118. Your oil platform supply vessel is fitted with a two-pass, fire-tube, oil-fired auxiliary boiler. What is the most practical way of determining if the outside surface of the boiler tubes are excessively scaled with hard scale deposits?

Measuring the outside diameter of the tubes directly indicates scale thickness, as hard scale deposits increase the tube's outside diameter. Other methods are indirect and influenced by multiple system variables, while visual inspection lacks the necessary quantitative accuracy.