Resources

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.

Metal Oxide Figure
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.

elec sensor pic
                                                                               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.

Optical sensor
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

- Sousan S, Koehler K, Hallett L, Peters TM. Evaluation of consumer monitors to measure particulate matterJournal of Aerosol Science, 107: 123-133, 2017 (Abstract, DOC. 25 KB)

- Kotsev A, Schade S, Craglia M, Gerboles M, Spinelle L and Signorini M. Next Generation Air Quality Platform: Openness and Interoperability for the Internet of Things. Sensors, 16 (403): 1-16, 2016 (Abstract, DOC. 23 KB)

- McKercher GR, Salmond JA, Vanos JK. Characteristics and applications of small, portable gaseous air pollution monitors . Environmental Pollution, 1-9, 2017. (Abstract, DOC. 24 KB)

- Sousan S, Koehler K, Hallett L, Peters TM. Evaluation of the Alphasense optical particle counter (OPC-N2) and the Grimm portable aerosol spectrometer (PAS-1.108). Aerosol Science and Technology, 50 (12): 1352-1365, 2016. (Abstract, DOC. 24 KB)

- Deng Y, Chen C, Xian X, Tsow F, Verma G, McConell R, Fruin S, Tao N, Forzani ES. A Novel Wireless Wearable Volatile Organic Compound (VOC) Monitoring Device with Disposable Sensors. Sensors, 16:2060, 2016. (Abstract, DOC. 23 KB)

- Kelly KE, Whitaker J, Petty A, Widmer C, Dybwad A, Sleeth D, Martin R, Butterfield A. Ambient and laboratory evaluation of a low-cost particulate matter sensor. Environmental Pollution, 221: 491-500, 2017. (Abstract, DOC. 24 KB)

- Manikonda A, Zikova N, Hopke PK and Ferro AR. Laboratory assessment of low-cost PM monitors. Journal of Aerosol Science, 102: 29-40, 2016. (Abstract, DOC. 23 KB)

- Jovasevic-Stojanovic M, Bartonova A, Topalovic D, Lazovic I, Pokric B, Ristovski Z. On the use of small and cheaper sensors and devices for indicative citizen-based monitoring of respirable particulate matter. Environmental Pollution, 206: 696-704, 2015. (Abstract, DOC. 23 KB)
 
- Holstius DM, Pillarisetti A, Smith KR and Seto E. Field calibrations of a low-cost aerosol sensor at a regulatory monitoring site in CaliforniaAtmospheric Measurement Techniques, 7: 1121-1131, 2014. (Abstract, DOC. 23  KB)
 
- Mead MI, Popoola OAM, Stewart GB, Landshoff P, Calleja M, Hayes M, Baldovi JJ, McLeod MW, Hodgson TF, Dicks J, Lewis A, Cohen J, Baron R, Safell JR and Jones RL. The use of electrochemical sensors for monitoring urban air quality in low-cost, high-density networks. Atmospheric Environment, 70: 186-203, 2013. (Abstract, DOC. 24 KB)

- Afzala A, Cioffia N, Sabbatinia L and Torsia L. NOx sensors based on semiconducting metal oxide nanostructures: Progress and perspectivesSensors and Actuators, B 171-172: 25-42, 2012. (Abstract, DOC. 23 KB)

- Aleixandre M, Gerboles M. Review of small commercial sensors for indicative monitoring of ambient gas. Chemical Engineering Transactions, 30: 169-174, 2012. (Abstract, DOC. 23 KB)