This little unit seems to be very accurate, the parallel mics at .499" as well. I've checked about ten places on the boiler shell and it appear to be checking at .375" /.370" in different locations. I was most concerned about the mud ring area. The paint must be removed and the surface must be smooth for it work accurately. Grinding the paint and a bit of steel while doing so is probably the reason for the .005" variance.
It comes with a rather large tube of coupling agent, just clean the area, put a small dab of agent on and as soon as the transducer "couples" it reads the thickness.
Will check thickness from .06"/1.5mm to 7.0"
It has 17 different presets for a range of materials. Aluminum , brass, stone, plastics, etc.
For $120 on Ebay, it looks like a pretty good investment. Of course this doesn't substitute an actual boiler inspection by a qualified person, but it gives one a basic idea of the condition of a boiler shell.
-Ron
. I like to see some actual data instead of simply relying on the exterior condition to judge the interior condition - and the sellers testimony.