292 CHEMICAL SENSORS: TECHNOLOGIES. VOLUME 4: SOLID-STATE DEVICES 3. CATALYTIC BEAD DEVICES The most widely available commercial calorimetric sensors are pellistors, or catalytic bead devices, as shown in Figure 7.1. The devices are fabricated by a method that involves hand fabrication. The core of the devices is fabricated using Pt wire, which has the advantages of being robust at high tempera- tures and having a useful temperature coeffi cient of resistance of 0.0038 (°C. (By measuring the resistance of a Pt wire at two temperatures, and dividing the difference by the resistance at the lower tem- perature, the temperature change can be calculated by dividing that ratio by the temperature coeffi cient of resistance.) Fine wire, of diameter 10­50 m, is used, with the advantage of lower power consump- tion at lower thicknesses coming with the trade-off of greater device fragility. The Pt wire is wound by an automated tool into a coil of precise dimensions. The Pt wire is coated with an alumina support material that is impregnated with a catalyst. The high effective surface area of this supported catalyst ensures that under diffusion-limited conditions, there is suffi cient active catalyst for combustion, even if some of the catalyst has become inactive. This helps to ensure long operating life of the device. The coated device is fired to a high temperature (500­800°C) to stabilize the catalyst and pre-age the device. The device is soldered into a package that may include filters and a flame arrestor. Typical characteristics of commercial devices include the following performance parameters. Sensitivity is commonly listed as covering the range between 0 and 100% of the lower explosive limit, with a minimum sensitivity at around 1­2% of the LEL. Operating temperatures are in the range of