A method of operating a pulse echo ranging system comprising a transducer assembly for providing transmission and reception of high frequency energy pulses at substantially different plural frequencies. The apparatus includes a housing with an interior space with first and second stages. The Eruberin A may be extracted from Pronephrium penangianum. Generally, the block table is stored periodically andor at system shutdown. The defect may then be classified as corresponding to the defect type whose signal profile has the highest probability. The flue heat exchanger may be a coil of tubing wrapped around flue section, such that the tubing picks up heat from the heated flue gasses.