Conductivity and metal detecting explained

maxxkatt

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One of the confusing concepts for newbies is conductivity in terms of metal detecting. Conductivity is a term that describes how easy electricity (electrons) flow through a metal. Low conductivity means the metal is more resistant to electron flow. A high conductive metal is a metal that offers a lower amount of resistant to electron flow.

What does this have to do with metal detecting? A higher frequency in a metal detector say 20 kHz to 40 kHz or higher is good for finding low conductive metals such as gold and tiny pieces of metal.

A lower frequency in a metal detector is better at finding high conductivity meals such as silver coins.

Silver is highest conductor, followed by aluminum and gold, then coper Brass (10% zinc), zinc, brass (40% zinc), nickel, tin, iron, lead, platinum and tungsten.

Seeing how close aluminum follows silver you now should understand why aluminum screw caps are so difficult to distinguish from silver coins. Both have high conductivity and are round. The two very characteristics that the design engineers use to ID silver coins.

A metal detector operating at below 10 kHz will find silver coins better than one operating at 20 kHz. A metal detector operating at above 20 kHz will find small gold particles better than one operating below 10 kHz.
You might ask what does this mean to me? When buying a metal detector, you should buy one with operating characteristic that best suits your type of hunting. If you are only hunting silver coins, then one operating at a lower frequency over a one operating at a higher frequency may be the better choice.

Likewise, if you are hunting gold you buy a gold detector that operates above 40 kHz to locate the smaller gold pieces. Another example is that when I am hunting civil war bullets is that with my Equinox 800 I can switch to a single frequency mode of 20 kHz or 40 kHz and tune my detector to hunting lower conductors like lead bullets or hopefully gold coins.

We have the best of both worlds which is a multi-frequency (Multi-IQ tm) like the Equinox 800. (I am using the 800 as an example because I own one and can write with more knowledge about its characteristics). The 800 can operate in the multi-frequency mode. This is where the Minelab designers have decide which mix of two frequencies it uses in each hunting modes. A common misconception about the 800 is that in the multi-frequency mode it operates in all frequencies from 5 kHz to 40 kHz. This is not true. The engineers have selected the best two frequencies and Minelab does not publish these exact frequencies. The do give you hints.

Park1 is better for coins (high conductors). Park2 is better for jewelry (high conductor alloys. Field1 is for coins and artifacts (coins, brass and iron).
Field2 probably the same mix of two frequencies as Field1 but with other settings changed.

Beach1 is good for lower conductivity targets such as gold jewelry (alloys) and small gold chains. Better for dry ocean beach sand.

Beach2 per the 800 manual uses a “very low” weighted multi-frequency combination to maximize ground balance for salt in the salt water. Better for wet ocean beach sand and hunting in the surf.

Also keep in minds each different mode in the Equinox 800 (and I assume other multi-frequency machines like the Simplex, Apex, Equnox 600 and Vanquish 540) have a different mixture of settings like recovery speed, iron balance, discrimination in addition to a different frequency combination.

What does all this mean to the 800 owner? When hunting a particular piece of property that you think is hunted out in silver coins, you can re-hunt the site with different single frequencies (lower) and recovery speeds, sensitivity and iron balance to essentially give yourself a completely different detector. In the past detectorists not using today’s more advanced detectors would have to hunt the “hunted out” sites with different detectors with different characteristics. This explains why you see on the forums lots of members owning more than a few different metal detectors.
 
Seeing how close aluminum follows silver you now should understand why aluminum screw caps are so difficult to distinguish from silver coins. Both have high conductivity

Aluminum is actually a low conducting metal. Having found nearly 1400 silver coins, I have never once thought I had a silver under the coil that turned out to be aluminum the size of a silver coin. Aluminum needs to be the size of a soda can to sound like a silver. YMMV, of course.

Aluminum is actually closer to low conductors such as nickel and iron than it is to silver. Conductivity is measured in siemens per meter; the opposite of ohms (resistivity).

This link provides the conductivity of various metals. While aluminum is high on the chart, it is the actual conductivity in siemens relative to silver that matters for the physics being discussed.

https://www.thoughtco.com/electrical-conductivity-in-metals-2340117
 
We always talk about conductivity, but that is only half the story. If we only look at conductivity, we can't begin to explain discrimination.

