CO2 sensors based on new measurement principle

This article originally appeared in the May'22 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

CO2, the base substance for all organic compounds, is a colourless, odourless gas consisting of carbon and oxygen. Plants use photosynthesis to convert it to oxygen together with water. CO2 is produced by cellular respiration and by the decay of animal and plant organisms. In human civilisation, it is also produced by the combustion of fossil commodities in industry, when heating buildings, or in vehicle combustion engines. This is causing the concentration of CO2 in the Earth’s atmosphere to rise, which drives the greenhouse effect and provokes climate change. By Marcel Saffert, Product Sales Manager Analog & Sensors, Rutronik.

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.

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.

Not only that, but coronavirus control measures such as home working and home schooling 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 minimise the impact of contamination such as dirt or dust.

This principle results in comparatively large sensors – Sensirion’s SCD30 measures 35mm × 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 miniaturisation 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.