synthetic graphite

[satdaveuk]

Dont know what happened there, it kept repeating are previous posts but seems to be ok now

Update.... its because i had it on save draft while i cooked my daughters dinner.

Regards

[satdaveuk]

satdaveuk,

If you're referring to spray foam, such as Great Stuff, don't bother. Here's my bad experience with it:

http://www.geotech1.com/forums/showp...6&postcount=10

I haven't tried it myself, but many have had good results with two-part foam.

Hi
Haha yes learned that lesson when making bottom coil shaft with 21mm plastic pipe, when driving the foam in you need to open back doors to allow the air to come out and the foam to flow, not to much needed because at least 25% for exspansion or otherwise pop, but never gooyey always solid so must be different stuff ive used.
But after saying that will heed your advise because ive not used it on coil housings before, only resin with dd coils for induction balance, wouldnt even attempt foam with those as shifting oviously accures enough to totaly knacker the balance of the two coils.

Regards
Dave

[baum7154]

Mid-point Shield contact

Satdaveuk, one other thought on the idea of a mid-point shield contact foil...It may give better detection consistency and symmetry due to equal resistance from the mid-point contact to each end of the shield at the gap. It may change the optimum shielding resistance a bit. A small point but possibly a change worth making.

Regards,

Dan

[golfnut]

Your right dave painting the shell and having a disaster burns a shell.

I already potted Rx coil too using thin epoxy!!


I got a couple of spare shells.

S

[satdaveuk]

Satdaveuk, one other thought on the idea of a mid-point shield contact foil...It may give better detection consistency and symmetry due to equal resistance from the mid-point contact to each end of the shield at the gap. It may change the optimum shielding resistance a bit. A small point but possibly a change worth making.

Regards,

Dan

Good comment, dont know if its actually a advantage or disadvantage having a 10mm copper strip midway, we can give it a trial, guessing may cause a problem due to its high conductivity, but who knows your points are very valid and certainly worth experiments.

Something else I was reading about with shielding was in high overall conductivity situations ie 0 ohms shielding, was using a pot down to deck and adjusting for optium performance in the field.
Thats something else worth trying but can see other problems arrising in the fact the big lump of metal is still stuck round the coil which is what we have got away from by using graphite, but still worth playing with for that extra mile.
Thanks for your interest Dan and keep up the experiments.

Regards
Dave

[satdaveuk]

Your right dave painting the shell and having a disaster burns a shell.

I already potted Rx coil too using thin epoxy!!


I got a couple of spare shells.

S

Ive still got my 3 shells, put the tx/rx coils in dry for the TGSL havent got any further due to bench space, The coils are prepared with the graphite, not bothered to tape these two, going stright in with just insulators to isolate from each other then wire CX caps and hot glue, theres only 400 micro henrys difference between the two so may have problems with nulling, hopeing to compensate with the CX but we will see.
In the past needed that magic 500uh difference to get optium performance .
Anyone had any luck with that sticky coil calculator on this forum working on windows 7??, its a bummer cant get it to work.

Regards

Dave

[baum7154]

Mid point shield contact strip

The way I envision this being accomplished is to put the gap opposite the coil ends with the shield contact strip between the coil leads with a hole in the shielding to accommodate the high side coil wire penetration. The ground side would be tied to the mid point shield strip.

If this configuration would not work then the shield gap would be as it is now and we would need a long wire from the mid point shield strip round the coil back to the connection with the ground side of the coil. This would end up with more copper in close proximity to the coil that what we have now.

I'll try all of this out on my next coil build and let the forum know but it may be a month or two.

[Woody.au]

Hi, After building over 100 test coils I can say 100% for sure that you should not be using Epoxy or any type of resin type paint and there is a good reason why. The resins are insulators and have to rely on intimate contact between graphite particles to maintain a conductive path. this is hit and miss and with thermal induced flexing can create a noisy coil. Here is the way to do it... use 60% Graphite powder and 10% carbon black and combine with a water based acrylic based paint. once it sets it will never crack and as the acrylic is converted into an electron carrier by way of activation with carbon black it makes a highly conductive matrix to lock the graphite particles as a very conductive coating. It allows one to obtain very low resistances with just one coating, if its too low just add water and thin it a little. aim for 60-80 ohms per centimeter for P.I and as high as 1000 ohms per cm for VLF.
Any resistive material between the coil and target will rob some power but its very hard to measure or even notice from practical experience.

