...Very interesting Gunnar. I like it. I've done a little rearranging and added a few comments of my own, here and there, where assumptions were made.
Thanks for your efforts. I hope this starts a serious discusson of this topic.
"I've had some talks whith my frend Mike, and he is amazed at how different gold rings can be all over the place with VDI numbers and on the discrimination. So I've done some research ===
PURE GOLD has an electrical conductivity of 77 thats very high .
This is based on the International Annealed Copper Standard, a measure of electrical conductivity. This scale compares the conductivity of materials to that of annealed copper, which ranks at 100% on the scale. The readings are expressed as "percent IACS." Since some materials are better conductors than copper, readings over 100% are sometimes encountered.
Commercially available pure copper is about 101% in the IACS scale, while silver is closer to 103%.)
...lets look at nickle. Remember pure gold is 77 on the IACS scale.
Now lets add an alloy of 25% silver and 1.8% nickle to gold and the conductivity drops -- plummets actually, to 14%.
We know copper has a high conductivity of 101%, but when 75% copper is alloyed with 25% nickle in the gold, the conductivity drops to 4.6%
This whole concept of alloys intrigues me, as I didnt know there were so many options for alloying. This explains why foreign made gold jewelry can differ from what I think of as "US Standard."
"Now think of your disc dial. First comes iron, which has a conductivity of 14-15%
Then it moves to foil, nickle tabs and then to zinc penny, etc."
I was looking for the complete IACS scale. Do you know where I can find it?
Depending on the carat and alloy used in rings, they can be all over the place. Some are right down with foil and some that have 10% zinc in them can be up there with the zinc penny. Thats why white gold is the worst offender.
A gold ring alloyed with 14% copper and 8.5 platinum and 4% silver and 1% zinc only has a conductivity of 11.5 even though you have 72% gold in the ring.
I'm assuming this is the normal composition of white gold?
Now the theory is that the lower your frequency the better your detector is at finding the higher conductivity targets like gold, silver, copper... while the higher your frequency, the better it is at finding lower conductivity targets.
And that theory generally holds true. But, there is no definitive frequency. We dont search for just one thing - we search for everything.
"George Payne, the master of modern detector design, has repeatedly stated that the best frequencies for detecting all targets were between 10 and 15 khz, meaning frequencies in that range would respond better on targets from low in the conductivity range to high in the conductivity range. He said the optimum frequency for single frequency detectors was 12.5 khz." - JB(MS)
Electrical conductivity as it relates to detecting also has to do with the phase response of a target, once the field signal is coupled into it.
Our VLF detectors can more rightly be called 'RF inductive coupling instruments.' VLF is just shorter, and so easier to stick on a label.
When any signal is inductively coupled to a conductive element, the intitiating RF signal is phase shifted in response to that effect. This is how we determine the identity of the target, as each type of metal couples RF energy in it's own way.
Added to this is the shape and size of a target, since our detectors essentially recognize the surface area of a target. These differeing shapes and sizes also have a dramatic effect on response. This is why pulltabs can be so different, even though they are mostly the same size and all made of nearly pure aluminum.
Since rings are single-turn inductive loops, they couple electrical signals very well. They are also of a large enough size so that they are readily picked up.
However, the wide range of alloy metals which go into them means that, even though they are great inductors, their response will be very closely associated with their composition. So, an alloy compostion that places a ring at 22% IACS will be strongly seen as just that.
Yet, it gets worse when you take alloy gold and finely work it into filligree or scroll/wire jewelry. Then you add the effects of shape and varying surface area to the equation, and it can get ugly - fast!
I found a beautiful 14K Scorpio charm last year, about the size of a nickel. But it hit dead hard on iron/foil range, and even then it was scratchy and popping what with all the little legs and the chain thrown into the mix.
The point of all this is that, if you are not accepting and recovering the range of targets between foil and screwcap, you will miss gold jewelry. You'll dig a lot of nonferrous trash and junk, but hey - that's life. You can't sugar coat that bitter pill.
[size=small]Thanks to JB(MS) for the quoted information.[/size]
