Crude coil sheilding paint

[6666]

If you hold the paper up to the light you can see how thinly painted patch BB is
and you can see some brush marks in the snail trail to.

The only real difference I can see between "marine" varnish and ordinary varnish
is marine varnish is supposed to be more flexible and UV resistive
but I can bend the paper over and it does not effect the resistance

[6666]

Well the varnish has dried and the results appear to be ok.
The long snail trail is roughly 56 inches or 1.4 meters long
and has a resistance of 7.83K AA, it is painted on very thick,
you can alter the resistance by varying the thickness of the paint

BB is 16K I painted this very thin

CC is 800 ohms

EE is 1.5K

[6666]

Today just for fun I tried making a crude shielding paint using ordinary wood varnish (not marine varnish) and graphite powder.

I used four teaspoons of varnish and two teaspoons of Pressol brand (made in germany) graphite powder,
mixed it up and painted some snail trails about half inch wide on some printer paper .
It still has to dry properly but some initial testing with the multimeter looks good,
more to report later.

[6666]

The Litz wire was given a single wrap of plumbers Teflon thread seal tape before winding the coil.

Wow Kev thats a Labor of luv, what process did you use to wind the teflon tape on ?

[6666]

TV balun cores maybe F29 material.

[golfnut]

Hi Kev,

What resistances are you seeing with your shield?
Is it for use on a Minelab PI, VLF, or other?



As mine is a vlf DD there is more conductive potting in central area at opposing ends of tis part is ~10k
on perimeter opposing sides ~20k

from centre to gnd pin on plug ~8k

Working well, got 7 small coins earlier, all thumnail or smaller.

Tested on really wet saturated lush short lawn this am and was ok very usable - as mentioned conductive potting is on outside and touches ground/grass everything.


S

[Kev]

Nice method, makes for a rigid waterproof coil of arbitrary shapes.

However, didn't the ML solder/ferrite patent have very small "component pin" sized ferrite beads, of high saturation flux material?

Hi ODM,
That maybe so, but after going through all my ferrite materials junk box the ones that gave a minimal signal response were the early miniature TV balun cores, almost undetectable. All other "beads" gave responses like pieces of iron. I need to find out what material was used for the old ribbon to coax baluns and obtain some small beads of the same, but for now I am just observing. It maybe that my machine doesn't sample early enough to achieve much benefit from this anyway. Who knows at this stage there maybe some advantage having both wires running through a homogenous core?

Cheers
Kev.

[ODM]

Nice method, makes for a rigid waterproof coil of arbitrary shapes.

However, didn't the ML solder/ferrite patent have very small "component pin" sized ferrite beads, of high saturation flux material?

[Altra]

Hi Altra,
The closed cell foam is covered with a couple of layers of ABS dissolved in MEK, this makes a smooth contiguous surface for the shield.
Once the plastic hardened I gave it a light buff with sandpaper to ensure the paint has something to grasp hold of.

Two other features of this coil may have added to its extreme sensitivity.
1) the Litz wire was given a single wrap of plumbers Teflon thread seal tape before winding the coil. This gives greater inter-winding insulation and resistance and lowered capacitance
2) the solder joints in the coil are covered by a ferrite core from a 75 to 300 Ohm TV balun. This is to evaluate a recent Minelab patent, and it give electrical isolation of the joints too.

Originally I used to paint the shield straight on a masking tape wrap, but I believe continuity was interrupted by movement and vibration etc., just as you feared with the cloth tape.

Cheers
Kev.

Thanks, that's quite a process. I sure you will feel good when you see the first gold, making it all worth it.

[Kev]

I have had very recent success (tonight) using graphite loaded epoxy.

I figure if your potting in epoxy, why not have the screen medium in the potting resin. No foil, mylar, sprays, just a short drain wire ~1-2".


S

Hi golfnut,
Thank you for your suggestion and congratulations on finding a workable solution.
I hadn't planned on epoxying this coil, it is to be eventually sealed up in an ABS housing, it is just taped together at present.
I will though investigate your suggestion for a waterproof coil.

