Bottom of cooling jacket: Not super thick
You should buy the right gel. I'm not convinced white lithium is a valid substitute.
I can understand questioning this, but so you will know..... I use a smooth cast iron brake rotor as my 'test coupon' to calibrate the sonic tester as I can put a caliper around it. The accuracy is as good as I expected, within the resolution of the unit, which is 0.1mm or .004". The readings there don't change dry, or with spit, or with the grease as a coupling.
The coupling gel is just to make the sonic impulse and reflections couple effectively between the material under test and the impulse head. Even the gel has a different propagation velocity versus the material under test, but it is so thin, the time delay between the impulse and the reflection return is not effected enough. And, with the far side of a casting being rough, the reflections are going to be diffused anyway, crating some uncertainly in the measurement even if the measurement-side surface is perfect. The sonic tester can only look at the earliest time or arrival of the reflections and say that is the distance.
Is there a reason that the use of a caliper at the same spot to get the same results does not seem like adequate confirmation? I'm not looking for .001" accuracy... I'm looking to see if this is thick or thin.
