MEMS gas sensors can meet the special needs of various applications. According to Memes Consulting, CO2 (carbon dioxide) concentration is a key indicator for measuring indoor air quality. Good air quality can make people in it feel better and more productive, while high CO2 concentrations represent poor indoor air quality, often due to insufficient air conditioning and air recirculation. This condition can lead to well-documented health problems such as Sick Building Syndrome (SBS), as well as other indoor toxins such as fine dust, mold, pollen, bacteria and even asbestos.

CO2 Limits Prevent Disease Over the years, greenhouse gas concentrations and CO2 levels in the environment have gradually increased due to human activities; today the values in the environment are just above 400 ppm (0.04%), which represents healthy fresh air. Indoors, CO2 concentrations up to 1000 ppm (0.1%) are still considered acceptable, and this can be achieved with a good supply of fresh air. From a scientific point of view, CO2 concentration values are critical to health, as higher CO2 concentrations can have long-term negative effects on the human body.

MEMS gas sensors can meet specific needs in a variety of applications

Health effects of high concentrations of CO2

Even if the CO2 concentration is in the range of 1000 ppm (0.1%) ~ 2000 ppm (0.2%), the poor air quality is obvious, and people will feel tired in such an environment. If CO2 levels continue to rise, people experience stuffy heat, headaches, drowsiness, decreased concentration, and increased heart rate. Therefore, the World Health Organization recommends that the indoor CO2 concentration be controlled within 1000 ppm (0.1%) as much as possible. The U.S. Environmental Protection Agency (EPA) clearly recommends outdoor air circulation to improve indoor air quality, and can use a heating, ventilation and air conditioning (HVAC) system that integrates heating, ventilation, and air conditioning.

CO2 sensors are essential in many applications

Considering the needs of a large number of applications, it is not surprising that market analysts expect the annual growth rate of the CO2 sensor market to reach double digits. According to the "Gas and Particulate Matter Sensors - 2018 Edition" report released by Yole, the gas sensor market size will reach $1 billion by 2022. CO2 sensors can be used to monitor indoor air to ensure better ventilation in homes, schools, offices and commercial buildings, thereby increasing focus and productivity. Smaller size sensors are also suitable for living areas and corresponding Internet of Things (IoT) devices such as digital assistants, smoke detectors, routers, air purifiers, and air conditioning systems, and can even be installed in laptops or monitors.

MEMS gas sensors can meet specific needs in a variety of applications

Gas sensor market size from 2018 to 2023

Pattern recognition can be used to determine the number of people in a room and people's daily activity levels. This information can then enable smarter buildings to make better air-conditioning decisions. In HVAC systems, CO2 sensors help reduce electricity consumption by up to 50%, which translates to 20% to 30% energy savings for the entire building. This is because in normal systems the air circulation is a timer mode that maintains a constant fresh air input (eg during working hours); management based on real CO2 measurements adjusts the fresh air supply according to the actual conditions of the room. Therefore, the daily work cycle of the HVAC system is greatly shortened, and the energy saving effect is obvious.

In fact, there are many other applications of CO2 sensors, such as vehicle CO2 monitoring to adjust the driving position or the air quality in the whole vehicle. In agriculture, CO2 sensors can be used to control CO2 concentrations in greenhouses for higher yields and cost savings. CO2 sensors can also be used in medical applications such as CO2 detectors (capnometry); CO2 detectors are a method of measuring the CO2 content exhaled by a patient in real time, which is particularly effective in the field of anesthesia.

Industrial cases include detecting CO2 leaks near CO2 gas sources such as dry ice reservoirs, oil storage tanks, and underground gas sources. Smart cities can also link CO2 emission sources to vehicle density for traffic management.

CO2 sensor technology today

At present, non-diffusion infrared (NDIR) sensors are widely used, especially in the field of smart buildings. However, due to their relatively large size and high price, NDIR sensors can only be used in limited fields. This sensor consists of an infrared light source, a sampling chamber, a filter, and a reference and absorption infrared detector to provide true, accurate CO2 measurements. However, aside from purely aesthetic reasons, NDIR sensors are not suitable for installation in mobile devices, thermostats, and other smart home components in life, mainly due to their high cost and low integration capabilities due to their form factor.

