Vallon VMH3CS Mine Detector

[Ferric Toes]

Thickness of the sample makes a big difference.

I found that a square of 1in wide copper tape used in EMC applications gave the same conductivity reading as a 1in square aluminium off a drink can. The copper tape is a bit thinner at 0.05mm and I have a whole roll of the stuff which is easy to cut up. I cut five identical 1in squares and first measuring one, I then stuck the next one on top, and so on, until five were stuck together. The tape is adhesive backed. Results with the Hocking probe are as follows.
1. 7.67% IACS... 0.05mm thickness
2. 27.00........... 0.10
3. 45.50........... 0.15
4. 53.40........... 0.20
5. 53.70........... 0.25

Adding more caused little change. If I get time over the next few days, I will plot the log/lin decay of similar samples. Also interesting would be to do the same exercise with 10mm squares of copper tape.

Eric.

[Skippy]

A quick bit of Googling reveals that the wall thickness is commonly 0.097mm to 0.12mm, though clearly it's going to vary a bit around the world. It's not unreasonable to assume that the development of better alloys will result in thinner walls.
I understand the problem with higher-strength alloys is that they don't extrude well, they crack, etc. So the search is on for a soft hard malleable strong alloy......

[green]

Hi Skippy, The can I have measures 0.07mm in the middle section, including paint and any inner coating. It contained Southern Comfort, Lemonade and Lime. The can says it was produced in Belgium. I'll look out for a discarded Coke can; don't drink it myself.
I can detect a 10mm square of the 0.07mm stuff at 5in with the Vallon and slightly discernable at 6in. I cut a piece 9mm square and couldn't detect it.

The Hocking Sigma 2000 Conductivity Meter has an inductive probe running at 250kHz so paint, corrosion, etc do not have any effect. One of the calibration pieces has a plastic film over it. You have to get the same reading as another piece without the film. Both read 100 as they are annealed copper which everything else is referenced to as a %.

Eric.

I tested a 9x9mm can side awhile back and got 2usec for the TC.

[Ferric Toes]

Intriguing, I'm tempted to try and make some measurements myself of conductivity. I will report back.
I've just measured the wall thickness of a small sample of cans with a micrometer, and they were all the same thickness over most of the middle 50% of the can, measuring 0.105 to 0.110mm thick....but this includes the paint on the outside, and the plastic? coating on the innards.

I would imagine any aluminium would be difficult to measure the characterisitcs of, due to the oxide coating?

Hi Skippy, The can I have measures 0.07mm in the middle section, including paint and any inner coating. It contained Southern Comfort, Lemonade and Lime. The can says it was produced in Belgium. I'll look out for a discarded Coke can; don't drink it myself.
I can detect a 10mm square of the 0.07mm stuff at 5in with the Vallon and slightly discernable at 6in. I cut a piece 9mm square and couldn't detect it.

The Hocking Sigma 2000 Conductivity Meter has an inductive probe running at 250kHz so paint, corrosion, etc do not have any effect. One of the calibration pieces has a plastic film over it. You have to get the same reading as another piece without the film. Both read 100 as they are annealed copper which everything else is referenced to as a %.

Eric.

[Skippy]

OK, I've done my can metal resistivity test. I carefully spiral cut a 3mm wide strip from the middle of a can. The total length of my sample was 1.85 metres, and area is 0.105mm x 3mm. I did a good 4-wire resistance measurement, and got R = 0.265 Ohms.
Putting all that into R = p.L/A gives
resistivity = 45.1 x 10e-9.
Comparing this to a 100% copper value of 1.724 x 10e-8 gives a conductivity figure of:
Conductivity =38.2% IACS.
This compares very well with the figure for 3004-series Al alloy in the table I linked to earler of 39 - 43%. 3004-series is a common can alloy, apparently.
So nothing unusual about the can alloy, seemingly.

