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Improving the Environment and the Balance Sheet of Smart Cities with Connected Sensing

By European Editors

Contributed By Digi-Key's European Editors

Air quality is a key concern to organizations like the World Health Organization (WHO), which links pollutants like particulate matter, ozone, and nitrogen oxides (NOx) with diseases and increased rates of early mortality. Air pollution is one of 10 environmental factors included by the WHO in its assessment of global disease burden, a platform for monitoring public health in various parts of the world and helping determine suitable responses. Within this project, the WHO has been able to assess the proportion of global disease that could be prevented by improving the environment, and has published country profiles for 192 member states.

Urban areas are the most likely to experience poor air quality. The WHO believes that life expectancy could be extended in poor areas of the world by a good 10 years if risks to health, including environmental risks and others like nutrition and clean water, can be improved. Even in more developed areas like Western Europe, the WHO believes a further five years of average life expectancy are achievable. Hence, many groups and communities worldwide stand to benefit from studying and working to reduce the global burden of disease.

In Europe, North America and parts of Asia, initiatives to monitor and improve air quality are well established. Action on air quality in the EU, for example, includes a Clean Air Program for Europe which is supported by new policies announced in 2013 that encompass various initiatives to reduce emissions from vehicles and other combustion processes. The European Environment Agency (EEA) maintains an air quality database, called Airbase, which gives public access to results based on data from monitoring sites across the EU. In other areas of the world, air quality data can be less intensively monitored. In locations worldwide there is room for improvement, and interest in collecting data for analysis continues to increase globally.

More pervasive air quality monitoring

Cost is an important barrier to extending air quality monitoring, whether from a low or high base. A high quality regulatory air quality monitoring station can cost tens of thousands of dollars. They are often intended to be in use for long periods, perhaps a decade or more, and can be large and difficult to move to another location if required. If only a small number of stations can be installed as part of an air quality monitoring project, the amount of data collected will be limited. Modeling and extrapolation must be quite extensive to enable conclusions to be drawn for larger areas.

On the other hand, smaller and more mobile sensors are available for prices up to only a few thousand dollars or sometimes considerably less, at the expense of reduced accuracy and precision. They can be inexpensive to run as some devices only need to have their batteries changed, and operating lifetime tends to be shorter than larger regulatory stations.

The emergence of lower cost sensors, combined with the internet of things (IoT), can enable a change in the way air quality is monitored. Extra sensing stations can be inserted at locations between larger reference AQMS equipment, increasing reliance on data and improving the accuracy of modeling and interpolation. These can be small, semi-permanent or portable monitoring stations that are easy to set up and can be ready to begin collecting air quality data very quickly.

Large networks of sensors, however, are known to generate vast quantities of data, challenging conventional collection and analysis methods. In this case, IoT technology could offer a solution by simplifying the capture of data from monitoring stations in the field and reducing operating costs, as well as leveraging Cloud computing resources to process, analyze and report the resulting information.

It is also worth noting that smaller authorities and NGOs (Non-Governmental Organizations) can take advantage of low-cost sensing stations and smaller scale Cloud platforms to setup small scale or localized monitoring schemes. For example, ones that are close to industrial sites, schools or traffic hotspots, for enforcement or lobbying purposes.

Cutting the cost of sensors and circuitry

New manufacturing techniques to reduce the cost of sensors can also help extend the deployment of air quality monitoring stations. SPEC Sensors has developed a Screen Printing Electro-Chemical process for manufacturing high-performance, low-power sensors in small packages for volume manufacturing and at much lower cost than conventional electrochemical sensors. Several variants are available for measuring pollutants such as ozone, methane, nitrogen dioxide, carbon monoxide, hydrogen sulfide, n-heptane and chlorine. They each have a compact 20 mm x 20 mm square outline only 3 mm high, with a central gas access aperture, and can be used in health or residential air quality monitoring, or for industrial monitoring. They are supplied calibrated, and have expected lifetimes of 10 years.

An interesting and worthwhile note on using electrochemical gas sensors is that in addition to ensuring the sensing face is protected against contaminants such as dust and oils, the housing material should be chosen with care. Use of some plastics such as ABS can reduce sensor response due to adsorption of the gas molecules. Materials such as stainless steel or polypropylene are recommended wherever possible, and inert materials such as EPDM (ethylene propylene diene monomer (M-class) rubber) used for any seals or diaphragms.

An electrochemical sensor is a three-terminal device that contains a working (sensing) electrode, counter electrode and reference electrode. The working electrode comprises a catalytic material formulated to react with the detected gas, which results in current flow proportional to the gas concentration. A potentiostat circuit is used to control the potential of the working electrode and convert the generated current to a measurable voltage. Figure 1 shows a sample circuit as recommended by SPEC Sensors.

Diagram of potentiostat circuit

Figure 1: Potentiostat circuit for measuring the response of an electrochemical gas sensor.

TI has created a gas sensor reference design and evaluation kit that uses the LMP91000 Analog Front-End (AFE) IC to interface to an electrochemical sensor. Using this device, which has an integrated transimpedance amplifier (TIA), simplifies circuit design and allows important parameters such as bias voltage and TIA reference voltage to be programmed without the use of external resistors. A temperature sensor is also included, which is a useful addition as ambient temperature can affect the response of the working electrode. Figure 2 illustrates this integrated solution to the challenges of designing the sensor AFE.

Diagram of Texas Instruments LMP91000 Analog Front-End (AFE) IC

Figure 2: The LMP91000 simplifies interfacing electrochemical sensors to electronic monitoring circuitry.

Another advantage of using the TI reference design is that the kit incorporates Bluetooth® Low Energy technology, leveraging the CC2541 2.4 GHz radio IC. A mobile app is also provided, allowing the user to immediately begin collecting sensor data and analyzing the results on a tablet or smartphone. The app performs the necessary post-processing of raw data, as collected via the LMP91000 AFE, and digitized and transmitted by the CC2541.

Conclusion

The IoT presents an opportunity to greatly expand air quality monitoring on a cost effective basis, subject to the availability of a new generation of sensing stations that are affordable and easy to deploy. The latest sensor and IC technologies enable the development of low-cost, miniaturized sensor modules for detecting pollutants such as ozone, methane and NOx that are of interest to health and environmental monitoring programs worldwide. By enabling pervasive monitoring to gather reliable air quality data, these modules could have an important role in helping prevent diseases related to poor outdoor air quality, and extend life expectancy everywhere.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

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European Editors

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Digi-Key's European Editors