Vaisala MGP260 Series, User Manual, Page: 16 / 92

2.4 Measurement principle
1
2
3
The Vaisala CARBOCAP
â
sensor used in
the probe is a silicon-based, nondispersive
infrared (NDIR) sensor for the
measurement of methane (CH
4
), carbon
dioxide (CO
2
), and humidity (H
2
O).
Figure 4 Probe cuvette with mirror and
sensor chips
1 Mirror
2 Cuvette
3 Sensor chips under TO5 packages
The sensitivity to gases is based on absorption of infrared light at a characteristic wavelength.
During measurement, infrared light is routed through the cuvette that contains the gas to be
measured. A mirror reflects the light from the cuvette to thermopile detectors that measure
the light intensity at a wavelength determined by a Fabry–Pérot interferometer (FPI) and a
band pass filter. One set of optics measures humidity and carbon dioxide, and a second one
measures methane.
The measurement consists of two steps: first, the FPI is electrically tuned so that its pass band
coincides with the characteristic absorption wavelength of the measured gas and the signal is
recorded. Second, the pass band is shifted to a wavelength where no absorption occurs in
order to get a reference signal. The ratio of these two signals, one at the absorption
wavelength and the other at the reference wavelength, gives the fraction of light absorption
from which the gas concentration is calculated. Measuring the reference signal compensates
the possible effects of sensor aging and signal attenuation due to dirt on optical surfaces,
making the sensor very stable over time.
MGP260 User Guide M212504EN-A
14
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