gold nugget simulation?

[Ferric Toes]

What does your meter read for a 1inch square and a 1/2inch square of regular strength aluminum foil? Would or does heavy duty read the same?

T-C = 0.072 x D x T x %IACS Skippy's formula reply#165. If TC is proportional to D and T and multiplier doesn't change, they should read the same?

That is an interesting experiment. I used 1" square pieces of regular foil (0.02mm). One square is too low to read. Two squares reads 1.8..............10 squares reads 25.00.........15 squares reads 38.0, and from then on it seems to be invariant. I have all the intermediate numbers up to 20 squares which is 0.4mm total thickness. Need to think about it more, but suspect it relates to the skin depth at 250kHz. (0.3mm at 15 squares).

Eric.

[green]

The 3.5mm and 5mm targets(around 2mm thick) I've tried with lead and solder calculate a multiplier around .03 instead of around .06 that I was expecting. Tried a test with AWG12 to see what the multiplier calculates. 2mm length calculates .023 for the multiplier. Including chart nugget simulation_3 where targets calculate .065 for multiplier. Same as before the 10 and 20mm length have the same TC. Tried vertical and horizontal, same slope. A coin on edge has a different slope than one laying flat, why doesn't the wire? Getting more questions than answers.

[Skippy]

Those formula(e) were based on thin test targets, backed up by coin tests, also pretty thin relatively. I suppose it's likely that when thickness gets to half the diameter, the circulating currents have other directions to flow in ... I'm guessing resistance goes down as thickness increases, but not as much as the simple model assumes. So R is bigger than expected, hence ( L / R) = time-constant, is smaller.
I'm sure I can get hold of some larger bits of ingot 60/40 solder, used in electric melting pots for 'dipping' wire-tinning. That would probably be a good material for testing thickness-related stuff. One for the other threads.

My unfinished 'nuggets' include some quite round ones, I'll try and test them before and after finishing. Attached picture of the little fellows:

[Carl-NC]

Re-reading some of this thread... I missed this before:

* Though Carl M. never backed up his reasons for using lead shot and solder blobs (he should've), I felt he was close, based on my tests with lead-free solder, and the fact he should know what he's doing ....

I use lead & solder purely for convenience with a splash of "close enough." Lead shot is easy to get and standardized, plus I can get 100 targets from 1 shell. Solder is easy to customize to any size/shape I want. Like Eric, I also use 1" foil squares with incrementing thicknesses to cover a broad range of taus.

As Carl has mentioned before:

Response conductivity is due to target metal type.
Response inductivity is due to target thickness.
Response strength is due to target surface area.

This is overly simplistic when dealing with metal detectors. Target tau is determined by skin depth, and if you look at the skin depth equation it has only conductivity and frequency (assuming rel. permeability is ~1), no mention of surface area. I believe this is valid when the target is much larger than the incident field, which is usually the case in non-destructive testing where the probe is small and the test target is usually large. In metal detectors the target is much smaller than the incident field so I assume there is a strong edge effect that makes target tau very dependent on target size. My books on NDT don't mention this and I've not run across any research articles that discuss it, so I am just beginning to look more in-depth at this.

All-in-all, I don't consider standardized targets to be all that complicated. Lead shot isn't 1-for-1 the same as gold nuggets, so that an 8-1/2 piece of shot (1-grain) isn't really equivalent to a 1-grain gold nugget. Besides, as Eric pointed out, gold nuggets vary widely, depending on exact alloy; there is no way to standardize on a "1 grain gold nugget." So what you do is simply pick a standardized target, like a piece of #6 shot. This is 2 grains, which might closely resemble gold nuggets in the 1-1.5 grain range. Plus several more larger sizes of shot. Then consistently use the same targets for all tests.

Besides using lead & solder, I also have a variety of real gold nuggets, ranging from sub-grain to 1 gram. Some found, some bought on eBay. Some nuggets are noticeably harder to detect than smaller pieces which again suggests that trying to create perfect equivalences is both futile and unnecessary.

[Skippy]

Thanks for that reply ,Carl. So largely based on experimental testing, which is good. Solder certainly is convenient, lots easier than sawing up coins.

