Hi Dennis, about the coils:
The 5" coil is for EXTREMELY trashy sites.
The 4.5 x 7" Coil is like a 5"/8" coil Hybrid, it will go into tighter places than the 5", and I'm sure it pinpoints better too. It has the depth of an 8" coil, and the Trash separation of a 5". Use it for EXTREMELY/HIGHLY trashy areas.
The 8" Coil is for HIGH trash Sites.
The 10.5" coil is for Moderate/Light Trash sites.
The 12.5" coil is for Low/Light trash sites.
The 14", 15" & 18" Coils are for sites where there's vitually no trash, nor many targets to be picked up, you know, to punch DEEP into the ground for those targets out of range from normal 8"-10.5" coils, or for Cache hunting.
(14 Inch EXcelerator)
(15" Coiltek W.O.T.)
(18 Inch EXcelerator)
I'd suggest you first get to know how trashy your sites are, for example, if it nulls constantly go for the 8" coil.
If it constantly nulls and you find yourself without treshold for several seconds get the 5" coil.
If you find yourself with nulling once in a while stick with the 10.5" coil, and can be used for cache hunting.
If you find yourself with almost no signals for a while get the bigger 12.5" coil since you will cover more ground and it will go deeper. It can be used effectively for cache hunting.
If you find yourself with hardly any signals at all get the bigger 14" or 15" coil. They will cover a lot more ground and will almost double your detection depth. These coils can be useful to find Caches easily.
Now if the site you're in you have a really hard time finding a signal or there's a possibility of finding a cache, get the 18" coil.
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Keep in mind that coils other than the 10.5" stock coil will give false depth readings, but they're usually 2"-4"+ off. I don't know how innacurate, maybe someone can chip in on this one, maybe Ralph from Sunray electronics can answer how 'off' are the reading. The only thing I know is that smaller coils will give a shallower reading, I don't know about bigger coils though. Hope this helps.

