Sensirion sensors SCD40 and SCD41 broke the size barrier and are the smallest CO2 sensors currently available. They measure only 10x10x7 mm, which is 10 times less than other CO2 sensors.
How Is It Possible?
SCD4x works on the photoacoustic principle.
The CO2 molecules in the measuring chamber absorb part of the radiation. The absorbed energy of the CO2 molecules excites mainly molecular vibrations, which leads to increased translational energy of the molecules and, due to the closed measuring chamber, to a periodic change of pressure in the chamber, which is measured by a microphone.
Molecules of other gases do not absorb infrared radiation of this wavelength and do not contribute to the change in pressure in the measuring chamber.
Changes in pressure are proportional to (number CO2 molecules) / (chamber volume). The more CO2 molecules are present in the measuring cell, the greater the pressure changes and the microphone’s output voltage. If we halve the volume of the measuring chamber, the number of CO2 molecules will also be halved, but the pressure changes will remain the same so that the sensitivity of the sensor will not change.
The sensitivity of the NDIR sensor is proportional to fractional absorbance
FA = 1-e ^ (- klx)
- k is the absorption coefficient for the specific gas and filter combination
- l is the equivalent optical path length between the lamp and detectors
- x is the concentration of the gas.
The FA can be plotted against x, as shown in the figure below. The FA value increases with x, but saturation occurs at high gas concentrations.
To measure concentration in range, let’s say 0 to 10 000 ppm (1%), we can choose k*l around 50, which leads to FA = 0,049 at 1 000 ppm. If we halve the length of the optical path, then k*l = 25 => FA = 0,025. The sensitivity at 1 000 ppm decreased by 49%.
Comparison of SCD30 and SCD4x
How to Start with SCD4x
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