Sensirion Breaks CO₂ Sensor Size Limitations

Sensirion is once again a pioneer in the innovation of environmental sensor solutions with the launch of the innovative SCD4x miniature CO₂ sensor, which occupies a footprint of just 1 square centimeter. Based on the principle of photoacoustic sensor technology, this disruptive innovation opens up new possibilities for more integration and applications by minimizing size while ensuring optimal performance.The SCD4x offers an unparalleled price/performance ratio, making it particularly suitable for volume production and cost-sensitive applications.

As people become more environmentally conscious, not only has the way they eat, dress, live and move changed, but building design has also been affected. In order to conserve natural resources (mainly for heating), modern house design often strives for high energy efficiency. This leads to buildings becoming more airtight and better insulated than traditional buildings. Airtight buildings are less likely to allow air exchange through walls, roofs, windows, cracks, etc., which reduces indoor air quality. Reduced indoor air quality, in turn, affects people's productivity and health and well-being. Therefore, indoors must be equipped with an effective ventilation system to ensure enough fresh air to create a healthy and efficient environment. Since ventilation systems require a lot of energy to regulate and introduce fresh air, it is important to ensure that the system is energy efficient. Ventilation strategies that control airflow exchange on demand enable this.

Humans are the main cause of increased indoor CO₂ concentrations as well as other pollution generation. Fresh air requirements vary depending on the number of people in the room as well as human activities (e.g., cooking, exercise, recreation). The higher the CO₂ concentration in an enclosed building where people are located, the lower the air quality. The CO₂ concentration is therefore regarded as one of the air quality indices and can also be used as a control parameter for ventilation systems - ventilating on demand based on indoor air quality measurements ensures both a healthy and comfortable environment and high energy efficiency.

Sensirion's new SCD4x sensors are revolutionizing the CO₂ sensor market based on the unique PASens® technology, which uses the photoacoustic measurement principle. PASens® technology uses the photoacoustic measurement principle, which, unlike today's common non-dispersive infrared (NDIR) technology, is independent of the size of the optical cavity, thus minimizing the size of the CO₂ sensor while maintaining sensor performance. Customers are able to realize a flexible, compact and cost-effective integration and are no longer prevented from using CO₂ sensors due to limited installation space. In addition, the cost structure of the sensor is optimized and the selling price is significantly reduced due to the significantly reduced number of integrated electronic components. Thanks to its miniature size and optimized cost structure, the SCD4x can be integrated into new products and high-volume applications such as compact ventilation systems, air exchangers, duct probes, air purifiers, thermostats, air conditioning units and air quality monitors.

Effects of increased indoor air CO2 concentration

Carbon dioxide is one of the main products of human metabolism. After food intake, carbohydrates, fats and proteins are converted to CO₂ in the presence of oxygen, which is subsequently re-transmitted through respiration. Although CO₂ is quickly diluted after it is exhaled, the concentration of CO₂ in confined spaces rises rapidly. This is especially true in high-traffic areas such as seminar areas and classrooms, as well as in small spaces such as the cabins of automobiles, where CO₂ concentrations may rapidly increase by a factor of ten in just a few minutes. Relatively location-independent atmospheric CO₂ concentrations are generally around 400 ppm, but may reach 5000 ppm in inadequately ventilated rooms. CO₂ accumulation complicates human metabolism, and drowsiness and difficulty in concentrating may occur at CO₂ concentrations of up to 1000 ppm.

Considering the special effects of CO₂ on human metabolism, it is necessary to measure CO₂ molecules selectively. For indoor CO₂, selective excitation of the relative oscillations of individual atoms by infrared light absorption is more appropriate. Figure 1 shows the different absorption bands of typical gases in the atmosphere.

The figure shows that CO₂ at a wavelength of 4, 26 μm has a very distinct absorption line and does not overlap with other gases, making it ideal for selective measurements. Unlike NDIR gas sensors (e.g. Sensirion SCD30), photoacoustic sensors do not need to detect transmitted light, but indirectly detect the amount of light absorbed by the gas through the photoacoustic effect. The photoacoustic effect generally refers to the increase in pressure of a gas when it absorbs light.

Molecules excited by infrared radiation transfer vibrational excitation to other molecules, which leads to an increase in the translational kinetic energy, i.e. a localized temperature increase. In a closed volume, an increase in the translational kinetic energy leads to an increase in pressure, which is captured by the pressure sensor. Figure 2 shows the general design of a non-resonant photoacoustic gas sensor.

PASens® Technology

The key components of the photoacoustic sensor include a closed photoacoustic chamber illuminated by narrowband infrared light, a microphone that detects changes in the pressure of the photoacoustic chamber, and a photoacoustic cell opening for gas exchange with the outside. A broadband infrared emitter emits a light source, which is passed through a bandpass filter to form narrowband infrared light.

The principle of determining CO₂ concentration by forming a signal from a photoacoustic sensor (e.g. SCD4x) is as follows：

• An infrared light source is turned on, releasing a narrow band of infrared light to illuminate the photoacoustic cavity, which excites the gas molecules to be measured (CO₂ in this case) to oscillate.
• After a period of time (usually a few milliseconds), the oscillations subside, the gas temperature rises, and the pressure rises.
• The pressure equilibrium time in the photoacoustic chamber is shorter than the time it takes for the oscillations of the gas molecules to excite and subside, so the pressure rise can be recorded using a microphone connected to the photoacoustic chamber.
• After 10 ms, the light source is turned off, the photoacoustic cell reaches thermal equilibrium with the environment, and the temperature and pressure subsequently drop, returning the system to its initial state.

In order to improve the signal-to-noise ratio of the generated signal, the above measurement operation needs to be repeated several times. Therefore, the tests were performed using a modulated light source that produces periodic pressure variations that can be treated as sound waves. Unlike the SCD30, which prefers a light bulb as its light source, the SCD4x uses Sensirion's MEMS-based light emitter. This allows for rapid modulation of the light source and greater long-term stability due to active control of the light source. To miniaturize the sensor and avoid environmental influences, all SCD4x components are mounted in a photoacoustic cell.

Summary

Sensirion is once again at the forefront of technological innovation in environmental sensing to create healthier and more efficient environments. the SCD40 breaks the size limitations of CO₂ sensors and continues to write the success story of the SCD30. the SCD4x innovative miniature CO₂ sensors offer the combination of size and affordability to integrate into a wide range of home devices and outperform conventional NDIR sensors. Pictured is a demonstration application/device that can be configured with Sensirion's innovative CO₂ sensor solution.

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