There is currently no comparable solution on the market - both true and accurate CO2 measurement and cost-effective. Although so-called eCO2 sensors can be used to detect various types of indoor pollutants, they are not a good substitute for NDIR sensors. The eCO2 sensor does not perform the actual measurement, it uses an algorithm to calculate the equivalent CO2 value. Its calculation assumes that the CO2 in the area is mainly generated by the people present. Therefore, eCO2 only provides estimates based on many assumptions. And the adjustment of indoor air quality based on this eCO2 value is only done based on information that may be inaccurate.

This situation can cause the air-conditioning system to consume unnecessary energy or not ventilate at all when air-conditioning is required. The result is likely to be that air quality cannot be effectively improved, and users will lose confidence in using this eCO2 sensor product.

MEMS gas sensor based on photoacoustic spectroscopy

Infineon has successfully developed a new CO2 sensor based on photoacoustic spectroscopy (PAS) with its design experience and experimental process of MEMS microphones. PAS is a physical method suitable for detecting gas components in mixtures, such as CO2 concentration in indoor air.

MEMS gas sensors can meet specific needs in a variety of applications

Schematic diagram of CO2 sensor based on photoacoustic spectroscopy

Photoacoustic spectroscopy is based on the fact that gas molecules only absorb light at specific wavelengths, and CO2 typically only absorbs light at a wavelength of 4.2 µm. An infrared light source with a filter provides energy to the gas in rapid succession in pulses of light of precise wavelengths. This results in rapid heating and cooling of the gas sample, which in turn results in thermal expansion and contraction. The resulting sound can be recorded and evaluated using a microphone, and used to calculate the CO2 content in the gas. The higher the CO2 concentration, the stronger the acoustic signal. The use of highly sensitive MEMS microphones as detectors enables significant miniaturization compared to NDIR-CO2 sensors.

Challenges in developing this sensor

The Infineon CO2 sensor integrates a photoacoustic transducer, detector, infrared light source and filter on a single printed circuit board. The sensor features a small microcontroller for onboard signal processing, complex algorithms, and MOSFETs for driving the infrared light source. The modulated infrared light source will radiate to the gas mixture in the sampling chamber. After CO2 absorbs infrared light, it heats and increases the pressure in the sampling chamber, and the pressure change can be measured by a MEMS microphone.

MEMS gas sensors can meet specific needs in a variety of applications

Infineon's compact Xensiv PAS CO2 sensor

During the development of the PAS-CO2 sensor, the main challenge was to push the performance of the microphone to the limit and minimize the system noise; i.e. to isolate the MEMS detector from external noise, so that only the CO2 molecules in the cavity were detected. pressure changes. Infineon modeled the response of the MEMS microphone before fabricating the prototype to validate the modeling results.

Advantages of Infineon's PAS CO2 sensor solution

Infineon's new Xensiv PAS CO2 sensor in 2019 uses the IM69D130 Xensiv MEMS microphone with a signal-to-noise ratio of 69 dB. The sensor is designed for scenarios with low self-noise, wide dynamic range, low distortion, and high acoustic overload point. Thanks to the above-mentioned advantages of the IM69D130, the gas sensor can measure the slightest pressure fluctuations, so even a small amount of gas is sufficient to accurately determine the gas concentration. Therefore, the sampling chamber can also be designed to be small. Compared to conventional, equivalent CO2 sensors (true CO2 measurement), the new sensor is more than 75% smaller in size. An integrated microcontroller converts the MEMS microphone output signal into a ppm reading, available via serial I2C, UART, or PWM. Direct ppm reading, surface mount, and simple design enable simple and fast integration with flexible production digitization. All internal components are developed and designed according to Infineon's high quality standards.

The sensor is very robust and has a measurement range of 0 ppm to 10,000 ppm with an accuracy of up to 5,000 ppm (±3% of reading or ±30ppm of accuracy). Its operating temperature range is 0°C to 50°C and relative humidity is 0% to 85% (non-condensing). The drift value is less than 1% per year (active self-calibration). In pulsed mode, the CO2 sensor is designed for a lifetime of 10 years. Xensiv PAS CO2 possesses these excellent properties, making it ideal for demand-driven ventilation control in smart buildings, and indoor air quality control in smart home applications.

MEMS gas sensors can meet specific needs in a variety of applications

Typical Applications of Xensiv PAS CO2 Sensors

Infineon is planning several variants of the sensor to meet the special needs of various applications, such as low power consumption in battery-operated applications, smaller size and lower cost in portable devices, and extremely demanding industrial applications Need to be more reliable. The development of other gas sensors is also planned by Infineon.