There was an article some years ago in Western and Eastern Treasures in Tech Talk about the depth indicated by most metal detectors. It mostly addressed air tests versus depth indicated on ground targets. It seems if a metal object has been in the ground some time, the metal leaches into the surrounding area ant presents a larger target to the MD, therefore a closer target reading for say a quarter underground. When you air test a quarter, the detector .reads it as a smaller target and indicates a truer depth or distance reading. I guess what I am saying is that depth readings are mostly incorrect depending upon leaching, mineralization and other factors. Sound volume is probably just as effective as meter readings on most objects. On the other hand, each machine reacts differently, even the same models. Then again, what do I know.