We're not trying to create 'perfect' equivalents, just not unrealistic rubbish ones. And ones that can actually be made/reproduced by others, without too much effort/expense.
[ you know how Tom Dankowski is fond of using rare, collectible, expensive US 1 Dollar gold coins as a test target .... that was my reasoning for my serious attempt at making the 'replica' dollars, in the post I linked to earlier. Test targets should cost 5 dollars maximum, preferably under 1 dollar, as you're going to want to bury a few of them at different depths, locations, maybe flat-on & on-edge, maybe near iron, plus an airtest one, and a spare.]

I will find the original photo of those dollar replicas and post it here. The 'Imageshack' link on Tom's forum thread is now dead.

[green]

Re-reading some of this thread... I missed this before:



I use lead & solder purely for convenience with a splash of "close enough." Lead shot is easy to get and standardized, plus I can get 100 targets from 1 shell. Solder is easy to customize to any size/shape I want. Like Eric, I also use 1" foil squares with incrementing thicknesses to cover a broad range of taus.

This is overly simplistic when dealing with metal detectors. Target tau is determined by skin depth, and if you look at the skin depth equation it has only conductivity and frequency (assuming rel. permeability is ~1), no mention of surface area. I believe this is valid when the target is much larger than the incident field, which is usually the case in non-destructive testing where the probe is small and the test target is usually large. In metal detectors the target is much smaller than the incident field so I assume there is a strong edge effect that makes target tau very dependent on target size. My books on NDT don't mention this and I've not run across any research articles that discuss it, so I am just beginning to look more in-depth at this.

All-in-all, I don't consider standardized targets to be all that complicated. Lead shot isn't 1-for-1 the same as gold nuggets, so that an 8-1/2 piece of shot (1-grain) isn't really equivalent to a 1-grain gold nugget. Besides, as Eric pointed out, gold nuggets vary widely, depending on exact alloy; there is no way to standardize on a "1 grain gold nugget." So what you do is simply pick a standardized target, like a piece of #6 shot. This is 2 grains, which might closely resemble gold nuggets in the 1-1.5 grain range. Plus several more larger sizes of shot. Then consistently use the same targets for all tests.

Besides using lead & solder, I also have a variety of real gold nuggets, ranging from sub-grain to 1 gram. Some found, some bought on eBay. Some nuggets are noticeably harder to detect than smaller pieces which again suggests that trying to create perfect equivalences is both futile and unnecessary.

Thanks for the reply

Target tau is determined by skin depth

What is your definition of skin depth? (Search)depth where current is 37% of surface current. I'm thinking tau is determined by target thickness. A 1x1inch piece of aluminum foil has a tau of about 1.8usec, 2 layers about 3.6usec. Did skin depth or thickness or both double tau? I'm always getting definition mixed when discussing PI's. A 9x9inch piece of aluminum foil has a tau 9times a 1x1inch piece measured with a 1.5 inch coil.

I wonder how easy it is for everyone to get #4 bird shot. Thanks again for reply #142, A PI should detect #4 bird shot at 2 to 3 inches with a 8inch coil. Still doesn't matter what targets we come up with if no reference, you have a reference. Other than #4 bird shot I don't, maybe I haven't searched enough. So what you do is simply pick a standardized target, like a piece of #6 shot. This is 2 grains, which might closely resemble gold nuggets in the 1-1.5 grain range.(from your reply)proves I haven't paid enough attention now I have 2 references? What distance should a good PI detect the #6 shot, what size coil?

A lot of this is learning something. Standard targets with reference detection distance is needed to determine if detector is functioning properly. My thoughts.

[Skippy]

I'm getting the feeling that pure lead is probably not going to be our 'metal of choice' for nugget replicas, something higher in conductivity is going to end up being more realistic, though that's the purpose of making things and testing them - find out what works. Plus, there's no harm in having two series of dummies, a low-conductivity series (eg. lead) and a higher conductivity series ( ?metal) .

I would have to think a bit if I wanted to get hold of lead shot here in the U.K. I don't believe there's a 'gun shop' anywhere near me, and I wouldn't be legally allowed to purchase ammo anyway.
Is lead still commonly used? I heard that a bismuth alloy was now commonplace, as it's more environmentally friendly? [ I know there's a fun science experiment you can do, levitating bismuth magnetically, it works because it's diamagnetic. Melted down shot is a common source of the metal.]

