Vallon VMH3CS Mine Detector

[garyq]

I checked a US nickel on my VHM3CS and get an air range of 15 inches for a definite response and 16 inches for an iffy response that you would likely miss. I think that is good for a coil which is about equivalent to an 8 inch diameter. That is why I want to make a 12in and see what I get then. What foil are you using, Carl? Is it baking foil?
I hope to get another VHM3CS shortly, which I will be happy to open up and look at the works.

Hello Eric,

Just some further food for thought as some time back I created 5 separate discs formed out of Iron(Ferrous), Lead , Brass, Aluminum and Copper all to the same dimensions as a US 5c (Nickel) coin.



In the pic attached you will also see results shown using DD coils on an earlier series PI detector and the results below were with a later series of PI detector.

Therefore these air tests results below with the discs & coins were with an 8” mono on that later series PI detector set in two different timings of Normal and Sensitive.

Discs.

Ferrous Normal 10.5” Sensitive 11”

Brass Normal 13.5” Sensitive 14”

Copper Normal 13” Sensitive 10”

Aluminum Normal 14” Sensitive 11.5”

Lead Normal 15” Sensitive 12.5”


Coins

US 5c (Nickel) Normal 15” Sensitive 16”

Australian 5c Normal 14” Sensitive 14”


I expected the Copper disc to perform better then it did although not sure what this type of tests prove, if anything at all.

Gary

[Ferric Toes]

Foil again

Having a reel of copper shielding tape, I decided to use that instead of aluminium foil. One of the benefits is that it has a non-conductive adhesive backing that make it easier to make a sandwich. The tape and adhesive measures 0.06mm thick. Without the adhesive, 0.05mm. I cut 5mm square pieces as accurately as I could and measured 1 square on my MVM at 10uS delay and in a uniform vertical field. I set the calibration control to give a reading of 20

For the next test, 5 of the 5mm squares were measured and gave a reading of 100. This shows that for a number of identical objects isolated from each other the reading increased in direct proportion to the number of objects. This was checked by measuring 2x, 3x, and 4x.

Neither the single square or the five squares together were detectable on the Vallon using their standard elliptical coil.
Next, five squares were mounted as a sandwich with the adhesive layer acting as an insulator between them. The reading had increased dramatically to 1790.

If the delay was increased to 20uS there was no signal from the 5 squares test, but for the sandwich the signal did not disappear until 40uS delay. A Vallon test now gave a range of 4 inches for the copper sandwich.

Eric.

[Ferric Toes]

Hello Eric,

In the pic attached you will also see results shown using DD coils on an earlier series PI detector and the results below were with a later series of PI detector.

I expected the Copper disc to perform better then it did although not sure what this type of tests prove, if anything at all.

Gary

Hi Gary, I expect that the copper disc has the longest time constant and that the detector you used does not have a long enough TX pulse to fully energise the disc. The same effect occurs with silver coins, where usually the range is less than one would expect. On the detector in question, does the copper disc give a descending tone?

Eric.

[KingJL]

Eric,
Regarding your post #260, "The Probes are Coming"; If you don't mind, what is the coiled cable you are using for your probes and from where can it be obtained?
Regards,
J L King

[Ferric Toes]

Eric,
Regarding your post #260, "The Probes are Coming"; If you don't mind, what is the coiled cable you are using for your probes and from where can it be obtained?
Regards,
J L King

Two years ago I bought some coils and shafts that appeared on ebay for the VMH3CS. They were sold as being scrapped because of various faults but I managed to get 3 good coils and some shaft sections that were useable. The coils were not compatible with any electronics that I had, but I hung onto them hoping one day to get a Vallon control unit to try out. Well that day arrived recently and I have been playing with it ever since.

What I am getting at, is that the coiled cable is from Vallon shafts and is not something that is generally available; which is a shame. I guess they have it specially made so as to fit inside their telescopic shaft with just the right length of coiled section. It is very tight and springy as the picture shows. Diameter is 3mm and the shield is lap wound rather than braid.

Fifteen years ago I used to buy a coiled microphone cable to use on probes. It was available from Maplin in UK but was discontinued and I have not found a source for it since then. It is 4.5mm diameter and neoprene outer, Again lap wound with polythene inner insulation. The main disadvantage was it's higher capacitance compared to coax; perhaps due to the semiconducting layer under the outer lap wound conductor to stop cable noise when moving the mic.

Picture shows Maplin cable at top and two pieces of Vallon cable. The shorter Vallon with two turns is out of the coil swivel so that you can rotate the coil 180 degrees without stressing the cable.

If anyone knows of a source of coiled coax or mic cable I would be interested to know.

Eric.

[KingJL]

If anyone knows of a source of coiled coax or mic cable I would be interested to know...

Me too!!

[6666]

If anyone knows of a source of coiled coax or mic cable I would be interested to know.

For short lengths try CB or Amateur radio curly cord microphones, just cut the mic and plug off, a bit wastefull but fairly cheap, if you have a disposals store nearby or online check them out for old taxi radios etc. Pilot head sets.

https://www.google.com.au/search?q=A...iled+mic+cable


http://www.kcb.co.uk/shop2/contents/en-uk/d121.html

http://www.thunderpole.co.uk/leads/m...6-core-3m.html

https://www.amazon.com/Replacement-M.../dp/B00AB2L3MG
http://www.ebay.com/bhp/cb-mic-cord

http://www.curlycords.com.au/curlyco...rly-cords-pvc/

[Sean_Goddard]

Made my own coiled cable from Belden "Tesoro search head cable".

