speaking of aluminum; conductivity

Hunting farm fields and pastures as I do, I am amazed at the TID range occupied by aluminum. It seems that aluminum of some size, thickness, degree of flatness or crumpled-ness, weathering, and shape can appear with practically any tone or TID your machine can spit out.
Which leads me to question how detecting machines measure conductivity, particularly as it relates to mass, ie, footprint x thickness. I want a machine that assigns a tight TID and tonal range to ALL aluminum, regardless of mass, orientation, and depth. Same for brass and lead.
How can I make good decisions about digging or walking when TIDs and target tones only tell me it could be almost anything. You see the trash the Hoover Boys dig to find their goodies? That's how I feel. I have numerous detectors and like some better than others. Some times. Some places.
But the most reliable feature any of them offers is to raise the coil and see which targets go silent at what height. That saves me digging more trash and missing less good targets than all of the other settings any machine offers. I think that's sad.

Case in point:
A four hour hunt of a small cut corn field Friday where V nickels, Barber quarters, and confederate bullets have been found. Using an AT Max and small coil, I found a 1936 Merc, a 1920 wheatie, a 1.5 inch square ladie's compact, and a long lost pocket knife (nothing left of the handles). And numerous shards of twisted metal, much of which was aluminum. The dime and penny had sweet sounds and tight pinpoints, but so did many of the trash items. Else, I wouldn't have dug most of them.

Edit: I meant to say something about the state of modulated audio development/offerings in detectors presently. I think the present offerings to we the consumers is pretty sad. Many detectors still can't communicate to us the relative depth or mass of a target. And those that do seem to do a poor job of it for the most part. Is it deep or is it small? is it shallow or is it large? I have a hard time accepting as fact, in this day and age, that a detector can't clue us in to both a target's size AND depth. How 'bout it, people?

What say you?

Wayne



Edited 1 time(s). Last edit at 11/04/2018 02:55PM by ncwayne.

Cant be done. Metal detectors have no idea what is below them. All you get is an indication of the electrical response of a target. That response is determined partly by the type of metal, but more about the size, mass, shape, surface condition and other factors. Every type of metal can respond anywhere on the detecting scale. The only reason we have a VDI scale is because coins and rings are generally a uniform size and (until you get to 2000 year old hammereds anyway).

This is why relic and ancient coin hunter don't really use a VDI except to try and fine tune how much iron response to try to weed out.

Yes I have always said the Analogs for the most part have more or better Modulated audio. And some like the CZs can be adjusted with the volume/ Audio boost control. With the exception of Minelabs Etrac/Explorer/ CTX which is also adjustable with volume gain setting.
But even non adjustable Analogs like Tesoros,Nautilus, and Whites MXT have great modulation in Audio.
Personally I won't use a Detector without it.
Noting like Hearing that soft sweet deep round type tone that You know is a keeper.

silverfiend Wrote:
-------------------------------------------------------
> .... The only reason we
> have a VDI scale is because coins and rings are ge
> nerally a uniform size ...


Good post. And the "uniform size" @ rings " that you allude to, would, at best, tell you it's ring/coin sized, at a certain TID. It is not going to tell you gold vs aluminum.

But when it comes to highly conductive coins, then yes: EVERY SINGLE USA dime and EVERY SINGLE USA quarter, etc.... Comes out of the mint at the exact same TID . Per its size and conductivity. The same can not be said of gold rings. They come off the printing press @ 10,000 different sizes, shapes, alloys, etc....

Tom_in_CA Wrote:
-------------------------------------------------------
> silverfiend Wrote:
> --------------------------------------------------
> -----
> > .... The only reason we
> > have a VDI scale is because coins and rings are
> ge
> > nerally a uniform size ...
>
>
> Good post. And the "uniform size" @ rings " that
> you allude to, would, at best, tell you it's ring/
> coin sized, at a certain TID. It is not going to
> tell you gold vs aluminum.
>
> But when it comes to highly conductive coins, then
> yes: EVERY SINGLE USA dime and EVERY SINGLE USA q
> uarter, etc.... Comes out of the mint at the exact
> same TID . Per its size and conductivity. The s
> ame can not be said of gold rings. They come off
> the printing press @ 10,000 different sizes, shape
> s, alloys, etc....


All newly minted USA quarters haven’t weigh the same always.

