Eric or anyone ?

[Anonymous]

Can someone please explain this one to me ?
I understand the difference between conductivity and conductance, but don't quite understand how it relates in this particular case.
If I take a large pulltab and a nickel and individually discriminate them both completely out on my Shadow X5 (induction balance unit), what is causing them to create a combined much higher "apparent" conductivity value when placed under the coil one on top of the other ? The two do not have to be touching to experience this effect, and if I do the same thing with two items of the same conductivity, they do not seem to combine to create a higher "apparent" conductivity due to any increase in conductance or signal strength.
Is there something in the dissimilarity of the two objects that is causing a weird distortion in the electromagnetic field of the coil, making it react as if the conductivity of the two as a single target were much higher than either of the constituents ?
Now when I go back and clip the circle of the pull tab thereby separating the circle, the tab will immediately give a higher conductivity reading, but when placed back under the coil together with the nickel, the two will no longer read at the same high level of conductivity.
Is it the difference in how the eddy currents move in relation to the solid "disc" of the coin and the "ring" of the pull tab that is causing these distortions, causing the variations in "perceived conductivity" as seen by the detector ?
Thanks in advance. This one has me stumped!!
Ralph

 

[Anonymous]

I think what your are describing is "target averaging".

 

[Anonymous]

How can you take 5 and 8 and get an "average" of 10 ? See my point ? The combined targets are producing an "apparent" conductivity higher than either of the constituents. They will do so when co-located one atop the other, but side to side give entirely different values.

 

[Anonymous]

As far as I know, a metal object will register higher conductivity as is get bigger/larger mass. Two objects will register an average of their conductivity but this will also be affected by their total mass put together.
Two metal objects on top of each other, or side by side, or any other combination as long as they are close enough together that they are within the coils detecting range will be averaged and seen as one target. The detection range of a mono coil is dome shaped. Anything within that dome as you sweep will be averged as one target.
That is to the best of my knowledge, which is limited.
Rgds, JT

 

[Anonymous]

[No message]

 

[Anonymous]

C&P from an answer to this question on another site:
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Here is the test results made with a Nautilus which is ideal because of its 0-100 vernier adjustment range.
Nickel alone discrims at 45
Ringtab flat discrims at 52
Ringtab folded discrims at 56
Ringtab/nickel combo with tab towards coil discriminates at65
Ringtab/nickel combo with ring towards coil discriminates at 67
.....That 0 to 100 discrimination scale on the Nautilus really brings out the fact that the ringtab/nickel combo reads about 10% higher than the ringtab alone which is the higher conductor of the two items.
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Simple "averaging" does not explain this phenomenon, since true averaging would result in a value "between" the lower and higher conductivity values, not well above the value of the highest constituent.
BTW, this is really not a VLF or PI question, as it pertains more to the targets themselves, and I've just been wondering what was causing such results in this particular case. My friendship with Eric over the years, and knowing his level of understanding in such matters just seems to lead me back to his forum(s) for answers to the really "tough ones"....... <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="">
Ralph

 

[Anonymous]

Hi Ralph,
I'm trying to think this one out, but a couple of questions first. What happens if the tab and the nickel are side by side but not touching? Do you get the same conductivity reading with the tab on top of the nickel, and vise versa? You say that when you cut the tab (breaking the ring), the conductivity reads higher. I would have thought it to be the other way round? Certainly on a PI, the decay would get faster, indicating more resistance and a lower conductivity.
Eric.

 

[Anonymous]

You're right, my mistake on the cut tab ring. The conductivity reading actually decreased rather than increased.
With the tab and nickel placed together, one atop the other, the same results occurred when taking a reading from the tab side or the nickel side. But if they are placed flat and side by side, right to left, then the apparent conductivity reading drops back within range of the two constituents (between the nickel and tab).
Even when the nickel and tab are one atop the other, they do not have to be touching to get the increase in apparent conductivity. But then the higher conductivity level falls off as the two objects are moved farther away from each other, though still in the position of one atop the other.
It might be interesting to try this same thing using a solid nickel and aluminum disc, and then again with a nickel and aluminum ring and see if the results were similar. I just still have the feeling that the dissimilar materials along with the dissimilar shapes (disc and ring) somehow are producing some unusual effect that mimics a higher conductivity level by the combination of the two objects.
Ralph