What is Ndir sensor?

Non-Dispersive InfraRed Sensor

Definition: Non-Dispersive InfraRed (NDIR) sensor is an optical gas sensor composed of infrared light source (IR source), optical path (Optics Cell), infrared detector (IR Detector), circuit (Electronics) and software algorithm (Algorithm). It is mainly used to measure compounds, such as: CH4, C2H2, C2H4, CO2, N2O, CO, CO2, NH3, VCM, CF4, SF6, ethanol, Freon, etc., and contains most organic substances, including volatile organic compounds (VOC).

NDIR sensors use a broad-spectrum light source as the light source for the infrared sensor, which is called non-dispersive because there is no beam-splitting grating or prism to split the light. The light travels through the gas under test in the optical path, through a narrow band filter, and reaches the infrared detector. The concentration of the gas to be measured is determined by measuring the intensity of the infrared light entering the infrared sensor. The intensity is strongest when there is no measured gas in the environment. When a measured gas enters the gas chamber, the measured gas absorbs part of the infrared light, so that the intensity of the light reaching the detector is weakened. By calibrating the degree of absorption of infrared light at the zero point and the measuring point and by scaling, the instrument is able to calculate the concentration of the measured gas.

Why use an NDIR sensor?

Firstly, common sensors on the market, such as Catalytic Combustion (CAT) Sensor, Electrochemical (EC) Sensor, and UV Photoionisation (PID) Sensor do not measure all gases, for example, CO2 cannot be measured but NDIR sensors are able to measure CO2. The most common gases measured by NDIR are CH4 and CO2. The most common gases measured by NDIR are CH4 and CO2.

The second aspect is the many advantages of NDIR sensors, which include:

1. Resistance to poisoning. The biggest enemies of CAT sensors for methane measurement are halides (substances containing fluorine, chlorine, bromine, and iodine), organosilicon compounds, and sulfides. When these substances are burned on the catalytic beads, the activity of the catalyst is discounted, which is reflected in the measurement data as a decrease in sensitivity.

2. No carbon accumulation: When the CAT sensor measures hydrocarbons with long carbon chains, it is easy to accumulate carbon because of insufficient combustion, resulting in the formation of a thin layer of carbon powder on the surface of the catalytic bead, which is reflected in the measurement data, that is, the zero point is raised and the sensitivity is reduced; the light source and detector of the NDIR sensor are all protected by the glass or filter and do not come into contact with the gas, so there will be no combustion phenomena occurring.

3. Oxygen is not required; CAT sensors require oxygen to participate in the oxidation reaction. But NDIR is an optical sensor, which does not need oxygen to participate.

4. Measurement of concentration can be up to 100% v/v. The signal characteristic of the NDIR sensor is: that when there is no gas to be measured, the signal strength is maximum, the higher the concentration, the smaller the signal. So it is easier to measure high concentration than low concentration.

5. Excellent long-term stability: The stability of the NDIR sensor basically depends on the light source. As long as you choose a good light source, the long-term stability will be excellent, 2 years without calibration can be done.

6. Wide temperature range: NDIR can be used in the range of -40℃ to 85℃ without any pressure.

7. Low maintenance cost. This is closely related to its long-term stability. It is very likely that the instrument is broken, but the sensors are still good.