The GTI 1500's all-metal mode is really just another discrimination set up. It's benefit is that it tends to knock out much of the noise and clutter response of a genuine all metal mode, while still responding to all targets beneath the coil.
In the Garrett line, only the 2500 and the CX Plus currently have genuine, non motion, all-metal operating modes. Instead, the others offer what is really a ZERO discrimination mode. I employ it when I want to determine how much iron is beneath the surface.
Iron can be a real monster, masking other targets so you can't detect them. It is far more common than you may think.
It can especially play havoc with the Garrett's, when you insist on running the SENS any where near the top end of the scale. By switching to ZERO discriminate, you get a tone or ID indication and so learn real quick if you are experiencing iron masking.
So what's the big deal about the other TRUE, all-metal mode? Simply put, it will detect everything beneath the coil, even doing so without motion of the coil. It is most commonly understood as a pinpoint mode by most folks unaccustomed to using it. Go to pinpoint on your 1500. You'll experience true, non motion all-metal operation that way.
Discrimination, on the other hand, was first designed to eliminate the effects of ground minerals. Then came iron discernment. Only later did discrimination become associated with ID'ing every pull tab on the planet. Before that, detectorists were mostly using all metal, period. We're talking the 60's, here.
In Steve's excellent response, he indicated the impracticality of hunting in true ALL METAL. Those early detectorists had it tough, picking up every sort of metal in the ground. There wasn't as much non-ferrous trash in those days, though, and so the rewards were often worth the all-metal hassles.
40 years later, things are a bit different. Unless on a clear field, deep woods site or wide beach with few targets, it is a near fruitless endeavor to hunt that way. In all the usual places we hunt, there is just too much common trash mixed with good targets to really be effective in all metal. Well, unless you have the patience of Job, that is. Most of us don't.
It is the deepest mode available, generally, with the trade off is your instrument responds to everything good or bad, deep or shallow. In practice, some discrimination is almost mandatory.
So why bother with all-metal? There are some times where the benefits of the all-metal function that cannot be denied.
One of these is to help discover old sites in remote locations. When you aren't sure just where a site may be, go to all metal and hunt until you start to find iron. Then switch to some discrimination. Iron is the most common trash item left by man, especially in the past, and it can be used to both identify and date a site. In England, Roman sites can be distinguished from later use sites simply by the types of iron refuse found.
Some other uses for all-metal are:
- Hunting at the sandy beach, where good targets may be at the extreme limits of discriminate detection.
- Hunting when a site is devoid of surface targets... and all that may remain are the deepest of the deep targets.
- Hunting so as to unearth every possible item from the soil at any location. This "scorched earth" method is masochistic at most common sites, but it is thorough.
95% of the time, the real trick is to use just enough discrimination, or the right combinations of it, for whatever site you are hunting. This includes Garrett's ZERO discriminate mode.
It's also the likeliest reason Garrett took non-motion, all metal modes off all but their two top-of-the-line models. Garrett seems to have deemed true, non motion all-metal to be unneeded by the rank and file user, and so leave it off their other instruments.
