Campbell Scientific EC155 CO2 and H2O Closed-Path Gas Analyzer and EC100 Electronics with Optional CSAT3A 3D Sonic Anemometer - Manual (Page 54)

EC155 CO
2
and H
2
O Closed-Path Gas Analyzer
11. Theory of Operation
The EC155 is a non-dispersive mid-infrared absorption analyzer. Infrared
radiation is generated in the larger block of the analyzer before propagating
through a 12 cm sample cell. Chemical species located within the sample cell
will absorb radiation at characteristic frequencies. A mercury cadmium
telluride (MCT) detector in the smaller block 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 − =
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 EC155, 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
, radiation 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 EC155 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-cell temperature and pressure) to give the CO
2
and H
2
O concentration for each chopper wheel revolution (100 Hz), filtered to
the user-specified bandwidth. The EC100 also synchronously measures and
processes data from an optional CSAT3A 3D sonic anemometer head.
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