Monitor CO2 Levels with Miniaturized Photoacoustic Sensors

Article By : Maurizio Di Paolo Emilio

Sensirion's new miniaturized CO2 sensor, based on photoacoustic sensing, is aimed at applications in IoT, consumer electronics, appliances, and HVAC market...

As high CO2 levels compromise human health, productivity and comfort CO2 sensors play an essential role in the monitoring of indoor air quality. Sensirion has introduced a miniaturized CO2 sensor device based on photoacoustic sensing. It’s offered in a package only 10 × 10 × 7 mm, making it appropriate for applications in IoT, consumer electronics, appliances, and the HVAC market. The sensor, designated the SCD40, can also measure temperature and humidity.

The CO2 factor
High indoor CO2 concentration typically is the result of human presence. Our body breathes in oxygen and emits CO2, which accumulates indoors if the environment is not properly ventilated.

Moreover, the dense thermal insulation of modern buildings indirectly contributes to the increase of carbon dioxide. For example, dense windows and doors that reduce consumption and heating or cooling costs do so at the cost of reduced exchange of air with the outside. The consequence is, therefore, an increase in CO2 concentration that leads to a continuous need for ventilation. It is in these cases that CO2 sensors are often used to regulate the ventilation system.

High CO2 levels compromise human health and productivity. Combining a CO2 sensor together with air exchangers and intelligent ventilation systems allows ventilation to be regulated in the most energy-efficient and human-friendly way. In addition, CO2 sensors play an essential role in monitoring indoor air quality and can therefore be integrated into air purifiers and intelligent thermostats and other smart home products.

Carbon dioxide concentrations above 1,000 parts per million (ppm) reduce productivity and can cause drowsiness in humans. At CO2 concentrations higher than 2,000 ppm, some people start to get headaches. Let’s consider that in a closed room, such as a crowded classroom, the presence of many people and poor ventilation can produce a level of carbon dioxide up to 5,000 ppm.

The New York Times said, “The higher the carbon dioxide, the worse the test-takers did; at 2,500 ppm, their scores were generally much worse than at 1,000 ppm.” Moreover, “without a specialized sensor, you can’t realistically know how much carbon dioxide is building up while you hunker down in a small room for a long meeting.”

Air exchangers and intelligent ventilation systems in the commercial and residential sectors use CO2 sensors to regulate ventilation in the most energy-efficient and human-friendly way. The integrated COsensor makes an enormous contribution to both air quality and energy savings. The air-conditioning electronics for a CO2 sensor also allow for monitoring the air quality trend, as well as for making decisions without relying on our senses (Figure 1).

“Public awareness for indoor CO2 levels is growing: There is an increasing number of initiatives from the public and private sector to monitor and counteract high CO2 concentrations,” said Marco Gysel, product manager of CO2 Sensors at Sensirion. “Most initiatives focus on classrooms, universities, and commercial office buildings, but there is an increasing demand for CO2 sensing for residential apartments as well. Authorities and companies are starting to realize that the reduced cognitive performance of students and workers comes at a high cost.”

Figure 1: Impact of CO2 on human decision-making performance (Image: Environmental Health Perspectives. 120(12). doi:10.1289/ehp.1104789)


The new SCD40 sensor and photoacoustic technology
The SCD40 miniaturized CO2 sensor offers new approaches to product design and will create the basis for a wide range of new sensing applications. Sensirion’s experience has enabled it to improve its latest innovations in CO2 sensor technology, offering a new device that is smaller than a factor of seven compared to its predecessor, the SCD30. The photoacoustic detection principle reduces the size of the optical cavity used in the SCD30 without compromising performance.

State-of-the-art CO2 sensors such as Sensirion’s SCD30 are based on the non-dispersive infra-red (NDIR) optical detection principle. The use of these NDIR sensors is limited to a few applications due to their size and cost.

NDIR-type sensors are optical sensors, which are frequently used in gas analysis. The main components are the infrared source with a wavelength filter, a sample gas chamber, and an infrared detector (Figures 2 and 3). By illuminating an infrared beam through a sample cell (containing CO2) and measuring the amount of infrared absorbed by the sample at the required wavelength, an NDIR detector can measure the volumetric concentration of CO2 in the sample.

The sensitivity of a sensor based on the NDIR principle is directly proportional to the optical beam path. A large reduction of the path leads to a compromise of its performance, which limits the miniaturization potential of this technology. In addition, sensors based on the NDIR principle do not have an economic BOM structure due to their size, structure, and a large number of discrete components.

