Abstract: In a flue gas scrubber for removing sulfur dioxide from flue gas and having a spray dryer vessel inlet duct defined by a duct wall, the apparatus may be used to measure in situ the wet bulb temperature of the flue gas in the dryer vessel inlet duct. A water nozzle and a temperature sensor are both located in the dryer vessel inlet duct. The nozzle sprays water on the sensor and the wet bulb temperature of the flue gas is continuously measured. The apparatus gathers wet bulb temperature data which may be used for process control or for determining whether a leak has occurred within the tubes of a coal-fired steam boiler.

Abstract: A sheath is used to house a conventional pH sensor within the sheath's interior. The sheath is exposed to a descending spray within a sulfur dioxide (SO.sub.2) absorber. A fill port collects a portion of the spray and the sheath accumulates the collected spray as a slurry sample within its interior. The pH sensor becomes immersed in the sample slurry. Slurry is discharged from the interior by way of drain notches and drain holes. Discharged sample slurry descends to enter a bulk slurry to allow recycling. A steady state of slurry collection and slurry discharge is reached such that the sample slurry is continuously refreshed. Elevation of the sheath over the bulk slurry prevents slurry from the bulk from entering into the slurry sample.