A Pellister (CxHy Sensor) is a heated sensor that measures total hydrocarbons. Also know as a catalytic or heated bead sensor, it consists of a specially matched pair of precision resistive thermal devices (RTDs) overlaid with two different coatings. The first is covered with a propriety catalyst that creates an exothermic reaction to the presence of targeted hydrocarbons. The second RTD is covered with an inert coating and acts as a reference. Both RTDs are heated to approximately 950 °F to increase the catalytic reaction rate.
In the presence of hydrocarbons the catalytic RTD will have higher heat value than the reference. The RTDs are tied to two legs of a "Wheatstone Bridge" to maximize the output and stability. Testo's unique cross calibration techniques essentially make the HC module blind to interference gases such as carbon monoxide and free hydrogen.
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