Hi Lamark,
My first thought on depth is to ask: are the coils the same diameter? Both DD or both Mono?
Jonathan Porter, DVD The GPX Factor, (GPX4000) mentions rain. Water assists conductivity; if you have wet ground layer
with dry above or below, it will cause erratic threshold. As well as limit depth, or cause you to recalibrate and constantly manual GB.
I also have an X-Terra 70. Where I hunt in NorCal, there is little point in tyring to use it after a rain. Sorry, take that back... be
prepared to dig lots of ground noise targets. It will find real targets, just hard to tell what is real and ground noise because the threshold
is terribly erratic.
Air tests: I do that to remind myself what a .5 gram nugget sounds like, but I've never tried any comparisons. Good question,
I would guess that too is a function of coil type and diameter vrs technology.
The X-70 runs an 18.75 DD 10X5 elliptical coil - the GPX-4000 typically runs the Commander 15X12 semi-elliptical mono loop.
DD for the X-70 to aid in mineralized conditions and large mono loop for depth and sensitivity for nugget shooting - not everyones
favorites... just mine.
Bill