Target ID More About Target Size, Than Type Of Metal

[PimentoUK]

Quote: "I find quite a bit of copper wire -- single stranded.  The gauge is in the 12-16 range.  They typically hit in the USA zinc penny and aluminum screw cap range."

Assuming that's American Wire Gauge, AWG, that is 2.05mm to 1.29mm. Mathematics tells me that would be the target frequency range 13 kHz to 34 kHz, respectively. On the ID scale, 13 kHz is above US 5c coins ( 16.5 kHz) but way below 'zinc cent' ( 5 kHz ) , and 34 kHz is down in the foil zone . So I'm unsure why you're getting your wire come in at '5 kHz'.

[strick]

On 1/22/2022 at 8:30 AM, green said:

Interesting topic. I sent some targets to someone that has 6000 and 7000 detectors. Targets included #4, #6, #8 and #9 lead shot. Both detectors could detect #4 and #6 shot not #8 or #9 shot. Guessing coil size might be some of the reason. Is being able to detect #9 shot important, if so at what distance? 

The 6000 can easily pick out gold the size  #8 bird shot at 2-3  inches deep... In my opinion the 7000 can as well but it sucks for this type of gold as it will tune out the gold or it double blips on it...its not a clear response like the 6000. Were talking stock coils here...The 6000 is a plug and play type detector. 

gold the size of #9 birdshot is dig-able as well but listen carefully lol 

The gpx 6000 is an amazing machine in it's capability of finding small sized gold...I liken it to a long range sniper rifle with carefully hand loaded rounds. But ask yourself before you buy... do you really want to find gold the size of fly poop? 

strick

[GB_Amateur]

5 hours ago, PimentoUK said:

Quote: "I find quite a bit of copper wire -- single stranded.  The gauge is in the 12-16 range.  They typically hit in the USA zinc penny and aluminum screw cap range."

Assuming that's American Wire Gauge, AWG, that is 2.05mm to 1.29mm. Mathematics tells me that would be the target frequency range 13 kHz to 34 kHz, respectively. On the ID scale, 13 kHz is above US 5c coins ( 16.5 kHz) but way below 'zinc cent' ( 5 kHz ) , and 34 kHz is down in the foil zone . So I'm unsure why you're getting your wire come in at '5 kHz'.

Got my Equinox set up on the bench for some coin dTID testing so I grabbed three pieces of (dug) scrap copper wire, each about 3 inches or so in length but in various orientations (~2.0 mm dia. w/hook half loop on one end, ~2.2 mm dia. straight, ~3.2 mm dia. L-shape).  The dTID readings on the Eqx were a bit broad but approximately 14-15, 15-16, and 18-20 respectively in Park 1, recovery speed = 4, gain = 17, 11" coil.  Targets were swiped about 2.5-3.5 inch above the center of the coil.  (For reference, our USA 5 cent hits 12-13 and our modern zinc cents around 21, if uncorroded/undamaged.)  So maybe those numbers are more in line with what you expect?

The lengths of copper wire I find can be all over the place, from half inch to a couple feet.  Not surprisingly the in-field dTID's vary with length and shape, too.

[PimentoUK]

Ahh, better data, thanks.

Your measurements are in good agreement with the scientific tests we did ( on the Geotech1 forum ).

I calculate  ( with Eqx TID )

2.0mm : freq = 13.5 kHz, TID = 14

2.2mm : freq = 11.3 kHz, TID = 16

3.2mm : freq = 6.1 kHz, TID = 20

There's a bit of leeway allowed, I'm converting from F75 readings to Eqx ones, I never got round to 'calibrating' the Eqx ID's, in part because they covered a rather small useful range.

In case anyone's slightly interested, here is the maths:

We found the target time-constant was proportional to the cross-sectional area of the wire, and proportional to the electrical conductivity of the metal, and the result was this formula:

TC = 0.029 * D * D * %IACS

where TC = time-constant ( in microsecs ); D = wire diameter ( mm );  %IACS = metal conductivity.

Example: 2mm diameter copper, TC = 0.029 * 4 * 102 = 11.8 usec

To convert time-constant into target frequency use this:

target freq, f = 1/ ( 2* pi * TC ) with TC in seconds

using the above TC as an example, this gives 13.5 kHz

It's a good straight-line match for wires up to 2.5mm, then it starts to drift away a bit , due to skin effect; the full diameter of the wire is not all seen. PI machines don't see this so badly.

But regarding: "Not surprisingly the in-field dTID's vary with length and shape"

I think you may have missed the main point of my earlier post : TID does NOT vary with wire length, unless you have very short lengths. It can depend a bit on shape, irregular objects commonly give erratic ID's.

