As far as questioning the test,it seems to be a very comprehensive test using minimally 12 checks per cylinder.We do 16 checks if we find thin areas.
Diffrent types of material have different inherent sound velosities,ie:steel velosity aprox.5918m/s and aluminium is 6350m/s.If the guage is not set correctly the measurements will be erroneous by a certain percentage.
There are 3 ways to calibrate a sonic meter.
1- Calibrate to a known thicknes/material
2-Calibrate to a known velosity
3- A 2 point calibration.
We generally use 1- calabration to the known thicknes, sincethe advent of added alloys and strengthen materials to the parent metal.ie: added nickel in cast iron.Cast iron having a sound velosity m/s of 4572 and nickel having a sound velosity of m/s 5639.
Depending on the amount of nickel interduced into the cast iron will negate using 2-calibration to a known velosity(chart value).The best way to measure a block is to find an area that can be measuered(known thickness)your probe should read the same value.
Other considerations are choosing the correct transducer probe size and diameter as well as a good copulant for seal.Keep in mind test should be done at nominal room tempertures.