For most of the 20th century, gas detection and analysis was relegated to the scientific instrumentation equipment found almost exclusively in laboratories. But since the turn of the century, sophisticated sensors capable of detecting groups of gases have come down in size and price allowing for the implementation of gas sensor IC designs in production volumes.

Today, the most popular applications for gas sensor chips relate to air quality monitoring, supporting the operation of home and building automation (HABA) systems, triggering air cleaners and purifiers, and providing data for lifestyle and home environment applications hosted on consumer devices such as smartphones.

Gas sensor chips in today’s applications typically use a micro-machined sensing element made from metal-oxide (MOX) material. In active sensing mode, the element is heated to a temperature between 150°C and 450°C. At these high temperatures, the resistance of the sensing element changes when exposed to various types of reducing or oxidizing gases. By measuring the change in resistance, a relative value for the concentration of gases in the ambient air may be calculated.

These single-element gas sensors offer considerable value in air quality monitoring applications. For instance, the MOX-based CCS811 gas sensor from ams (Figure 1) provides two key benefits:

  • a calculated equivalent total volatile organic compounds (eTVOC) value measuring the relative concentration in ambient air in parts per billion (ppb)
  • a calculated equivalent carbon dioxide (eCO2) value measuring the relative concentration in ambient air in parts per million (ppm)

Figure 1 The ams CCS811 evaluation kit, combining a sensor board (right) with an USB-to-I2C bridge board (left)

Through the application of software algorithms in a device such as an air purifier or a smart thermostat, an indication of air quality or an overall air quality score may be derived from the eTVOC measurement of a broad range of reducing gases. In addition, the eCO2 measure provides a reliable proxy measure for human occupation of a confined space, since people’s exhalation tends to raise the concentration of indoor atmospheric CO2 unless counter-balanced by inflows of fresh air from outside.

These general measures of relative gas concentrations are successfully used in the automatic regulation of demand-controlled ventilation (DCV) and air cleaning systems. This indicates the value which may be derived from the application of air quality monitoring technology – but the potential for even greater value is still to be realized.

Progress depends on the development of gas sensing technology in four domains. This article outlines the focus for gas sensor manufacturers’ development efforts today and describes the new and improved applications that the technology will support in future.

Driven by customer need

The force driving the fast pace of research and development at sensor manufacturers is customer demand for better ventilation and air cleaning systems, which operate automatically. To meet this demand, the technology of gas sensing needs to improve in the following ways:

  • greater selectivity
  • greater accuracy and precision
  • greater sensitivity

The need for selectivity arises from the different response of the human body to different gases. The CCS811 gas sensor from ams, a device operating across a large installed base, can provide a measurement of total volatile organic compounds (VOCs) – not of any one VOC. The term VOCs represents a broad category of gases: their effects can be limited to mild discomfort but with no effect on health. This is true, for instance, of human-generated gases that have an unpleasant odor, such as bad breath. But the VOCs category also includes harmful chemicals such as benzene, a known carcinogen in humans and a component of tobacco smoke and the exhaust fumes from automobile engines (Figure 2).

Figure 2 Exhaust fumes from vehicles are a cause of poor quality in urban areas.

A gas sensor that can discriminate between benign but odoriferous VOCs and toxic VOCs would enable the development of improved air quality monitoring systems that could graduate the urgency and severity of their signals to users depending on the relative harmfulness of the air, and improved HABA systems that can automatically evacuate harmful gases from indoor spaces without intervention from the user.

The accuracy and precision of gas sensor measurements affects the user’s perception of the value of the information provided by air quality monitoring devices. Today, gas sensor ICs can support the provision of a broad index which classifies air quality as fresh, moderate, or poor. The accuracy of the measurements of VOCs and CO2 concentrations is sufficient to ensure that this three-band index is consistent and accurate over time and over variations in ambient temperature and humidity.

But the user might be left in doubt about how to respond to a ‘moderate’ air quality score. Is the air almost fresh, so no action is required? Or almost poor and declining fast enough to warrant triggering urgent action to ventilate the area? More accurate and precise measurements enable the provision of more useful indications to the end user. 

Greater sensitivity will allow monitoring devices to pick up even very small concentrations of very harmful or very uncomfortable gases, improving value to the end user. Achieving this involves several key criteria.

[Continue reading on EDN US: The priorities for gas sensor development]

Paul Wilson is a senior marketing manager at ams.

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