The detection method of CO2 concentration is roughly divided into chemical methods and physical methods. The methods for detecting CO2 concentration include titration, thermal catalysis, gas sensing, and electrochemical methods. These are chemical methods. These methods generally have problems such as high price and poor universality, and the measurement accuracy is low. Physical methods include ultrasonic methods, Gas Chromatography, and numerous methods for detecting by means of optics. There are also methods of combining chemical and physical like photoacoustic spectroscopy. Absorption spectroscopy is based on the fact that gas molecules of different chemical structures absorb different degrees of radiation at different wavelengths, and CO2 gas molecules strongly absorb infrared light of a specific wavelength.
At present, various CO2 sensors for detection mainly include solid electrolyte type, barium titanate composite oxide capacitor type, and conductivity change type thick film type. These sensors have poor selectivity to gases, are prone to false alarms, require frequent calibration, and are used. Shortevity, such as short life. The infrared absorption type CO2 sensor has the characteristics of wide measuring range, high sensitivity, fast response time, good selectivity and strong anti-interference ability. Therefore, this design uses an infrared absorption type CO2 sensor.
1 sensing principle
The infrared absorption type CO2 gas sensor is based on the principle that the absorption spectrum of the gas differs depending on the substance. The chemical structures of different gas molecules are different, and the absorption degree of infrared radiation of different wavelengths is different. Therefore, when infrared radiation of different wavelengths is sequentially irradiated to the sample material, the radiation energy of certain wavelengths is selectively absorbed by the sample material and becomes weak, generating infrared rays. Absorption spectrum, so when the infrared absorption spectrum of a substance is known, the absorption peak of the substance in the infrared region can be obtained therefrom.
When the same substance has different concentrations, there are different absorption intensities at the same absorption peak position, and the absorption intensity is proportional to the concentration. Therefore, by measuring the influence of the gas on the wavelength and intensity of the light, the concentration of the gas can be determined.
According to the Bill Lambert's law, the relationship between the output light luminous intensity I, the input light luminous intensity I0, and the gas concentration c is
I=I0exp(-αmLc) (1)
Where: αm is the molar molecular absorption coefficient; c is the gas concentration to be tested; L is the working length of the light and gas (sensing length). Transforming equation (1), you get:
2 instrument design block diagram
The digital signal output by the front-end sensor communicates with the STM32 in serial mode. The instrument is designed for three-channel acquisition. The relay circuit is used to select the sensor channel. The STM32 displays the concentration value on the LCD screen. The LCD screen has a touch function. LCD display interface, call related button program, select sensor channel and save as U disk data. The block diagram is shown in Figure 1.
Figure 1 Instrument design block diagram
3 sensor selection
DYNAMENT's premier carbon dioxide sensor was selected. This sensor uses non-dispersive infrared principle to detect gas. It includes long-life tungsten infrared light source, optical channel for diffused gas, a pair of temperature-compensated infrared principle thermoelectric exchange detection elements, and semiconductor temperature. The sensor and the electronic circuit for processing the signal of the infrared thermoelectric exchange detector are convenient and quick to use, as shown in Fig. 2 is the outline package diagram of the carbon dioxide sensor.
Figure 2 Carbon dioxide sensor outline package
Stirrer Mixer And Shaker,Magnetic Stirrer,Laboratory Hot Plate,Stirrer With Hot Plate
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