Hi rjw4law and Richard,
Not bad at all Richard, let me try filling in some detail.
I use the 75 for prospecting, that means using the all-metal motion mode a good deal of the time. The ground mineral in some areas can be both highly mineralized, and also demonstrate considerable variability.
High ground mineralization: As ground mineralization increases, the "window" of ground balance tolerance decreases; that's to say the GB must be kept very close to true ground balance compared to more lightly mineralized areas for best performance. There is less "forgiveness" from very high mineral ground in terms of staying properly balanced.
As you know, when we ground balance the unit, we balance to a certain height above the ground. In high iron mineral especially, lifting or lowering the coil beyond that GB height can cause the detector threshold to increase or decrease in sound. An example would be the spurious ground noises resulting from searching uneven rocky areas where the coil is constantly being lifted and lowered, or coming into close contact with many rocks containing various levels of iron mineralization. A fast threshold retune speed in conjunction with "autotracking" is a good way to reduce these distractions.
Variable mineralization: In some locals the ground balance can increase or decrease dramatically within a few yards. An example would be coming into contact with negative hot rocks; such rocks, for example magnetite, can demonstrate much higher GB comp points than the general terrain. On the other hand, coming into an area with "rusty" or weathered iron mineralized rocks can cause the GB to drop immediately by as much as 10 to 15 full units. Fast autotracking can ensure the unit stays balanced as much as possible under these conditions, and again... in conjunction with fast threshold retune speed, autotracking can help limit "ground noise".
That's not to say that "locking" the auto GB switch into the forward position provides "autotracking" per se. I don't know what to call it. What I do know is that the unit can be adjusted way off ground balance, and then tested for example, on a piece of magnetite (with a GB of "89"). Manually set the GB to anything you like, say "50". Make one sweep across the magnetite and a return sweep; the ground balance will adjust that quick...the detector is perfectly balanced at 89. Regardless of whether it's called autotracking or not, I call that kind of performance good enough for me.
I hope the above explains why I was looking for a way to be able to keep the trigger switch locked in a forward position for prolonged time periods. Yes, you lose ID ability in that trigger-forward position, but once a target is located, simply pull the trigger into the centre position. This eliminates any chance of "tracking" into the target, while re-establishing visual target ID.
Jim.
