Vallon VMH3CS Mine Detector

[Ferric Toes]

Eric, can you give a relative rate of "rapid change of amplitude" that you experienced? I have assumed the fastest rate to be more than 10 seconds, but this is a total "assumption". This is an area where I have no experience or data to use as a basis for comparison.

Difficult to answer, but at a sweep speed of 1m/sec the coil could encounter, within the 1m, say, three high viscosity areas and three low. The 'rapid change of amplitude' is observed at the output of the detectors first amplifier. This is then sampled, integrated, balanced, and subtracted to give a dc output that does not change throughout the sweep. This is similar to earth field cancellation where you are sweeping over magnetised rocks. A good detector will give no output but if you observe the first RX the signal is leaping about quite dramatically. I would say milliseconds rather than seconds depending on pulse rate and coil diameter.

Eric.

[Ferric Toes]

Eric maybe it was at "Beggary Hill" at a guess.

Hi Gary, That's the place.

[STAVROSNU]

Dear Eric,

please read your private messages.



Best Regards

Stavros

[Skippy]

On the subject of 'standard test targets' that are low-conductivity and small-ish, I'm quite fond of the Norwegian/Swedish 10 Ore coin from the 1980's. They are cupro-nickel, small (15 mm) and thin, too. They target ID below the UK CuNi 5 pence and CuNi sixpence, on a VLF. Their time-constant is about 3.4 microsecs.
I know you're thinking that it's obscure, but they are quite common, essentially worthless, and easily found on eBay. I've picked up some from 'world coin' mixes at car boot sales (=flea market)
https://www.ngccoin.com/price-guide/...289-duid-94533

[6666]

Eric maybe it was at "Beggary Hill" at a guess. Hi Gary, That's the place.

Thanks will have a look there next time I'm up that way.

[Ferric Toes]

Foil and Coil Tests

Wanting to do some waveform tests free from noise, I constructed a small Fig. 8 test coil. This was made of of two windings left over from some industrial sensor work a while back. The windings are about 350uH each and by putting two, one on top of the other, I managed to achieve 1.36mH, which I didn't expect to work on the Vallon as it is rather low. Additionally the resistance was too high at over 6 ohms due to the thinner wire used. I was quite surprised therefore, on power up test, that the coil checked OK and the Vallon was alive.

The next thing I wanted to do was some more tests on kitchen foil that I had started earlier. I cut a piece 1in square (25.4mm) and found that I could detect that at about 1 in above one side of the coil. Wanting to start with a size of foil that was just not detectable, I cut the foil down to 20mm square. [From here on I stuck with metric for the foil]. I then folded the piece in two to give 20 x 10mm and found that it was just detectable above the coil. Second fold and the range was an inch. Third and fourth fold and the range was up to 1.5in. Fifth and sixth fold and the range was dropping but still 1in when the foil was 2.5mm x 2.5mm. The folded thickness ended at 1mm.

This is interesting because the mass of metal remains constant while the surface area reduces and the thickness increases in proportion. This begs the question on how realistic is the kitchen foil test in relation to what we are looking for? The sensitivity of industrial metal detectors, are tested by using ferrous and non-ferrous spheres as this gets rid of the orientation problem. Metal cubes and cylinders are also used, where l = d in the case of a cylinder.

I shall be using this small fig. 8 coil to look at the object signals at low levels free of noise, although I shall use my preamp across the coil as the Vallon one is inaccessible.



What looks like a solder splash is the folded piece of foil.

Eric.

[Skippy]

I'm not a fan of folded foil, due to lack of consistency. The foil itself is one variable, there are different thicknesses out there. In approx 2003 I constructed a couple of 'electrostatic lifter' flying machines. (cheap, very entertaining and quite dangerous)

Keeping the weight of these things down is useful, and some economy foil from a Poundland type shop was 65% the thickness of a standard supermarket foil. I don't have any notes handy on what thicknesses I measured then. It would be informative if you could measure your foil thickness, eg. by measuring a 20-layer thick sandwich.
I've just measured two samples from my kitchen now:
Regular supermarket foil: thickness = 9.5 micrometers
A large roll of catering industry foil, feels a bit heavier: thickness = 11.3 micrometers
So a 19% difference.

Also the foil folding process will create inconsistent connections between layers, not helped by aluminiums' insulating oxide layer.

[Monolith]

Foiltest, my take.


With a single layer of foil, let's say 20x20mm, the target response is relative to the surface area and the magnetic field density.
As the foil is very thin, the skin effect also plays a major role.
Since this is a very short TC target, an early sampling time is important .
When we fold the foil, the relative surface area exposed to the field(lines), diminishes, but the TC increases.
If the sampling delay is a bit longer than ideal, for the short TC target, the response will increase with the longer TC.


One unknown factor with folded foil, is the actual conductivity between the layers. Aluminum oxidizes instantly when exposed to the oxygen in the air. Aluminum oxide has an extreme high resistance, but as the oxide layer is very thin, it is easily damaged by friction.
Therefore we have conductivity along the edges of the folds, as it is a single piece of foil, but we also have a certain, unknown amount of conductivity between the layers. It gets very complex.