"How detecting machines measure conductivity, particularly as it relates to mass"
Mass/weight is irrelevant, detectors don't /can't measure that.
What they do measure is a combination of electrical resistance and inductance. Lead, silver, gold, aluminium all conduct electricity, so they all have resistance and inductance, so they all give a signal to a metal detector.
"I want a machine that assigns a tight TID and tonal range to ALL aluminum, regardless of mass, orientation, and depth."
I'm sure you've read enough threads on here about this subject, there's even one currently at the top of the recent list at the moment, titled 'Aluminum".
It can't be done.
"A four hour hunt of a small cut corn field ... "
Hunting cultivated land is a large part of European detecting. You dig everything non-ferrous. The "Hoover Boys" are doing it right, maybe they've hunted on this side of the Atlantic ...

Regarding the "small and shallow" vs "large and deep", it's likely that some improvements are possible. I'm sure there are distinguishable 'signatures' for different depth targets. But how useful is it to the guy holding the detector? If you knew it was shallow, you wouldn't need to dig a 8 inch diameter hole, like you may if you thought it was deep. And knowing at what depth to expect the target may help you avoid damaging it with ill-informed digging.
Have you tried holding the coil at an angle to the ground, particularly when in pinpoint mode? Tipping the coil obviously moves the sweetspot over, and by comparing the flat-on target location with a tilted-coil reading, preferably from both directions, it may give clues to depth. I've tried it, but not with much success, I admit.



Edited 4 time(s). Last edit at 11/04/2018 09:42PM by Pimento.

When I first started out detecting in the 1990's I had the Garrett GTI 2500. When it was carefully pin pointed, (de-tuned) over a target, then re-pin pointed at full strength the sizing feature actually worked pretty well. Size B were almost always a coin, even our Canadian crap rang in as coins, almost never fooled by crushed cans, small bits, etc as long as they were no deeper that approximately 6".
Unfortunately the machine had a lot of other shortcomings, ability to handle the ground without overloading being the worst.
I always wondered if that system could be improved upon in an updated, ergonomically friendly unit with a better ground balancing system. Too bad Garrett didn't pursue it.

[www.ncbi.nlm.nih.gov]

tnsharpshooter Wrote:
-------------------------------------------------------
>
>
> All newly minted USA quarters haven’t weigh the s
> ame always.


Perhaps. But ask yourself : How many of those, coming off the mint-press, share the same conductivity, on a size-per-size basis , as an aluminum object ? The answer is: Zero. The quarter, despite slight fluctuations, will share the spectrum zero times with aluminum objects, on a size-per-size basis.

But then ask yourself : If we were asked the same question of typically occurring park-style aluminum vs typically occurring gold ring, the answer would be "all of them" share the same TID , on a size-per-size basis

tnsharpshooter Wrote:
-------------------------------------------------------
> [www.ncbi.nlm.nih.gov]
> 38/


Interesting. But no bearing on the subject at hand here

Just some more speculations on shape-showing , blah blah. Nothing to do with identifying composition.

A quote from the link , that their machine will :

" roughly identify different metal shapes with varied widths "

This is nothing new . Various touted advertised machines now, claim to show shape even now. And cool 3d images of the "spike-depiction" of how it looks in the earth. And while your link might be an improvement on that, the devil will be in the details. The "devil in the details" is the pixel size (in whatever type "picture" they choose. Whether 2d, 3d, etc...). Nor does it address that a millions objects can share that same "spike" diagram. Nor will address to the topic of this thread, which, as I understand it, is gold vs aluminum

They even say themselves in your link :

" ...Also, we hope this metal detection system can be extended to identify a variety of metals with different geometries.... "

Tom_in_CA Wrote:
-------------------------------------------------------
> tnsharpshooter Wrote:
> --------------------------------------------------
> -----
> > [www.ncbi.nlm.nih.gov]
> 52
> > 38/
>
>
> Interesting. But no bearing on the subject at han
> d here
>
> Just some more speculations on shape-showing , bla
> h blah. Nothing to do with identifying compositio
> n.
>
> A quote from the link , that their machine will :
>
> " roughly identify different
> metal shapes with varied width
> s "
>
> This is nothing new . Various touted advertised m
> achines now, claim to show shape even now. And co
> ol 3d images of the "spike-depiction" of how it lo
> oks in the earth. And while your link might be an
> improvement on that, the devil will be in the deta
> ils. The "devil in the details" is the pixel size
> (in whatever type "picture" they choose. Whether
> 2d, 3d, etc...). Nor does it address that a mill
> ions objects can share that same "spike" diagram.
> Nor will address to the topic of this thread, whic
> h, as I understand it, is gold vs aluminum
>
> They even say themselves in your link :
>
> " ...Also, we hope this metal de
> tection system can be extended to identify a varie
> ty of metals with different geometries.... "[/colo
> r]


Wonder if a detector was indeed made that could show shape and size only and depth capabilities medicore (not composition/conductivity), would detectorists (you) have one ??
Or just a detector that shows round disc shaped period ?? But can’t distinguish ferrous from nonferrous.

