CIRAS-3 Operation Manual V. 1.09
41
Both H
2
O and CO
2
molecules have diverse absorption spectra, so we use two prominent absorption
peaks, seen below at 2.6 and 4.26 µm, respectively. CIRAS-3’s electronics could be considered the fifth
component, which processes raw analog-to-digital (A/D) information from the IRGAs detectors, accurately
translating this information into gas concentrations.
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 CIRAS-3 automatically corrects these FGB effects.
CIRAS-3’s 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 CIRAS-3 is that recalibration is not a routine (annual) maintenance task.
The factory calibration ranges of 0-2000 µmol mol
-1
CO
2
and 0-75 mb water vapor are ideally suited for
most typical applications.
Factory linearization of the IRGA cells is standard, but slight differences between IRGAs are inherent due
to the uniqueness of optical filters and reflection characteristics of the cells - this is common to all
differential analyzers. Still, the Reference and Analysis cells should be made to match a standard such
as Zero air, and CIRAS-3’s Auto Zero function corrects for nearly all changes that result in calibration