Gas sensors for measuring VOC and fine dust

10/31/2018 Know-How

Today, people spend around 90% of their time indoors, be it at home or in an office, restaurant, cinema or gym. The quality of this indoor air can be impaired significantly by harmful gases and fine dust. Maria Alejandra Salazar Martinez, Product Sales Manager Analog & Sensors at Rutronik, explains how gas sensors in smart home devices can help improve air quality.

Substances from carpets, rugs, and furniture, paints, adhesives, solvents, cleaning agents, cosmetics, and from people themselves all fill the air in a room. Many of them, such as acetone, ethanol or xylene, are potentially harmful chemicals known as VOCs (volatile organic compounds). They can have a negative impact on our health if their concentration exceeds 220ppb (parts per billion). The possible consequences of an excessively high VOC content are headaches, dizziness, dry and irritated mucous membranes, reduced productivity and concentration.

An excessively high VOC content can be prevented relatively easily, for example by opening windows or using air conditioning, ventilation or special air purifiers. In order to improve air quality in the long term, the main sources of VOC emissions must be eliminated wherever possible. But what are they? And when is the VOC content actually too high? To be able to answer these questions, it is necessary to know their concentration. Respective gas sensors are available for this purpose.

Most gas sensors, which also detect VOCs, only demonstrate insufficient long-term stability, i.e. their sensitivity decreases relatively quickly and their response time becomes longer and longer. The supplier Sensirion was able to identify siloxanes as the reason for this. These chemical compounds of alternating silicon atoms and oxygen atoms are used in cosmetics, detergents, silicone oils as well as for freeze drying. Siloxanes are harmless to humans but damage the sensor element of metal oxide gas sensors.

Sensirion has therefore developed a siloxane-resistant metal oxide gas sensor with unprecedented long-term stability. To achieve this goal, the supplier combined its specific metal oxide technology with a multi-pixel platform. The platform integrates four sensor elements with differing sensor materials in a very small 2.45mm x 2.45mm x 0.9mm DFN package featuring an I2C interface and a fully calibrated air quality output signal. The gas sensor <link _blank external-link-new-window "open internal link">SGP</link> thus enables single gas as well as multi-gas measurements. The sensor elements that are not required can be used to compensate for interfering gases.

This enables smart home devices such as ventilation or air conditioning systems to switch on and control themselves intelligently according to the air quality. This also makes them particularly energy efficient. If a baby monitor with integrated VOC measurement always displays the current value, babies and toddlers can be protected from an excessively high VOC content and VOC sources can be localized.

Environmental sensors even go one step further. In this case, the following are measured:

  • VOC
  • Humidity
  • Temperature
  • Air pressure
  • Fine dust
  • Possibly other parameters

Smart home and IoT devices that take all these values into account can further improve air quality, provide maximum comfort, and help protect our health. At the same time, their energy efficiency improves.

The gas sensor <link _blank external-link-new-window "open internal link">BME680</link> from Bosch Sensortec is a combined sensor for VOC, ambient temperature, relative humidity, and air pressure in a small 3mm x 3mm x 0.95mm package. It therefore enables:

  • Indoor air quality measurement
  • Volume and air flow measurement
  • Warning regarding dryness
  • Warning regarding high temperatures
  • Demand-oriented and energy-efficient control of smart home devices such as HVAC equipment

Even the actual gas detector is energy efficient: Although not all sensors operate in measuring mode all the time due to intelligent algorithms, the BME680 still delivers very quick and up-to-the-minute values. It is therefore also suitable for mobile devices, e.g. fitness trackers or smart watches.

A somewhat different approach to environmental sensors is taken by Sensirion: Besides temperature, humidity, and VOC, the key parameters for the supplier include fine dust and CO2.

Half of fine dust contains particles measuring 2.5 µm in diameter (PM2.5), the other half consists mainly of even smaller particles. PM2.5 are among the most hazardous air pollutants as they can penetrate human lungs, cause allergic reactions (asthma attacks), and promote cardiovascular diseases.

The fine dust sensor SPS30 from Sensirion is based on the laser scattering measurement principle. Together with high-quality components, it enables precise fine dust measurement over the entire service life of the sensor of at least eight years. Smart home devices for monitoring air quality can thus be implemented to help prevent long-term harm caused by fine dust. The gas detector SPS30 will be launched in the fourth quarter of 2018.

Along with VOCs, CO2 is a decisive indicator for air quality. According to DIN EN 13779, indoor air is deemed good up to a CO2 content of 800ppm and considered poor with values above 1400ppm.

The CO2 and RH/T (relative humidity/temperature) <link _blank external-link-new-window "open internal link">sensor module SCD30</link> from Sensirion measures the CO2 content very accurately thanks to Sensirion's CMOSens® technology for IR detection. In doing so, the sensor module automatically compensates for long-term drifts using the dual-channel principle. The sensor module also features an integrated humidity and temperature sensor that models and compensates for external heat sources without additional components. The SCD30 therefore enables smart home devices with accurate and stable monitoring of CO2, temperature, and humidity.

Find components at <link _blank external-link-new-window "open internal link"></link>.

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