Vaisala GMP231 - Manual (Page 17)

Chapter 2 ___________________________________________________________ Product Overview
VAISALA ________________________________________________________________________ 15
Operating Principle of CO
2
Measurement
The Vaisala CARBOCAP
®
sensor that is 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).
1 = Cuvette.
2 = Mirror.
3 = Window.
4 = Sensor chips.
1403-160
Figure 3 CARBOCAP
®
Sensor of the GMP231
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.
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3
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