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Author Topic: Metal Detection ...how do they do it ?  (Read 9077 times)

Offline neilep

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Metal Detection ...how do they do it ?
« on: 19/10/2007 16:03:50 »
Hi Peeps Of Treasure Chesting Digging Joy !

I'm neil...how are ewe ?..i'm Ok...still have an ear infection though !!..not nice !!
Thanks for the chit chat !!

see this metal detector:




Looks cool don't it ?..I always wanted one...never had one though.....I'd like this one !!

How On earth does it do what it does ?

..but I also want to know how it differentiates between different types of metal too ?




I don't know.....I heard that this forum was a good polace to ask questions about stuff so I thought I'd ask ewe guys.


 

Offline daveshorts

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Metal Detection ...how do they do it ?
« Reply #1 on: 19/10/2007 23:35:07 »
I think some of them work by having two oscillators, one of which involves the coil at the bottom, these two are tuned to be at the same frequency. What you normally hear is the difference between the two oscillators. so if it isn't near anything metallic you don't hear anything.

When you put the coil near a piece of metal you will get electric currents moving in the metal, these produce magnetic fields of their own which make it harder for current to move through the coil. This alters the speed of the oscillator, and you hear the difference as a squeal of varying pitch.

Different sized bits of metal and objects with different resistivities will be sensitive to different frequencies, and the sound will change differently as you move it across the metal. So you could change the overall RF frequency and see what the effect is. I don't know if modern metal detectors do anything more sophisticated, like sending in more than one frequency at once your ears would probably be good at interpreting the information.
 

Offline elegantlywasted

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Metal Detection ...how do they do it ?
« Reply #2 on: 19/10/2007 23:43:37 »
My experience with metal detectors is that they dont really detect metal, just a change in density. I worked at a meat packing plant and all of our products needed to pass through a metal detector, and in my training they told us that the variety of machine we used scanned the meat (each meat had a different density) and any anomalies in the density reading were read as bits of metal.

upon rereading that, I'm not entirely sure it made sense...
 

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Metal Detection ...how do they do it ?
« Reply #3 on: 20/10/2007 00:13:57 »
My experience with metal detectors is that they dont really detect metal, just a change in density. I worked at a meat packing plant and all of our products needed to pass through a metal detector, and in my training they told us that the variety of machine we used scanned the meat (each meat had a different density) and any anomalies in the density reading were read as bits of metal.

upon rereading that, I'm not entirely sure it made sense...

No, it doesn't make sense, and won't make sense to people who use metal detectors for finding coins etc. in the soil.

Ultrasonics could be used for density.
 

Offline elegantlywasted

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Metal Detection ...how do they do it ?
« Reply #4 on: 20/10/2007 00:17:41 »
I was always told it was a metal detector, but it is quite probable that it could have been an ultrasonic device
 

Offline techmind

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Metal Detection ...how do they do it ?
« Reply #5 on: 21/10/2007 19:10:40 »
see this metal detector:

How On earth does it do what it does ?

..but I also want to know how it differentiates between different types of metal too ?


The general principle of metal detectors that the inductance of a coil of wire changes when metal objects are brought into its proximity. For an air-cored coil, the sensing range will be comparable to the diameter of the coil - but will depend significantly on the size of the target object.

Inductance is a tricky concept to describe, but relates to the way energy is exchanged between a current-flow and a magnetic-field (any current-carrying wire creates its own magnetic field). A straight piece of thick wire has very low inductance, a coil of a few turns has slightly more inductance, and coil with many turns has much more inductance.
See also Wikipedia on inductance: http://en.wikipedia.org/wiki/Inductance


Anyway...

Ferromagnetic metals (principally iron, but also nickel and cobalt) will increase the inductance of the coil (because they act like an electromagnet and "amplify" the magnetic field), provided the measurement frequency is fairly low (few 10's Hertz). Other metals, such as copper or aluminium (or iron at higher-frequencies, say >10kHz), will decrease the inductance (because induced eddy-currents in those materials will create opposing magnetic fields). By measuring whether the inductance increases or decreases in the presence of a specimen, possibly at different measurement frequencies, your metal-detector could classify some metal types.

There are several ways to measure inductance of a coil, but typically you wire it up as part of an L-C (coil-capacitor) oscillator circuit; the frequency of oscillation decreases when the inductance increases and vice versa. For maximum sensitivity you need to detect very small changes, so systems of balenced/differential coils may also be employed.
 
« Last Edit: 21/10/2007 23:23:50 by techmind »
 

Offline Karen W.

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Metal Detection ...how do they do it ?
« Reply #6 on: 22/10/2007 08:54:43 »
I love metal detectors we found tons of fun treasury goodness as children! LOL and adults.. I bought two for my preschool children when we were studying metals magnetism etc.. they used the detectors to decide which objects were metal, then we testes tons of things we found with magnets, hee hee hee! IT was very fun!!!!
 

Offline Chloecray

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Re: Metal Detection ...how do they do it ?
« Reply #7 on: 16/07/2013 08:25:31 »
I do not know much about the theory of it. Found this somewhere else, the discrimination of metal detector work by by identifying diifferent frequency that transmitted from diverse metals. Many gold detectors use this to distinguish out trash metal such aluminum or lead. Seems make sense.
« Last Edit: 16/07/2013 09:13:18 by CliffordK »
 

Offline MrVat7

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Re: Metal Detection ...how do they do it ?
« Reply #8 on: 08/02/2014 02:44:53 »
I think it works on principle of electromagnetic induction .
 

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Re: Metal Detection ...how do they do it ?
« Reply #8 on: 08/02/2014 02:44:53 »

 

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