In a standard round coil, your signal, a magnetic field, going down (and up) from the coil during the TX time is shaped like an ice cream cone. In a Double D, it is shaped like a broad axe head. Both have the tiny end down (or up) at the furthest point from the coil. Your targets are "lit" by this field and develop a tiny magnetic field of their own based on the type of metal and the shape. In ferrous metals, the field is built up inside the object which turns it into a tiny magnetized item for a millisecond or two. In non ferrous metals, the field is built up on the surface in the form of eddy currents which also act like a magnet. The ferrous items have a larger field with fuzzy edges while the non ferrous items have a sharper, more defined image shape.
Now to the guts of the question. The sensitivity controls the size of the received signal's first stages of amplification. The higher the sensitivity, the greater the field sensed signal amplification. Imagine that your receiver section is an eye. When you turn it down, it has the effect of narrowing the field of view on your coil. This allows you to "see" an object with much greater clarity without losing too much depth. You don't lose the depth because your receiver has no such limitation of "shape" in its ability to receive a signal. It doesn't care if the signal came from Mars. It will still indicate a target as long as the target's field is strong enough to reach the RX coil.
The discrimination takes a look at the target signal's shape, return strength and frequency shift (if any) to determine what the most likely metal or shape of the object being sensed. No matter how good your discrimination is, machines are only right around 75% of the time. Worse yet, when you filter out the non ferrous items of aluminum, you also filter out the smaller gold and silver items. Not finding nickels? They are notched out with pull tabs, as are small gold and silver rings. If you're not finding too many of those, this is why.
Your sensitivity setting can be helpful in screening out the noise from water in the soil, plant growth, black sand and variety of other things in the ground which create a symphony in your headset. Keeping it reduced a bit and cranking it up for those tough to pinpoint items is what I suggest. Get the extraneous noises to go away and you will be better able to pick out targets from the quagmire down there. Use minimum discrimination or none at all. I know it can be a pain to dive down and recover every nail, hairpin and pop top out there but remember one thing. When you dig those out of the way and check again, you will occasionally find a really valuable item. Us old guys know this from experience. The younger crowd reading this learns it the painless way.
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« Last Edit: February 11, 2011, 02:08:10 pm by GoldDigger1950 »
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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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