Steve,
I just needed to maximise my time in the water, especially when perfect water hunting conditions don't last long. The amount of bottlecaps was taking up too much of my precious time and energy....these were the main culprits to my PI and some of them do sink down deep and then lose their iron "harsh audio".....the shallow ones I can usually ID okay.
Also, I've done extensive tests (depthwise) between a friend's Excalibur and my high-end PI and to tell you the truth, the depth performance was just about the same and on many gold rings was neck and neck.
I placed items in lengths of PVC pipe (one end capped) and then filled them with dry sands, wet sands....the pipes were then buried into the beach sand perfectly flush with the top of the surface. I then detected over them, changing between detectors, different lengths of pipe, dry sand, wet sand....and so on. I know myself that the tests were a good indication of the detector's ability. Tests on another beach may produce different results but on my local beaches the results were an eye-opener to me. The PI may pull in front on items that have been buried for a while (halos, etc), but most of my summer finds are recent drops. During winter and after the iron targets have disintegrated to nothing (and are not being replenished) then I may reach for the PI only if I'm satisfied that their is a depth advantage.
Again, all of this if for my local conditions and personal experiences.
I know the Excal will do the job as well as the PI even in moderate black sands.
Regards,
Tony.