20 · CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 1: GENERAL APPROACHES the platinum wire filament (Figure 1.15a), still used in some gas detection instruments. However, the "bead" construction is the most effective and promising. The "bead" (see Figure 1.15c) takes the form of a ceramic pellet supported on a platinum wire coil. Usually, pellistors consist of small-diameter Pt spirals (approximately 20 turns) surrounded by a porous refractory bead, typically alumina impregnated with a precious metal catalyst such as palladium, thorium, or another catalyst. A noble metal acts as a catalyst to promote exothermic oxidation of flammable gases. The presence of catalyst coating reduces the temperature needed to achieve a stable signal for hydrocarbons. In this case the sensitive element is normally electrically self-heated to between 400 and 800°C for methane and other gases. This helps in extending the life of the filament. We need to note that pellistors are sensitive only to flammable gases and vapors. Hot-wire sensors (pellistors) are traditionally used to detect the percentage of the lower explosive limit (% LEL) levels of flammable gases and vapors. The LEL is usually expressed as percent by volume of combustible gas in air. Of course, pellistors have been used for detection of parts-per-million concentrations of some gases and vapors, but in this case they require active sampling and other instrument design consider- ations. Moreover, the pellistors are nonselective sensors (Korotcenkov 2007). 9. CONDUCTOMETRIC SENSORS Conductometric sensors have a simple structure and their operating principle is based on the fact that