Introduction

We usually breathe carbon dioxide (CO ₂) is spitting out. In addition, by burning fossil fuels, etc. CO ₂ is occurring. It consumes a large amount of fossil fuels and has a lot of CO ₂ has increased by emitting CO ₂ Greenhouse Gases 1 Also known as a species. In recent years, globally this CO ₂ Increasingly, there is a demand for concentration measurement.

Considered to be a greenhouse gas CO ₂ At first glance, it seems that it has little effect on the human body, but there are research reports that the higher the concentration, the more people complain of symptoms such as increased fatigue, difficulty concentrating, drowsiness and headache. Based on these research reports, there is a movement to monitor indoor air quality and reduce the impact on the human body.

In addition, in recent times COVID-19 (COVID-19) Avoidance of the three Cs is required due to the occurrence of One of the measures to prevent infectious diseases is to improve enclosed spaces with poor ventilation, but as a method to check whether ventilation is sufficiently performed, the Ministry of Economy, Trade and Industry CO ₂ Concentration indicators are provided. To determine if this indicator is met, CO ₂ A concentration measuring instrument is required.

In this article, we will discuss the indoor CO ₂ We will explain the mechanism of the sensor used in the concentration measuring instrument and explain how to measure it correctly.

How the CO2 sensor works

General semiconductor formula CO ₂ the sensor is big 2 There are different methods.

One is NDIR (Non Dispersive InfraRed): non-dispersive infrared absorption) method, the other is photoacoustic (Photoacoustic) method. Both methods are CO ₂ It is an optical detection method that utilizes the principle that molecules absorb light of a specific wavelength.

In the main method of the NDIR method, the path through which the reference light passes and CO ₂ of the path of light absorbed by 2 By using the paths and obtaining the light intensity difference of each path, CO ₂ Calculate the concentration. Since it is necessary to secure an optical beam path and the sensitivity is proportional to the path length, the disadvantage is that the sensor size increases as the sensitivity increases.

On the other hand, in the photoacoustic method, CO ₂ Molecules emit light of specific wavelengths (4.26um wavelength) energy increases when absorbing CO ₂ Using the principle of molecular vibration, CO₂ By picking up the vibration of molecules as sound with a high-performance microphone, CO ₂ Calculate the concentration. There is no need to secure an optical path, but the technology to realize it with semiconductor parts has not been established, and until now, it was a technology that was used only in measuring equipment used in laboratories. This time, the technology to realize it as a semiconductor sensor has been established, NDIR Smaller size than the method CO ₂ I was able to enable sensing.

both types CO ₂ is detected directly, so the correct CO ₂ Concentration measurement is possible.

[NDIR method]

[Photoacoustic method]

Mechanism of gas sensor

There are semiconductor gas sensors that can detect gas, like _CO2 sensors. The main measurement method is called the MOX (metal oxide) method. Heated MOX reacts with volatile organic compounds (VOCs) such as hydrogen molecules to change the resistance value. Calculate the gas concentration based on

This sensor cannot distinguish between different types of gases and responds to all VOCs. And in some cases, this gas sensor that reacts to VOCs is used as a CO ₂ sensor. Both VOC and CO ₂ are gases, but since the gas type cannot be determined, it is not possible to determine whether CO ₂ is being detected correctly when calculating the CO ₂ concentration with this sensor.

Difference between CO2 concentration and CO2 equivalent

As mentioned above, some gas sensors have CO ₂ As a concentration measurement is possible, CO ₂ equivalent(CO ₂ eq) There is a sensor that claims a method.

CO ₂ What is eq H. ₂ based on concentration CO ₂ estimated the concentration CO ₂ Concentration measurement method.
"CO ₂ As explained in the section on how the sensor works, CO ₂ the sensor directly CO ₂ Accurate concentration measurement is possible because molecules are measured.

on the other hand, CO ₂ eq Then H. ₂ On the basis of CO ₂ Due to the method of estimating the concentration, the surrounding H. ₂ or VOCs include H. ₂ will react to around VOCs In an environment where the gas sensor is VOCs include H. ₂ from CO ₂ To estimate the concentration, a high concentration VOCs in the environment actually CO ₂ Despite the low concentration of CO ₂ If the concentration is high, the sensor will misidentify.

Summary

There are CO ​₂ sensors that directly measure CO ₂​ ​​_and gas sensors that simulate CO ​₂ on the market. method. Sensors that can accurately measure CO ₂ concentration are optical (NDIR or photoacoustic) sensors, and MOX gas sensors convert CO ₂ concentration from H ₂ concentration, so CO ₂ concentration can be measured correctly. I can't.

The Ministry of Economy, Trade and Industry has also issued the "Guidelines for Selection of Carbon Dioxide Concentration Measuring Instruments", so please refer to them.
Ministry of Economy, Trade and Industry: https://www.meti.go.jp/press/2021/11/20211101002/20211101002.html

again, SENSIRION at the company CO ₂ As a sensor for concentration measurement, NDIR method of SCD30,semiconductor CO ₂ A photoacoustic system that has succeeded in miniaturization as a sensor SCD40 said, CO ₂ We provide sensors. These sensors have CO ₂ In addition to the automatic self-calibration function required for concentration measurement, it also has a forced calibration function, which allows you to easily determine the calibration point in a short time. CO ₂ It is possible to carry out concentration measurements.

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