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How does an NDIR CO2 Sensor Work?

 
NDIR stands for “nondispersive infrared”. Today, it is the most common sensor type for the measurement of CO2, carbon dioxide, concentrations in an environment. NDIR sensors use a tube with air, an infrared lamp, an optical filter, and an infrared (IR) light detector. The air inside the tube is a sample of the air in the environment of the sensor. This means that the air inside the tube is constantly being replaced by new samples, allowing for real-time measurements.
 
 NDIR CO2 sensor
  

Working principle of NDIR CO2 sensors
The infrared lamp directs waves of light through the tube. Simultaneously, the air inside the tube moves toward the optical filter, which is set up in front of the IR light detector. The detector measures how much of the light can pass through the filter. 
 
This is an accurate indicator of the CO2 concentration in the air sample: the CO2 molecules absorb infrared light (photons). However, they do this at only one unique wavelength: 4.26 microns. They allow other wavelengths to pass through.
 
The remaining light wavelengths hit the optical filter and are absorbed by it. The detector then reads out how much of the infrared light of the wavelength that CO2 molecules can absorb, was not blocked by the CO2 molecules in the tube. The detector can thus identify “signatures” of CO2 molecules in the air samples.
 
The sensor then calculates the difference between the amount of light that the infrared lamp emitted and the amount of light that the detector received. This difference is an accurate representation of the amount of CO2 molecules in the air sample because the CO2 molecules have absorbed the “missing” light.
 
This measurement is converted to micro voltages. These micro voltages can then be converted to different outputs, like values, but also output control signals like an analogue 0-10 Volt signal or (digital) Modbus RTU communication.
 
Advantages of NDIR sensors
NDIR sensors are often small. At Sentera, we integrate them into a compact encasing. They cost little to manufacture or operate, partly because of their small size, but also because they need few components to operate. Even though they have very low operating costs, they are very precise and stable. Because of this stability, they are durable with a lifespan of up to 15 years.
 
Sensor calibration and sensor drift
Over a certain period, CO2 readings will change. When a room has not been in use for a long time, for example after a weekend, we can assume that the actual CO2 concentration will have dropped back to around the outdoor CO2 concentration. However, the measured CO2 values may not drop to these levels. This is called sensor drift.
 
Sentera CO2 sensors use the ABC Logic self-calibration algorithm. ABC stands for Automatic Background Calibration. This algorithm automatically corrects the drift in the NDIR in normal indoor applications. The algorithm knows that the outdoor CO2 concentration is about 400 ppm.
 
The algorithm uses the lowest measured CO2 concentration over a certain period to adjust the sensor. For example, if the period is set to 180 hours, the sensor will look at the lowest concentration of CO2 in that period. If that value is 450 ppm, the sensor will adjust the readings by 50 ppm. If the sensor measures 850 ppm, 800 ppm will be the value shown. This value will be closer to the actual CO2 concentration because of the unavoidable sensor drift.
 
This algorithm eliminates the need to manually recalibrate the sensors. Sentera sensors are thus maintenance-free and have an exceptionally long life expectancy
 
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