In reality what the detector looks at is Admittance. This is a number (a complex number actually) that has two components. The first component is Conductivity, what is being discussed. The second component is called Reactance. If the target had no reactance, the received signal from the target would always be in phase with the transmit signal. As the target's reactance increases, the phase shift between the transmit and received signal increases. This phase shift forms the basis on which the detector discriminates targets.
 
We always talk about conductivity, but that is only half the story. If we only look at conductivity, we can't begin to explain discrimination.

In reality what the detector looks at is Admittance. This is a number (a complex number actually) that has two components. The first component is Conductivity, what is being discussed. The second component is called Reactance. If the target had no reactance, the received signal from the target would always be in phase with the transmit signal. As the target's reactance increases, the phase shift between the transmit and received signal increases. This phase shift forms the basis on which the detector discriminates targets.

Got it. An item's Conductivity Admits me to React!

I'm kidding. This is all way over my head.
 
A couple of things...

Gold is not a low conductor. Gold is highly conductive. Debased gold, however, decreases in conductively as the karat numbers get lower. In terms of conductivity, 24k > 22K > 18k > 14k > 12k. And to muddy the waters a bit some 14k gold is more conductive than other 14k gold depending upon the base metals used in the alloy. This of course is true for 22k, 18k, etc.

As far as lower frequencies and silver coins go, the big issue is VDI numbers. A silver quarter reacting to a 18Khz signal is going to produce a higher VDI number than a 5Khz signal. Because of this, higher frequncies can cause larger silver targets to wrap into the negatives, where as lower frequencies are less likely to do that. This isn't as big a problem with White's machines since they normalize their VDIs at 6.592Khz. (Disclaimer: I don't have a good understanding of how Minelabs work, so I do not know if they suffer from this).

For the purposes of hobby metal detectors, 5Khz will find a silver coin just as well as 22Khz will. The lower frequencies may have an advantage on deeper targets, but, over all there is not a huge noticable difference. I use a 3VI so I can hunt 2.5Khz, 7.5Khz, and 22.5Khz, or all 3 at once. I have found silver coins using all three frequencies (as well as all at once). When using multi-frequency, I use the lowest frequency data as the basis whether to dig or not (if it is in the negatives, it is definitely iron).
 
Aluminum is actually a low conducting metal. Having found nearly 1400 silver coins,.......

We always talk about conductivity, but that is only half the story. If we only look at conductivity, we can't begin to explain discrimination............

There's a great video related to this discussion of phase and discrimination and I can't seem to find it on youtube. Here's a different video that's shorter and gets right to the point, but doesn't show as many metals or go into some other technical details.

https://www.youtube.com/watch?v=fnwgf5RrhTg
 
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Ahhh...here it is. I doubt you'll find a better 12 minute video on the science of hobby metal detecting. He covers how DD coils function, phase shift and it's relationship to VDI, conductivity, and explains why different metals with the same conductivity behave/react differently in the magnetic field.

https://www.youtube.com/watch?v=EcuTsifSgBs
 
So the equinox is only running 2 frequencies in multi? If that's true then the v3i should be better running 3 frequencies at once.

I have no idea where MaxxKatt is getting the info that Minelab's Multi-IQ uses a combination of two frequencies for each hunting mode. He says himself that Minelab doesn't publish that information.

Regardless, whether something is "better" or not has to do with what the software does with that frequency information, the audio design, other features, etc.... It's a sum of the parts.
 
...........This is where the Minelab designers have decide which mix of two frequencies it uses in each hunting modes. A common misconception about the 800 is that in the multi-frequency mode it operates in all frequencies from 5 kHz to 40 kHz. This is not true. The engineers have selected the best two frequencies and Minelab does not publish these exact frequencies. The do give you hints.............

Do you have a source for this info?
 
...The 800 can operate in the multi-frequency mode. This is where the Minelab designers have decide which mix of two frequencies it uses in each hunting modes. A common misconception about the 800 is that in the multi-frequency mode it operates in all frequencies from 5 kHz to 40 kHz. This is not true. The engineers have selected the best two frequencies and Minelab does not publish these exact frequencies.

