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Frequency More About Target Size, Than Type Of Metal


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#8 1 grain lead shot air tested on my 14" DD coil at 1" in Cancel/Normal.

#9 .75 grain lead shot was touching the left edge of the 14" DD in Cancel/Normal.

#8 1 grain lead shot air tested on my 11" mono coil at 1.5" in Normal.

#9 .75 grain lead shot air tested on my 11" mono coil at .5" in Normal.

Sensitivity was on 5 due to EMI. Results might be better on 10 or in Auto Plus (12)

I don't have any #8 or #9 steel shot to test.

Green, I also don't want to keep hijacking this VLF thread with PI GPX 6000 test results.

Start a new thread if you want.

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Similar results to what I saw Jeff, although the #9 lead was within 1 cm of the coil, which is virtually having to touch the coil to get a good response over the noisy threshold, if that and that was in manual 10 and auto+.  I think it'd be very rare you'd find a #9 lead pellet with the GPX unless you're a real coil scrubber with very flat ground with no stones or grass to lift the coil off the ground.  It'd have to be a fresh landed shot as it doesn't take much for a pellet to sink down 1cm, however in NZ most modern shot is steel which gives a much better response than lead, the lead shot are the older pellets and they tend to be in the soil, not on top of it. 

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On 1/21/2022 at 4:45 AM, Steve Herschbach said:

Maybe more going on there I think than we know about, like 5 kHz not optimal as a multi pick? Both Nokta and XP picked 4 kHz. EMI?

4 to 40 in multi is becoming a thing.

Interesting you suggest EMI.  The "L" was VERY quite when I spent an entire day with it detecting various sites and parks in San Francisco.  One spot was specifically selected due to it's high concentration of EMI which forces one to drop the sensitivity on the EQX to the mid teens for any kind of stability, but you loose the depth required to get to silver coins.  The day I tested the EQX, I had to reduce its sensitivity to 17 to be usable, whereas the L could run at 28-30 and remain stable,  quite even, sometimes to the point of "tap tap is this thing on?" moments and I'd run it over the eyelets in my boots to make sure it was running, and it always was 🙂

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On 1/21/2022 at 4:34 PM, PimentoUK said:

Another non-intuitive target is the long-and-thin type. Such items include wire ( copper or brass, typically ), plain finger-rings that are broken ( C-shape ), copper/bronze/aluminium nails.

Metal detectors induce circulating electrical currents in the target that tend to form loops as large as they can, without straying too far from 'circular'. So for a wire, these current loops are little more than the wire diameter in size, they don't go up-and-down the length of the wire at all. There are many of these loops along the wire, so they all contribute to giving a stronger response to the detector, but the 'target frequency' is still that of a very short snippet of wire, no more than twice the diameter.
So wires tend to have very a high 'target frequency' , that's independant of their length. This can make them hard to detect.
An extreme example : among my electronics junk, I have some test coils that are much like the windings inside a detector search-coil. 100 turns of 0.2mm enamelled copper wire, over 20 grams of highly-conductive metal in total. But place one 5mm from a detector coil, it is invisible. The target freq is about 2.5 MHz, so it's not going to match any commercial machine.

A few years ago at a San Francisco park scrape, I got a seriously wonky signal on my Racer2.  It was big, not a can big but spread out, trying to size it was difficult, it sounded good'ish but it was seriously funky.  I dug the target to uncover a piece of larger gauge copper wire probably deposited from the 1906 Earthquake refugee camp times.  Imagine a wire coat hanger unraveled and twisted into a distorted quasi figure eight shape.   As I pulled it out, I was like, no wonder the signal was so funky.   Imagine my surprise when I re-scanned where I'd just pulled the wire from and got a banging 90's signal that turned out to be a Barber quarter.  That was a great park scrape.

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On 1/21/2022 at 5:45 AM, Steve Herschbach said:

Maybe more going on there I think than we know about, like 5 kHz not optimal as a multi pick? Both Nokta and XP picked 4 kHz. EMI?

4 to 40 in multi is becoming a thing.

Sounds like a good candidate for Nox update 4.0!

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Time constants for some targets I have. Target frequency=160kHz/us TC, 2us TC=80kHz.

Expected time constants for gold chain?  lead shot, gold nuggets and gold chain

gold targets.jpg

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Wondering how a VLF detector VDI indicator works. If I compare the decay time constant for a piece of regular strength aluminum foil 10mm square and a piece of #9 lead shot with a PI detector there isn't any difference. Does a VLF detector show the difference?

 

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