Operating principle of CO2 measurement - GMP231

GMP231 User Guide

Document code
M211501EN
Revision
E
Language
English (United States)
Product
GMP231
Document type
User guide

Vaisala CARBOCAP® 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).

Figure 1. CARBOCAP® sensor of the GMP231
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Cuvette
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Mirror
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Window
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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.

Figure 2. CO2 measurement inside the incubator
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Chamber wall
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Chamber interior
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Light source
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Band pass filter
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Ambient air (400 … 1000 ppm CO2)
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Light absorbed by CO2 in the incubator gas
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Thermopile detector
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Fabry-Perot interferometer
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Sapphire window. Separates the sensor’s active components from the measured environment.
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Gold-plated mirror