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I am looking at being able to detect aluminium (or other cheap metal if suggested) with ultra high specificity and sensitivity in the presence of steel, and possibly titanium.

The piece of aluminium would be a cord of around 3mm by 5cm.

Would a metal detector be able to be tuned for this purpose?

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  • \$\begingroup\$ A phrase for this is "metal discrimination". Metal detectors can use discrimination to tell different metals apart. \$\endgroup\$
    – Passerby
    Oct 5 '13 at 17:06
  • \$\begingroup\$ Can you explain what you mean by a "cord"? Is the sample in motion? How close can you get to it? Are the samples alloys or pure metals? What do you mean by "ultra high specificity"? \$\endgroup\$
    – Joe Hass
    Oct 5 '13 at 17:12
  • \$\begingroup\$ It is a cord or band within a piece of surgical cotton gauze, placed to make it x-ray detectable. I was wondering if a metal detector could detect this inside the body without being confused by the steel clips etc used inside the body. (I'm a surgeon) \$\endgroup\$
    – gcgregory
    Oct 5 '13 at 18:31
  • \$\begingroup\$ so it would have to work through body tissue \$\endgroup\$
    – gcgregory
    Oct 5 '13 at 18:32
  • \$\begingroup\$ ultra-high specificity, ie no false positives \$\endgroup\$
    – gcgregory
    Oct 5 '13 at 18:35
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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.

Will this technique be able to tell the difference between aluminium and iron? Yes

Will this technique discriminate between iron and titanium? Almost certainly

Will this technique be able to recognize a regular and repeatable mass of aluminium and discriminate it from titanium? Probably, but tests would need to be carried out to see how easy that is, given the likely signals from various metal types.

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  • \$\begingroup\$ pressed enter by accident, it sounds obvious, but the carbon in steel or the chromium in stainless will not confuse the ability of the detector to distinguish? \$\endgroup\$
    – gcgregory
    Oct 5 '13 at 19:56
  • \$\begingroup\$ @gcgregory - Stainless at 300kHz is easily distinguishable from steel when the size is less than about 2.5mm diameter. From memory it's easier to distinguish larger pieces at lower frequencies. Whatever solution you come up with you'll need to trial it at different frequencies to optimize the final method. \$\endgroup\$
    – Andy aka
    Oct 5 '13 at 20:47
  • \$\begingroup\$ @gcgregory body tissue is slightly conductive compared to metals and by its shear bulk compared to small metal pieces the effect it gives to metal detectors is known as "bulk effect" and can look like stainless steel. SS has a mainly "resistive" phase angle because it's non-ferromagnetic properties tend to cancel out its much weaker ferromagnetic properties hence it can look like tissue (well chicken carcasses anyway (been there, and failed to detect SS contaminants). \$\endgroup\$
    – Andy aka
    Oct 5 '13 at 23:11
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If you have control over the gauze, I would consider using RFID instead of simply detecting the metal. Small waterproof RFID tags with reasonable range are easily available - consider microchipping pets, for example.

False positives will be almost non-existent (unless someone takes tagged gauze too close, which is an issue with pretty much any system like this), and you should be able to differentiate between different items - write 'gauze' on one tag, 'sponge' on another for example.

I'm not certain whether the range would be adequate, though. I don't think you would get more than 20-30cm, so you may need to e.g. wave a wand over. Building a scanner into an operating table might be feasible, though.

(yes, I realise this is a rather old post)

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