Do you have a source for this info?

Maxxkatt is essentially correct - it has been determined through oscilloscope analysis that the Equinox transmits two primary frequencies in each multi mode. Carl Moreland, who was a senior engineer at Whites and First Texas has broken the results down like this:

Park & Field modes: 7.8k + 39k
Beach modes: 7.8k + 18.2k
Gold modes: 40k + 5k

I’m having a hard time finding good links that show this information concisely. A lot of the stuff gets pretty technical and is for folks with larger hat sizes than me, but this is the best I could track down so far:

Geotech Forum: Minelab Equinox 800 Frequency (requires creating a free user account if you don’t already have one)

Detectorprospector.com forum: Multi IQ Spectrum Test

Detectorprospector.com forum: Minelab Multi-frequency

Regardless of the transmitted frequencies, the real secret sauce is how the received signals are processed and weighted.
 
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Maxxkatt is essentially correct - it has been determined through oscilloscope analysis............

Huh....I don't know how I missed that info. It's the first time I've seen it mentioned on these boards. In fact, it seems like the "how many frequencies" topic has been debated many times.

Anyway, I'm not surprised at all that it's not using all five frequencies in multi-frequency mode, but I would have put my money on three. My guess is that they found no additional benefit of using a middle frequency in addition to blending a higher and lower frequency.

I've had some clues based on where EMI is hitting in the single frequency modes and how audible it was in Multi-Freq mode. For example, I have noticed situations where I've heard no EMI in Park 1 Muti-Freq and yet it was hitting hard across the the middle frequencies. That made me wonder about the absence of those frequencies in the Multi-Freq mix.

It's also not surprising that the frequencies they chose to blend are not exactly the same frequencies they wanted to make available for a range of single frequency operation.

As everybody else concluded, the devil is in the details of what happens to those signals after they've been received. I also don't think it's clear whether and how harmonics are being received and processed. In the end, a detector is the sum of its parts and what they did with the Equinox truly set a standard of design and performance. But, there's also a reason why I have three detectors and use all three regularly.
 
Silver is highest conductor, followed by aluminum and gold, then coper Brass (10% zinc), zinc, brass (40% zinc), nickel, tin, iron, lead, platinum and tungsten.

Seeing how close aluminum follows silver you now should understand why aluminum screw caps are so difficult to distinguish from silver coins. Both have high conductivity and are round. The two very characteristics that the design engineers use to ID silver coins.


Aluminum does not follow silver. Silver is most conductive, followed by annealed copper, copper, gold, and THEN aluminum.
 
Yes lower frequencies do penetrate the ground and other solids better. Real world examples are hearing mostly the bass notes from your neighbor's loud stereo or those coming from cars who occupants love rap music.

The US Navy communicates with ultra low frequencies from large buried antennas to their sub under water. But the communications are very slow compared to their pop up near the surface sat transmission in the much, much higher frequencies and can transmit a lot of data in a very short period of time in contrast to the ultra low frequencies.

The post who said it is really the sum of the parts that makes it effective or something like that is correct. The Equinox and most modern multi-frequencies (at one time) detectors are in a sense a computerized signal processor to deliver to your ears and VDI what the signal processor thinks is the right tone, volume and VDI reading.

My sources are two individuals on two other metal detecting forums who I have come to learn really know their stuff and do not guess when offering opinions unless they indicate first they are just guessing.

This is how I learn more about this hobby. When I have a technical question that I cannot find a good explanation for my question elsewhere I go to these two technical sources in private messages.

I consolidate and share this information with others on this forum who might find it interesting. I in no way claim to know or totally understand a lot of things these two sources tell me. I do understand most of it, but I am not an electrical engineer or even close. I was a 32G20 in the Army and understand some of the fundamentals and took 3 physics courses when completing my BS degree at GA Tech. But that just basically gives me a little bit of background for trying to understand the theories behind our hobby. Does all this info help? It is fun to learn, but nothing replaces getting my butt out of this chair and into the field with my 800 or 540. Of course since I am into Civil War relic hunting in Georgia, my starting point is good research when trying to find available Civil War relic hunting. in my opinion the most successful hunters do their research and hunt.
 