[green]

Yes, I have started to do similar tests myself and it will be interesting to compare results. I am using my Magnetic Viscosity Meter to plot decay from 10uS - 50uS in 5uS steps. I will do a log signal vs time as you do. I have some reservation about aluminium from cans. It is very springy and obviously an alloy. Aluminium varies tremendously in conductivity depending on what the other alloy components are, but maybe all Coke cans are the same. I cut up a can from another drink assuming it would be the same and I can detect a 10mm square with the Vallon. I will report back in more detail later.

Hi Eric
A couple charts I posted awhile back. The 1x1 inch aluminum foil folded twice has a TC about twice the 1x1 in foil. That way the material would be the same and one target TC would be less than 2 and the other smaller greater than 2. If you can detect the twice folded and not the 1x1 inch we could experiment with a little smaller size to start with, maybe 20x20mm. I don't have a mic. to measure thickness but could cut a large enough piece of the foil to weigh on my reloading scale to verify we are using the same foil thickness. I found folding the foil around a needle worked good. Looking forward to test comparisons. Thanks. The charts give .001 inch thickness, not a accurate measurement. Probably need to test a few pieces to check for repeatability once you have decided on a size.

[Skippy]

Intriguing, I'm tempted to try and make some measurements myself of conductivity. I will report back.
I've just measured the wall thickness of a small sample of cans with a micrometer, and they were all the same thickness over most of the middle 50% of the can, measuring 0.105 to 0.110mm thick....but this includes the paint on the outside, and the plastic? coating on the innards.

I would imagine any aluminium would be difficult to measure the characterisitcs of, due to the oxide coating?

It's possible the severe extrusion process changes the structure of the outer (and inner) layers, such things as very few voids/defects, long thin grains/crystal. It certainly plays a significant part in the resulting mechanical strength, I'm sure. Drawn wires, for example, can be several times stronger than the bulk material. Stainless steel bicycle spokes (about 1.5 - 2mm diameter) have UTS figures of 1250 MPa, compared to around 430 MPa for a big lump.

[Ferric Toes]

For such a common everyday product, the alloy used in drinks cans is surprisingly advanced. It is very high strength, and an enormous amount of design thought has gone into the shape/manufacturing process, all because keeping the can weight as low as possible is vital. The can typically costs 5 - 10 times that of the contents.
Aluminium alloys do vary in resistivity, but it's quite likely the can alloys are very similar, plus there aren't that many manufacturers / manufacturing plants around the world.
This table lists a fair few Al alloys:

http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/

The wall thickness of the can varies - it's progressively thickened towards the bottom, so don't take your sample from too far down.
I guess it's worth putting a micrometer on the samples you're using, just to see what differences in thickness exist.

I have a Hocking conductivity meter and it gives a reading in IACS% as in your chart. The centre of the aluminium can measures 6.6 and towards the top and bottom it rises to 7.0. A 10mm x 10mm square from the middle still measures 6.6. This low reading probably partly due to the thinness of the metal. A nickel by comparison is 5.5 IACS

[Skippy]

For such a common everyday product, the alloy used in drinks cans is surprisingly advanced. It is very high strength, and an enormous amount of design thought has gone into the shape/manufacturing process, all because keeping the can weight as low as possible is vital. The can typically costs 5 - 10 times that of the contents.
Aluminium alloys do vary in resistivity, but it's quite likely the can alloys are very similar, plus there aren't that many manufacturers / manufacturing plants around the world.
This table lists a fair few Al alloys:

http://eddy-current.com/conductivity-of-metals-sorted-by-resistivity/

The wall thickness of the can varies - it's progressively thickened towards the bottom, so don't take your sample from too far down.
I guess it's worth putting a micrometer on the samples you're using, just to see what differences in thickness exist.

[Ferric Toes]

Hi Eric
Charted amplitude vs target distance for the four aluminum targets with a Rx(two 200mm round, figure eight) Tx(oval surrounding Rx). The longer sample time with increased integrator gain helped with the 40x40mm can but not the other low TC targets. I would be interested in the smallest piece of aluminum can side the Vallon can detect to see if the TC needs to be greater than 2usec. I will chart the TC if you give me the size. Thanks

Yes, I have started to do similar tests myself and it will be interesting to compare results. I am using my Magnetic Viscosity Meter to plot decay from 10uS - 50uS in 5uS steps. I will do a log signal vs time as you do. I have some reservation about aluminium from cans. It is very springy and obviously an alloy. Aluminium varies tremendously in conductivity depending on what the other alloy components are, but maybe all Coke cans are the same. I cut up a can from another drink assuming it would be the same and I can detect a 10mm square with the Vallon. I will report back in more detail later.

