Vaisala CARBOCAP GMP231, User Manual, Page: 14 / 92

2.4 Operating principle of CO
2
measurement
Vaisala CARBOCAP â sensor used in the GMP231 is a silicon-based, non-dispersive infrared
(NDIR) sensor for the measurement of gaseous carbon dioxide. It is especially designed to
tolerate high temperatures in standby mode, up to +195 °C (+383 °F). This allows it to be used
in applications that utilize high temperature for heat sterilization. The operating temperature
range of the sensor is 0 … +70 °C (+32 ... +158 °F).
2
3
4
1
Figure 3 CARBOCAP â sensor of the
GMP231
1 Cuvette
2 Mirror
3 Window
4 Sensor chips
The sensitivity to carbon dioxide is based on absorption of infrared light at a characteristic
wavelength. The light is emitted by a novel, silicon-based microchip emitter into the cuvette
which contains the gas to be measured. After reflection from the mirror, the light intensity is
measured with a thermopile chip at a wavelength chosen by a micromechanical Fabry–Pérot
interferometer (FPI) and a band pass filter. A hermetically sealed sapphire window is used to
protect the sensor chips from moisture and contamination. A heater chip is utilized to prevent
condensation in normal operation.
The carbon dioxide measurement consists of two steps: First, the FPI is electrically tuned so
that its pass band coincides with the characteristic absorption wavelength of carbon dioxide
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 carbon dioxide concentration is calculated. 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.
GMP231 User Guide M211501EN-E
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