Apogee Instruments MO-200 Benutzerhandbuch - Seite 10
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Sensor Calibration
Although the MO-200 reports the relative percent oxygen in air, it will respond to absolute oxygen concentration. Changes in
barometric pressure and temperature cause changes in absolute oxygen concentration, and as a result, changes in sensor
signal output. This causes apparent changes in relative oxygen concentration, even though the relative amount of oxygen
remains constant. Changes in absolute humidity (water vapor pressure of air) cause changes in absolute and relative oxygen
concentration, as water vapor molecules displace and dilute oxygen molecules. Therefore, the MO-200 oxygen meters are
not calibrated at the factory and must be calibrated by the user. It is recommended to calibrate the MO-200 before taking a
series of measurements, or under conditions where drastic changes occur in pressure, temperature, or humidity during
subsequent measurements.
Because the MO-200 is easily calibrated to ambient atmospheric conditions, the following information regarding the effects
of barometric pressure, temperature, and humidity on the MO-200 may not be pertinent to all users. It is provided as a
reference for users who wish to make the corrections without recalibrating the meter.
Effect of Barometric Pressure on Oxygen Concentration
The ideal gas law, equation (1), shows that absolute gas concentration increases by 0.987 % at sea level for every 1 kPa
increase in pressure (1 kPa / 101.325 kPa = 0.00987). For a sensor that measures absolute gas concentration, but is calibrated
to read out in relative units, a 1 kPa pressure increase at sea level results in an apparent oxygen increase of 0.207 % (0.00987 *
20.95 % = 0.207 %) and an apparent relative oxygen concentration of 21.157 %. Relative gas concentration didn't really
increase, but absolute concentration, which is what sensors measure, did change. This shows up as an apparent change in
relative concentration.
Due to lower barometric pressure at higher elevations, the percentage increase in absolute gas concentration per kPa
increases with elevation. For example, at an elevation of 1378 m (Logan, Utah), barometric pressure is approximately 86 kPa
and absolute gas concentration increases by 1.16 % for every 1 kPa increase in pressure (1 kPa / 86 kPa = 0.0116). Again, for a
sensor that measures absolute gas concentration, but is calibrated to read out in relative units, this results in an apparent
oxygen increase. In this example, 0.247 % for every 1 kPa increase in barometric pressure (0.0118 * 20.95 % = 0.243 %) and an
apparent relative oxygen concentration of 21.193 %.
A barometric pressure correction should be applied to all oxygen sensors that are calibrated to read relative oxygen
concentration. The equation to correct relative oxygen measurements for barometric pressure at any elevation is:
where O
is measured oxygen concentration [%] (apparent oxygen concentration), P
2M
time of calibration, and P
M
pressure (P
, in kPa) for a given elevation is calculated from:
B
where E is elevation [m]. In order to make a barometric pressure correction on gas measurements, it must be continuously
measured as it changes over time (see Apogee webpage for a barometric pressure sensor that can be used for continuous
measurements of barometric pressure: http://www.apogeeinstruments.com/barometric-pressure/). The typical annual
barometric pressure range is approximately 4 kPa, or the average pressure for a given elevation +/- 2 kPa.
The apparent effect of barometric pressure on relative oxygen measurements, based on calculations from equation (2), is
plotted in the figure below for 1378 m elevation to show the significance of measuring and correcting for barometric
pressure. If not accounted for, barometric pressure fluctuations show up in oxygen measurements as a change in relative
oxygen concentration because sensors respond to absolute oxygen concentration, but are generally calibrated to read out in
relative units.
is barometric pressure [kPa] at the time of the current measurement. Approximate barometric
=
−
P
101
.
325
101
B
P
=
C
O
O
2
2
M
P
M
E
−
−
.
325
1
1
44307
.
69231
is barometric pressure [kPa] at the
C
. 5
25328
10
(2)
(3)