where:
P = irradiance after passing through the optical path,
P
o
= initial irradiance, ε is molar absorptivity,
c = analyte concentration, and
l = path length.
In the IRGASON, radiation is generated by applying constant power to a tungsten lamp, which
acts as a 2200 K broadband radiation source. Specific wavelengths are then selected using
interference filters located on a spinning chopper wheel. For CO
2
, light with a wavelength of
4.3 µm is selected, as it corresponds to the molecule’s asymmetric stretching vibrational band.
For H
2
O, radiation at 2.7 µm, corresponding to water’s symmetric stretching vibrational band, is
used.
The IRGASON gas analyzer is a dual-wavelength single-beam analyzer. Rather than using a
separate reference cell and detector, the initial intensity of the radiation is calculated by
measuring the intensity of nearby, non-absorbing wavelengths (4 µm for CO
2
and 2.3 µm for
H
2
O). These measurements account for any source and detector aging, and window
contamination.
The chopper wheel spins at a rate of 60 revolutions per second, and the detector is measured 512
times per revolution, resulting in a detector sampling rate of 30.72 kHz. The detector is
maintained at –40 °C using a three-stage thermoelectric cooler and is coupled to a low-noise
preamp module.
The EC100 electronics digitize and process the detector data (along with ancillary data such as
sample temperature and pressure) to give the CO
2
and H
2
O densities after each chopper wheel
revolution (60 Hz). These are then filtered to the user-specified bandwidth. The EC100 also
synchronously measures and processes data from the IRGASON sonic anemometer.
12. References
Fleagle, R. G. and Businger, J. A.: 1980,
An Introduction to Atmospheric Physics
, Academic Press,
Inc., New York.
IRGASON® Integrated CO
2
/H
2
O Open-Path Gas Analyzer and 3D Sonic Anemometer
57
Summary of Contents for IRGASON
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