Vallon VMH3CS Mine Detector

[kingswood]

That's correct for pure gold.
Conductivity of gold is 4.10 x 10^7 S/m.
Aluminium = 3.50 x 10^7 S/m
Lead = 4.55 x 10^6 S/m

Gold is the most conductive of the three, but when found as a nugget there are other impurities that restrict the flow of eddy currents. So even though gold is highly conductive, nuggets are considered to be of low conductivity. This is due to both the impurities and their small physical size.

Thanks for this

[kingswood]

A question about PI sensitivity !
So I think that I may have been thinking about sensitivity all wrong!...I was of the belief that sensitivity affected detection depth...then I read this: "Turning down the sensitivity level enables you to hear weak responses to deep targets as the audio distraction caused by the above-mentioned interferences is reduced" and "Many enthusiasts believe that Sensitivity sets the detector's transmit power, and "More Sensitivity = More Detection Depth" as well as "Less Discrimination = More Detection Depth". However, these are huge MISCONCEPTIONS".
Is this actually the case with a PI detector like the VMH3CS, or is that for VLF's??!! Confused !!!

[kingswood]

More reading !! So, reducing the sensitivity basically effects how large the target needs to be to cause a response, correct?....

[kingswood]

Doing a little reading today...came across a mine detector report. They compared the VMH3 and the VMH3CS UXO against steel, chrome and aluminium balls. If I am reading the graph correctly, the "regular" vallon did better than the UXO on aluminium balls!
They said: "Special cases are visible for the other detectors and aluminium (Figures 23, 24, 26) where it seems as if the crossing will appear much later and the curves are almost parallel".
Interesting
http://bookshop.europa.eu/hu/metal-d...-NA-22534-EN-N.

[Ferric Toes]

Doing a little reading today...came across a mine detector report. They compared the VMH3 and the VMH3CS UXO against steel, chrome and aluminium balls. If I am reading the graph correctly, the "regular" vallon did better than the UXO on aluminium balls![ATTACH]39456[/ATTACH]
They said: "Special cases are visible for the other detectors and aluminium (Figures 23, 24, 26) where it seems as if the crossing will appear much later and the curves are almost parallel".
Interesting
http://bookshop.europa.eu/hu/metal-d...-NA-22534-EN-N.

That is an interesting report, which I will study further. On a quick look, it wasn't clear if the VMH3CS UXO ran a different program or was the standard unit but with the 60cm UXO coil.

I have just finished the winding for the 15in coil (38cm). Inductance and resistance measure good and the next job is to shield it with Scotch 24. Hope to be able to test within the next week.

Eric.

[kingswood]

That is an interesting report, which I will study further. On a quick look, it wasn't clear if the VMH3CS UXO ran a different program or was the standard unit but with the 60cm UXO coil.

I have just finished the winding for the 15in coil (38cm). Inductance and resistance measure good and the next job is to shield it with Scotch 24. Hope to be able to test within the next week.

Eric.

It will be interesting to see how the 15" performs.
I am not sure about the software of the Vallon UXO...
The report is very good reading....They tested the machines against the PMA-2 minimum metal mine that contains only a 0.08g aluminium firing pin....The Vallon detected this at 180mm...Interestingly, then it recorded less detection of the 2mm aluminium ball (that would be the equivalent weight to the PMA-2).. They concluded that this is because the detector is detecting the surface of the object..
I then would assume from this, that the shape of a gold nugget would effect how well the Vallon detects it...For example a 0.5g round nugget may be less detectable than a 0.5g thin flat nugget...I think

[Ferric Toes]

The coil that shouldn't have worked, but did.

I made a shielded winding for a 15in coil to suit the VMH3CS but made a mistake that I should have been aware of, having made so many coils before for standard PI's. It's all about measuring inductance. For many years, I used a Thandar TC200 meter which operates at 1kHz. This seemed good enough and still works fine, but a couple of years ago I bought a digital LC meter where the range setting is automatic, except that you have a HI/LO button depending on the size of the inductance you hope to measure. There is an overlap between HI and LO which enable me to measure 1-2mH on either range. The difference is that the measurment frequency changes depending on the inductance and whether you are on the HI or LO setting. For small inductances on LO it can be 100kHz while on HI it can be 10kHz.

The problem became apparent when measuring the inductance of the 15in coil where I started with 50 turns and took some off until it was about 1.5mH at 43 turns. This was on the basis that when the coil is taped up to pull the wires closer together, the inductance goes up, which it did. It reached 1.9mH which I thought was about right. I measured a stock Vallon coil for comparison and it read 2.0mH. This is where things were going wrong as the meter was for the LO inductance setting and running at 90kHz when the button was pushed that shows the frequency of the measurement.

I continued, oblivious to the mistake, and added the shielding which in this case was woven copper fabric tape. I then measured the inductance which had shot up to 2.8mH which was a large jump for no apparent reason. There were a few other changes I had made, one was to use 7/0.2 pvc wire instead of the enamelled 0.53mm solid wire that Vallon use. Additionally I used 1.5m of RG58 coax instead of the thinner Vallon stuff, so was fully expecting the coil to require some major tweaking.

