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Metal detecting enthusiast here!

How does a change in frequency help to detect gold better? I keep reading that one manufacturer's metal detector works at 18KHz to better detect gold. Yet another manufacturer is about to release a new coil that operates at 15KHz, 30KHz, and 55KHz which will locate gold even more effectively. Why does an increase in frequency help to locate gold with a metal detector?

Can someone explain that?

Here is a link to the coils in question.

http://md-hunter.com/new-xp-22-5-hf-coil-two-of-them-for-xp-deus-novelty-2016/

Link to the manufacturer's website and page that talk about their new coils: http://www.xpmetaldetectors.com/blog-detection/en/update-deus/deus-product-information-and-news/

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  • \$\begingroup\$ Are you sure that its about the actual metal? All I ever heard about is depth and size of objects that somewhat correspond to frequencies. \$\endgroup\$
    – PlasmaHH
    Jan 16, 2017 at 13:41
  • \$\begingroup\$ A link to what you read would be good. \$\endgroup\$
    – Andy aka
    Jan 16, 2017 at 13:42
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    \$\begingroup\$ If I was trying to sell a new metal detector into a crowded market, I might just be tempted to say that it's different to the others, and that it detects gold better. Just sayin'. There are metals that increase the field, iron, nickel, and ones that decrease the field, conductive loops of much anything else. The difference between artefacts of brass, copper, alli, gold will be overwhelmingly due to their size and shape, and little to do with their conductivity. \$\endgroup\$
    – Neil_UK
    Jan 16, 2017 at 13:50
  • \$\begingroup\$ @Andyaka, I added a link to the coils in question. They're going to be released sometime soon. XP is a very reputable name in the MD community. \$\endgroup\$
    – Anton
    Jan 16, 2017 at 13:55
  • \$\begingroup\$ I wonder if that screw between coil and shaft isn't metal. \$\endgroup\$
    – Janka
    Jan 16, 2017 at 13:56

1 Answer 1

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A quote from the linked page says this: -

Designed primarily for gold prospecting

And this raised my eyebrow straight away. See the picture of the search head: -

enter image description here

Anyone who has prospected for gold in the pay dirt will not want an open frame search head. What happens is that when you get a signal (hopefully gold), you move your search head to one side and, with a small trowel, start digging and putting a trowel's worth of pay-dirt on top of the search head, discarding each trowel-worth of dirt when it doesn't register a reading. Thus an open-frame search head is not that good for doing prospecting.

So, my initial thought that this was marketing BS is somewhat reinforced.

Why are low frequencies favoured and what problems might you get with higher frequencies? A bit of a back-story: -

Traditional metal detectors use "discrimination" to select between ferrous and non-ferrous materials. All that discrimination does is shift the neutral inductive balance so that ferrous materials produce a smaller signal (up to a certain object size) whereas non-ferrous metals instantly produces a bigger signal. This method has been used for years but, since technology in metal detectors in the food industry (for instance) have measured a signal by its phase angle and amplitude (a much more complex method), the treasure hunting metal detectors are playing catch-up and may indeed be able to properly discriminate between ground effects and small pieces of metal when operating at much higher frequencies.

Ground effects are a problem but generally, the effects produce phase angles indicating that the signal is purely resistive i.e. the ground takes energy and converts it to small amounts of heat. Good-conducting metals produce an "inductive" effect that registers a phase angle of around plus or minus 90 degrees. As you go higher in frequency the ground effect phase angle spreads out and starts to encroach on the phase angle produced by metals.

So it wouldn't surprise me that if decent tech is used you can push the operating frequency higher. Pushing the operating frequency higher means a higher induced eddy current in the "gold" and a better disturbance in the oscillation field. This basically boils down to better signal to noise ratio and hence smaller particles can be detected.

Will it detect gold rather than silver or copper or aluminium? No, look at this electrical resistivity table: -

enter image description here

For a given sample size/shape it will be very difficult for a metal detector to distinguish between the top four conducting metals.

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  • \$\begingroup\$ Your knowledge of EE is impressive! Perhaps you should go to work for XP and give them a hand! \$\endgroup\$
    – Anton
    Jan 16, 2017 at 14:55
  • \$\begingroup\$ I'd rather go gold hunting but the only really interesting places left are Alaska and Australia and I'd rather stay in the UK. Having said that I did a bit last October in North Carolina (where gold was first found in the US) at the John Reed gold mine and Thermal city. en.wikipedia.org/wiki/Reed_Gold_Mine and thermalcitygoldmine.com \$\endgroup\$
    – Andy aka
    Jan 16, 2017 at 14:57
  • \$\begingroup\$ Are you into metal detecting? \$\endgroup\$
    – Anton
    Jan 16, 2017 at 15:10
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    \$\begingroup\$ I've designed food and pharmaceutical metal detectors but my gold mining experience was purely around hiring equipment to process the pay dirt and automatically sieve the gold. I have some interest in MDs but not enough to buy or build one! \$\endgroup\$
    – Andy aka
    Jan 16, 2017 at 15:22
  • \$\begingroup\$ I designed this monster but remember it's for a food line and conveyor belt: w3.inlineprosess.no/images/ip-no/produkter/instrumentering/… \$\endgroup\$
    – Andy aka
    Jan 16, 2017 at 15:26

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