  Sensing technology has been widely investigated and utilized for gas
detection. Due to the different applicability and inherent limitations of
different gas sensing technologies, researchers have been working on different
scenarios with enhanced gas sensor calibration. This paper reviews the
descriptions, evaluation, comparison and recent developments in existing gas
sensing technologies. A classification of sensing technologies is given, based
on the variation of electrical and other properties. Detailed introduction to
sensing methods based on electrical variation is discussed through further
classification according to sensing materials, including metal oxide
semiconductors, polymers, carbon nanotubes, and moisture absorbing materials.
Methods based on other kinds of variations such as optical, calorimetric,
acoustic and gas-chromatographic, are presented in a general way. Several
suggestions related to future development are also discussed. Furthermore, this
paper focuses on sensitivity and selectivity for performance indicators to
compare different sensing technologies, analyzes the factors that influence
these two indicators, and lists several corresponding improved approaches.
