they do not occur rapidly so it is not always
necessary to measure and correct for pressure.
At sea level, the pressure averages 101.3 kPa and ranges
± 2%. The pressure extremes are uncommon. Barometric
pressure varies less than ± 1% on about 90% of the days
of the year. When the sensor is in air the reading could
change from about 20.7 to 21.2 % over a period of
several weeks, assuming typical pressure changes.
EFFECT OF
TEMPERATURE: In a constant oxygen
atmosphere, the mV output of the sensor would change in
proportion to a change in the absolute temperature.
Since room temperature is about 300 K, a 3 degree
Celsius temperature change would change the output by 1%
(from 20.9 to 20.7%).
However, this unit has a temperature compensation
circuit that corrects for about half of the temperature
effect. For exact measurements, the sensor has either a
thermistor (-TM) or a type-K thermocouple (-TCK) inside
that can be used to calibrate and more precisely correct
for the effect of temperature.
Visit the Programs & Manuals section for a sample
program written for a Campbell Scientific, Inc.
datalogger to automatically monitor and compensate for
changes in temperature while using the sensor.
EFFECT OF HUMIDITY:
The graph below shows an example of humidity
dependency. The sensor is not influenced by humidity,
but its output decreases as O2 is displaced
by water vapor molecules in the air.
CLICK IMAGE FOR LARGER VIEW.
UNDERSTANDING OXYGEN IN AIR
