SCD4x - The Smallest CO2 Sensor

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

You will need SEK-SCD41-Sensor, SEK-SensorBridge and ControlCenter software for your Windows, Linux or Mac PC.

We have included both Sensirion CO₂ sensors, SCD40 and SCD41, in our stock range.

If you need more information about Sensirion products, please contact our sales representative for your region or send us an email to sensirion@soselectronic.com; we will be happy to help you.