Bottom of cooling jacket: Not super thick

A note - if the surface of the media being tested is "as cast", your results may vary... Most transducers made for a curved surface will not sit very well on an as-cast finish. I would suggest if you tested on the cast finish you use a bur or sanding roll and smooth it out. Then re-test. The coiupling gel is not "filler". The transducer halves have to contact the surface properly or the reading will be suspect.