Apogee Instruments MO-200 Manual do Proprietário - Página 11
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Effect of Temperature on Oxygen Concentration:
The ideal gas law, equation (1), shows that absolute gas concentration decreases by 0.341 % for a 1 C increase in temperature
from 20 C (1 K / 293 K = 0.00341). For a sensor that measures absolute gas concentration but is calibrated to read out in
relative units, a 1 C temperature increase from 20 C results in an apparent decrease of 0.0714 % O
%) and a relative oxygen concentration of 20.878 %. As with barometric pressure, to obtain accurate oxygen measurements
with a sensor that is calibrated to read relative oxygen concentration, a correction should be applied to compensate for
temperature effects. The equation to correct relative oxygen measurements in air for temperature effects is:
where O
is as given above, T
2M
(note that temperatures in equation (4) must be in K). The effects of temperature on relative oxygen measurements, based on
calculations from equation (4), are plotted in the figure below to show the significance of measuring and correcting for
temperature. If not accounted for, temperature fluctuations show up in the measurement as an apparent change in relative
oxygen concentration because sensors respond to absolute oxygen concentration, but are calibrated to read out in relative
units.
O
2
is air temperature [K] at calibration, and T
C
A) Barometric pressure and absolute oxygen
concentration at 20 C as a function of
elevation. Equation (3) was used to calculate
barometric pressure. B) Effect of barometric
pressure on apparent relative oxygen
concentration. Oxygen sensors respond to
absolute oxygen concentration, but are often
calibrated to yield relative oxygen
concentration. As barometric pressure
fluctuates, absolute oxygen concentration,
and thus oxygen sensor output, fluctuates
with it, producing an apparent change in
relative oxygen concentration if this pressure
effect is not accounted for. It is assumed the
sensor was calibrated at 86 kPa, and the solid
line shows how the apparent relative oxygen
concentration is dependent on barometric
pressure.
T
=
M
O
2
M
T
C
is air temperature [K] at the time of measurement
M
11
(0.341 % * 0.2095 = 0.0714
2
(4)