“With regards to miniaturization, the NDIR technology seems to reach a limit for CO2 sensors, as the sensor sensitivity is directly proportional to the optical beam path length and thus the sensor size,” said Gysel. “Sensirion is always aiming at disrupting sensor markets by making components smaller and more price-effective without compromising performance. For CO2 sensing, we identified the photoacoustic technology as the most promising approach: In addition to reducing the size and the cost of CO2 sensors, this technology allows for SMT assembly to replace arduous through-hole soldering. These three factors combined have the potential to open up new CO2 sensing markets. Personally, I believe that the photoacoustic technology has the potential to replace NDIR as the standard CO2 sensing technology over the next five to 10 years.”

Figure 2: SCD30 Technology (Image: Sensirion)


Figure 3: NDIR principle (Image: Sensirion)

The new SCD40 is based on Sensirion’s photoacoustic PASens technology. The photoacoustic detection principle allows miniaturization of the sensor without compromising performance. This is because the sensitivity of the sensor is independent of the size of the optical cavity. By also using Sensirion’s CMOSens technology for miniaturization, it was possible to combine the two technologies and create a new type of sensor (Figure 4).

Figure 4: Size comparison of NDIR (SCD30) and PASens Technology (SCD40) (Image: Sensirion)

The photoacoustic principle is relatively simple: A modulated narrow-band light signal at 4.26 µm corresponding to the absorption bands of CO2 molecules is emitted in a small enclosed space. The CO2 molecules in the measuring cell absorb part of the irradiated light. The absorbed energy of the CO2 molecules mainly excites molecular vibrations, which results in an increase in translational energy, causing a periodic change in pressure in the measuring cell, which can be measured with a MEMS microphone.

“Upon absorption, the energy of the photon is first transferred to the CO2 molecule and subsequently to the surrounding molecules,” said Gysel. “The absorbed energy results in a microscopic pressure increase. Since millions of absorption events take place inside the optical cavity, the pressure increase becomes a macroscopic phenomenon. By modulating the IR emitter, we induce a pressure increase and decrease with a well-defined frequency — which is nothing more than an acoustic sound wave. While the frequency of the sound is given by the IR emitter modulation frequency, the amplitude of the sound is proportional to the CO2 concentration.”

The microphone signal is then used to measure the number of CO2 molecules in the measuring cell and can be used to calculate the CO2 concentration.

Figure 5: Functioning of PASens technology. On the top of the sensor, there are dust filters. (Image: Sensirion)


Figure 6: SCD40 sensor (Image: Sensirion)


Figure 7: SCD40 sensor with its features (Image: Sensirion)

“This amplitude of the photoacoustic signal can be measured with a MEMS microphone,” said Gysel. “The CO2 concentration is then calculated using the built-in processor by means of advanced signal-processing algorithms. The beauty of the photoacoustic measurement principle is that the sensor sensitivity is mostly independent of the optical cavity size. Thus, we can use this technology to shrink down the sensor size without compromising on sensor performance.”

The SCD40 represents a combination of sensing and MEMS technology by combining minimum size and maximum performance. The SCD40 opens up many new possibilities for integration and application. It offers a measurement range of 0 ppm to 40,000 ppm, fully calibrated and linearized output, and digital I2C interface.

“Perhaps the biggest asset with the SCD40 is that we design and produce all the critical components in-house,” said Gysel. “This allows us to realize the highest performance while keeping a cost-effective BOM structure. For example, the actively regulated IR emitter that is based on our CMOSens technology ensures the highest long-term stability and is significantly more cost-effective than existing off-the-shelf products.

“Sensor accuracy is very important for two reasons,” he added. “One the one hand, it enables our customers to design products with a superior performance. On the other hand, some customers need high accuracy to be compliant with norms and standards — this is very critical in the HVAC market, for example. The accuracy of our SCD40 is specified as ±30 ppm plus 3% of reading, which is among the best accuracy that can be found on the market. Another key specification is the sensor lifetime of 10 years that demonstrates our high standards regarding sensor reliability.”

The CO2 problem is very much felt. According to the World Health Organization, more than 5.5 million people worldwide die from air pollution each year. Many cities have integrated sensors to map air quality into existing infrastructures. Indoor air pollution can have a number of consequences for our health. To keep the concentration of contaminants low, there must be air circulation and exchange with the outside world. Intelligent indoor and outdoor sensors can not only save energy but also improve the quality of life.

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