SO.sub.2 sensor

A device for measuring SO.sub.2 concentration in a flowing gas stream comprising:

• a. an oxidation catalyst selected from the group consisting of Ru, Rh, Pd, Os, Ir, Group VIB metal oxides, and Group VIII metal oxides and adapted to oxidize oxidizable components of the flowing gas stream but not SO.sub.2, said catalyst being situated to oxidize said oxidizable components in the flowing gas stream prior to contacting the flowing gas stream with a catalytic sensor element;
• b. a catalytic sensor element comprising a catalyst suitable for oxidizing SO.sub.2 to SO.sub.3 in the presence of oxygen and a temperature measuring device, where the catalyst is placed about as a layer over the temperature measuring device so that the catalyst is in physical and thermal contact with the temperature measuring device and where the catalyst is in contact with the flowing gas stream, and the heat produced by oxidizing SO.sub.2 raises the temperature of the temperature measuring device substantially proportionally to the SO.sub.2 concentration in the flowing gas stream;
• c. a reference element adapted to detect the ambient temperature of the flowing gas stream in the vicinity of the catalytic sensor element;
• and
• d. a flowing gas temperature controller for controlling the temperature of the flowing gas stream prior to its passage over the catalytic sensor element to operating temperature.