What resistances are you seeing with your shield?
Is it for use on a Minelab PI, VLF, or other?

The reason I used the low density closed cell foam to isolate the shield from the winding was to reduce any absorption attenuation or hindrance to the field lines.
I have noticed that embedding the coil in epoxy or having shields close to the windings causes a marked loss in sensitivity. This is predominately with Minelab PIs but for a robust truly waterproof coil, potting is the best compromise.

In line with what you mention I at one time had a coil of mine leak so that the shield had continuity with the water the detector become much quieter and may have had better responses to targets, but it died before I could fully investigate it. It maybe time to look further into it.

Cheers
Kev.

[Kev]

Hi Kev,
Looking good! You have come along ways since experimenting with battery electrolyte.
I understand your coil constuction. But I am wondering what material you are using on the final layer? The layer the shield paint is applied on?
Years ago ml used some kind of cloth tape on the large mono coils. I always worried about expansion or contraction effecting the conductivity.

Hi Altra,
The closed cell foam is covered with a couple of layers of ABS dissolved in MEK, this makes a smooth contiguous surface for the shield.
Once the plastic hardened I gave it a light buff with sandpaper to ensure the paint has something to grasp hold of.

Two other features of this coil may have added to its extreme sensitivity.
1) the Litz wire was given a single wrap of plumbers Teflon thread seal tape before winding the coil. This gives greater inter-winding insulation and resistance and lowered capacitance
2) the solder joints in the coil are covered by a ferrite core from a 75 to 300 Ohm TV balun. This is to evaluate a recent Minelab patent, and it give electrical isolation of the joints too.

Originally I used to paint the shield straight on a masking tape wrap, but I believe continuity was interrupted by movement and vibration etc., just as you feared with the cloth tape.

Cheers
Kev.

[golfnut]

There is a carbon product callled Graphene and acheives really low ohmic values.

S

[golfnut]

I have had very recent success (tonight) using graphite loaded epoxy.

I figure if your potting in epoxy, why not have the screen medium in the potting resin. No foil, mylar, sprays, just a short drain wire ~1-2".



I mixed 25g of 100um graphite with 20g of thin-ish epoxy. Thick but has flow.

Trowel application - popsicle stick works well - brush useless.



It is exposed on mine and will be my scratch cover as well. Top up if wears thru.

I had read that the conductive paint has to be potted over - in my tests earlier it is fine with conductive epoxy exposed. Tested earlier on damp grass and clover no chatter an falsing - Im quite encouraged.

It went from totally unusable on wet or damp grass/clover to close to perfect . It ground balances properly and more crisp too.

It goes on like demerera sugar or tarmac, and settles to reaonably smooth. Before full cure squeegee it with a damp jaycloth encourage it to lie down flat.

You could leave this recessed and later scratch it up and por nice shiny potting compound on top, if looks are your bag.

S

[Altra]

Hi Kev,
Looking good! You have come along ways since experimenting with battery electrolyte.
I understand your coil constuction. But I am wondering what material you are using on the final layer? The layer the shield paint is applied on?
Years ago ml used some kind of cloth tape on the large mono coils. I always worried about expansion or contraction effecting the conductivity.

[Kev]

160ohm seem lowish already

Hi Davor,
Minelab PI are very very fussy when it comes to coils and the shielding must be right if you want to build an operational coil, let alone one that surpasses stock standard.
The graphite card Minelab use has a resistance of 40 to 45 Ohms per inch, and the coil above measures 35 to 40 Ohms per inch so it's in the ballpark.

Of course from a frequency domain perspective, from which I believe you are viewing this, it would be way too low, so maybe for VLF you'd need to water this paint down a bit.

I tested the coil at a beach which is littered with volcanic rocks, and it performed superbly, super sensitive while stable, now I need to test it on the goldfields which I intend to do within the week.
The spacer on this coil between the windings and shield is a very low density foam which has a very low loss tangent, and I think this is contributing to the excellent sensitivity and
depth it is achieving. Here's hoping it will also cope with highly mineralised ground.

Cheers
Kev.