If it was too easy...doubt if hobby would last...not an answer just a realization....

tnsharpshooter Wrote:
-------------------------------------------------------
....
>
> Wonder if a detector was indeed made that could sh
> ow shape and size only and depth capabilities medi
> core (not composition/conductivity), would detect
> orists (you) have one ??
> Or just a detector that shows round disc shaped pe
> riod ?? But can’t distinguish ferrous from nonfer
> rous.

Good musings. Even if pixel sizes got down to 1000th of inches (such that we're not looking at blotchy images), there another problem we'd be facing: That "shape-showing" assumes the object (coin or ring or whatever) is lying flat in the ground. The moment you add the slightest bit of tilt, is the moment you can kiss shape-showing goodbye. A round ring or coin now becomes an elliptical oblong shape, eh ?

And lest someone think that, when it comes to rings : "What are the odds that it's necessarily tilted in the first place ??" (ie.: That Tom_in_CA is bringing up a ridiculous exception) : I knew a guy, back in the 1980s "silver rush" days, that had worked hundreds of silver out of a certain old park in a big city in CA. He .... Like all the other guys md'ing there, had been sloppy and (gasp) passed foil and tabs, in their quest for old silver.

One day, he reasoned that "we must have missed a lot of gold rings" and "we are missing old nickels". So he made it his mission to do the following experiment : He gridded off a certain large area, that had been especially productive for old silver. And for the next year, a time or two every single week after work, he'd stop by for an hour or two. And methodically worked out EVERY SINGLE SIGNAL. No matter how much it pained him . And he kept careful log-books, from which to create graphs, pie-charts, statistical studies, etc.... Plotting locations, item ID, depth, etc .... of every item.

By the end of the year, he DID INDEED have a small handful gold rings, a few gold charms, etc... Albeit with a punishing ratio of junk. And he DID INDEED bag a variety of buffalos, a few V's, etc... (all of which were sickly brown, corroded, and worthless). He concluded that if gold rings had been his objective: That he should have simply gone to the beach. Doh!

BUT AN INTERESTING SIDE OBSERVATION AROSE FROM HIS STUDY : Since he'd been keeping meticulous notes of depth, he was carefully digging/extracting each item. And he made an interesting notation: Whenever he got a gold ring, with ANY sort of "crown" (eg.: Stone setting. Or a ring where one end was heavier than the other, in some way, like a school class ring) , that invariably the ring was tilted towards the heavy end ! In some cases, almost perpendicular. Doh! Only perfectly uniform bands had no tilt. HMmmm. So as you can see, shape showing would not do much good for a lot of rings, in a turf environment where this tendency starts to happen.

Another problem with shape showing is that our main nemesis (ring tabs) are ALSO "ring-shaped". Doh !

It is my understanding that with all else being equal between targets, the larger target will produce a higher target ID. That ID is a measurement of conductivity. Therefore, conductivity increases with the size of the target.

When I speak of the measurement of conductivity as it relates to mass, I mean how much higher should the conductivity read for a similarly shaped target that is twice the size of another of equal metallic composition? Two lead balls, one twice the size of the other. Two brass buttons, one half the size of the other. Two pcs of aluminum, one twice the size of the other. How much difference should be expected in the conductivity of each target? And is the conductivity measurement directly proportional to the target ID? IOW, should the target ID be double the value for a target that is twice the size of another of equal metallic composition? I know it isn't double the value, but is there a precise relationship in measured conductivity based on size alone, all other target aspects being equal? I mean a silver dime might read a 90 and a silver quarter might read a 94. A silver half might be 96 and a silver dollar a 98 (hypothetical). Hey, they're all silver. Basically the same metal. The only difference is their dimensions and weight (one might say mass). So, hasn't mass played a part in the determination of target IDs for each coin? That's what I'm asking. And is there anything more a detector can do with the raw data returned from the target to determine its relative size/weight/mass, or make a decent guess at it, than what is presently available?

Wayne

I have dug aluminum can slaw hoping for a nickel then re-scanned the hole and found a dime which the can slaw obviously had masked. So like iron it masks targets too.

