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.