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Discriminate, Discriminate, Discriminate...but What Exactly??


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The Gold Monster is sparking my interest in VLF to supplement my detecting arsenal. 

But I have a question - what exactly does a discriminating machine discriminate out?  

I am assuming ferrous and non-ferrous, which in my simple terms means magnetic and non magnetic.  Please correct if wrong.

So on the iron/ferrous side do we have - nails, tin, bottle caps, metal buttons, please add things in here. Where do meteorites fit in?

On the non ferrous side we obviously have gold. But do we also include lead, aluminum, I think most coins aren't magnetic (never tested them), please add things in here. 

With VLF am I still going to be digging every bit of lead, aluminium can and coin?  Or does is narrow down to just gold?  

I have done a brief search on here that got a lot of hits for the word discriminate but nothing specific.  I also know I could google a lot of these answers but I know the very experienced on here will be able to give a very succinct list of what is discriminated out and I can only assume that this is a question many other newbies would be interested to know the answer for.  

I hope you can help. 

Adam.

 

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G'day Northeast. 

This will help understand it a bit : Not a total listing but enough to give you the idea

 

4.  Ferrous metals are metals that consist mostly of iron and small amounts of other elements. Ferrous metals are prone to rusting if exposed to moisture. Ferrous metals can also be picked up by a magnet. The rusting and magnetic properties in ferrous metals are both down due to the iron. Typical ferrous metals include mild steel, cast iron and steel. Examples: 1.Mild Steel. 2.Cast Iron. 3.High Carbon Steel. 4.High Speed Steel. 5.Stainless Steel. Rusting. Magnetism.

Some examples of Non-Ferrous Metals are:

  • Aluminium & Aluminium Alloys
  • Copper
  • Gold
  • Silver
  • Brass
  • Lead
  • Zinc
  • ETC.

The Gold Monster 1000 doesn't narrow it down to just gold.

A combination of detect modes ( Gold/iron reject & All Metal ) coupled with the "Gold Chance Indicator" will hopefully help narrow it down some what though.

 

Cheers,

Ashley

 

 

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4 hours ago, Northeast said:

So on the iron/ferrous side do we have - nails, tin, bottle caps, metal buttons, please add things in here. Where do meteorites fit in?

Iron, stony-iron and most chondrite meteorites are ferrous, so when using the Gold Monster 1000 to search for them, you'll want to use the all-metal mode. 

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Short answer - it is fairly easy to separate ferrous from non-ferrous, and that is what many prospecting machines attempt to do. There is no way to reliably tell one type of non-ferrous metal from another non-ferrous metal, so you have to dig all the lead bullets, aluminum, etc. to get the gold. Now some details.

Metal detectors are electromagnetic devices. They therefore "see" items that can either conduct electricity (are conductive) or have magnetic properties, or both together.

The most basic obvious difference is the split between items with magnetic properties and items that do not have magnetic properties. We tend to think in terms or ferrous and non-ferrous but it is a bit more complicated than that. Stainless steel for instance can be very conductive but have minimal magnetic properties. It is maybe a bit more to the point to think in terms of "things that rust" and "things that don't rust". Common ferrous items that rust are fairly easy for detectors to deal with, but even then thin flat steel and items with holes, like washers, can present issues. What most prospecting type VLF detectors attempt to do is separate ferrous from non-ferrous. All items like gold, copper, lead, aluminum, silver, etc will read as "good" items on a machine like a Minelab Gold Monster.

Coin detecting machines try to go a step farther. The more conductive an item is combined with the larger the item is can be measured by a metal detector. Aluminum, lead, and gold are less conductive than copper or silver and so read lower on a metal detector identification scale. However, a small gold item reads lower than a large gold item. A small copper item reads lower than a large copper item. A large enough gold item can read like a small copper item. Therefore you cannot tell one metal from another. If every item were exactly the same size and shape you could sort metal directly by their conductivity, but the fact that larger items conduct electricity better than small items throws that out the window. Just like a large copper wire carries electricity better than a thin copper wire, size matters as much as basic conductivity.

However, for certain man made items, like a dime, you can calibrate a response that identifies a dime. The catch always being something else may also read as a dime.

whites-v3i-target-vdt-scale.jpg

Even more technical, from How Metal Detectors Work

"The resulting received signal will usually appear delayed when compared to the transmitted signal. This delay is due to the tendency of conductors to impede the flow of current (resistance) and to impede changes in the flow of current (inductance). We call this apparent delay "phase shift". The largest phase shift will occur for metal objects which are primarily inductive; large, thick objects made from excellent conductors like gold, silver, and copper. Smaller phase shifts are typical for objects which are primarily resistive; smaller, thinner objects, or those composed of less conductive materials.

Some materials which conduct poorly or not at all can also cause a strong signal to be picked up by the receiver. We call these materials "ferromagnetic". Ferromagnetic substances tend to become magnetized when placed in a field like a paper clip which becomes temporarily magnetized when picked up with a bar magnet. The received signal shows little if any phase shift. Most soils and sands contain small grains of iron-bearing minerals which causes them to appear largely ferromagnetic to the metal detector. Cast iron (square nails) and steel objects (bottle caps) exhibit both electrical and ferromagnetic properties.

It should be pointed out that this discussion describes an "Induction Balance" metal detector, sometimes referred to as "VLF" Very Low Frequency (below 30kHz). This is the most popular technology at the present time, and includes the "LF" Low Frequency (30 to 300kHz) instruments made for prospecting.

Discrimination

Since the signal received from any given metal object exhibits its own characteristic phase shift, it is possible to classify different types of objects and distinguish between them. For example, a silver dime causes a much larger phase shift than an aluminum pull-tab does, so a metal detector can be set to sound off on a dime yet remain quiet on the pull-tab, and/or show the identification of the target on a display or meter. This process of distinguishing between metal targets is called "discrimination". The simplest form of discrimination allows a metal detector to respond with an audio output when passed over a target whose phase shift exceeds a certain (usually adjustable) amount. Unfortunately, with this type of discriminator the instrument will not respond to some coins and most jewelry if the discrimination is adjusted high enough to reject common aluminum trash for example pull-tabs and screw-caps."

A few articles on this website that may help:

Metal Detectors With Reliable Target ID Numbers

FORS Gold, F75 & V3i Tone And VDI Tidbits

The Ferrous/Non-Ferrous Overlap

Tune Out Nails, Your Will Miss Gold!

Target ID / VDI Numbers For Gold Nuggets And Gold Jewelry

Visual Comparison of Some Target ID Scales

Target ID Normalization Details page 4-3 of V3i Advanced Manual

Here is another chart showing typical target overlap issues. Ferrous and low conductive/small items near top, more conductive/larger items near bottom.

makro-racer-2-target-id-vdi-chart.jpg

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Ashley, Lunk and Steve - thank you. I really appreciate the time/effort you have taken to respond. 

And Steve, your response in particular - I was happy just to know the what but find myself now knowing the why...and actually understanding it :ohmy:

Hopefully it will answer the question for someone else too. 

Thanks again :wink:

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