Gas sensors
Gas sensors are devices that can convert information such as the components and concentrations of gases into a signal that can be analyzed by personnel, instruments, or computers. According to the literature, the first use of gas sensors is associated with mining some 200 years ago, whereupon detection of methane leaks was tested by bringing canaries and open-flamed torches to the mining tunnels. When the bird became silent/dead or the torch (together with the unfortunate miner carrying the flame) exploded, the presence of methane was obvious. However, it is reasonable to think that sensors were applied even earlier than that, because winemaking (or other fermentation types of food/drink production) yields a carbon dioxide byproduct, which due to its higher molecular mass (44.0 g/mol and thus density) than that of air (28.8 g·mol−1 for 21% O2 and 89% N2) accumulates in cellars (often used as cold rooms for the processes) and can cause choking of humans entering the cellar. According to current practices, a simple burning candle is used to see whether its flame is extinguished by the formed CO2. A catalytic sensor, the invention of O. Johnson in the 1920s, in which platinum was used to detect combustible mixtures of hydrocarbons in the air is considered to be the starting point of the modern era of gas sensors. Since then, the field has grown enormously with thousandsapers published and dozens of conferences organized in the field every year indicating that chemical sensors and gas sensors have become a truly significant branch of science and technology today.
Monitoring the concentration level of gas components is very important in many fields. For instance, the presence of sulfides (e.g., H2S) and acetone in respiration are potential indicators of oral diseases and diabetes, respectively; elevated NH3, CO, and H2 concentrations in the environment or the production plants of industrial processes (e.g. in the production of fertilizers) can warn of malfunctions and prevent hazards caused by leakage of these toxic and flammable gases; high levels of SOx and NOx in the air are key indicators when assessing air quality, and the list of examples may continue endlessly. Therefore, in our modern societies, gas sensors play essential roles in various technological production/control processes and in their safety, including environmental pollution monitoring, agriculture and food industries, and are also widely used in national defense and medicine/healthcare among many others.
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