Carbon dioxide is one of the central products of the human metabolism, with which with the food taken up coal hydrates, fats and proteins are converted under supply of oxygen among other things in CO₂, which is delivered again over the respiration. While this is quickly diluted outside, CO₂ concentrations can increase rapidly in closed rooms. In particularly strongly frequented areas, like e.g. seminar areas or classrooms, in addition, in particularly small interiors, like for example a car cab, the CO₂-Konzentration can quite increase tenfold within only a few minutes. While the atmospheric CO₂ concentration, relatively independent of location, is around 400 ppm (parts-per-million), up to 5000 ppm can be reached indoors with insufficient ventilation. The accumulation of CO₂ complicates the metabolism; already at a CO2 concentration of 1000 ppm sleepiness and concentration disorders can occur.

Due to the specific effect of CO₂ on the human metabolism a selective measurement of this molecule is justified. For CO₂ here the selective excitation of relative oscillations of the individual atoms, which can be achieved by absorption of infrared light, is suitable. Figure 1 shows the different absorption bands of typical gases occurring in the atmosphere.

Here it turns out that CO₂ with 4,26 μm has a very pronounced and with other gases to a large extent overlap-free absorption line, which is well suitable for a selective measurement. In contrast to NDIR-based gas sensors, such as the SCD30 from Sensirion, photoacoustic sensors do not detect the amount of transmitted light, but the amount of light absorbed in the gas. This is done indirectly, using the photoacoustic effect: This effect generally describes the increase in pressure after light absorption in a gas:

Molecules excited by IR-radiation transfer the vibration excitation to other molecules which leads to an increase of the translational energy, i.e. to a local temperature increase. In a closed volume, the increase of the translational energy leads to an increase in pressure, which can be detected by a pressure transducer. The general design of a non-resonant photoacoustic gas sensor is shown in figure 2.

The key components of a photoacoustic sensor are a predominantly closed measuring volume, which is illuminated with narrow-band IR light, a microphone to detect pressure changes in the measuring volume and an opening of the measuring cell to the environment, which allows gas exchange with the surroundings. The narrow-band IR light is usually generated by a broadband emitter, which shines the emitted light through an optical band-pass filter.

With a photoacoustic sensor such as the SCD4x, a signal is generated that determines the CO₂ concentration as follows

1. The IR light source is switched on and emits narrowband IR light into the measurement volume. This leads to vibrational excitations of the gas molecules to be measured, here CO₂. 
2. After a short time (typically a few milliseconds) the oscillations subside, which leads to an increase in temperature, which in turn leads to an increase in pressure.
3. Compared to the excitation and de-excitation times of the gas molecules, the pressure inside the measuring chamber balances out quickly, so that the pressure increase can be recorded by a microphone connected to the measuring chamber.
4. After several 10 ms the light source is switched off, whereby the temperature and thus the pressure decreases due to thermalization of the measuring cell with the environment and the system returns to its initial state.

To increase the signal-noise ratio of the generated signal, the above measuring cycle is repeated several times. For this purpose the light source is modulated, the generated periodic pressure changes can then be considered as sound waves. In contrast to the SCD30, for which a light bulb was the first choice, a MEMS-based emitter developed by Sensirion was chosen for the SCD4x. This can be modulated quickly and has better long-term stability due to active control. For miniaturization and to protect the sensor from environmental influences, all components of the SCD4x are installed inside the measuring cell, see Figure 3.