synthetic graphite

[varxo]

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[reiv]

What this "spider" has common with this thread ?

[satdaveuk]

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.

Hi
You need to shield the whole coil not just the bottom because otherwise your open to EMI interference which is a big problem for metal detectors making sure you leave the magic 5-10mm gap of course.
The 1-2k per inch ( depending on size coil) seems to be the typical value with most builds in most locations around the world but for optimum performance you need to experiment as whats good in one location is not so good in the next.
Also whats important is keep the graphite constant throughout the coil, its better to apply to much and then lightly sand to a smooth finish with required resistance than like yours where you have graphite not fully covering (note left hand side of picture), Also I cant see any gap it appears you have a completed circuit which is no good, correct me if I am wrong'
To read the graphites true reading at 1" intervals you need to lightly sand anyway otherwise your reading through the glaze which will give you false readings.
Lightly sand between each coat of mixture otherwise your into all sorts of problems both electrical and mechanical which will cause falseing.

Its both easy and effective if you follow the procedure put on here but, if you start to stray from it IE using water based glues/paints etc then your in trouble because its no good for this application.
The varnish types used with great success have been Hardglaze outside varnish and Yacht varnish.

[reiv]

Thanks for replay Dave

I will start from the begining for better understand. First I wind the coils check them if they can null, after that I tape them with paper tape, glued copper scraps with cable to make connect with shield, then I paited them mixture water based glues/paints with graphite (now i know that is wrong) with 5-8mm gaps, and finally i taped them once again with paper tape. Then i glued them in coil shell, picture bellow shows it.
It appers disaster soe, i didn`t want to throw out this coil to trash, just want to rescue this coil soe i shield only bottom cover (i know better will by all) but coils already was glued to shell..
I was done shield like You wrote- 1st layer 2 days off, then light sandind check resistance, 2nd layer, 2 days off and second light sanding. Graphite is constant from all cover with 8mm gap, visible at north of the previous picture.
I just want to ask is it normal that i can take treshold only at "5" and sensitivity at "8" on the weet snow and grass, or I have to make another coil ?

[satdaveuk]

Thanks for replay Dave

I will start from the begining for better understand. First I wind the coils check them if they can null, after that I tape them with paper tape, glued copper scraps with cable to make connect with shield, then I paited them mixture water based glues/paints with graphite (now i know that is wrong) with 5-8mm gaps, and finally i taped them once again with paper tape. Then i glued them in coil shell, picture bellow shows it.
It appers disaster soe, i didn`t want to throw out this coil to trash, just want to rescue this coil soe i shield only bottom cover (i know better will by all) but coils already was glued to shell..
I was done shield like You wrote- 1st layer 2 days off, then light sandind check resistance, 2nd layer, 2 days off and second light sanding. Graphite is constant from all cover with 8mm gap, visible at north of the previous picture.
I just want to ask is it normal that i can take treshold only at "5" and sensitivity at "8" on the weet snow and grass, or I have to make another coil ?

If your detector is working with reasonable stability and finds the goods at a reasonable depth then yes you have a good build you have more to play with settings on a quieter site.
Worth bearing in mind that if you were to go detecting with minimal minerals within the soil content and no overhead power lines, Tv, Radio,Phone, etc transmitters around you would be wacking the sense maybe full up and be able to establish a nice crisp smooth threshold.
And also bare in mind in those very same ideal situations you could make a second coil with higher resistance property's with the Faraday shielding to give you a bit more edge far as depth.
Its a bit of catch 22 situation with coils because its a very fine line between producing Faraday shielding that blocks unwanted signals allowing wanted to pass, or over shielding which reduces the signal response to wanted signals.
When doing your testing your better off going somewhere open in the middle of no where, leave your mobile phone in the car or at home, check your shoes or boots for metal content if so change them, keep keys etc in your top pockets well away from front end of detector and you will notice a very big difference in your results.
Dig some holes and bury different coins etc and let us know your results.
Far as snow don't worry about it, some of my detectors actually respond better in snow and frosted ground.
Your going the right way about it with real ground tests, avoid comparing with anyones air tests because it doesn't prove anything or any help to how well a metal detector will work the way its designed for.
You done a neat job of that coil like the way you have mounted TX/RX capacitors within the search head which means you can easily make interchangeable coils.
All the best

[satdaveuk]

Hi Everyone
I have been contacted a couple of times regarding the paper tape issue so I thought its worth pointing out here encase any others fall into the trap of using masking tape in place of hospital paper bandage tape.
Do not be tempted into using decorators masking tape because its non porous.
Hospital paper tape is like very thin blotting paper with a strong adhesive backing which allows the graphite mix to bind with it leaving a strong sturdy finish.

