Gold chains.

[JBM]

My friends had their new Quattro out today and it will not detect a gold chain.

I used my Explorer MK11 and I could get a sound but not of very good quality in all metals.
I tried it lincked in a circle and in a ball also strched out in a straight line.
Have you had any experience with this problem.
Thanks,

Jerry.

 

[Cody]

A gold chain can be very difficult to detect. The links in conjunction with eddy current flow causes the chain to be rejected. It depends on the size of the chain and if it is just chain links. It is also rejected in all metal because it looks like soil minerals but in some soil where the chain looks something like a hot rock can be detected very clear. In some ways it is similar to how a nail will give a silver sound due to the flow of eddy current around the axis of the nail.

These detector reject soil minerals and trash targets based on a time constant so if a target has a TC the same as the soil or trash that is set to be rejected then the chain is rejected. It really does matter what we do with the chain as far as how it is positioned as it will still be rejected or just detected. The chain starts to appear to have the same resistance as the soil which is most often much greater than any metal target. This is because of the self inductance of the links and the fields combine to "buck" the electromagnetic field of the TX coil. This has been a problem for all of these detector since day one but is kind of shocks a user when discovered.

 

[Ace]

A small coil like a 5" or 8" might solve this problem, try the Sunray coils, I've heard and read great things about these coils; Just curious JBM, was it a thin chain? What karat was it? Maybe those things can have some effect on the detector.

 

[Bill_in_MA]

Cody,
I know that some past posts had also said that gold chains and small gold objects would produce a weaker signal due to the fact they have a higher frequency where the peak Resistive effect occurs. With the Explorer front end filters set up to anaylze the range from 1.5k to 100k small gold falls at the upper end of the range and thus the Explorer is not optimised for small gold.
Thoughts......

 

[Cody]

Metals have what is called the frequency of the metal, signature frequency, etc, but boils down to a metal can be simulated with by an inductive/resistive circuit. The inductance is charged through the resistance of the metal. The self inductance charge and discharge through the resistance is frequency sensitivity. (As you know the other side of the coin where resistance is concerned is conductivity.) The charge and discharge is supports eddy currents which is how a target is identified. Gold will respond better to a higher frequency and small gold, say the size of a BB, will be detected better with a higher frequencies and a smaller coil. This all woks out fine but if we take a small link chain a foot long and coil it up we have a lot of gold but it still will be rejected many times. If we are in all metal then the gold chain looks like soil minerals in that it blends in with the soil.

The Explorer does not have front end filters as such. The receiver coil is direct coupled to 6 demodulators. They work in pairs of two to output three signals. The outputs of the demodulators are then filtered and averaged for three time domain signals that are process by the microprocessor. A frequency domain detector is much more concerned with filtering so we see as many as 6 filters in some of them.

The Explorer looks at the outputs of the demodulators, averages the outputs, filters out any ground noise, and then uses sensitivity, manual or semi-auto, to control the demodulators so the Explorer is not going to have soil mineral problems. What is often posted as soil mineral problems is actually discrimination of refined iron, non-linear electronics, Q of coil changes as it is moved over the ground, and ageing of electronics. This can be corrected simply by using semi-auto or manual which adjusting one leg of the demodulators. The end results are three ground balanced signals that consist of long, short, and medium time constants that are used to id the metal.

The links and self inductance of gold chain will oppose the electromagnetic field of the coil so the end results are a long time constant. The soil has a TC of about 1us so the gold chain has a long TC which causes it to look like the soil. The bucking of the transmitter generated magnetic field by the eddy current in the links is based on Lenz

 

[Bill_in_MA]

Thanks for that explanation Cody. I understand it a lot better now. I appreciate the level of detail that you go into and find it anything but boring. The more ways your hear a difficult concept or at least one that is new and you're not familiar with - the easier it is get a handle on it in order to understand it better and make it your own. Kinda' like pinpointing aye - we narrow it down and shrink it to the ultimate point and there we are - left with that golden nugget or kernel of wisdom - Thanks !.

 

[wpaxt]

Cody,

Very informative post. However, if 50KHZ to 60KHZ is ideal for small, fine gold, and the Explorer covers the range from 1.5 to 100, then that range should be covered, right?

If not, couldn't the user have a selection mode on the machine to specify a certain frequency? Say in a situation where you knew you were looking for a fine gold chain in a certain area? Not being versed in the science of this, I would still imagine that this would present problems in high mineral content areas such as saltwater beaches, correct?

Thanks for enlightening us on the how & why of the gold chain issue.

Bill (S. CA)

 

[Cody]

Glad you posted as it gives me a chance to correct something I often do and that is say short time constant when I should say long time constant so let me correct this right off. Precious metals have a long TC and iron has a short one.

You question about the 1.5 kHz to 100 kHz is a good one as it would seem that this would cover the range for fine gold. My guess is that the Explorer would do a good job with nuggets if we use a small coil in the 3

 

[JBM]

Thanks all for your input especially Cody.
These threads are priceless.
This will help a number of friends as well as myself.
Regards,

Jerry.

 

[wpaxt]

Got it, Cody. One wonders if at some point a multi-frequency machine like the Explorer could have a "selectible frequency" feature allowing the user to channel all of the power into one single frequency. Probably asking too much of the machine, since it isn't really fair (or possible) to make a machine that is all things to all people. But it sure would be nice to be able to find a few of the many gold chains that I know are hiding from all of us on the salt water beaches here in CA.

Bill