Chance Pi problem....¿¿¿????

[bernte_one]

hello baum

dfbowers has made and instruction pdf for vacuum forming
or write him

if you are from germany
for good material look here i orde there some times
http://www.s-polytec.de/
write the shop owner and ask for 1,5mm abs plattes he makes good prices

regards

[baum7154]

Chance update

Thanks for the reply.

Has any one played with the value of the 390 ohm coil damping resistor. What is the best way to optimize it?

[baum7154]

After doing a bit of reading it appears that the 390 resistor is only an adjunct to the active damping provided by the IRF9640 mosfet. I wonder if there is any point to trying to optimize the 390 ohm damping resistor? It still seems that an optimized value might benefit detection of small gold in the low microsecond ranges. Has anyone used another value for the damping resistor in this detector?

[baum7154]

Chance coil shielding

I just shielded my 8" spider coil last night using .001" copper foil cut into 2cm strips and wrapped toroid style on the coil. I have about 6mm/.240" urethane foam spacing between the coil windings and the copper shield. The Guard Interval setting on the unshielded coil operated at '19' before but now only begins to run at 33 or 34 depending on other settings. That said the unit does still detect gold and other metals as before. I'm wondering if I should try some other shielding materials such as metallic ribbon or metalized mylar. I have some ribbon that runs about 7 ohms per inch and it does not give much of a response when held close to the detector coil. Is it better to have a high conductivity faraday shield like the copper or something less like the ribbon? ANy help with this is appreciated as I want to get this coil finished and tested in the field.

Regards,

Dan

[satdaveuk]

If you read the other forums down bottom of page ie german forum who spent alot more time on the chance project you will find the best results were when using graphite faraday shielding at 1k-2kohms per sq inch.
Copper shielding works to well as proved by my experiments I made last year, its like trying to work a radio in a car without a aerial, shielding the signals we need.
Mylar works well providing you can obtain some that has continuious conductivity after winding, my findings were its very fragile, hit and miss, but others had sucess or so im told.
Best findings were, synthetic graphite mixed 1:1 hardglaze varnish splash of turps which turns it into a gun metal grey paint, 2 coats , with 10mm copper strips either end.
Its not only robust you can add if required resistance not met or subtract by lightly sanding.
Keep meaning to do a thread on this procedure but havent got round to it yet


Regards

[der_fisherman]

If you read the other forums down bottom of page ie german forum who spent alot more time on the chance project you will find the best results were when using graphite faraday shielding at 1k-2kohms per sq inch.
Copper shielding works to well as proved by my experiments I made last year, its like trying to work a radio in a car without a aerial, shielding the signals we need.
Mylar works well providing you can obtain some that has continuious conductivity after winding, my findings were its very fragile, hit and miss, but others had sucess or so im told.
Best findings were, synthetic graphite mixed 1:1 hardglaze varnish splash of turps which turns it into a gun metal grey paint, 2 coats , with 10mm copper strips either end.
Its not only robust you can add if required resistance not met or subtract by lightly sanding.
Keep meaning to do a thread on this procedure but havent got round to it yet


Regards

Are you therefore implying that a high resistance is best for the shielding, or a low one? Do you measure from end to end of the shielding? Thanks for your answer.
Regards
Andy

[satdaveuk]

Yes Andy
With the experiments ive done in real conditions over here you dont want anywhere near short circuit, theres a fine line between shielding that take cares of unwanted intrusions and that of wanted ones, although must stipulate that most experiments were done with induction balance format.
The coil in my understandings and research is basically acting as a very nice antenna which is very capable of picking up crap that we dont want, but on the otherhand we are looking for are cake and eating it, and thats where the happy medium comes into play.
Not just looking for a good constant overall shielding, but looking for a shield that wont block the wanted information hence the 1-2kohm per sq inch resistance, proven by me in english soil.
The golden goose to success is keep it constant throughout the coil as best as possible.

Regards

[satdaveuk]

Just a food for thought.
Noticed posts in the past where constuctors have disected manufactured coils and have stated that the faraday shielding was open circuit.'
Wonder if infact it was?... maybe there meters didnt run up to high enough resistance to check it, or maybe they went by gospel expecting continuity of a few ohms.
Its been in the back of my mind for quite some time.
In all proberbiltys there was nothing wrong with the coils it was down to design factor.

Regards

[baum7154]

The Faraday shield must be kept as an open loop or little to nothing will get out. This is a different issue than the resistance of the material the loop Is made from. So what I am getting from this discussion is that it is better to have the material be of high resistance from end to end of the open loop faraday shield.

It is odd that my coil does act very close to normal with the low resistance fool shield. It seems to detect my gold ring at approximately the same distance as before shielding. I will have to try the graphite shield to see if it improves performance

[baum7154]

coil shield

Was the graphite shield you wrote of in the form of a flat sheet placed above & below the coil, or was it in the form of a ribbon wound around the search coil? I have an 8" spider coil and 6mm of urethane foam thickness surrounding it with 2 cm wide copper ribbon wound (toroid style) over the foam with about 40% overlap of turns. Is this how you would go about shielding using your graphite material? What is the best substrate to apply the graphite to...paper, parchment, nylon fabric, ribbon, mylar, etc.?

Thanks for the help!

