Introduction to the principle of NDIR carbon dioxide sensor

1 Introduction

Environmental field : In recent years, carbon dioxide is the main gas that causes the greenhouse effect, so it has attracted widespread attention. Normally, the content of carbon dioxide in the atmosphere is about 0.04vol%. According to research, when the concentration of carbon dioxide in the air is about 3vol%, the human body will experience rapid heartbeat and shortness of breath; when the concentration reaches 5vol%, the human body will produce symptoms such as dyspnea and tinnitus.

Medical field : In addition, as a product of the human body's metabolic cycle, carbon dioxide has a concentration range of 4vol% to 5vol%, which can directly reflect the physiological state of the human body. The respiratory carbon dioxide gas concentration monitoring module has become a key part of many clinical medical monitoring equipment.

2. Classification

There are mainly the following types of carbon dioxide gas sensors: infrared absorption type, electrochemical type, thermal conductivity type, mass sensitive type and semiconductor type. The current market situation of various types of carbon dioxide sensors is as follows:

• Infrared absorption carbon dioxide sensors are the most widely used. (Many are sold on Taobao)
• Electrochemical principle carbon dioxide alarms are very common in the market. (no research)
• The thermal conductivity carbon dioxide sensor is mainly used to detect the concentration of carbon dioxide with a large range and a percentage content. (Sensirion in Switzerland has one on sale)
• Mass-sensitive carbon dioxide sensors are currently only in the research and development phase. (haven't seen it before)
• The semiconductor carbon dioxide gas sensor is not popular in the industry at present due to its great influence on temperature, and its application is seldom. (haven't seen it before)

3. Principle of infrared gas sensing

3.1 Lambert-Beer law

This law describes the relationship between the degree of absorption of infrared light by the measured substance and the thickness and concentration of the measured substance.

For example, when a beam of monochromatic light passes through an absorbing medium, part of the light energy will be absorbed by the medium, and the transmitted light intensity will decrease accordingly, as shown in the figure:

The formula is expressed as follows:
$$I=I_0{e}^{-\; inKCL}$$
:
$I_0$——incident light intensity, the light intensity before the infrared rays pass through the measured gas,
$I$ ——Outgoing light intensity, the light intensity of infrared rays after passing through the gas to be measured,
$C$ ——concentration, the concentration value of the gas to be measured,
$L$ ——optical path length, the length of interaction between infrared light and measured gas from the infrared light source to the detector,
$K$ ——absorption coefficient, which depends on the absorption line of the measured object.

Note that the physical formula described by this law is idealized. In the actual sensor test, the optical path length $L$ will not always be a single value, the wavelength band of the peak emission intensity of the light source will also change, the transmittance of the filter is related to the center wavelength of the filter, and the performance of the sensor will also be affected by the ambient temperature and Effect of gas pressure. Therefore, in practical applications, we need to modify the Lambert-Beer law and add compensation algorithms. Introduce two coefficients$a$ and$n$ modified formula, obtained:
$$I=I_0{e}^{-a{C}^n}$$

aa $a$ and$n$ is two fixed values, which are determined by the target gas and specific device design, and can be determined bycalibration.

3.2 Detection principle of non-dispersive infrared (NDIR) method

The detectors currently used for gas detection can be divided into spectroscopic and non-spectral. As shown in Figure 2-2, the former is to separate the monochromatic light through the spectroscopic system and then introduce the light into the gas chamber, and then detect the light intensity by the detector; the latter is to directly input the infrared light with continuous spectrum into the gas chamber by the light source , through the filter installed on the detector to selectively absorb the required wavelength band.

In practical applications, spectroscopic detectors can separate monochromatic light with a narrow band because of their high precision spectroscopic equipment, and are usually used in laboratory spectral analysis instruments. However, due to their complex spectroscopic systems and large volumes, It is impossible to share the same gas chamber, so non-spectral detectors are more commonly used in miniaturized and portable applications. This is why the detection methods of carbon dioxide sensors we see in Taobao are almost all non- spectral infrared (NDIR) detection .

3.3 Concentration, temperature and humidity calibration

After the preparation of an infrared gas sensor is completed, it needs to be calibrated to obtain the formula:
$$1-\frac{\text{ Act }}{\text{ Zero }\cdot \mathrm{Ref}}=\mathrm{Span}\left(1-e^{-a{C}^n}\right)$$
Span, a, n values. Among them, Act, Zero and Ref are the voltage values ​​measured by the infrared detector, and C is the gas concentration. Span, a, n values ​​are obtained by calibrating the gas with known carbon dioxide concentration. When using the sensor, we can measure the output voltage of the detection element to obtain the Act, Zero and Ref voltage values, and then deduce the gas concentration C through this formula.

The temperature and humidity test is to obtain the calibration curve of the sensor under different temperature and humidity environments, and to check whether the environment will affect the output of the sensor. First, the concentration calibration of the sensor at different temperatures is carried out. The purpose of this step is to explore how different temperatures will affect the output of the sensor, and perform temperature compensation through the data obtained from the calibration. Secondly, the concentration calibration of the sensor under different humidity is carried out to perform humidity compensation.

3.4 Research on response time

The response time of an infrared gas sensor is usually related to the gas chamber structure (optical path length), light source modulation frequency and gas flow rate. How to improve the response time of the sensor requires experimental research on these three factors to obtain the response time curve of the sensor under the corresponding factors.

4. References

[1] Yang Yina, Hao Jiangan, Zheng Xiaohong. Research progress of carbon dioxide gas sensor [J]. Journal of Functional Materials and Devices, 2022, 28(03): 187-199. DOI: 10.20027/j.gncq.2022.0027. [2]
Yuan Bo. Preparation and Research of CO_2 Concentration Sensor Based on NDIR Principle[D].University of Electronic Science and Technology of China,2019.