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Environmental Monitoring Systems: A Review

Published in : IEEE Sensors Journal (Volume: 13, Issue: 4, April 2013)
Authors : Anuj Kumar, Hiesik Kim, and Gerhard P. Hancke
Summary Contributed by:  Jayraj Mulani

Economic and industrial growth, technological advancements, and infrastructure developments have transformed the lifestyle of humans. Unfortunately, the overall developments have led to an alarming rise in environmental pollution and greenhouse gas emissions, adversely affecting the environment and human health. Studies show that constant monitoring of the environment using Environmental monitoring systems have made it possible to reduce greenhouse gas emissions. It has made the persons conscious and concerned about the air quality and indoor physical environment.

An environmental monitoring system consists of a data acquisition unit, graphical interface unit and a communication unit. The sensors take regular measurements (Smart Transducer interface) that are subsequently translated into a perceivable representation (Network Capable Application Processor) and transferred to the application processing unit for analysis. The communication interface between STIM and NCAP is called Transducer Independent Interface (TII).

A transducer is simply a device to convert physical or chemical information into an electrical signal (Sensor) and vice versa (Actuator) for measurement and control. A smart transducer (STIM) comprises an array of sensors/actuators, a microcontroller, and a communication network as one unit.

A significant section of pollutants is gaseous, making a gas sensor vital for STIM. Gas sensors measure the concentration of gases by detecting their ionization potential, which is unique for each gas. Electrochemical, solid-state, infrared and photoionization are some of the most popular technologies used for gas sensors.

A Solid-state or Semiconductor sensor consists of a metal oxide film, heating element and an insulating medium. The absorption/desorption of gas on metal oxide surface induces a change in conductivity of semiconductor which can be measured. The heater is required to raise the temperature of the semiconductor film, which leads to optimal sensitivity and response time.

In an electrochemical gas sensor, contact with gas molecules causes an electrochemical reaction in the sensor electrode, resulting in an electron flow between electrodes. This electric current is linearly proportional to the gas concentration.
The photoionization sensor uses an ultraviolet light source to ionize ions of gas molecules, which can then be detected and measured.

An Infrared sensor uses the property of gases to absorb infrared light at specific wavelengths. The decrease in transmitted light is measured to quantify the gas concentration.

The STIM is the sensing module of the environmental monitoring system. The data received needs to be processed, adapted for reading and transmitted to the server. A Network Capable Application Processor (NCAP) is a basic communication node of the system. On one end, it controls the STIM; on the other, it processes, converts, and transmits data to the server and remote controls the STIM.

The utility of a sensor and the monitoring system is determined by functional parameters such as sensitivity, selectivity, detection limit, response time, and recovery time. Fabrication costs, sensor stability and durability, and energy efficiency are also deciding factors.

The environmental monitoring system still needs extensive research and studies as the available sensors have limitations and challenges to address. There is a need to develop an autonomous, energy-efficient and real-time environmental monitoring system that can rapidly and reliably detect and analyze pollution sources.

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