CO2 limit values prevent symptoms
The concentration of greenhouse gases and thus also the CO2 value in the environment has slowly increased over the years due to human activities; today it is slightly above 400 ppm (0.04 %) – a value that represents healthy fresh air. Indoors, CO2 values up to 1000 ppm (0.1%), which can be achieved with a good supply of fresh air, are still considered acceptable. From this concentration on, it becomes critical for health from a scientific point of view, because higher values have negative long-term effects.
Even in the range from 1000 ppm (0.1 %) to 2000 ppm (0.2 %), poor air quality is noticeable and you feel tired. From this level on, people experience the air as stuffy, and headaches, drowsiness, reduced concentration and increased heart rate can occur. Therefore, health organisations recommend keeping the CO2 value indoors below 1000 ppm (0.1%) if possible. Specifically, the EPA recommends the supply of outside air to improve the indoor air quality, for example with an HVAC system (heating, ventilation, air conditioning).
CO2 sensors indispensable in many applications
Given the large number of applications, it is not surprising that analysts expect annual double-digit market growth for CO2 sensors. CO2 sensors are used to monitor indoor air and ensure better ventilation in homes, schools, offices and commercial buildings, boosting our ability to concentrate and our productivity. Smaller sensors are also suitable for living areas and for corresponding IoT devices, such as digital assistants, smoke detectors, routers, air purifiers or air conditioning systems. Even the installation in laptops or monitors is conceivable.
Pattern recognition can be used to determine the number of people in a room and their daily activity level. This information can be used to make decisions for better air conditioning in building automation. In HVAC systems, CO2 sensors reduce power consumption by up to 50 %, which can mean energy savings of 20 % to 30 % for the entire building. In normal systems, the air circulation is kept constant in timer mode, for example during working hours. On the other hand, a control based on true CO2 measurement regulates the supply of fresh air on the basis of the actual occupancy of the room. This leads to a shorter daily duty cycle of the HVAC system and thus to considerable savings.
But there are many other applications, including in vehicle CO2 monitoring to regulate the air quality in the driver’s cab or in the entire vehicle interior. In agriculture, the sensor is used to control the concentration of CO2 in greenhouses in order to achieve higher yields and cost savings. Sensors are also used in medical applications including capnometry, a method for measuring the CO2 content of a patient’s exhaled air in real time, especially useful in the field of anaesthesia.
Industrial use cases include the detection of CO2 leaks in the vicinity of CO2 gas sources such as dry ice reservoirs, storage tanks or underground gas sources. Smart cities can correlate CO2 emission sources to drivers’ density for traffic management.
Todays’ CO2 sensors
Today NDIR (non-dispersive infrared) sensors are widely used, especially in building automation. However, they are relatively large, expensive and therefore can only be used to a limited extent. Such a sensor, consisting of an IR light source, sampling chamber, spectral filter as well as reference and absorption IR detectors, provides true and accurate CO2 measurements. However, in addition to purely aesthetic aspects, it is not suitable for installation in mobile devices, thermostats or other smart home components in the living room, primarily because of its higher cost and lower integration capability due to its form factor (significant size).
There are currently no comparable solutions on the market that can perform such true and accurate CO2 measurements and that are cost effective at the same time. Although there are so-called eCO2 sensors to detect various indoor pollutants, these are not good alternatives to NDIR sensors. An eCO2 sensor does not perform real measurements; it uses algorithms to calculate an equivalent CO2 value. This involves assuming that the CO2 level is caused primarily by the persons present. As a result, it provides an estimate based on numerous assumptions. Consequently, with this eCO2 value, the regulation of the indoor air quality can only be performed on the basis of this potentially inaccurate information.
This leads to air-conditioning systems consuming an unnecessary amount of energy or not ventilating properly at all, precisely when it is needed. As a result, air quality is not effectively improved and users lose confidence in products that work with such eCO2 sensors.
MEMS-based photoacoustic spectroscopy
Thanks to its experience with MEMS microphones and through experimental processes, Infineon has succeeded in developing a new CO2 sensor based on photoacoustic spectroscopy (PAS) – a physical method that is suitable for detecting gas components in a mixture and, for example, determining the CO2 concentration in indoor air.
Photoacoustic spectroscopy utilises the fact that gas molecules only absorb light with a specific wavelength; in the case of carbon dioxide, this wavelength is 4.2µm. An infrared source with an optical filter supplies the gas with energy in a rapid succession of light pulses at precisely this wavelength. This leads to the rapid heating and cooling of a gas sample, which in turn leads to thermal expansion and contraction. The sound generated by this can be recorded with a microphone, evaluated and used to draw conclusions about the amount of CO2 in the gas. The higher the CO2 concentration, the stronger the signal. The use of a highly sensitive MEMS microphone as a detector allows for significant miniaturisation compared to NDIR-CO2 sensors.
Challenges in sensor development
The Infineon CO2 sensor integrates a photoacoustic transducer with detector, infrared source and optical filter on a printed circuit board. The sensor has a small microcontroller for on-board signal processing, sophisticated algorithms and a MOSFET for operating the infrared source. A modulated IR light source radiates onto the gas mixture in the sampling chamber. The CO2 present absorbs the IR light, heats up and increases the pressure in the sampling chamber and these pressure changes can be measured by a MEMS microphone.
A major challenge in developing a PAS-CO2 sensor was to push the performance of the microphone to its limits and minimise system noise, i.e. to isolate the MEMS detector from external noise so that only the pressure change originating from the CO2 molecules in the chamber is detected. Infineon modelled the MEMS microphone response before prototyping some units to validate the modelling results.
Features and advantages
Infineon’s new Xensiv PAS CO2 sensor features the IM69D130 Xensiv MEMS microphone with a signal-to-noise ratio of 69 dB. It is designed for applications where low self-noise, wide dynamic range, low distortion, and high acoustic overload point are required. Thanks to the IM69D130, the slightest pressure fluctuations can be measured in the gas sensor, so that even a small amount of gas is sufficient for the exact determination of the gas concentration. As a result, the sampling chamber could also be designed small. Offering true CO2 measurements, the new sensor is more than 75 % smaller than conventional CO2 sensors with comparable performance parameters. The integrated microcontroller converts the signal at the MEMS microphone output into a ppm value that is available via three interfaces: the serial I²C, UART or PWM interface. The direct ppm readings, surface mount capability and simple design allow easy and fast integration with flexible production numbers. All components are designed and produced in-house to Infineon’s high quality standards.
The extremely robust sensor covers a measuring range from 0 ppm to 10,000 ppm with a measurement accuracy in the range up to 5000 ppm (± 3 %) at ± 30 ppm. It operates in a temperature range from 0°C to 50°C at a relative humidity of 0 % to 85 % (non-condensing). Drift is less than 1 % per year (with active self-calibration). In pulsed mode, the CO2 sensor is designed to last ten years. This makes the Xensiv PAS CO2 ideal for demand-oriented ventilation control in building automation and for controlling indoor air quality in smart home applications.
Infineon is planning several sensor variants to meet very specific requirements in various applications, such as low power consumption in battery-powered applications, smaller size at lower cost for portable devices, and even more reliable variants for extremely harsh industrial applications. Sensor variants for other gases are on the roadmap too.
About the Author
Hicham Riffi is Senior Manager for Product Marketing Environmental Sensors at Infineon Technologies – www.infineon.com