[baum7154]

Shielding materials

Thanks for the info Woody. I have used polyvinlyacetate mixed 50/50% with powdered graphite by volume. I will definitely try your suggestion on the next coil I build and shoot for about 200 ohms per inch to use on my Chance PI detector. If you read my previous posts about a mid-point foil contact on the Faraday shield ,as opposed to contact at one end , I'd like to hear your thoughts on the concept. I also have been thinking of using Litz wire for my coils. I am focused on small gold detection with this detector.

Regards,

Dan

[satdaveuk]

Hi woody
Thats interesting comment, can I ask what makes/types ratios of mixtures you were using when doing these hundreds of coil tests with synthetic graphite and resin fix.
The reason why I ask is because I certainly havent had any cracking or degrading with the DD coils set in 2 part resin, If had been its pretty ovious wouldnt still be using it, only a mear 23 sets compared to your hundreds, but still a good count.
My coils have been floged to death by me and my customers since early this year using my formulars and had no problem todate.
Synthetic graphite that I use bought locally in the UK is 99.9% carbon, hence highly conductive, talked to the technical deparment of that company before any experiments took place and they assured me with the type of resin I was using together with fiberglass would not react in anyway with it, and todate they have been correct .
Before knocking resin mixtures suggest you look at the different types thats manufactured, together with different specifications, then come back to me and tell me you tryed them all.
The only thing you quoted on that my experiments marry up to is the 1kohm, which todate can only comment for here in england as soils in various countrys will demand more/less.
Im not knocking your input because theres always something better round the corner infact will proberly try your ideas to see if theres any benifit, What gets up me goat is the way you blanket everything on my tecniques which was months of experiments before posting here, as a no go without really knowing what your talking about because the elements im using oviously isnt what your were using in your hundreds of coil tests.

Regards
Dave

[golfnut]

Feedback on my latest coil.

27DD Georgi shell - I used an fat pencil - drew all over the inside so it all looked silver/grey - took a while

I tacked 4x 8cm fine bare wires to the graphite using silver heater repair paint commenned the ends like a star point for gnd point.

I then mixed ground up carbon to a grey xylene car plastics primer and gave generous coat.

puy the coils onto this and epoxy pot.

So shell conductive, no conductive material over windings when viewed from beneath.


No falses on wet Grass, decent sens on test coins in garden.

So if you like, the coil has non metallic shield on 3 sides.

Seems good.

S

[satdaveuk]

Feedback on my latest coil.

27DD Georgi shell - I used an fat pencil - drew all over the inside so it all looked silver/grey - took a while

I tacked 4x 8cm fine bare wires to the graphite using silver heater repair paint commenned the ends like a star point for gnd point.

I then mixed ground up carbon to a grey xylene car plastics primer and gave generous coat.

puy the coils onto this and epoxy pot.

So shell conductive, no conductive material over windings when viewed from beneath.


No falses on wet Grass, decent sens on test coins in garden.

So if you like, the coil has non metallic shield on 3 sides.

Seems good.

S

Thats good result Bob
Did you take any pictures of procedure?
Am i right in understanding you only painted shell with what ever and left underneath unshielded ? Tell me more because id like to try it out.
Many thanks

[satdaveuk]

Heres another coil made for the minipulse 3 runs a bit unstable in the garden so will try it out in a field.

The thin very very large one is one of two for experiments with the TGSL tryed mylar again so I could take off and save the wire but again had the same problem as before there was open circuites everywhere so shall replace with foil just to try them out because im curious what results would be. who knows may get another 12" lol, if I did id market them haha

Regards

Dave

[satdaveuk]

Ready for testing out in the field the coil is just hot glued all round, if it works ok then proberly do what Baun sugested just thin coating of resin mix over it to fix permanent and protect coil.
The base and lugs was cut out of my old business sign, very light.

[reiv]

Hi everybody I bought synthetic graphite too and decided to make a shield for my Idx Pro concetric coil, here are my tryes:

1. First I used pattex water glue to wood with some waterbased acrylic paint and water, I had nice surface with 1.5k Ohm resistance per cm.
But in the field, at weet snow it was total disaster.. To get detector stable I can take the treshold only at "1,5" (0-10 scale) and sensitivity at "6" soe it was no sense.

2. Second tray, i use the same mixture like Satdaveuk-varnish with graphite (2 layers) - painted bottom of coil, and get 1.5k per cm too. Results are much better, now in the wood on weet snow i can take treshold at "5" and sensitivity at "8", with this setup on my 10'' CC coil I can shoot 15mm diameter coin at 29cm in the air, witch I think it`s ok.

Satdaveuk: How do You think can i say that my shield is ok? Or I have to wait for snows went of and tray it on the weet grass. Now in the field i can`t take treshold much than"5-6" unless i do not touch detector at all because he is getting unstable.