Yes, I am also on the Tech forum to learn about and build my ow detectors.

Although a difficult read the Patents from the various detector companies do tell what is going on inside a detector.
Been reading the MineLab patent that much of the NOX is based on and it is very interesting. The Multi=frequency is used for a few internal adjustments like TX power, Coil compensation and Ground balance.
Then for target analysis.
This is in patent US7432715
 
One of the confusing concepts for newbies is conductivity in terms of metal detecting. Conductivity is a term that describes how easy electricity (electrons) flow through a metal. Low conductivity means the metal is more resistant to electron flow. A high conductive metal is a metal that offers a lower amount of resistant to electron flow.

What does this have to do with metal detecting? A higher frequency in a metal detector say 20 kHz to 40 kHz or higher is good for finding low conductive metals such as gold and tiny pieces of metal.

A lower frequency in a metal detector is better at finding high conductivity meals such as silver coins.

Silver is highest conductor, followed by aluminum and gold, then coper Brass (10% zinc), zinc, brass (40% zinc), nickel, tin, iron, lead, platinum and tungsten.

Seeing how close aluminum follows silver you now should understand why aluminum screw caps are so difficult to distinguish from silver coins. Both have high conductivity and are round. The two very characteristics that the design engineers use to ID silver coins.

A metal detector operating at below 10 kHz will find silver coins better than one operating at 20 kHz. A metal detector operating at above 20 kHz will find small gold particles better than one operating below 10 kHz.
You might ask what does this mean to me? When buying a metal detector, you should buy one with operating characteristic that best suits your type of hunting. If you are only hunting silver coins, then one operating at a lower frequency over a one operating at a higher frequency may be the better choice.

Likewise, if you are hunting gold you buy a gold detector that operates above 40 kHz to locate the smaller gold pieces. Another example is that when I am hunting civil war bullets is that with my Equinox 800 I can switch to a single frequency mode of 20 kHz or 40 kHz and tune my detector to hunting lower conductors like lead bullets or hopefully gold coins.

We have the best of both worlds which is a multi-frequency (Multi-IQ tm) like the Equinox 800. (I am using the 800 as an example because I own one and can write with more knowledge about its characteristics). The 800 can operate in the multi-frequency mode. This is where the Minelab designers have decide which mix of two frequencies it uses in each hunting modes. A common misconception about the 800 is that in the multi-frequency mode it operates in all frequencies from 5 kHz to 40 kHz. This is not true. The engineers have selected the best two frequencies and Minelab does not publish these exact frequencies. The do give you hints.

Park1 is better for coins (high conductors). Park2 is better for jewelry (high conductor alloys. Field1 is for coins and artifacts (coins, brass and iron).
Field2 probably the same mix of two frequencies as Field1 but with other settings changed.

Beach1 is good for lower conductivity targets such as gold jewelry (alloys) and small gold chains. Better for dry ocean beach sand.

Beach2 per the 800 manual uses a “very low” weighted multi-frequency combination to maximize ground balance for salt in the salt water. Better for wet ocean beach sand and hunting in the surf.

Also keep in minds each different mode in the Equinox 800 (and I assume other multi-frequency machines like the Simplex, Apex, Equnox 600 and Vanquish 540) have a different mixture of settings like recovery speed, iron balance, discrimination in addition to a different frequency combination.

What does all this mean to the 800 owner? When hunting a particular piece of property that you think is hunted out in silver coins, you can re-hunt the site with different single frequencies (lower) and recovery speeds, sensitivity and iron balance to essentially give yourself a completely different detector. In the past detectorists not using today’s more advanced detectors would have to hunt the “hunted out” sites with different detectors with different characteristics. This explains why you see on the forums lots of members owning more than a few different metal detectors.

Someone is bored.:laughing:
 
Low khz is good for depth and ID on modern milled high conductor silver in young countries like the US where the coinage is consistent.
Across europe for example, silver coinage varies in quality, much of it was debased, and lots of it fall into the low conductor foil range.
This is why higher freq detectors are so popular in the UK. Where a lot of folk in the US would disc out or ignore digging say a small signal reading 5 or 6 on an Equinox, we would dig it as it has the chance of being a small silver coin.
 
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