Target ID More About Target Size, Than Type Of Metal

[Geotech]

Nice post Steve. US classic coinage provides an excellent example. We have 4 coins (dime, quarter, half, dollar) that are all the exact same alloy: 90% silver, 10% copper. Yet have different responses due to size & thickness. And a nickel that is larger and thicker than a dime yet has a much lower phase response, showing that metal type can also play a strong role. But, yeah, any alloy can end up just about anywhere. I'm reminded of the big aluminum token I once found that had the exact same TID as a silver dollar.

 

[Cal_Cobra]

Steve you bring up some interesting points with the size vs conductivity discussion. 

Although I started park hunting, I mainly relic hunt these days.  I run my EQX in F2 not because it's supposed to be hotter on a certain type of metal, but because it's more sensitive to smaller targets.  This opens up a lot of types of targets as well as targets on the fringe/edge/etc.  But if it's more about target size vs conductivity, in theory using the Gold modes on the EQX vs Park or Relic modes for relic sites should work better eh 🤔

The college of hard knocks taught me that the only test that really matters on a detector is that detector in your hands at your sites. There's no such thing as inert soil in California or Nevada where I enjoy detecting.  Inert Florida soil doesn't tell us much out here about how a detector will operate in the field.  It's a bit akin to the nail board test, it's one metric we like to see our detectors pass, but it's only one of an endless variety of variables we're up against.

[PimentoUK]

The relationship between shape/size , metal and its conductivity, and target frequency is potentially quite complex, and results in plenty of variety, that will easily surprise anyone not well educated on the topic.
Most metals we dig are not 'laboratory grade' pure elements, they are either impure, or intentionally-created alloys, often with unknown properties. Alloys can have vastly different properties to their constituent elements ... far from intuitive.
For example .900 fine gold ( or 22ct / .917 alloy ) has an electrical conductivity about one-sixth that of pure gold, even though the 10% that isn't gold is copper, one of the best conductors. The conductivity of .900 gold is near-identical to pure tin, which is a pretty poor conductor if you look.
Likewise, the cupro-nickel alloy used in the US 5c coin, and many other world coins, is an especially lousy electrical conductor. It's widely used as electrical resistance wire, in heaters, kettles, hairdryers, electronics resistors. But it's 60% copper, not really what you'd expect.

And corrosion can make a big difference to real-world conductivity. It's one of the reason some of our ancient silver coins read quite low down the ID scale. Despite them originally being .925 Sterling Silver, the copper tends to leach out, leaving the remaining metal porous and sponge-like, so electricity doesn't pass easily through it. Low conductivity means low ID.

An example where physical shape gives non-intuitive behaviour is finger-rings ( or any ring shape item, like washers, for example ). It turns out that changing the diameter of a finger ring has hardly any affect on the target frequency / ID value. Scientifically speaking: if you double the diameter of a ring, you double the electrical resistance around the 'loop' , that's pretty obvious. What's less obvious is that the inductance of the loop also almost doubles, too. These two doublings pretty much cancel out, as ( inductance divide by resistance ) determines target ID. This can explain why very small copper washers can give impressively high ID values. It can even be tested practically - fabricating copper wire rings of different major diameter, but the same wire gauge is not too hard. Sweep them over the coil and the results can be seen.

[Jeff McClendon]

This is a fantastic topic. Since I usually use an Equinox in my area, I get lots of questions from fellow detector users about which mode to use.....

This is just from my experience so it could be totally wrong. The frequency used in my detecting area which like KOB and Cal Cobra has soil that is definitely not inert, really matters in terms of what frequency can actually penetrate far enough to hit deeper targets. SMF tech like Multi IQ really helps but single frequency detectors in certain ranges work too. The rule of thumb would say a lower frequency would penetrate farther on say, coin sized US targets. That is just not the case here. Frequencies in the vicinity of 20 kHz or a bit higher seem to be able to get through the magnetic ground fog created by all of the magnetite and volcanics in the soil in my area. Detectors running below 12 kHz get absolutely no depth here. Even some approximately 13 kHz VLFs (MXT/MX7, T2 F75 are great examples) that ordinarily are incredibly deep don't do well at all here.

So it seems to me that the amount and type of iron and volcanic content of the soil mineralization also determines what the best frequency/SMF frequencies may be for a certain area which may be determined by the size of the frequency wavelength. Using Multi IQ tech which seems to make the ground more invisible using the Equinox certainly helps here too.

[Geotech]

22 minutes ago, PimentoUK said:

Alloys can have vastly different properties to their constituent elements ... far from intuitive.

Good point... here is a graph I recently did for ITMD3 showing the conductivity of copper-nickel alloys, from pure copper to pure nickel. As you can see, practically the whole alloy range is worse than pure nickel despite the addition of copper. This quirk is true of many simple alloys.

[mh9162013]

Thanks for the great post; definitely learned a lot. 👏

[Steve Herschbach]

1 hour ago, Geotech said:

Good point... here is a graph I recently did for ITMD3 showing the conductivity of copper-nickel alloys, from pure copper to pure nickel. As you can see, practically the whole alloy range is worse than pure nickel despite the addition of copper. This quirk is true of many simple alloys.

 

Gold alloys especially. Almost nobody detects pure gold, but alloys that are all over the map.

Regardless of the metal type, as long as it and the shape is the same, the larger it is, the higher it reads. Throw in differences in shape and purity as pimento notes, and it’s a Wild West crapshoot. These gold nuggets vary wildly in purity, shape, and porosity. It means some small nuggets read very high, and some big stuff very low, the big nugget reads like a silver bar.

Australian and California gold is relatively pure and clean, so the numbers climb nicely with size. Alaska gold is crap by comparison, lower purity ( yiu can see the color difference), lots of quartz inclusions, etc. The numbers are all over the place.

 

[Cascade Steven]

Geotech:  I've been away from the forum for a while and just dropped in on this very interesting conversation.  I have a question about conductivity and apologize in advanced if I missed the answer elsewhere.  When you talk about conductivity of metals, do you refer to electrical conductivity or magnetic conductivity as in magnetic susceptibility?  Thanks

[PimentoUK]

Steven : It's electrical conductivity ... how good would it work if you made an electric cable out of it ?

Copper, silver = great choices

bronze = not that good

lead, tin = going to need pretty thick wire

cupro-nickel, stainless steel = they will be glowing red all the time

Here's a decent reference table for many 'industrial' metals/alloys:

http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/

 

[Erik Oostra]

18 hours ago, Steve Herschbach said:

You have to know enough to read between the lines when it come to marketing, and in this case it is really telling you lower frequency for larger stuff, higher frequency for smaller stuff. Again, does not matter if it is silver, copper, or gold. It's all about size.

What you're saying here applies to Equinox update 3.0.. According to Minelab adding 4 kHz to the single frequencies "enhances the detection of large deep targets, particularly those found in parts of Asia".. I'm not sure why Minelab focuses on Asia, but it does reinforce their message that lower frequencies are for larger stuff.. Like you said: "It's all about size".. 

This does have me thinking about update 4.0.. if we ever get one.. maybe they'll go for a higher frequency next time? say 45 kHz for small shallow targets to bring the Equinox inline with the Gold Monster.. now that would be good.. I'd definitely get that update.. 

Thanks Steve for always explaining this stuff so well..