The Acid Deposition Rate Probe (ADR) was originally developed during the 1960’s by the Central Research Engineering Laboratories of the C.E.G.B. in England. The technology has been successfully employed within the United States power generating industry for the past twenty years and is a device for simulating cold gas path metal surface conditions in fossil fueled boilers. Under conditions of stable boiler operation, the ADR is placed within the flue gas stream immediately in front of the air pre-heater heat recovery elements. During each thirty minute test cycle, the ADR will accumulate condensed sulfuric acid (from SO3) and particulate matter, including alkaline material such as magnesium oxide from fuel treatment systems. The test effectively duplicates the collection, corrosion and fouling conditions experienced with the existing boiler firing conditions at air pre-heater and adjacent duct area metal temperature regimes.