Wrap it tightly around a 12.5mm copper pipe and bake it at 140 degrees C for 30 mins then remove and place directly in the freezer wait a hour. BINGO coiled cable.

No good for search heads though as the L an C components change as the cable expands and contracts.

[Ferric Toes]

For short lengths try CB or Amateur radio curly cord microphones, just cut the mic and plug off, a bit wastefull but fairly cheap, if you have a disposals store nearby or online check them out for old taxi radios etc. Pilot head sets.

https://www.google.com.au/search?q=A...iled+mic+cable


http://www.kcb.co.uk/shop2/contents/en-uk/d121.html

http://www.thunderpole.co.uk/leads/m...6-core-3m.html

https://www.amazon.com/Replacement-M.../dp/B00AB2L3MG
http://www.ebay.com/bhp/cb-mic-cord

http://www.curlycords.com.au/curlyco...rly-cords-pvc/

Thanks for all those links and there are lots of curly cords but - they are 4 way, 6way, with some shielded, but no coaxial that I could find. If my preamp had a balanced input, then some of those may be OK. At the moment I am stuck with unbalanced and need a coaxial type.
Straight coaxial mic cable is readily available and I once tried some as an alternative to r.f. coax. All seemed to work OK in the workshop but as soon as I took the detector outside, I started to get spurious signals. I subsequently found that the mic cable was microphonic. As I swept the coil from side to side the cable was tapping against the shaft, causing the problem. The cable was Van Damme, so not low end stuff.

The Vallon coiled cable is good because with my belt mounted version of the detector, I can have some on a probe and reach out without having a metre or more dangling all over the place. Also, with a standard coil on a shaft the cable can extend just enough for scanning, and then retreat back into the top shaft when extra cable length is not required.

Eric.

[Ferric Toes]

Made my own coiled cable from Belden "Tesoro search head cable".

Wrap it tightly around a 12.5mm copper pipe and bake it at 140 degrees C for 30 mins then remove and place directly in the freezer wait a hour. BINGO coiled cable.

No good for search heads though as the L an C components change as the cable expands and contracts.

Hi Sean,
I'm not sure what the Tesoro search head cable consists of, but the Maplin curly cable that I have is OK for PI detectors? The length of Maplin cable in my picture measured 2uH for the screen and 3uH for the core end to end. Certainly the L varied if I stretched it out or compressed it, about 0.5uH. However, with the forward current down the core and the return current back up the shield the inductance will cancel to a considerable degree. By shorting the screen to core at one end and measuring the inductance at the other, I now got 1.5uH and saw no significant variation if I stretched the cable or compressed it. About 0.01uH.
That particular length (about 1.5m when extended) measured 328.7pf and showed no change when stretched or compressed. The big problem is its high value of capacitance 218pf/m as against 82pf/m for RG58.

Eric.

[KingJL]

The big problem is its high value of capacitance 218pf/m as against 82pf/m for RG58.
Eric.

I might try Sean's method for creating coiled coax with some Belden 9221, 7/39 braided center (30 awg), 75 ohm, 17.3pf/ft (~58pf/m), 2.5mm outer diameter.

[Chet]

Here is some more information; note step 4 is to turn the wrap inside out to get a tighter spring.


Have a good day,
Chet


http://www.engineering.com/Library/A...led-Cable.aspx

[DOOLEY]

thought i'd chip in..............

I do headphone cable the same way sean suggested,
works well.



temperature wise I guessed it,
warm the cable gently and evenly with a heat gun on low until the cable goes
"shiny",
then its about right.........

then leave on the former you've wrapped it around till cool.

I use an 8mm long wooden dowel,
cable wound and taped at each end,
ends up just like that hard to find headphone curly cable.

[6666]

Thanks for all those links and there are lots of curly cords but - they are 4 way, 6way, with some shielded, but no coaxial that I could find.

Sorry I did not pick up on that you only wanted coax cable.

[garyq]

Hi Gary, I expect that the copper disc has the longest time constant and that the detector you used does not have a long enough TX pulse to fully energise the disc. The same effect occurs with silver coins, where usually the range is less than one would expect. On the detector in question, does the copper disc give a descending tone?

Eric.

Hello Eric,

In regards to your question about he Copper disc and did it give a descending tone then YES it did in both Normal and Sensitive timing.

Btw the Aluminum and Brass discs also gave a descending tone in both timings however the Lead disc gave a rising tone in both timings.

The Ferrous disc gave a rising tone in both timings.

Just for the record both timings also gave a rising tone on US 5c and Australian 5c coins.

However when testing on an Australian 1923 Threepence which contains 92.5% Silver, 7.5% Copper it gave a descending tone in both timings.
But when testing an Australian 1956 Threepence which contains 50% Silver, 40% Copper, 5% Zinc, 5% Nickel then Normal timing gave a rising tone and Sensitive timing gave a descending tone.

Maybe of further interesting when testing the same detector with the 8" mono coil on Gold nuggets of these weights 0.16 gram, 0.44 gram, 1.60 gram, and a 6.76 gram then the Normal timing gave a descending tone on all of them whereas the Sensitive timing gave a rising tone on all of them.

Gary