Campbell Scientific EC150 CO2 and H2O Open-Path Gas Analyzer and EC100 Electronics with Optional CSAT3A 3D Sonic Anemometer - Manual (Page 16)

EC150 CO
2
/H
2
O Open-Path Gas Analyzer
The algorithm uses the following environmental parameters to control the
heater:
• Analyzer body temperature, measured inside the source housing
(heater control does not allow the body temperature to drop below
ambient air temperature)
• Ambient relative humidity (in humidity greater than 80% heaters will
try to maintain internal temperature 2 degrees warmer than ambient)
• CO
2
signal level (1 min average CO
2
signal level; below 0.7 will
cause the heater to turn on maximum power until the signals recover)
• Average slope of the CO
2
signal level over 1 min
• Standard deviation of the CO
2
signal over 1 min
4.8 Theory of Operation
The EC150 is a non-dispersive mid-infrared absorption analyzer. Infrared
radiation is generated in the upper arm of the analyzer head before propagating
along a 15.0 cm (5.9 in) optical path as shown in FIGURE 5-1. 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 − =
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 path length.
In the EC150, 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
measurements, light with a wavelength of 4.3 µm is
selected as that corresponds to the asymmetric stretching vibrational band of
the CO
2
molecule. For H
2
O, the symmetric stretching vibration band is 2.7
µm.
The EC150 is a dual wavelength, single-beam analyzer. This design eliminates
the need for a separate reference cell and detector. Instead, the initial intensity
of the radiation is calculated by measuring the intensity of nearby, non-
absorbing wavelengths (4.0 µm for CO
2
and 2.3 µm for H
2
O). These
measurements mitigate measurement inaccuracy that may arise from source or
detector aging, as well as for low-level window contamination. For window
contamination that reduces the signal strength below 0.8, windows should be
cleaned as described in Section 8.3, Cleaning Analyzer Windows.
The chopper wheel spins at a rate of 50 revolutions per second and the detector
is measured 1024 times per revolution, resulting in a detector sampling rate of
51.2 kHz. The detector is maintained at −40°C using a three-stage
thermoelectric cooler and is coupled to a low noise pre-amp module.
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