Apogee Instruments MO-200 Owner's Manual - Page 12
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Effect of Humidity on Oxygen Concentration
As absolute humidity in the atmosphere increases, water vapor molecules displace and dilute other gas molecules. This
causes the signal output of a gas sensor to decrease. The water vapor effect on relative oxygen concentration as a function of
relative humidity (RH) and at a constant temperature is a linear decrease with increasing RH, as shown in the figure below.
Conversely, the effect as a function of temperature at constant RH is a curvilinear decrease with increasing temperature,
essentially the inverse of the slope of vapor pressure curves from a psychrometric chart. Even though water vapor molecules
dilute and displace oxygen molecules, and thus cause an actual and not an apparent decrease in relative oxygen
concentration, humidity effects are often accounted for to yield relative oxygen concentrations for a dry atmosphere. The
equation to correct for humidity effects is:
where P
is barometric pressure at calibration [kPa], e
C
vapor pressure [kPa] of air at calibration. Vapor pressures in equation (5) are calculated from:
where RH is in % and e
is saturation vapor pressure [kPa] of air calculated from air temperature (T
S
In soil environments relative humidity is generally between 99 and 100 %, unless the soil is extremely dry (below the
permanent wilting point of -1,500 kPa). Thus, the water vapor effect can be accounted for as a function of temperature by
correcting oxygen measurements based on the shape of the curve for 100 % RH in the graph below.
(
+
P
e
=
C
AM
O
O
2
2
M
P
is vapor pressure [kPa] of air at the time of measurement, and e
AM
RH
=
e
e
A
S
100
T
18
.
A
=
e
. 0
61121
exp
S
257
Effect of temperature on apparent relative oxygen
concentration. As with barometric pressure, absolute
oxygen concentration, and thus oxygen sensor
output, varies with temperature. As temperature
changes, relative oxygen concentration remains
constant at 20.95 %, but an apparent oxygen change is
measured if the temperature correction is not applied
to relative measurements. It is assumed the sensor was
calibrated at 20 C, and the solid line shows how the
apparent relative oxygen concentration is dependent
on temperature.
)
−
e
AC
C
T
−
A
678
234
5 .
.
+
.
14
T
A
12
(5)
is
AC
(6)
, in C):
A
(7)