Method for determining deposit buildup

A method of determining low density deposits on the inner surface of a boiler tube comprises the generation of ultrasonic energy which is directed from the outer surface of the tube to the inner surface of the tube. The energy is reflected back and forth between the inner and outer surfaces with each reflection of the inner surface loosing energy by attenuation into any existing low density deposit. The first and fourth reflections are measured and digitized for analysis to determine the amount of attenuation. The amount of attenuation is representative of the low density deposit on the inner surface of the tube. The tube is preferably empty of water to avoid transmission of ultrasonic energy through water in the tube.