Jason,
First I will assume you are using the standard 9 inch concentric medium frequency coil. Just to review, each depth arrow is about equal to two inches. Set the sensitivity to 15 and do an air test outdoors with a few different coins and see if they read distance wise correctly. Keep in mind that you should always make sure the coin is centered, especially in the ground.
Next locate a a trash free area in your yard and GB the machine making note of the GB number. Lay each coin on the ground and slowly sweep over the ground noting if the distance looks correct. Ball park distance is OK for this stage. Now if it's appropriate, dig some measured holes in a trash free area at like 3, 7, and 11 inches, and lay the coins flat in the holes. Back fill a little at a time, and compact the soil as you work your way back up out of the hole.
If the distance suddenly on the buried coins is way off, then you may have highly mineralized soil, or it may be mixed in with very small iron or foil. Once again make sure to pinpoint and center up on the targets.
A helpful check is to GB the machine, put it in All Metal, and begin to increase the sensitivity while bobbing the coil like GB'ing. Note the point that the machine starts to spit, if sensitivity can only be increased to say 10 then it may point to high mineral or trash mix.
Hope this helps.
BarnacleBill
