threw your name in there cause i know you've done a fair number of blocks. in response to the block thread i decided to dust of my UT mic and start playing with it.

was looking for some general input from people's experience.

some time ago i took a couple pieces of block (they may be from different blocks don't remember) and machined one to .123" and the other to .230". i didn't have a surface grinder at the time so i just milled them but they're close enough (i am going to grind them now).

when i set the velocity on one i find that it's off by 3-5 thousandths on the other. while i don't like this (like it's really gonna matter) i'm not surprised because they may be different compositions plus i'm calibrating a radius probe on a flat surface which i question the wisdom of. i rock the probe to get the minimum reading then use that.

a friend of mine that owns a machine shop has a UT mic and the standards it came with are pieces of cylinder so the radius isn't an issue. without a complete circumference piece of a cylinder off the top of my head i can't think of any way to get a radiused piece of uniform thickness to calibrate the velocity

another thought is about the couplant in the past i have used the couplant that came with the mic but found it must have some water or other corrosive properties.

where i used to work there was a ton of NDT going on and iirc sometimes they just used different forms of grease (white grease i believe)

i tried some lithium grease tonight and it seemed to work fine, i'd rather use this to prevent any corrosion

looking at the sheets i've seen over the years it seems most people take 12 readings in each hole which seems a little tedious. the mic i have (and i would think most do) has a scan feature where you can start at one end and drag the probe to the other it will then display the minimum thickness measured. i'm not 100% certain of the scan rate but iirc one thing i read said it pulsed at 200HZ but i don't know if it reads every pulse.

i kinda like the idea of scanning each quarter of the hole as opposed to taking three different readings

only concern i have is wear on the transducer but don't think that would be a major issue using grease and only enough pressure to maintain coupling

thoughts/comments?