Vallon VMH3CS Mine Detector

[Chet]

Mine has the year 2010 on the serial no. plate. There is also a 5 and a 10 punched on a final inspection label, so could be May 2010.

Hi Eric
Here is one on eBay that shows a punched date of 7/2006.
Chet

http://www.ebay.com/itm/201580006429...%3AMEBIDX%3AIT

[Ferric Toes]

Eric,

Your use of Scotch 24 is very good to see as I discovered this stuff many years ago when doing independent coil research. Lead shields seems to have fallen into safety and health issues these days. I see that you chose to spiral wrap the double layer Scotch 24 around the coil. Another method is to cut it in a single layer a bit narrower than the coil cross section bundle circumference and put it on the coil as a U-shaped shield with the open seam on the top of the coil. This keeps the wire mesh shield from forming a shorted cross section loop around some coils that may be detected at low delays. I used this U-shaped shield method along with Teflon insulated coil wire for an 11" 300 uH coil used on your CS6PI low power PI design that I modified to operate at 13.5 KHz at about a 7.5 uS delay. This gets me about 14" on a U.S. Nickel. The spiral wrap shield method, however, may not be an issue with this VMH3CS mine detector coil where low noise and not coil speed is the main issue.

Thanks for the update.

Joseph J. Rogowski

This is the first time I have used the Scotch 24 tape. It looks as though the Vallon coil has it spiralled around from what I could see by scraping off the potting in a dud coil. Doing the same gave the best chance of ending up with a working coil. The manual says that the unit checks L, C, and R of the coil on power up.
I re-measured the resonant frequency as my earlier measurement did not take into account the capacitance of the coiled cable inside the telescopic part of the shaft. This alone adds an extra 290pf to the coil and cable in the lower shaft. The Vallon coil with total cable measured 156kHz and my 11inch coil was 153kHz. Since my coil works, this difference must be acceptable.

A U shaped shield of the Scotch 24 must be difficult to hold in place. I have always used total wrap round, even on fast coils, as the collapsing magnetic field on switch off does not induce eddy currents in a concentric shield. On the other hand flat shielding does have eddy currents so you have to use thinner conductive material.

[Ferric Toes]

Hi Eric
Here is one on eBay that shows a punched date of 7/2006.
Chet

http://www.ebay.com/itm/201580006429...%3AMEBIDX%3AIT

Yes, that is the label I was looking at. Interesting that the 2006 model does not have the 'pinpoint' button.

[daverave]

Hi Daverave
The Ground Balance was off. The test was in my back yard with the coil lying on a 1.5” thick board lying on low mineralized ground. The 1.5” represents the average scan height over cobbles, pine needles, leaves and branches encountered in the California gold country. I just retested by ground balancing beside the board (at 7 on the control) then placed the coil back on the board. It surprised me that it only lost 0.5” in depth. I know that is loses much more when on high mineralized ground in the field.
Have a good day,
Chet

what was the in air depth of the tdi sl on nickel...sorry to go off topic on this but im very interested to know.

[Chet]

what was the in air depth of the tdi sl on nickel...sorry to go off topic on this but im very interested to know.

Hi Daverave
In an air test with the Ground Balance off I get 9.5" and 10.5". That is the same as when the coil was setting on the 1.5" board on the ground. In an air test with the Ground Balance set at 7 it still gets 9.5" and 10.5". It appears that it only loses about 0.5" with the Ground Balance on my back yard soil test. Just a quick note this is a TDI SL. A friend had a TDI PRO which we once compared in the field and the PRO was a little better than the SL.
Have a good day,
Chet

[daverave]

im very supprised cause ive seen a video on youtube showing that a white's tdi pro can do 18" on a nickel in wet sand....the tdi sl seems a lot less sensitive than the pro...once again sorry for going off topic.

[Ferric Toes]

Did a simulation in spice. Don't know if it means anything since I couldn't enter coil diameter. Compared to coils, 300uH(1MHz resonance) and 1.5mH(170kHz resonance). Target TC 3.3usec which could be close to a .3gm nugget. Could see target decay on both simulations. Tested a 6x6mm piece cut from the bottom of an aluminum coke can(TC=3.1usec) and a piece 6x6mm cut from the side(TC=1.3usec) awhile back. Couldn't see the target decay on the 1.5mH coil simulation with a target TC of 1.3usec.

