CO2 Sensors Based on New Measurement Principle - Time to clean the air

Impact on Humans

In small quantities, carbon dioxide is perfectly safe for human beings. At higher concentrations, however, it can prevent the lungs from taking in oxygen and can cause a variety of symptoms, depending on the amount of CO₂ in the air. With CO₂ of between one and three volume percent in the air, concentration may be impaired, fatigue may develop, and there may be an increase in heart rate or blood pressure. CO₂ content of five volume percent upwards may cause dizziness, headaches, shortness of breath, and ultimately unconsciousness.

This is why DIN EN 13779 defines four categories of indoor air quality based on carbon dioxide concentration. It classifies CO₂ levels below 800ppm (parts per million) as good, levels up to 1,000ppm as being of medium-good quality and levels of 1,000ppm upwards as medium. When CO₂ exceeds 1,400ppm, the air quality is deemed to be poor. At the workplace, employees must not be exposed to more than 5,000ppm of CO₂ over eight hours.

CO₂ and Coronavirus

In the midst of the coronavirus pandemic, studies have examined whether there is a direct link between the concentration of CO₂ and aerosols, which could be a trigger for COVID-19 infection when containing a viral payload. Based on current knowledge, there is no such link. Even so, a higher CO₂ content is indicative of poor indoor air, which also commonly entails elevated aerosol concentrations—so there certainly is an indirect link between CO₂ and aerosols. There are therefore two good reasons to be consistent in employing ventilation measures: improved well-being and greater performance as well as a reduced risk of infection by coronavirus.

Not only that, but coronavirus control measures such as home working and homeschooling as well as business and restaurant closures are causing most people to spend more time at home. Because buildings are increasingly being better insulated to satisfy modern energy efficiency standards, there is very little ventilation taking place. This is why it is more important than ever to take note of the CO₂ content of indoor air.

Avoiding Stuffy Air

Suitable sensors are used to measure CO₂ concentrations, for example for CO₂ warning lamps. In classrooms, they can be used to provide a simple, visual indication when concentrations are too high and the windows should be opened. In smart home systems, they provide values that are used to automatically trigger ventilation measures or warning signals.

This sensor data can also be used for other information, for example to determine how many persons are currently present in a room. An algorithm is used here to compare the average increase in CO₂ levels generated by human respiration against the measured increase in CO₂ concentration.

In the food industry and food logistics, controlled regulation of CO₂ concentration can actively affect product quality, because CO₂ can speed up or slow down the natural aging processes of fruit and vegetables. CO₂ content also has an effect on plants and animals. By detecting and adjusting this value, producers can use this to their benefit.

NDIR Measurement Technology

The SCD30 sensor from Sensirion is a proven solution for measurements of this type. It detects CO₂ concentrations using NDIR measurement technology (nondispersive infrared spectroscopy) with a high precision of ±30ppm +3% of the measurement value in a measurement range of up to 40,000ppm.

NDIR measurements are based on an infrared radiation source and two optical filters placed opposite one another with two detectors in a tube. The radiation source emits a wavelength that is solely absorbed by CO₂ molecules. Air flows into the tube through an opening, and the CO₂ molecules present therein absorb part of the radiation. The opposite detectors measure the resultant change in radiation intensity. The second detector provides a reference measurement to minimize the impact of contamination such as dirt or dust.

This principle results in comparatively large sensors—Sensirion’s SCD30 measures 35 mm × 23mm × 7mm—but due to the high precision of its measurements, it has still been a key product in CO₂ detection for many years.

Photoacoustic System

Sensirion has now introduced a successor—the SCD4x—which satisfies all requirements relating to miniaturization and reduction in power consumption. It is based on the new photoacoustic sensor technology, which does not require a minimum distance between the radiation emitter source and the sensor. This means that the SCD4x measures just 10mm × 10mm × 6.5mm and is still cheaper than its predecessor. However, the other measurement technology reduces the precision to ±50ppm +5% of the measurement between 400 and 2,000ppm (SCD40) or ±40ppm +5% of the measurement between 400 and 5,000ppm (SCD41). As with its predecessor, the SCD30, the measurement range is 0 to 40,000ppm.

Infineon also offers a CO₂ sensor based on photoacoustic measurement technology. With the size of 14mm × 14mm × 7.5mm and a precision of ±30ppm +3% of the measurement, the sensor is available since mid-2021, according to the manufacturer.

The small size and superior value for money make these new sensors especially compelling for applications in the smart home, IoT, automotive, HVAC, food, and consumer goods fields.

Photoacoustic measurement technology is based on narrow-band light that matches the absorption bands of CO₂ molecules. This means that it has precisely the wavelength range in which its electromagnetic radiation is absorbed by CO₂ molecules. The light is emitted into the measuring cell of the sensor, and the CO₂ molecules absorb part of the light. The energy that this produces causes oscillations in the CO₂ molecules, which increases the pressure in the measuring cell. A microphone measures this pressure difference, which enables conclusions to be drawn on the number of CO₂ molecules present in the measuring cell and thus on the CO₂ concentration in the air.

With this measurement technology, the low drift of the detector signal in common measurement ranges offsets the reduction in measurement precision. As the CO₂ concentration rises, so, too, does the drift. With NDIR technology, the exact opposite is true—the drift of the detector signal is more pronounced particularly where CO₂ concentrations are low.

Conclusion

It has never been as important as it is now to be aware of CO₂ concentrations in the air, because it not only affects the well-being and health of people, but can also help to contain the spread of coronavirus infections. The food production industry and logistics sectors as well as animal and plant breeding activities can actively influence the quality of their products using CO₂ levels. Tried-and-true CO₂ sensors benefit from high measurement precision. More recent models cater to demands for smaller sensors and lower costs, although measurement precision does suffer as a result.

Find components at www.rutronik24.com.

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