[satdaveuk]

What i did was after luming coil with wax string and then many coats of hardglaze varnish was wraped with selfamalgamateing tape, then a layer of hospital type paper tape, which is just thin paper tape, very obsorbent, then 10mm copper tape either end leaving the normall 5mm gap between them, after that just painted the graphite directly onto the tape , coats as required after each one dryes out then lightly sand for smooth finsh if required, then another layer of insulation tape.
I used the paper tape because the graphite mix binds well to it.
My findings with styreen housings for projects were better than exspected, using same procedure it seemed the longer it was on the housing the more solid it was, to remove it you would proberly need a hammer and chisel or lots of sanding.

Regards

[satdaveuk]

Far as using foil as I stated in another thread it depend what gauge you have used, I found that using the very cheap foil that normally discounted the type that tears very easily is the better type to use, but using good quality is a no no because again its to agressive.
Try the graphite method, you will be pleased, the more you play with it the better you will get, but i advise you the graphite till mixed is messy, very light, it goes everywhere

Regards

[baum7154]

Thanks for the clarification of the graphite application process. Do you use the synthetic graphite I see reference to elsewhere in these forums? Is the varnish you use a urethane based product or is it more like shellac or lacquer? I am going to fiberglass over the shielded coil so it won't be re-entereble once it is encased

I am anxious to get this shield in place on my coil and test the results. The current shield of .001" copper foil does appear to diminish small gold response but does respond well to my gold ring. Since my intent is to use the detector to find small gold I hope this shield will do the trick.

Do you know what the resistance of your shield measures end to end? You stated 1 to 2 K ohms per inch so I take that to mean per running inch around the coil? Therefore my 8" coil should be about 25 to 50 K ohms from end to end. For best sensitivity would you say I should try for the higher end of the range?

Thanks,

Dan

[baum7154]

Graphite Shielding

I just read about Blaster Graphite Dry Lube spray in a post on Coils section of the Tech Forum. I went out and got some and ran a few tests on a 10.8" X 1" strip of paper medical tape with 1cm copper foil contacts across each end. Spray was applied from a distance of about 10" to the adhesive side of the tape covering about 3" wide in a pass. Overspray was about 1" either side of the 1" tape.

4 single passes = 3700 ohms

This stuff does adhear to the surface but will rub off. I would recommend coating it with a spray-on flexible coating. I'd follow that with a wrap of cloth medical tape and fiberglass.

Spraying the graphite on seems to give an even coating but it is probably more messy than painting it on. I also think the paint will be more durable and fill the surface differences of adjacent tape edges wound over the coil better than the spray.

[satdaveuk]

I use synthetic graphite as recomended by a large graphite company here in the uk as its 98.9% min carbon content.
Its actually particals left over while producing carbon brushes for electric motors etc.
Once collected they further process it into a very very fine powder which is lighter and smoother than talc, its so fine it will enter the paws of your skin and feels like silk.
Anyway due to the high carbon content its very conductive so gives room to be mixed with other components and still maintaining good conductivity.
Findings over here were 1-2kohms per inch did the job without noticable sence drop, no falsing, anything below the 1k kept the machine quite but noticable lack of sence, going much over the 2k started to get chatty and useless.
Alot depends on location trial and error, all my tests were done in same location pasture land fairly high iron content, electric pylons in the far distance.
Almost all experiments were based round Ivonics TGSL on two builds although did make a untuned coil for the whites IV and findings were the same.
As a very good guide if you were to make a 10" coil pick 2k which = 200kohms overall from end to end mount on perspex or upvc, or even wood base, setup and hot glue.
Then go out and play.
Doing it that way if required you can simply use hair dryer to melt glue and remove coil, then either add/ remove graphite and off you go again.
I have read in forums where they add the graphite to the apoxy mix, which is all very well if you get the mix right first time, if not your knackered.
The reason why I went down this path was because was able to measure at intervals throughout the coil for regular constant continuity ie if i read 1.5kohms on the first inch , which then needs to be there after throughout the coil, If by chance the following inch is a bit low, just sand a nats whisker off with fine sandpaper and so on until you get a nice well balanced shielding.
The constant value throughout a coil shielding no matter what resistance you choose is more often than not overlooked, which is a very bad mistake because your opening the gateway to falsing and putting it down to materials used rather than the way its been installed.
So the best way forward is use 2k per inch and you will be well impressed, if your experiencing falsing etc, which i very much doubt then apply another coat to reduce resistance.
In alot of other countrys ie parts of USA maybe able to go up some, wether you will actually benefit from it remains to be seen, let me know.
Also worth keeping in mind is the fact that under shielding can give very similar symptoms to over shielding with alot of metal detector builds, so theres a bit more to it than meets the eye.
By the way the varnish I used was hard glaze type used for exterior wood, cant give anymore information on it because doesnt say on the tin, what I do know is its at least 15 years old and still very usable.
When replacment is required if know longer available will try boat varnish.
Also remember the thinner is turps which is short for turpintine, not the ozone substitute which has different properties, although may work its not proven by me.

If any of you want to hang fire for a few weeks will be building a coil using graphite for the PI Mini Pulse 3 project , its delayed due to some software and hardware issues, but getting there, will be interesting to see the outcome and could save a few of you alot of hassel.

Regards