I have just found some thin lead sheet, it was adhesive-backed originally, for adding weight to golf clubs. And some thicker strip that's for making decorative 'leaded glass' windows. All my other lead sheet is thick, 1.6mm or 1/16th inch approx.

[Carl-NC]

Target tau is determined by skin depth

What is your definition of skin depth? (Search)depth where current is 37% of surface current. I'm thinking tau is determined by target thickness. A 1x1inch piece of aluminum foil has a tau of about 1.8usec, 2 layers about 3.6usec. Did skin depth or thickness or both double tau? I'm always getting definition mixed when discussing PI's. A 9x9inch piece of aluminum foil has a tau 9times a 1x1inch piece measured with a 1.5 inch coil.

Yes, skin depth is the eddy current 37% decay point. δ = 1/sqrt(σµπf) is the standard equation for skin depth and includes no size dependency. Yet you already see that size matters. With the 1x1 foil, a 2x thickness ~doubles tau, skin depth is the same.

I wonder how easy it is for everyone to get #4 bird shot. Thanks again for reply #142, A PI should detect #4 bird shot at 2 to 3 inches with a 8inch coil. Still doesn't matter what targets we come up with if no reference, you have a reference. Other than #4 bird shot I don't, maybe I haven't searched enough. So what you do is simply pick a standardized target, like a piece of #6 shot. This is 2 grains, which might closely resemble gold nuggets in the 1-1.5 grain range.(from your reply)proves I haven't paid enough attention now I have 2 references? What distance should a good PI detect the #6 shot, what size coil?

What is a "good" PI detector? SMPI? TDI? GPX4500? And, yes, what coil? It's gonna vary a fair amount, and offhand I don't have any hard numbers.

I'm getting the feeling that pure lead is probably not going to be our 'metal of choice' for nugget replicas, something higher in conductivity is going to end up being more realistic, though that's the purpose of making things and testing them - find out what works. Plus, there's no harm in having two series of dummies, a low-conductivity series (eg. lead) and a higher conductivity series ( ?metal) .

The actual metal doesn't matter much. You can create targets that mimic the tau and depth of gold nuggets with lead, copper, aluminum, silver, etc. I'd pick something that's easy to deal with. Lead shot is already spherical and roughly the sizes you want. Lead solder can make any size you want. Another alternative is split shot used for fishing; I don't like it as much because it's physically split, but either squeeze it together or slice it in half and it works just fine. Aluminum foil works well, too. I use not only 1x1 targets but also 10mm and 5mm squares. They can be stacked in thickess to emulate larger nuggets.

[green]

Yes, skin depth is the eddy current 37% decay point. δ = 1/sqrt(σµπf) is the standard equation for skin depth and includes no size dependency. Yet you already see that size matters. With the 1x1 foil, a 2x thickness ~doubles tau, skin depth is the same. I'm missing how Target tau is determined by skin depth if doubling thickness doubles tau and skin depth is the same.



What is a "good" PI detector? SMPI? TDI? GPX4500? And, yes, what coil? It's gonna vary a fair amount, and offhand I don't have any hard numbers.[/FONT][/FONT]

[/LEFT] I still like trying a 19mm(3/4inch)piece of copper wire, AWG22, AWG19 andAWG16. Tau doesn't change with an error in cutting length. Easy target to make. Some may have already, if not I have some and would be happy to mail them. Anyone interested I would be interested in detection depth, detector used, coil size and description(settings used, ?). Thinking of asking at a couple other forums. Not sure AWG22, 19, and 16 are the best sizes to use. Expect to see some differences, detector used, S/N being some of the causes.


The actual metal doesn't matter much. You can create targets that mimic the tau and depth of gold nuggets with lead, copper, aluminum, silver, etc. I'd pick something that's easy to deal with. Lead shot is already spherical and roughly the sizes you want. Lead solder can make any size you want. Another alternative is split shot used for fishing; I don't like it as much because it's physically split, but either squeeze it together or slice it in half and it works just fine. Aluminum foil works well, too. I use not only 1x1 targets but also 10mm and 5mm squares. They can be stacked in thickess to emulate larger nuggets.