I plugged it in to one of my re-builds in a belt mount enclosure and surprise, surprise, it worked with one noticeable difference; after the self test the detector took a while to zero down, although it could be speeded up by pushing the C button. Detection range seemed excellent but no actual measurements were taken at this stage. I then went back to the inductance problem as I couldn't accept the 2.8mH being correct.

The Thandar meter gave 1.8mH and the digital meter 1.76mH provided I pressed the HI inductance button where it was operating at 11kHz. Next I measured a stock Vallon coil which was almost spot on 1.5mH and the measuring frequency 12.5kHz. I believe I have stated 2.0mH for the Vallon coil in some recent posts due to this measurement confusion, but it is certainly 1.5mH that is the correct figure.

I suspect that the overlong settling time is due to the inductance figure being too high at 1.8mH, so I shall reduce it to 1.5mH and test again. The resistance using 7/0.2 wire plus the 1.5m of RG58 is 4.6 ohms as against 4.0 ohms for the stock Vallon plus all the cable to the electronics. My resistance will come down a bit anyway when I take 2-3 turns off.

The lesson for me was - always check that the frequency for checking the inductance of PI coils should be 12kHz or less as considerable errors can occur in the measurement.

I have high hopes of making a good 15in coil, but the downside is that it can only be used with a re-built unit where I have used a commercial connector and a Whites shaft. There is no way I can replicate the Vallon bottom shaft and swivel arrangement.

Eric.

[Carolina]

Mr Foster. Could you help me some how with the proper sequence of dismantling my Vallon VMH3CS? Thank you

[Ferric Toes]

Hi Carolina,

Look out for a PM. I have written up only about one third of the dismantling instructions as yet, but it allows you to take a peek inside the box. There are several possibilities for going wrong as you get further into the dismantling procedure, so I have to be cautious what I write. All who have a go, do it at their own risk.

Eric.

[Ferric Toes]

15in coil - Air Tests

I made a second winding as the first one only worked at night . It seems to be a temperature effect, but still investigating. The second winding was a bit more accurate i.e. much closer to 1.5mH and with Scotch 24 mesh shielding as Vallon use. This seems reliable throughout the day.

The following measurements were made at 5am as EM noise is then negligible. Measurement is for two LEDs lighting up. Default sensitivity in 'normal'.

US nickel .............................18in

Guinness beer can .................40in

UK 50p coin .........................24in

10gm nugget .......................13in

1gm nugget .........................8in

I'm happy with those initial results , and more tests will follow when the coil is potted and ready for the great outdoors.

Eric.

[daverave]

I made a second winding as the first one only worked at night . It seems to be a temperature effect, but still investigating. The second winding was a bit more accurate i.e. much closer to 1.5mH and with Scotch 24 mesh shielding as Vallon use. This seems reliable throughout the day.

The following measurements were made at 5am as EM noise is then negligible. Measurement is for two LEDs lighting up. Default sensitivity in 'normal'.

US nickel .............................18in

Guinness beer can .................40in

UK 50p coin .........................24in

10gm nugget .......................13in

1gm nugget .........................8in

I'm happy with those initial results , and more tests will follow when the coil is potted and ready for the great outdoors.

Eric.

Hi Eric....wow they are really good depths with your 15" coil

[kingswood]

Very impressive results Eric
I am off to the coast with the family for a couple of days over Easter...plan to test the Vallon on a beach or 2
I will let you know how they like Australian beaches

[Carolina]

Thank you sir!!!

[bbsailor]

I made a second winding as the first one only worked at night . It seems to be a temperature effect, but still investigating. The second winding was a bit more accurate i.e. much closer to 1.5mH and with Scotch 24 mesh shielding as Vallon use. This seems reliable throughout the day.

The following measurements were made at 5am as EM noise is then negligible. Measurement is for two LEDs lighting up. Default sensitivity in 'normal'.

US nickel .............................18in

Guinness beer can .................40in

UK 50p coin .........................24in

10gm nugget .......................13in

1gm nugget .........................8in

I'm happy with those initial results , and more tests will follow when the coil is potted and ready for the great outdoors.

Eric.

Eric,

What performance difference did you notice or measure between using the woven copper fabric shield versus the Scotch 24 shield?

Thanks

Joseph J. Rogowski

[Geo]

I made a second winding as the first one only worked at night . It seems to be a temperature effect, but still investigating. The second winding was a bit more accurate i.e. much closer to 1.5mH and with Scotch 24 mesh shielding as Vallon use. This seems reliable throughout the day.

The following measurements were made at 5am as EM noise is then negligible. Measurement is for two LEDs lighting up. Default sensitivity in 'normal'.

US nickel .............................18in

Guinness beer can .................40in

UK 50p coin .........................24in

10gm nugget .......................13in

1gm nugget .........................8in

I'm happy with those initial results , and more tests will follow when the coil is potted and ready for the great outdoors.

Eric.

Hi Eric.
Is the Guinness beer can a 330ml ??


Thank you.