Masking tape is total non absorbent so graphite mix would not even stick to it let alone bind.

Regards

[reiv]

Yes, I confirm this info My previous shield with masking tape not works at all.. When i look at him with magnyfying glass i saw many cracks-because graphite mix can`t stick to this kind of tape.
Regards

[Davor]

How about white glue? seadoggy tested it with success.

[reiv]

I don`t know what kind of white glue You are talking, if white wood glue -I checked it, mixed with graphite and paint on to masking tape-maany false signals. I can`t by sure if it was because of glue, tape or both, it is waterbased glue soe it maybe not good. For me best work gives graphite with varnish

[satdaveuk]

Hi Davor
If the white glue you refer to is any form of glue thats water based (PVA type) my findings were when first applied everything seemed successful which I mentioned on here many moons ago, but after a few weeks cracks appeared which rendered it useless.
Just stick with the yacht varnish applied on the hospital bandage tape and you will be well pleased .
When shielding styrene housings a good robust finish is spray with plastic insulation like used on anode caps of the older TVs to prevent corona discharge or similar to damp start used on engine spark plugs, will post pictures later to show you some finished projects using this technique as it works well.

[satdaveuk]

Hi Reiv
Pleased to here of your success
The only people up to date who are having problems or had problems is where they have deviated from the spec of components advised
In the early days I tried many glues/paints with various mixtures which some from the off set were failures and some months down the line, which varied from mechanical to electrical or both.
PVA water based was the very first component I tried mixing with Graphite because I have a gallon of it and its cheap as chips, problems that were found was first of all getting a constant overall even resistance and weeks went on it got worse and then eventual hairline cracking ac-cured.
The tests were firstly all carried out on 12"x2" lengths of styrene which I use for MD coil and electronic housings.
It was after that once finding a good reliable mixture that I carried on with shielding coils and housings which are successful every time and still going strong.
Once you have done a few you will find that 2 coats and a third where the copper strips are applied does the trick every time, just remember the magic 48hrs between each coat.
With project housing shielding I do inside and out 3 coats then give the outside a sanding with fine wet and dry as used by car body shops which gives a nice smooth machined gun metal gray colour, after that spray a couple of coats with plastic seal spray as used by us TV engineers CRT tubes to stop corona discharge or other wise known as damp start used on engine spark plugs, its robust and gives a excellent finish.
When I assemble MD coil using the the plate method IE 2 8" round surfaces and center 7.5" or so I normally drill the hole for the cable johnny, then drill another hole between the sandwich and the cable outlet, Thread the earth wire between the two and solder that to the copper stripe which we applied in the grove, then graphite, followed by the plastic spray and then paper tape, remembering to leave the hole clear to thread start and finish of your coil wire.
Tips: once you have threaded your start wire alongside your already inserted earth wire insert some sleeving twice the diameter of your coil wire because thats for the end return of your coil wire otherwise it will have got covered whilst winding.
I personally sleeve all three wires anyway if using laminate wire to be on the safe side.
Using the PVC spray on the Graphite is to make the surface more robust because your winding wire into a grove where you will be catching the sides, maybe not a must but better to be on the safe side also its good because you can easily change wire amount of windings so easily till your fully satisfied before the final paper and graphite application on top edge of the windings.
Tip: when drilling hole between grove and cable outlet, give yourself plenty of room something like 5mm hole because why make life difficult when threading the end wires through.
Once you finished wires can easily be terminated at the base of the Johny for coil lead out, which i normally a dab of hot glue to keep in place whilst screwing the outlet in place.

Doing it this way you don't need to shield all the housing, just the grove and finally the edge when coil is wound which mimics the way we do when shielding direct on to coils, its just every things put on in reverse and can be easily undone if required.

When I got time will show some pictures.
Regards

[langleylocator]

Fascinating, subscribed,and bookmarked. im going to have to read all of this several time to let all the brilliant work and info sink in thanks for all of your hard work