[green]

The other factor is the diameter of the search coil and the coupling factor to the target. If I made an 8in, or less, coil for the Vallon, I reckon I would reach a point where the 1inch square of foil would be detectable. The same thing happens in reverse, if you increase the target area, then the two way coupling improves and lifts the amplitude of the whole decay curve. I know I can detect a bar of chocolate (same thickness foil) at about 3ft.
I will cut up an aluminium can today and test that. More likely to be a beer than a coke though.

Hi Eric
Charted amplitude vs target distance for the four aluminum targets with a Rx(two 200mm round, figure eight) Tx(oval surrounding Rx). The longer sample time with increased integrator gain helped with the 40x40mm can but not the other low TC targets. I would be interested in the smallest piece of aluminum can side the Vallon can detect to see if the TC needs to be greater than 2usec. I will chart the TC if you give me the size. Thanks

[Ferric Toes]

Hi Chet

The coke can is 40x40mm and foil is 38.1x38.1mm. The 10x10mm coke can has the same TC as the 38.1x38.1mm foil, thickness does make a big difference. I used 1.5x1.5 inch foil because that is what Eric used in his testing. Still don't why Eric could detect the 1.5x1.5 inch foil and not the 1x1 inch foil with the TC's being not too far apart.

The other factor is the diameter of the search coil and the coupling factor to the target. If I made an 8in, or less, coil for the Vallon, I reckon I would reach a point where the 1inch square of foil would be detectable. The same thing happens in reverse, if you increase the target area, then the two way coupling improves and lifts the amplitude of the whole decay curve. I know I can detect a bar of chocolate (same thickness foil) at about 3ft.
I will cut up an aluminium can today and test that. More likely to be a beer than a coke though.

[green]

Hi Green


That is a great chart. The thickness of the 1.5” = 38.1 mm Coke can versus the thin 40 mm aluminum foil shows a big difference in amplitude / signal strength. And a big difference in the time constant is also displayed. It shows why the thickness of the metal is so important to the detection process.


Have a good day,
Chet

Hi Chet

The coke can is 40x40mm and foil is 38.1x38.1mm. The 10x10mm coke can has the same TC as the 38.1x38.1mm foil, thickness does make a big difference. I used 1.5x1.5 inch foil because that is what Eric used in his testing. Still don't why Eric could detect the 1.5x1.5 inch foil and not the 1x1 inch foil with the TC's being not too far apart.

[6666]

There were two machines that were 'the best', depending on what you wanted to do. Aquastar was the most powerful and fully submersible, Goldquest SS V3 was the latest and continued to 2010

Hi Eric, yes poorly worded on my part, I should have said does the Goldquest give the best response to targets you typically find on the beach such as coins etc.

[Chet]

Hi Eric

Thanks for testing the 1x1 aluminum foil. Wondering why you couldn't detect the 1x1inch but could detect the 1.5x1.5 inch aluminum foil. I charted their time constant plus a couple others. A 156kHz resonance, 1.5uH coil requires a 735 ohm damping resistor to critical damp. Decay time constant=.0015/735(2usec). Could the reason be, the 1x1 foil TC(1.7usec) is shorter than the coil decay TC(2usec) where the 1.5x1.5 foil TC(2.4usec) is longer than the coil decay TC(2usec)?

Hi Green


That is a great chart. The thickness of the 1.5” = 38.1 mm Coke can versus the thin 40 mm aluminum foil shows a big difference in amplitude / signal strength. And a big difference in the time constant is also displayed. It shows why the thickness of the metal is so important to the detection process.


Have a good day,
Chet