Think cutting a piece of wire 19mm long would be easier and more repeatable than other targets. Learning something playing with Skippy's targets. Mostly I know less than I was thinking I knew. Plan on more testing.
.

[Ferric Toes]

Think cutting a piece of wire 19mm long would be easier and more repeatable than other targets. Learning something playing with Skippy's targets. Mostly I know less than I was thinking I knew. Plan on more testing.
.

Here is some more testing. This time on lead tape 0.19mm thick and 25.4 x 25.4mm square. %IACS readings are - 1off no reading, 2off 2.2, 3off 3.85, 4off 5.06, 5off 5.51, 6off 5.51 and so on for more squares. Total thickness for 5 squares = 0.95mm

Next is copper tape; solid type 0.07mm thick and 25.4 x 25.4 mm square. %IACS readings are - 1off 7.75, 2off 27.5, 3off 46.8, 4off 56.9, 5off 57.0 and so on. Total thickness for 5 squares = 0.35mm

Note as long as tape sample covers the sensor end, the area does not matter i.e. 0.5" square will measure exactly the same as a 1", a 2", or a 6" or more.

Eric.

[green]

Here is some more testing. This time on lead tape 0.19mm thick and 25.4 x 25.4mm square. %IACS readings are - 1off no reading, 2off 2.2, 3off 3.85, 4off 5.06, 5off 5.51, 6off 5.51 and so on for more squares. Total thickness for 5 squares = 0.95mm

Next is copper tape; solid type 0.07mm thick and 25.4 x 25.4 mm square. %IACS readings are - 1off 7.75, 2off 27.5, 3off 46.8, 4off 56.9, 5off 57.0 and so on. Total thickness for 5 squares = 0.35mm

Note as long as tape sample covers the sensor end, the area does not matter i.e. 0.5" square will measure exactly the same as a 1", a 2", or a 6" or more.

Eric.

Interesting. Looks to me %IACS readings are proportional to target thickness(approximate)if target thickness<2*skin thickness. Wonder how skin depth relates to a PI, would 100kHz relate to sampling the decay at 10usec? Does any of this make sense?

[Carl-NC]

Note as long as tape sample covers the sensor end, the area does not matter i.e. 0.5" square will measure exactly the same as a 1", a 2", or a 6" or more.

Oh good, that's what I expected. Somewhere I have an AutoSigma, just need to find it.

[Carl-NC]

Interesting. Looks to me %IACS readings are proportional to target thickness(approximate)if target thickness<2*skin thickness. Wonder how skin depth relates to a PI, would 100kHz relate to sampling the decay at 10usec? Does any of this make sense?

Yes, the very early part of the exponential slope is fairly linear-looking so it appears as if readings are proportional to thickness.

Target tau is directly related to classical sinusoidal phase angle by τ = tan(Φ)/(2πf). That is, if you hit a target with a 10kHz sinusoid and you see a phase response of 50°, then the tau of that target is about 19us. The 50° would be the phase response reported by a VLF detector, though it's usually scaled to some other range, like 0-100.

Oops, gotta run, more later...

[Carl-NC]

Continuing...

Phase is related to skin depth by



BTW, this phase does not include the extra 90° that is generated by the EMF, it is the EMF-to-eddy phase shift. A detailed look at everything that is going on would a very long discussion.

With some substitution, the relationship between tau & skin depth is



It appears that tau is dependent on frequency but there is also a frequency term embedded in δ, so the end result is that tau is independent of f.

[Skippy]

Post #208 is how I'm determining target time-constant with my VLF, a 13kHz Fisher F75. I've calibrated the VDI, so that it's pretty accurate for most targets, and for items very low-down the conductivity scale ( 15 - 20 VDI ) I can still do direct internal I and Q demod signal measurements.

I had plans to investigate Skin Depth at some point, I was contemplating using finite-element methods for the modelling, as any maths involving Bessel Functions is best avoided.
Here's a wiki page on the effect:
https://en.wikipedia.org/wiki/Skin_effect