 

[GB_Amateur]

2 hours ago, PimentoUK said:

But regarding: "Not surprisingly the in-field dTID's vary with length and shape"

I think you may have missed the main point of my earlier post : TID does NOT vary with wire length, unless you have very short lengths. It can depend a bit on shape, irregular objects commonly give erratic ID's.

Ah, thanks for repeating that.  I just checked a wound up (not like a coil, though) piece of the ~2.0 mm version.  (You didn't expect me to just take your word for it, did ya?  😁)  I didn't measure the lenght but guessing 2-3 ft.  It was right at 14-15.  So the signal strength increases but the dTID stays the same.  Good to know; I'll try and remember better than I read.  😏

[Jeff McClendon]

On 1/22/2022 at 9:30 AM, green said:

Interesting topic. I sent some targets to someone that has 6000 and 7000 detectors. Targets included #4, #6, #8 and #9 lead shot. Both detectors could detect #4 and #6 shot not #8 or #9 shot. Guessing coil size might be some of the reason. Is being able to detect #9 shot important, if so at what distance? 

Lead #8 and #9 birdshot is easily detected with the GPX 6000. I don’t own a 7000 so can’t speak about that. I also have no experience with modern #8 steel birdshot.

My GPX 6000 will detect #8 lead birdshot with its DD coil too. So, .1 gram to .05 gram gold is no problem for the GPX 6000 or the Equinox 800 using its gold modes if it is within 2” to 3” of the surface.

[phrunt]

The GPZ 7000 for me can't detect #9 shot with the standard coil, and can only just detect it near touching the coil when using a smaller coil like an 8" Coil.  In a real world situation you wouldn't find much #9 shot, although it will detect it with the smaller coils.

Here is how the GPZ responds on a number #4 and #6 lead shot with the standard coil, the GPX 6000 with 11" coil easily beats this.  This is with the GPZ settings maxed out which I show in the video.

This is how the GPZ responds on the same lead shot pellets when using a much smaller 8" coil, the difference is very noticeable.  I've tested every coil I own in the same way and generally the smaller the coil the more sensitive to the smaller pellets, the exception is concentric coils where a 15" coil is very competitive with a 10" coil on small pellets and I believe this is because it has a central receive winding that's quite small giving it good sensitivity.  Modern steel shot gives a much better response on every coil than this lead shot does.

[PimentoUK]

Quote: "So the signal strength increases but the dTID stays the same.  Good to know"

Maybe I should add some pictures to that post, to make it less 'wordy'.

If you feel experimental, try testing a 10mm ( 3/8" ) length of wire. Same result, I hope. The test samples we used were 19.0mm ( 3/4" ) long, for consistency. When the wire gets thin, it's not easy getting a strong enough signal to measure properly. So two alternative approaches were tried : multiple 19mm lengths, or a single longer length, both worked fine.

Here's an example of a real-world target giving the 'wire' problem:

this is the fixing pin off a Roman bow brooch, made of bronze, found by a guy on a UK forum. He saw it sticking out the soil, ran his French stick over it, to be met with silence. Only after some fiddling with the settings and pretty much scraping the coil over it did he get a poor beep, with a super-low ID value. The two problems here: the diameter ( about 1.7mm estimate ) and the metal : bronze, AND corroded. Now if it were nice pure copper, it could ID in the 5c 'nickel' range. But the unknown metal conductivity is going to be anywhere from 1/5th to 1/20th of that. This pushes up the target freq to 100 kHz, 200+ kHz, so even an 18kHz machine will struggle to see it, in motion-mode, anyway.

[green]

15 hours ago, Jeff McClendon said:

Lead #8 and #9 birdshot is easily detected with the GPX 6000. I don’t own a 7000 so can’t speak about that. I also have no experience with modern #8 steel birdshot.

My GPX 6000 will detect #8 lead birdshot with its DD coil too. So, .1 gram to .05 gram gold is no problem for the GPX 6000 or the Equinox 800 using its gold modes if it is within 2” to 3” of the surface.

Any thoughts, why some GPX 6000's can detect #9 lead shot and others not? What detection distance are you getting for #9 lead shot with your GPX 6000?

[phrunt]

I've only had access to one GPX 6000 and it only detected my #9 lead shot on a stick when it was touching the coil and then only just and only on certain locations of the coil on the left side from memory.  The pellet fell off the stick and was lost during this testing so unfortunately it ended there and I was unable to do the recorded video test like I've done with other coils.  I need to find another #9 lead shot and start again with every coil as I don't trust all lead shot to react the same.  The ones on a stick were sent to me by Geotech for testing purposes so I was very disappointed to lose the #9 off the stick.