WMA-5 Operation Manual V. 1.01
10
The gas sample is of course a mixture of gas molecules, and this can present problems in terms of
accurate detection of concentrations of a specific gas, such carbon dioxide. This effect,
foreign gas
broadening
(FGB), must be corrected to ensure accurate measurement of gas concentrations. With FGB,
the CO
2
gas in the IRGA cell is somewhat diluted by the increased air volume induced by water vapor.
This effect is about 0.1 µmol mol
-1
CO
2
mb
-1
H
2
O. The presence of water vapor also causes an increase
in infra-red absorption, which is detected as an apparent increase in [CO
2
]. This is of a similar magnitude,
but opposite to the dilution effect, and the WMA-5 automatically corrects these FGB effects.
The WMA-5 IRGAs are quite stable owing to their construction, calibration and thermal environment, but
various circumstances can cause apparent changes over time. Some changes may require recalibration,
although one of the strengths of the WMA-5 is that recalibration is not a routine (annual) maintenance
task as a result of our innovative “Auto-Zero” function. Our Auto-Zero function corrects for nearly all
changes that result in calibration drifts. It minimizes effects on span (gas sensitivity), of sample cell
contamination, lamp ageing, changes in detector sensitivity, amplifier gains and reference voltages.
Measurements are ratioed to the Zero reading before IR absorbance is determined. From the
relationship between absorbance and concentration determined in the factory for each instrument, and
the current calibration factor, the sample concentration is determined.
The WMA-5 detector is optimized
for this CO
2
waveband (4.26 µm)
