Infrared Gas Sensors’ Optical Working Principle
1- What are the Gas Detection Sensor Types? What is a Catalytic Sensor?
Various sensor technologies are used in flammable and explosive gas measurement systems. Catalytic and infrared are the most common of these. Catalytic-based gas measurement is the oldest and simplest system type that is being used. Catalytic sensors detect gas by burning hydrocarbon group flammable and explosive gases (methane, butane, propane, etc.) and produce an electrical output depending on the quantity. In the presence of flammable gas, the sensor part that converts the heat generated by burning the gas reaching the sensor to electricity is called ‘Pellistor’. Detection is made by measuring the temperature, which changes depending on the amount of gas in the environment, with a comparison circuit. With prolonged or high exposure to the gas, the lifespan of the pellistors is shortened or deteriorated. This situation, which is called poisoning, brings with it the necessity of continuous and regular calibration.[1]
2- What is Infrared Sensor? What are the advantages?
Infrared (IR) gas measurement technology is a new and modern technology comparing to the other methods, and their use in the industry is becoming more common. While the catalytic gas measurement system detects all flammable/explosive (methane, butane, propane, etc.) gas in an environment, it detects a gas for a single gas (just like methane) in the infrared gas measurement system. In addition, while catalytic gas measurement systems directly react with flammable/explosive gas; this is not the case with infrared gas measurement systems. Therefore, the operating life of the infrared gas measurement system is longer which makes it more cost-effective in the long-run.
3- How Does the Infrared Sensor Work? Infrared Sensor Working Principle
Infrared sensors are used in infrared gas measurement systems. Thermopile, which is one of the infrared sensors, produces very little electrical energy against infrared heat and radiation. Typically, this system consists of an infrared light source and an infrared light sensor. The infrared (IR) detection method is based on the absorption of infrared heat/radiation at specific wavelengths. Generally, two-channel thermopiles are used, but there are also single- and multi-channel thermopiles. The first channel (sensing channel) is at the absorption wavelength, the second channel (reference channel) is outside the absorption wavelength, and the minimum absorption point of other gases is chosen. (Figure 1).
3.1- How to Detect Methane Gas with Infrared Sensor?
The transmittance of methane gas, a hydrocarbon group gas, to infrared radiation is shown in Figure 2. When methane gas is exposed to infrared radiation, it absorbs some wavelengths of IR radiation due to its molecular bonds; It appears that at 3.3-3.4 µm it transmits only 20% of infrared light. Therefore, if methane (flammable/explosive) gas is present between the source and the sensor, the radiation level transmitted to the sensing element decreases and there is less light absorption in the sensor channel and less electrical output depending on the amount of gas. Absorption and electrical output are not affected in the reference channel by the presence of flammable/explosive gas, since an optical filter other than the absorption wavelength in the reference channel is selected. The gas concentration can be detected by comparing the relative values between the electrical output of the two channels.
3.2- How to Detect Carbon Dioxide Gas with Infrared Sensor?
The transmittance point in the infrared spectrum for carbon dioxide gas was determined as 4.26 µm. Figure 3 shows an example measurement principle graph. According to the carbon dioxide gas concentration in the environment, at 4.26 µm, the molecular bonds of the gas absorb the infrared radiation and the amount of radiation reaching the sensor decreases. There is no absorption at the reference wavelength of 3.91 µm. After the calibration, the amount of concentration in the environment is measured depending on the difference between electrical current outputs.
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