Wayne:
What a target ID machine is doing is measuring a characteristic of the target, known as the 'corner frequency' ( or the break frequency, natural frequency, -3dB frequency .. ). It then indicates that in some heavily-manipulated way on a 00 - 30 , 15 - 99 etc display. These are normally 'reversed', so the lowest corner freq targets read at the top end, the highest corner freq targets at the bottom end.

Corner frequency varies with size ( in all 3 dimensions) , the electrical resistivity of the metal, and to some extent the shape -- for example, rings are different to solid objects. Folded-up items are different to flat ones.

For a simple-shaped object like a coin, corner frequency is inversely proportional to: the diameter; the thickness; the electrical conductivity of the metal.
So for example, if you doubled the thickness of a coin, and made it 50% larger in diameter, it's corner frequency would fall to:
1 / (2 x 1.5) = 0.333 = 33.3%, for example falling from 3 kHz to 1 kHz.
If you made the coin from pure lead instead of 90% 'coin' silver, it's corner freq would increase ( 86 / 8.4 ) = 10.2 times, for example increasing from 3 kHz to 30.6 kHz. [ 86% and 8.4% are the IACS conductivity values for 90%silver / lead ]
And for fun, if you made it 3 times the diameter, 3.4 times thicker, and from lead, it would ID exactly the same as the original silver coin:
( 86 / 8.4 ) / ( 3 x 3.4 ) = 1.00. A large lead disc reads the same as a small silver coin.

It gets more complex for targets that aren't 'thin', so comparing round lead musket balls of different diameters would show up small errors in the above maths.

There's other factors to complicate matters. The detectors operating frequency has an effect, a 25 kHz machine will see targets differently to a 5 kHz machine, particularly larger targets.

ncwayne Wrote:
-------------------------------------------------------
> It is my understanding that with all else being eq
> ual between targets, the larger target will produc
> e a higher target ID. That ID is a measurement of
> conductivity. Therefore, conductivity increases wi
> th the size of the target.
>
> When I speak of the measurement of conductivity as
> it relates to mass, I mean how much higher should
> the conductivity read for a similarly shaped targe
> t that is twice the size of another of equal metal
> lic composition? Two lead balls, one twice the siz
> e of the other. Two brass buttons, one half the si
> ze of the other. Two pcs of aluminum, one twice th
> e size of the other. How much difference should be
> expected in the conductivity of each target? And i
> s the conductivity measurement directly proportion
> al to the target ID? IOW, should the target ID be
> double the value for a target that is twice the si
> ze of another of equal metallic composition? I kno
> w it isn't double the value, but is there a precis
> e relationship in measured conductivity based on s
> ize alone, all other target aspects being equal? I
> mean a silver dime might read a 90 and a silver qu
> arter might read a 94. A silver half might be 96 a
> nd a silver dollar a 98 (hypothetical). Hey, they'
> re all silver. Basically the same metal. The only
> difference is their dimensions and weight (one mig
> ht say mass). So, hasn't mass played a part in the
> determination of target IDs for each coin? That's
> what I'm asking. And is there anything more a dete
> ctor can do with the raw data returned from the ta
> rget to determine its relative size/weight/mass, o
> r make a decent guess at it, than what is presentl
> y available?
>
> Wayne

Pimento gives a great post to address your question.

The only things I would add, is that everything you're saying is already incorporated into most md'rs minds, when going to evaluate signals. Eg.: That even though a soda can and quarter might give the same "94" TID , yet we all can suspect the difference. Because of the size difference. However, I would not want to put this known effect into the hands of the detector to decide. I would want my brain to decide. Since, as Pimento says, it's not quite so simple. Ie.: co-located targets that give you subtle clues that it's two side-by-side targets. If left to the machine, it might classify that as a single large target, hence not a coin.

Also, while your illustration of dime vs quarter vs half vs silver dollar is a very measurable thing (size vs ID, etc...) yet ... when it comes to aluminum VS gold, I'm afraid that a gold ring and a tab and foil wad can all have the same size/shape. Oh sure, the weight-to-density is different. But last I checked, no machine can take them out of the ground and weigh the competing items. Doh !

Well, why didn't somebody just say so? <g>

Oh, you just did!

Thanks, Pimento!!

Actually, that's very helpful. It tells me that for all the bells and whistles, metal detectors are still very crude machines and not very much different, one from another.
All the more reason for the mfrs. to differentiate in the market place with lighter weight, more comfortable styling, screen legibility, waterproofness, user adjustables, intelligible audio, and precise metering.

