Furnace waste gas sampling

An apparatus for sampling waste gas flowing along a furnace waste gas duct, comprising an elongate probe for insertion into the duct and comprising an elongate sample tube having an open inlet end through which waste gas to be sampled is drawn for passage along the tube and out of the duct and a water-cooled elongate extraction conduit surrounding at least a part of the sample tube, the open inlet end of the sample tube being located within the water-cooled elongate extraction conduit, the extraction conduit having an open end through which waste gas is drawn from the flow of waste gas passing along the duct and an opposite end which is substantially closed to the egress of waste gas, filter means being provided at or adjacent the open inlet end of the sample tube to sieve out entrained particulate matter from the waste gas to be sampled, means for sensing the waste gas temperature along at least a part of the distance between the open and closed ends of the elongate extraction conduit, wherein a decreasing temperature profile is established in the waste gas between the open and closed ends of the elongate extraction conduit, the sample tube being located in relation thereto such that the filter means is maintained at a temperature above the dew point temperature of the waste gas.