Campbell Scientific IRGASON Integrated CO2/H2O Open-Path Gas Analyzer and 3D Sonic Anemometer - Manual (Page 53)

IRGASON ® Integrated CO
2
/H
2
O Open-Path Gas Analyzer and 3D Sonic Anemometer
The sonic virtual temperature, in degrees Celsius, is given by Eq. (9), where γ
d
= 1.4 and R
d
= 287.04 J·K -1 ·kg -1 .
T
c
R
s
d d
= −
2
273 15
γ
.
(9)
11.2 IRGASON Gas Analyzer
The IRGASON gas analyzer is a non-dispersive mid-infrared absorption
analyzer. Infrared radiation is generated in the upper arm of the analyzer head
before propagating along a 15 cm optical path. Chemical species located
within the optical beam will absorb radiation at characteristic frequencies. A
mercury cadmium telluride (MCT) detector in the lower arm of the gas
analyzer measures the decrease in radiation intensity due to absorption, which
can then be related to analyte concentration using the Beer-Lambert Law:
cl
o
e P P ε − =
(10)
where P is irradiance after passing through the optical path, P
o
is initial
irradiance, ε is molar absorptivity, c is analyte concentration, and l is
pathlength.
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; thus,
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 100 revolutions per second, and the
detector is measured 512 times per revolution, resulting in a detector sampling
rate of 512 kHz. The detector is maintained at –40 o C using a 3-stage
thermoelectric cooler and is coupled to a low-noise pre-amp 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 (100 Hz). These are then
filtered to the user-specified bandwidth. The EC100 also synchronously
measures and processes data from the IRGASON sonic anemometer.
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