Theories about coil-shielding

[Henrikras]

Okay Steve.. We can hope that they are what Japan i know for.. Nice People and Very helpful.

[golfnut]

Yes looks good.

I asked them about price and delivery and if I can get a sample.. It is Japan, so may be slow..

[Henrikras]

I'm in If you can get something Gulfnut - Of course some cost is okay.
Just made the shielding at a 15" coil inside with my own mixture - graphite powder and propanol alcohol. - Looks nice.
Not proper dryed up yet to measure. Looks like low resistance, I like that.
Henrik.

[golfnut]

No I just surfed and found the site and the product.

I will ask them for a sample - Or we both could and compare results?

Steve

[Henrikras]

Maybee you can ask them one small test packets ... 4 kg If it wouldn't work in coil work.
Did you find out the cost of this ?

[golfnut]

This looks promising but hard to obtain

http://www.lion.co.jp/en/chem/produc...n/carbon01.htm

[golfnut]

Really good science there Henrick.


I made some using low viscosity epoxy and did 40% graphite 60% epoxy by mass and got 8kohms/sq - not great.


I think the particle dont touch each other very well - like a pile of bricks..


My theory is a smaller particle - filler is required to bridge the graphite 'Chunks'


If I get say 30um carbon black I will try with my 120um graphite and binder like thinned cellulose car paint, or maybe acetone. Steve

[Henrikras]

Graphite test

Made a small test... See pictures.
I a CD plastic cover, clean first with acetone, then one lay with the mixture. And wait 5 hours to it dryed up.
I made 3 drainwires at the left and the right at each CD cover.. (Same type I'm using in the coil work)
This alcohol I got is buyed in a drugstore, ist used to clean Medical tools, but I Allways used it to clean good old VHS videos videohead. It seem to make the mixture nice and smooth.
They could Be some other types of alcohol getting the same good results. !!
Its a good idea to use a brush with many hairs... To stiff brush will not Be good.
Henrik.

[Kev]

Howdy,
On my PIs I've had very good results with the conductive paint from Stewmac, I think it's actually Aquadag, it's almost identical to what Nugget Finder and Coiltek use. I actually water it down a bit, it goes a very long way, and easily produces the 70 to 50 Ohm per inch my Minelab requires.

On my VLF coils I'm getting good results using mylar from inside large capacitors. I'm not sure if they are Polycarbonate, or Polyester. I'm using some from work that were being tossed out. They are X2 caps, so they are designed for mains voltage which is 230V where I reside, and thus have high insulation resistance. Which seems to mean that the mylar is a reasonable thickness, in fact it is almost see through, but much thicker compared to low voltage types.
In these caps the layers are twinned up so I first separate them and then wind one layer on the coil conductive side up, and then do a second layer conductive side down which overlaps the join of the lower one. This gives around 50 Ohms per inch, which is way below George Payne's recommendations, but since the current has to wend its way around a maze of curls and turns it seems to not generate much of any extra noise in the machine.



Worth an experiment anyway, it may just give you a real hot loop. One things for sure, it makes for a very fine screen, much better than cooking foil, which I believe along with others here, causes way too much signal attenuation.

It will work on PIs as well but it requires a bit more work for the GPXs.

Old computer PSUs maybe a good source.

Cheers
Kev.

[golfnut]

If one had a metallic sheild and new what the air distances were. The user could add a resistor across the end / gap -- when the R value got too low the distance to detect would suffer.

This isnt quite the same as discussed but may give first order pointers. S

[golfnut]

Yes it is insulator, not carbon black. I have graphite, but wanted carbon black and graphite mix trial.

Toner will be going back on the bay.



I would like to know what the mechanism is for the falsing, with inadequate screen.

It may be that leaked E Field from Tx coil is coupled to the Rx coil using the dielectric properties of the soil

it may be

Tx signal phase shifts with proximity to gnd
OR
Rx signal initial phase may skew in sympathy with proximity

[Henrikras]

That was some interesting and valuable information Skippy

[Skippy]

@Golfnut: Toner is regular plastic, and an insulator. That is why static electricity attracts it to the paper, and then a heated roller melts it onto the surface permanently.
Graphite powder is easily obtained, a bit of Google searching soon turns up suppliers. It's quite cheap from some distributors, too.

[Skippy]

I think the key to obtaining the correct amount of shielding is to understand the effects of too much shielding. We know that way-too-much will affect sensitivity - the Faraday cage is TOO good, TX signals can't get out so easily, RX can't receive so well. But how much loss can we tolerate? We know that there is an "Inverse 6th" power law affecting raw air detection depth ( I use inverse 5.5 power in all my calculations, as I think the typical detection depth is not far enough away for the 6th power approximation). So, as an example, if our shielding loses 10% of the signal, that's 10% of TX, 10% of RX, total loss = 20% approximately. This equates to a depth loss of about 3.8%, example: 10mm at 250mm distance. This is an acceptable loss, and it could be recovered possibly, if RX gain were increased by a modest 20%, this could be completely recovered, with little side-effects.
If we had heavier shielding, say 20% loss, it becomes more serious, 20mm loss on a 250mm target, and +55% gain (TX or RX) would be needed to recover it. This is probably the limit, any heavier shielding and we have problems. I think from Henrik's figures, he possibly has this level of shielding on his recently modified coil.
But if we have too light shielding, and we only lose 2%, say, the depth loss is just 0.8%. We are probably not getting the best shielding here.
George Payne, in one of his online documents about coil-building, liked a value of 10 K Ohms / square, but not lower than 1 K Ohms. I suspect the lower figure of 1 K / sq is the value we should be looking at, maybe 500 Ohms is viable, maybe 2K is where we are not trying hard enough.
One problem is measuring an irregular surface, to find out the surface resistivity. A seperate "test-piece" might be one solution.
The other thing to consider is the shape of the Faraday cage. Would a complete circular case be worse than a spider-type case, where the cage is closer to the coil wire?
Maybe an experiment worth doing is to make a single TX coil (circular would be easiest) and accurately measure field strength with a sniffer coil. Then enclose it in a graphite Faraday cage, and measure the difference. Repeat for different resistivities, different distances from the coil etc.

One final thought I had: I saw a link, in an earlier post, to graphite paint with VERY low surface resistivity, 50 Ohms / sq. This would be great to use as a "drain wire". One of the recognised problems with an actual wire is the painting of it with graphite. I imagine painting high-resistance (1K) graphite onto low-res (50 Ohms) would work well. You would have to avoid complete loops (like you would with a wire) and you still have to make a wire connection somehow, unfortunately.

[golfnut]

I recently bought some printer Toner for Ricoh commercial copier/printer - I was hoping it was Carbon Black.


I had intended to mix it with the graphite powder. It was unfortunately NON conductive.


Im interested to mix graphite with carbon black powder for more repeatability