How does a metal detector tell ferrous from non-ferrous?

And why, sometimes, is it not able to? (detector signals non-ferrous but what you dig is iron)

And how does it assign a degree of ferrous-ness? (ferrous reading on e-trac, for example)

Does the detector use the magnetic property of the ferrous target in any way?

Before I posted these questions I went here:
[electronics.stackexchange.com]

and found this:
Metal detectors can, in their basic form, discriminate between ferrous and non-ferrous metals. Basically, a ferrous metal will normally tend to increase the inductance of the "search-coil" whilst non-ferrous metals will reduce the inductance of same coil.

The most basic metal detector is a BFO type and its search-coil inductance forms a tuned circuit oscillator with a parallel capacitor - the oscillator frequency rises in the presence of non-ferrous metal and usually falls with ferrous metal.

Inductive balance metal detectors tend to be the main type used by beachcombers and these have a fixed oscillator coil and two "outer" receive coils that are wired antiphase producing zero net signal in the presence of no metal. The outer winding signals get amplified and used as the means of detecting metal. Like a BFO, inductive balance detectors can discriminate between ferrous and non-ferrous metal by the phase shift of the signal received.

Simple metal detectors (to discriminate against nails and other non-significant ferrous metal) will force an imbalance in the coils electronically and, ferrous metal will decrease the imbalance-signal whilst non-ferrous (likely to be more valuable) will increase the imbalance-signal. This will trigger a buzzer or light to indicate to the "user" the presence of a non-ferrous metal.

That's about as far as it goes on beachcombing detectors but the food and pharmaceutical industry have gone a step further. The received signal is processed and two parameters are extracted. These are the resistive and reactive components of the received signal and can tell you a fair bit about the make-up of the metal.

In these more sophisticated metal detectors, it is easy to discriminate between (say) 1mm iron and 1mm aluminium or 1mm brass - each will have there own signature relationship (resistive to reactive relationship) and, the overall amplitude can tell you how close the "foreign" object is to the search coils. It's fairly easy to recognize the difference between 1mm Fe and 2mm Fe based on phase angle alone - the amplitudes could be similar if the 1mm metal was close to the search head compared to the 2mm metal but there will be something like a 10º phase angle difference at a running frequency of 300 kHz. Different frequencies can be used to exploit different things.

Unfortunately there is some overlap in phase angle (resistive to reactive relationship) between different metals at different sizes but this can usually be overcome by knowledge of the overall amplitude.



Does anybody care to explain or refute any of this?

Thanks.

Wayne

ncwayne Wrote:
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> And why, sometimes, is it not able to? (detector s
> ignals non-ferrous but what you dig is iron)

You'd have to be more specific on this one, I use a CZ3D and never dig iron unless I chose to dig iron.


> And how does it assign a degree of ferrous-ness? (
> ferrous reading on e-trac, for example)

NASA Tom will have more definitive answers to this however,
it is my understanding that the detector analyses the decay of the electromagnetic field generated around the target which is registered back through the receiver coil in order to i.d. a target.

>
> Does the detector use the magnetic property of the
> ferrous target in any way?

Well yes it does..... any time you sweep your coil ( transmitter and receiver ) over a ferrous or non-ferrous target an electromagnetic field is generated through and around the skin of the target. The field varies based upon the properties of the target.
The electrons of the target are disrupted at differing degrees while this electromagnetic field is generated as well as the amount of decay from that same magnetic field is then sampled and received from the receiver coil and finally analyzed.

Again, NASA Tom would be your best bet to fully explain it with full clarity.

Up to my ____ in Pulltabs, Grant



Edited 1 time(s). Last edit at 03/23/2018 09:40PM by Up to my ____ in Pulltabs, Grant.

ncwayne

just be thankful we're not still stuck using BFO machines!

Not sure when you 1st picked up a detector but when I started in the 70s many guys (including myself) still used them (BFO's) because they were cheap and readily available (especially on the used market) VS TR units with disc and then TR/VLF machines with the capability to handle ground mineral that were just coming into play

it didn't take me long to read up on Garretts new tech of that time in western/eastern mags and know that I had to have a TR/VLF machine with disc but my 1st one was a BFO piece O crap. Literally most had around 3" at max for depth on coins in good dirt and no way to know what was under the coil unless you dug

BFO (beat frequency oscillator) metal detectors used two oscillators, each of which produced a radio frequency. One of these oscillators uses a coil of wire that we call the search coil. The second oscillator uses a much smaller coil of wire, and is usually inside the control box and is called the reference oscillator. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the coil is moved over or near a piece of metal. These metal detectors are easy to build at home from kits and/or from a kit with some mods added.

