VLF vs. PI conductivity scale

[Anonymous]

I'm familiar with VLF's conductivity scale starting with iron,steel,lead,nickles,gold,pulltabs,zinc pennies,copper pennies, dime,quarter,half and silver dollar. However I just heard that gold is low conductivity to a PI and steel is considered high conductivity. Could someone give us an idea what a PI conductivity scale looks like. Thanks.

 

[Anonymous]

Hi John,
Eric might want to confirm this, but it is my feeling that conductivity is conductivity, with properties being the same with either type of detector.
What makes PI "hot" to iron or steel is the magnetic properties involved. The magnetic properties allow for a stronger signal in ferrous metals because it already has it's own "current" so to speak, even prior to having "more current" induced into it by the detector. It's not as noticeable with VLF induction balance machines because of the way they operate, i.e. transmitting and receiving a constant signal and reading the conductivity to identify the type of metal while also comparing phase shift properties to identify ferrous properties.
Ralph

 

[Anonymous]

The new Garrett machine makes a high low sound for low conductive metals and a low high sound for high conductive metals. So this is why I'm asking about a conductivity scale as a PI sees it vs. the VLF. And point well taken that conductivity is conductivity. I guess I'm interested in the application of conductivity as the PI circuit sees it and how it compares to the VLF that I'm so used to. Thanks.

 

[Anonymous]

I read the same post as you did. Although I understood what he was saying, it was not correct.
On the conductivity level it would go something like this. Iron/steel, steel alloy, lead, gold alloy jewelry, aluminum, gold, copper, silver.
I may be a little out of whack on the order of the silver, copper, 24 k gold. Alloy mix of any combination, lowers the conductivity. Mixing silver, and copper produces a product of low conductivity even though both products started out as high conductivity.
A VLF detector sees it in about the same order. With a PI detector, it looks at the eddy current returns from the target. Iron/steel has a long eddy current, compared to lead that has a short return. Because of this it

 

[Anonymous]

Bill,
Could you then give me a new scale. One with the eddy current values from longest to shortest?

 

[Anonymous]

.....wouldn't actually tell you much, if there were such a thing. Two objects of identical metals can have various eddy current "decay rates", based on their shape, size, and cross-sectional mass. As an example, suppose you had two 14 karat gold rings. One is very tiny in cross section, while the other is very large and thick. The larger, thicker one would have a longer eddy current decay time than the smaller one would, even if they are made of the same exact alloy composition.
In a similar scenario, suppose you have two identical rings, both the same weight and cross sectional mass. If one is "intact" while the other is broken or separated at some point, the intact ring will produce a "longer lasting" eddy current than will the broken ring.
What a PI machine does is "receive" this eddy or "remaining" electromagnetic current within the metal object after the TX signal stops and the receiver circuit is switched on.
The reason that ferrous objects react so much more strongly than non-ferrous is because of a ferrous metals ability to retain an eddy current for a longer period of time (known as a longer decay curve).... actually the induced current from the TX function of the PI "IN ADDITION" to it's natural magnetic properties.
Another example might be two gold nuggets, both of identical weight, but one being rather flat in cross section while the other is more of a "chunky" shape. The "chunky" nugget will invariably give a better signal because of it's ability to produce a longer lasting eddy current.
Back to your original question though, "conductivity" per se is the same in any two metal objects of identical compositional make-up. If you had an 18 karat gold BB shot and an 18 karat gold bowling ball of the identical alloy, both would have the same relative "conductivity", while the larger of the two would have a proportionally higher "conductance" or ability to carry a greater amount of current due to the increase in the mass of the larger object.
So you can see that "conductivity is conductivity", and relates the same with either pulse induction or induction balance type detectors. You might say, however, that the PI works off of a combination of both conductivity AND conductance, while most VLF-IB discriminating machines base their discriminating capabilities only on relative conductivity, or compositional make-up within the metals themselves. That is also why even with the best of VLF-IB machines, larger objects, no matter what their composition or alloy, can easily overload the discrimination circuits.
Remember too, that "RELATIVE conductivity" is only an arbitrary set of numbers, using annealed copper at a certain temperature as a "basis" value of 100. Every other metal is assigned a number off of that basis to indicate either a greater or lesser conductivity "in relation" to the copper.
Hope this is of some help in explaining how conductivity relates to PI.
Ralph

 

[Anonymous]

Hi Ralph and John,
Here is a useful page for comparing conductivities.
Eric.

 

[Anonymous]

Why does a high conductive metal such as copper when alloyed with 10% nickle, produced such a low conductive product. That math does'nt seem to add up.

 

[Anonymous]

Hi John,
I don't know the physics of this, but it is a definite fact. I would guess that it has to do with the bonding of electrons in the alloyed materials. An electrical conductor has electrons that are free to move under the influence of an emf. If the electrons are less free to move then the metal is less of a conductor.
Eric.

 

[Anonymous]

The answer lies in a property of the metals called relative permeability. Basically, it is the ability of a material to accept and concentrate a magnetic field. Generally, the conductivity and relative permeability go hand in hand but that is not always the case. Ferrites are ceramic materials that are non-conductive but have a high relative permeability. The combined relative permeability of two very good conductors can actually be less than that of the original conductors when you alloy the metals together. Hope this helps.

 

[Anonymous]

The physics of permeability has to do with bound electrons, whereas electrical conductivity has to do with free electrons. To my knowledge there are no high-conductivity metals or alloys which also have high permeability. Superconductors have zero permeability.
Magnetic metal glass has very high permeability, and electrical conductivity somewhat lower than that of the corresponding crystalline alloy.
I don't know of any basis for a generalization that high permeability and high conductivity tend to go together, other than that among ferrite materials, the electrical resistivity tends to be lower with the higher permeability formulations. However, there are so many contrary examples, that even this is not a usable "rule of thumb".
An earlier post raised the question, Why do alloys have lower electrical conductivity than their constituents? I haven't studied the matter, but I suspect it has to do with electrons being scattered by nonuniformities.
--Dave J.

 

[Anonymous]

Hi Dave,
Are you saying something akin to the current "slowing" when passing from higher conductivity electrons to lower conductivity electrons, and then not being able to "speed back up" when passing again from low conductivity electrons to higher conductivity electrons ? In other words, the lower conductivity places a "drag" factor into the equation ?
Eric and I talked about some tests he had done on gold karat alloys several years ago (AU/CU) and somewhere in my files I have an article of similar sorts showing the alloying of gold and copper where the conductivity of the alloy begins to drop to a point of about 50/50 au and cu, at which time the conductivity begins to rise again to the point of again having a pure metal, in this case from pure au to 50/50 and back to pure cu. I always looked at it as someone running a foot race from asphalt to tar and back to asphalt with the tar still on their feet..... (grin)
Ralph

 

[Anonymous]

I was thinking of it in terms of scattering due to a distorted crystal lattice. However I don't know if this way of thinking about it is actually a good explanation.
Perhaps someone with an in-depth knowledge of the physics of conduction at the atomic level, can provide us with a good answer.
--Dave J.

 

[Anonymous]

Solid state physics is not my area, but I think of as being like the moguls event in skiing. You have a field of humps to maneuver through. In a pure metal the humps are all the same size and uniformly spaced. It's not easy getting through, but once you get your rhythm you can go a long way before you fall down. When you add another metal to the mix you get different size humps in the field. Then there is no simple rhythm that can get you through, so you fall down more often.
The skier of course represents an electron and the humps are the wavefunction of the metal alloy.
Robert