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Discriminating And The Drag Down Effect


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When it comes to disc setting we all have our preferred settings and we all carry on swinging the coil all day long without a care in the world, but after reading the Nokta fors gold thread, I got to wondering how many people adjust their Disc setting to allow for the Draw down effect caused buy hot ground, Most just get lazy and grab a rusty nail and wave it frantically over the coil and twist the knob til it sounds like a Goose farting in the fog and off we go til lunch time moaning to our mate that we got nothing, So then I got thinking about using the target we are after IE a rust stain Nugget in hot ground, So if we are lucky enough to find a small rust stained nugget then that would be the perfect item to use to tune our Disc settings with,

As Steve pointed out on the other thread this draw down effect can be as much as minus 20 to 30 on the VDI on a machine like the MXTs, Using the standard settings in the relic mode around 2.8-2.9 the MXT will knock out most junk but when using the relic mode for prospecting the normal thing to do is set the machine up so it just knocks out nails which is at around "2" on the dial but even with rust coated nuggets and hot ground this might be way to high,

I started testing and making notes of the where the dial starts to take effect and also what happens when you back it Off, The first thing I noticed was if you use a very rusted decaying 2" nail and by starting at disc Zero and turn it up to about 1.9 on the dial and at that setting the MXT will make Goose FF sounds with a VDI reading down from -10 ish down to -43. Now Once you have it set to that if you want to make that nail now make a High tone you will have to turn the Disc right down to 1.5 which will allow VDI reading of around the high -40s although you had to take it up to 1.9-2.0 to get it to make the low tone so Add the draw down of the ground effect and that the nuggets might be iron stained means that you have moved your chance of ever finding that nugget even further away,, There is a reason for the Iron probability meter on the GMT and in the manual they tell you to dig targets that read in to the 80%+ iron readings, Because if that nail can give you a reading of -25 add on to that That the ground can drag down the VDI reading by -30 or more that will give you a total reading of -55 yet you have set you disc to knock out nails that come between -25 to -40 then that would be Nugget is -15 below your Disc level which means you won't dig or hear it,

This brings to mind a mate in OZ who went prospecting in the centre of OZ in an area where 5K was having fits because of the junk, Anyway he took his MXT there to sort through the Iron and the hot rocks using it in the prospect mode In Tracking he was getting GND readings of 95 which is about as high as it goes and Yes he did find Gold, Although a Pi might of been the better choice which means digging the junk as well.

Good luck, john
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