TR, VLF and VLF/TR may be a bit confusing but they are all transmitter/receiver detectors. The terms TR and VLF are merely used to provide an easy way to distinguish the difference between the two. In electronics, VLF (as we all know) stands for Very Low Frequency (usually 3 - 30 kHz), but with metal detectors the term is usually used to indicate a "mineral-free all metal" mode of operation. TR stands for Transmitter/Receiver, but usually indicates a "non-motion discriminate mode" of operation. VLF/TR usually indicates "mineral-free all metal" and, "mineral-free discriminate" operation. Although they had their place, the higher frequency TRs were generally not competitive with the (at the time) latest VLF/TR's with silent search motion discriminators as they are/were unable to handle the problems of mineralization in the ground. TR's gave way to the superior ground effect control in/of the VLF imachines. TR's are normally restricted to use on non mineralized dirt or salt water beaches and in salt water itself as they are/were capable of ignoring the effects caused by conductive salts when properly tuned or did really well in good dirt too. Since TR's cannot ground compensate for mineralization, they are incapable of obtaining the depth achieved by the newer VLF type units in mineralized dirt limiting them to finding only shallow surface targets when used in mineralized areas. But, the newer VLF/TR detectors of that time did ground compensate, thus accomplishing much greater depths of detection when mineralization is/was present.

One of my all time favs from back then was the master hunter series from Garrett and especially around 1983 when the (Master Hunter ADS III VLF/TR Deepseeker with controls on side, and toggle switch) was introduced - it was a heavy beast with like 6 batteries and a pain to set up proper but once balanced and ready = I pulled handfuls of silver time and time again in parks, schools, house sites, vacant lots, churches, etc (masses of barber, fair amount of seated, tons of mercs and roosies) from every place I ever used a BFO machine prior because they were (the TR/VLF's) getting around 7"/8" depth all day/everyday on coins in my dirt and you didn't have to dig every signal just to see what it was UNlike with the BFO's which slowed you down because of digging so much trash plus the majority having had poor depth.

Side note: When running a TR with disc or a VLF/TR in TR mode with disc = they are/were able to see right thru iron (just like the old Compass machine/s). You can stick an iron barn spike on top of a quarter and the TR mode will ignore the spike and sound off on the quarter!

Hope Tom chimes in on this one = best thread I've seen on here in months

Wayne, that web site you quoted is about 40 years out of date. In a VLF detector, targets create an imbalance that produces both an amplitude and a phase. A purely magnetic target (like ferrite) produces a phase of 0°. Eddy targets produce phase responses from 90° (salt/foil) to 180° (big honkin' silver). Iron is a combination of magnetic & eddy responses and, depending on which part of the response is stronger, can fall anywhere from 0° to 180°. The composite magnetic/eddy response for a given piece of iron can also vary with depth so a piece of deep iron might show primarily an eddy response, making it indistinguishable from non-ferrous. Dig it up and wave it near the coil, and it clearly responds as iron. Finally, the composite response also varies with frequency, but in a way that's different from just an eddy target. So a multi-frequency detector often does better at iron ID. But deep iron can still fool even a multi-freq.

MichiganRelicHunter Wrote:
-------------------------------------------------------

> Side note: When running a TR with disc or a VLF/TR
> in TR mode with disc = they are/were able to see r
> ight thru iron (just like the old Compass machine/
> s). You can stick an iron barn spike on top of a q
> uarter and the TR mode will ignore the spike and s
> ound off on the quarter!
>
> Hope Tom chimes in on this one = best thread I've
> seen on here in months


Here I am ! What a trip down memory lane. There was some confusion early on, in those days. Because when the vlf/trs were introduced (like the groundhog, and 5000 D series I), the "TR " was, of course, the disc. mode. So sometimes the term "TR" became synonymous with "discriminate". However, other earlier machines, like the 77b and 66TR were also "TR". Albeit all-metal TR.

When I started: The school chum who'd gotten me into this, was swinging a 77b. And I quickly ran out and bought a used 66TR. His 77b was better and smoother though. But even by 1975 standards, both were quickly becoming dinosaurs. Doh! Pretty soon, a person was lost if he didn't have a TR disc.

But as thick as things would have seemed to be: We (or at least my buddy and I) didn't have the brains or know-how to do awesome sites in those days. It was just school-yards for mercs , wheaties and buffalos. We weren't doing relic hunting (ghost towns and stage stops and ruins). Weren't doing beach storms (didn't know about that, nor had driver's lic or cars to take us to the beach). Didn't know about old-town demolition sites (never occurred to us, and would have been scared of fences). Blah blah blah. So as virgin as things were, yet it's all relative: The machines and disc. and depth was primitive. So we'd knock ourselves silly for a 3-silver day in a park or school. Some things never change, eh ? haha

MRH good memories of the ole days. I can well remember hearing a BFO approaching me in the woods while I was pulling 2" minie' balls with my trusty MetroTech.

Now we see why it takes so long to make a breakthrough in detector development - thanks Geo!

Rick Kempf
Gold Canyon AZ- where there is no gold