Thanks, Tom in CA for your remarks, also.

One more question; is there any room in metal detector engineering for out of the box thinking or will we always be restricted to the box?


Wayne

Tom_in_CA Wrote:
-------------------------------------------------------
... when it comes to alum
> inum VS gold, I'm afraid that a gold ring and a ta
> b and foil wad can all have the same size/shape.
> Oh sure, the weight-to-density is different. But
> last I checked, no machine can take them out of th
> e ground and weigh the competing items. Doh !


But mass, or thickness does affect (or can affect) TID response. It doesnt seem that all have the ability but Minelab FBS does, and like other multi-freqs can too. The affect is very noticeable both in-ground and air tests. A full weight band new silver dime measures several points higher than a well worn and thin dime. The thinner the dime, the lower the number. Same metal, same diameter, only difference is the thickness and mass.

NO, mass does NOT have anything whatsoever to do with it. The thinner coin has different electrical characteristics. Specifically: it's inductance is essentially unchanged, but it's electrical resistance to current flow has increased, due to the reduced cross-sectional area the currents flow in. It's basically like getting a length of 1 mm diameter copper wire, and measuring its resistance with a meter. Then sandpapering the wire down, until it's 0.9 mm diameter, and re-measuring it. In this case, the cross-sectional area will go down to ( 0.9 x 0.9 ) = 0.81 times what it originally was, and the measured resistance goes up by ( 1 / 0.81 ) = 1.234, or +23%.
Applying this to the coin: if its thickness is worn from 0.80 mm to 0.75 mm, the resistance increases +6.67%, and the 'corner frequency' increases by +6.67%, for example changing from 3 kHz ( new ) to 3.2 kHz ( worn ). This could move its target ID lower by a point or so, depending on machine.

ncwayne Wrote:
-------------------------------------------------------
> .... is there any room in metal dete
> ctor engineering for out of the box thinking or wi
> ll we always be restricted to the box?
>

The engineers are ALWAYS trying to think "outside the box". After all, you've heard the old saying of : Build a better mousetrap at the world ($$) beats a path to your door.

It has little to do with anyone's effort, or lack-thereof, to "think outside the box". It has everything to do with the laws of physics. There can come a point where no amount of "smaller and faster" solves a problem (as was the case of computer evolution). There is some things that are simply limited by the laws of physics. Remember: You are pumping that signal through solid ground. Which is, by nature, a solid "fog" that the machine has to see through. Unlike a photograph and film where you take a picture. Or fish sonar that sees through the medium of water (which offers zero resistance). In this case, there's solid ground to deal with. So you can only pump in so much signal and only receive back so much data.

When I said: And for fun, if you made the new 'coin' 3 times the diameter, 3.4 times thicker, and from lead, it would ID exactly the same as the original silver coin: ( 86 / 8.4 ) / ( 3 x 3.4 ) = 1.00. A large lead disc reads the same as a small silver coin.
..it wasn't just made-up. I actually cast a similar lead disc for some experimental work over on Geotech1. Now I've done some measurements and some maths, here's the results:

The lead disc is 60mm diameter, and 5mm thick, (though it's not exactly perfect, being more 5.25mm in the middle, and 4.75mm at the edge, and the edge profile is a bit rough )

The silver US coin it ID's closest to is in fact a 25c 'quarter'. On an F75, I get 25c reading 83, and the lead disc = 81.
This 'translates' to : 25c 'corner freq' = 1.6 kHz and lead disc corner freq = 1.8 kHz, a 12% higher figure.

Now the maths:
The lead is thicker than the coin by ( 5mm / 1.307 mm) = 3.826 times
The lead is larger in diameter by ( 60 / 24.26 ) = 2.47 times
And, as mentioned in the earlier post, conductivity of 90% silver / lead = ( 86 / 8.4 ) = 10.24

So the 'theory' corner freq of the lead disc = "25c corner freq" x 10.24 / ( 3.826 x 2.47 ) = 1.6 kHz x 1.084 = 1.73 kHz

A pretty darn good match, you'll agree, 4% off, considering my disc isn't made by the US mint...

‘Blinding me with science” - I love it - thanks Mr. P.

[youtu.be]

Rick Kempf
Gold Canyon AZ- where there is no gold



Edited 2 time(s). Last edit at 11/07/2018 12:08AM by lytle78.