Metal-Oxide sensors

Semiconductor metal oxide sensors consist of one or more oxides from the transition metals. Commercially available gas sensors are mainly made of SnO₂ in the form of porous pellets or thick or thin films deposited onto an alumina or silica substrate. The sensing properties are based on the reaction between the semiconductor metal oxide and oxidizing or reducing gases in the atmosphere which lead to changes in conductivity. This change in conductivity is measured over a pair of interdigitated electrodes embedded into the metal oxide. A (mostly platinum) heating element is used to regulate the sensor temperature. The sensors have to be heated to 200 to 400 degrees Celsius to increase sensitivity and decrease response time.

Schematic metal oxide gas sensor

Electrochemical sensors

Electrochemical sensors operate by reacting with the gas of interest and producing an electrical signal proportional to the gas concentration. The sensor consists of a sensing electrode (also called working electrode), and a counter electrode separated by a thin layer of electrolyte. Gas that comes in contact with the sensor diffuses through a hydrophobic solid polymer membrane, eventually reaching the sensing electrode surface. The sensing electrode either oxidizes or reduces the target gas with the counter electrode balancing the generated current. These reactions are catalyzed by the electrode materials specifically developed for the gas of interest. However, the sensing electrode potential does not remain constant due to the continuous electrochemical reaction taking place on the surface of the electrode causing a deterioration of the performance of the sensor over an extended period of time. Consequently, a reference electrode is placed within the electrolyte in close proximity to the sensing electrode. The reference electrode anchors the working electrode at the correct bias potential. The value of the bias voltage applied to the sensing electrode makes the sensor specific to the target gas. With a resistor connected across the electrodes, a current proportional to the gas concentration flows between them. The current can be measured to determine the gas concentration. Because a current is generated in the process, the electrochemical sensor is often described as an amperometric gas sensor or a micro fuel cell. When gas concentrations are measured in the ppb range, the generated currents can be as small as a few nano-amperes.


                                                                               Schematic electrochemical gas sensor
Optical sensors

A low pulse is output from the sensor when the light receptor detects light scattered by particles. The particle concentration can be estimated based on a manufacturer provided curve of concentration versus the percentage of time the sensor is reporting a low pulse. Higher sensitivity versions of optical particle counters go beyond using “percent time” as the indicator but quantify based upon the strength of the light scattering detected.

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PM sensor operation diagram

References:

• http://www.everyaware.eu/ , EveryAware, Enhance Environmental Awareness through Social Information Technologies
• US EPA Community Air Sensor Network (CAIRSENSE) Project, Quality Assurance Project Plan - Original Draft