Catalytic bead sensors are solid-state detectors usually employed to detect combustible gases.  CATS are capable of detecting the presence of gas by measuring the change in resistance of a conductive material as a combustible gas oxidizes on a catalyst surface. In the most common design, two platinum coils are embedded in a bed of alumina and the voltage difference across them is measured using a Wheatstone bridge circuit configuration. One wire contains a catalyst that makes it an oxidative environment while the other wire, a reference, inhibits oxidation. When an oxidizable gas is present in the sensor, oxidation occurs at one wire and not at the other. This induces a voltage difference across the wire and an alarm is triggered to indicate the presence of a gas.  Sensors are typically calibrated to methane gas and correction factors are applied for the detection of other gases of interest.  In order to obtain the highest degree accuracy and precision, gas-specific calibration is required.  As gas sensors, CATs can be employed in a variety of areas including mining, emergency response, and any other application where the presence of combustible gases is of concern.    As gas sensors, most CAT devices are hand held or portable units.  The technology has been in place for more than half a century and many manufacturers are producing CATs.  Units cost is typically under $25,000.