NDIR = Nondispersive InfraRed, by shining an infra-red beam through a sample cell (containing CO or CO2), and measuring the amount of infra-red absorbed by the sample at the necessary wavelength, a NDIR detector is able to measure the concentration of CO or CO2 in the sample
These sensors work on the principle of measuring the attenuation of infrared radiation (with a specific wave length) in the air. The sensors consist of an infrared radiation source, a light-water pipe and the infrared detector with the appropriate filter. The signal from the infrared detector is further amplified and then using other electronics evaluates the attenuation of the radiation, which is caused by carbon dioxide, and on this basis the actual concentration of CO2 in the air is calculated.
For simplification – the more CO2, the more the infrared radiation is attenuated, and electronics in the sensor can evaluate it.
NDIR sensors are generally more accurate, longer-time stable, they measure concentration from zero, and can measure high concentrations of CO2.
These sensors typically consist of an electrochemical cell with a solid electrolyte (=solutions or melt that conduct an electric current). This cell is heated to the ignition of additional working temperature. The electrodes article chemical reactions similar to those of the fuel cell where oxygen is consumed and the electrodes formed article electromotive force. By measuring the electromotive force using special electronics are then determined the concentration of CO2 in the air. The main advantage of these sensors is high sensitivity and excellent selectivity to carbon dioxide. They are with somewhat lower durability and accuracy than NDIR sensor, but is still sufficient for use in ventilation technology.
Sensors working on the electrochemical principle up from about 400 ppm. These sensors usually have a built-in auto-calibration function that provides an automatic recalibration of the sensor to the fresh air. This eliminates the aging of the sensor and ensures a long-term stability of parameters.
Electroacoustic sensors operate on the principle of frequency ultrasound evaluation of changes in the mechanical resonator. By means of electronics the change in the frequency of the ultrasonic waves and on the basis of the change of frequency depending on the concentration of CO2 in the air it is determined the actual concentration of CO2.
The main advantage of these sensors is the long-term stability without the need of recalibration.
Sensors of all types typically have a continuous voltage output (0-10 V) or current output (0-20 / 4-20 mA), through which pass the information about the value of the concentration of CO2 in the air to the superior ventilation system.