[Ferric Toes]

I'm not a fan of folded foil, due to lack of consistency. The foil itself is one variable, there are different thicknesses out there. In approx 2003 I constructed a couple of 'electrostatic lifter' flying machines. (cheap, very entertaining and quite dangerous)

Keeping the weight of these things down is useful, and some economy foil from a Poundland type shop was 65% the thickness of a standard supermarket foil. I don't have any notes handy on what thicknesses I measured then. It would be informative if you could measure your foil thickness, eg. by measuring a 20-layer thick sandwich.
I've just measured two samples from my kitchen now:
Regular supermarket foil: thickness = 9.5 micrometers
A large roll of catering industry foil, feels a bit heavier: thickness = 11.3 micrometers
So a 19% difference.

Also the foil folding process will create inconsistent connections between layers, not helped by aluminiums' insulating oxide layer.

Folding a larger piece of foil such that I have 16 layers and using a micrometer in the centre, I get a thickness of 0.254mm. This give a thickness for one layer of 0.01587mm (Waitrose standard foil). It does not appear that connection between layers is important as in a uniform field perpendicular to the plane of the foil, no eddy currents will flow in the cross section i.e. between layers. Previously, I have tried the same thing with stacked coins and found that the time constant progressively increased even if cling film was between each coin.

Eric.

[Ferric Toes]

On the subject of 'standard test targets' that are low-conductivity and small-ish, I'm quite fond of the Norwegian/Swedish 10 Ore coin from the 1980's. They are cupro-nickel, small (15 mm) and thin, too. They target ID below the UK CuNi 5 pence and CuNi sixpence, on a VLF. Their time-constant is about 3.4 microsecs.

You have to be careful with the UK 5p as sometime after 2001, they changed from Cu Ni to plated steel. Totally different response.

Eric.

[Skippy]

I did say " UK CuNi 5 pence ", and I was making the assumption that when the 5p was mentioned previously in this thread, it was referring to the small one, not the 1 shilling sized one. So in the post you've just quoted, I meant the 1990 - 2010 version. They changed to Ni-plated steel in 2011 (though not released into circulation until 2012)

Just for clarity, I'm in the United Kingdom, not Ukraine, and I'm familiar with U.K coinage.

I'm going to change my 'Location' entry, you're not the first person to make this mistake, I've previously had a Personal Message from someone asking where in Southern Ukraine I was. It might be an idea to change your 'Location', too, Eric, as you also have 'UK'.

[Ferric Toes]

I did say " UK CuNi 5 pence ", and I was making the assumption that when the 5p was mentioned previously in this thread, it was referring to the small one, not the 1 shilling sized one. So in the post you've just quoted, I meant the 1990 - 2010 version. They changed to Ni-plated steel in 2011 (though not released into circulation until 2012)

Just for clarity, I'm in the United Kingdom, not Ukraine, and I'm familiar with U.K coinage.

I'm going to change my 'Location' entry, you're not the first person to make this mistake, I've previously had a Personal Message from someone asking where in Southern Ukraine I was. It might be an idea to change your 'Location', too, Eric, as you also have 'UK'.

Sorry; my reply was poorly worded. I know you are in the United Kingdom and when I said "You", it was for persons in general. Many persons in my neck of the woods are surprised when I show them that 'you' can attract some UK coins with a magnet when they appear to be made of non-ferrous metal. You certainly said "UK CuNi 5p" and I was fully aware of that. I did not know in what year the steel cored 5p came into circulation as I had virtually dropped out of metal detector work in 2011 until recently, so I learned something today.

Eric.

[green]

Folding a larger piece of foil such that I have 16 layers and using a micrometer in the centre, I get a thickness of 0.254mm. This give a thickness for one layer of 0.01587mm (Waitrose standard foil). It does not appear that connection between layers is important as in a uniform field perpendicular to the plane of the foil, no eddy currents will flow in the cross section i.e. between layers. Previously, I have tried the same thing with stacked coins and found that the time constant progressively increased even if cling film was between each coin.

Eric.

Hi Eric, You got .01587mm. Skippy got.0095mm and .0113mm. I gave it a try with some Reynolds kitchen foil(about6x6 inch piece). I have a Vernier calipers for thickness so I went with 64 layers to help with the resolution. (.0175mm) I measured and calculated the area before I folded the foil. Weighed the foil after folding and calculated thickness. (.022mm) Was hoping the two measurements would be closer. Don't know if I'm compressing the sample when I measure thickness or made an error somewhere. Wondering if someone could try both ways to see how close they come.


.

[Skippy]

You need flat foil before you start, no creases, wrinkles etc. Then fold carefully, to keep the flatness. Keep it compressed during the folding, you could try some heavy weight application, like your bodyweight, to help compress it. That should give a low 'void' level. And use a gentle squeeze with the verniers. I used a regular flat anvil micrometer, so I have the potential to 'do it up tight', but I found it wasn't really beneficial, it maybe reduced thickness (of 20 ply) by up to 0.005mm, eg. 0.225 to 0.220mm, but some of that might be the micrometer play/bending.

Your weight/density experiment is a curious alternative. You would need seriously good scales to get the weight right? For the density, I would suggest using that of pure aluminium, as I understand foil Al is 98% Al, and 2% Fe, which aids ductility.

Re: the measured thicknesses.
Eric's foil is from Waitrose, which is more upmarket than most supermarkets, maybe that accounts for why their foil is a thicker type?

Wikipedia tells me that typical 'standard grade' household foils range from 10 to 18 micrometres in thickness. My 9.5 um sample does seem fairly thin, though I'm sure I've had thinner.

[green]

My error. Un wrapped the foil. Wrote the wrong size for the weight calculation. Calculates .016mm instead of 17.5mm for the thickness measurement. Makes more sense, probably not compressing it enough.