I have not used Spice myself, although I have done simulations on Electronics Workshop. Hence, I am not quite clear what your simulation shows. The blue trace shows coil current (IL1) which grows linearly to a max. and then a damped fall, which is obviously slower for the higher inductance. The white trace I presume is coil volts with also a damped fall, but what is the linear decay that occurs at the lower level? I presume it is the target decay which would normally be exponential on a linear scale. The di/dt of the TX current is lower for the high inductance, but we seem to be getting more signal at late time; or am I reading this wrong?
The Vallon detector appears to kick start the current in the coil presumably from the flyback voltage in the previous reverse pulse. The current is then substantially flat for the last 40uS of the 50uS total pulse. There is a possibility that, rather than just a damping resistor, they use a form of dynamic damping to reduce the coil current quicker.

[green]

I have not used Spice myself, although I have done simulations on Electronics Workshop. Hence, I am not quite clear what your simulation shows. The blue trace shows coil current (IL1) which grows linearly to a max. and then a damped fall, which is obviously slower for the higher inductance. The white trace I presume is coil volts with also a damped fall, but what is the linear decay that occurs at the lower level? I presume it is the target decay which would normally be exponential on a linear scale. The di/dt of the TX current is lower for the high inductance, but we seem to be getting more signal at late time; or am I reading this wrong?
The Vallon detector appears to kick start the current in the coil presumably from the flyback voltage in the previous reverse pulse. The current is then substantially flat for the last 40uS of the 50uS total pulse. There is a possibility that, rather than just a damping resistor, they use a form of dynamic damping to reduce the coil current quicker.

I'm still trying to learn so I don't know if the spice model means anything. The Vallon has 1amp peak current So I changed the simulation until I got 1amp peak with the two coils(fast, 1MHz, and slow, 170kHz resonance). You mentioned detecting a .3gm nugget so I estimated a 3.3usec TC for the target. I plotted linear log(straight line for exponential decay, lower level is the target decay) to see if I could see the target decay with both coils which I could. So I think the Vallon coil could detect the nugget with a normal PI but not power efficient. The target does break at a higher voltage on the decay curve with the faster coil. I tried the simulation with the 156kHz resonance coil and the target decay still showed.

[Ferric Toes]

VMH3CSEF

Looks more the part for beach and nugget hunting with the 11in search head.
The coil winding has yet to be mounted are then there are two stages of potting.

The swivel is held in place by two M8 nylon bolts and one stainless steel bolt and nut (non-detectable) There will also be adhesive sealant between the underside of the swivel base and the coil shell.

The cable in the shaft goes through the swivel with three O ring seals and a two turn coiled section to allow free adjustments to the coil angle. The coil wire ends will come up into the recess on the top of the swivel in which there is a little PC with two pins. This is where the cable and coil are joined. The circular cover with O ring then seals everything in.

The VMH3CSEF should be ready to test on the beach by the weekend.

[rickb]

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[Ferric Toes]

error, all invalid attachments.

Thanks. All should be OK now.

[Chet]

Hi Eric
Great work!!!
Chet

[Ferric Toes]

im very supprised cause ive seen a video on youtube showing that a white's tdi pro can do 18" on a nickel in wet sand....the tdi sl seems a lot less sensitive than the pro...once again sorry for going off topic.

This is an effect that has been reported many times, in that objects in wet (seawater) sand can be detected at noticeably greater ranges than in air. The electrical conductivity of saltwater wet sand, although very much less than metal, actually aids the transmission of the magnetic signals to and from the target. Hopefully, this will be true of the VMH3CS as well.

[daverave]

This is an effect that has been reported many times, in that objects in wet (seawater) sand can be detected at noticeably greater ranges than in air. The electrical conductivity of saltwater wet sand, although very much less than metal, actually aids the transmission of the magnetic signals to and from the target. Hopefully, this will be true of the VMH3CS as well.

Hi Eric....thanks for that info that makes it clear why things are detected deeper in wet sand than in air that VMH3CS look a very nice unit and high quality build...be interesting to see the depth you get with your coil that you have made.

[Chet]

About three years ago I took my new TDI SL to a well hunted California beach. I found over a hundred targets; most were quite deep. Some nickels were over a foot down. That is the only time that I have hunted a beach with a Pulse Induction detector. The beach is hunted constantly by local people with VLF detectors that discriminated out the aluminum tabs and iron. I did not find any coins except nickels that discriminate close to aluminum tabs. Most of the finds were aluminum tabs, nickels, hair pins, sinkers and fish hooks. I only found a few junk rings and miscellaneous pieces of jewelry. I did a lot of deep digging and was very impressed with the performance of the new TDI SL at that time.


Have a good day,
Chet