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  • #31
    Another good thing about it is by doing it my way you have a simple method of checking that you have a good over all even run of shielding, by lightly sanding after the mixture has gone off gives you the same as a carbon track on a pot or preset so by applying conductors both ends ie copper tape with enough copper exposed for testing each end then off you go around the coil.
    Two coats takes you into reliable kohms per sq inch.
    Providing you do the fine sanding after its gone off after say 2-3 days you can bring the resistance down applying more coats.
    Todate as stated previously the kohms per inch is magic on the TGSL which is two coats and a light sanding.
    Let me put it this way, If I can reduce falsing to almost nil without loosing depth, you would have to agree the shielding is doing its job.
    I have proved with no shadow of doubt that thats where the problem lies with those who have used mylar blanket, because its patchy far as continuity, hence why the quick fix is running copper wire around the lenth of the coil to make up for the o/c with every crease within it, some that cannot be seen by the naked eye but can be detected before copper wire applied with a simple ohms meter.
    The kitchen foil which most of us have used for years, if applied correctly is constant, ie little falsing but the draw back been its to good as it also screens out things we need to detect.
    So doing extensive tests in practical terms in real situations is why ive come to this conclusion.
    The reason why ive not posted yet is because for instance the formular Ive used has now been on test for nearly three months, no cracking or change in resistance either on the coils or styreen housing infact it like granet, would have to be hammered off with a chisel, and is above my expectations
    However when first applying for the first 48 hrs its up and down like a yo yo, so was hoping to give it say 6months then tell all rather than jump the gun.

    Like all projects experiments on here it will always be ongoing but feel theres a big leap been made here far as DIY shielding of coils and housings.

    Warm regards

    Comment


    • #32
      Coil resistance

      Do you know what the resistance of your shield measures end to end on the coil? You stated 1 to 2 K ohms per inch so I take that to mean per running inch tested and adjusted around the coil? However the resistance across a given inch of 1 to 2 K doesn't give an idea of the overall resistance end to end because The total area surrounding the 8" coil might be 25" X 3". I'd just like an idea of that resistance end to end. For best sensitivity would you say I should try for the higher end of the range?

      Thanks,

      Dan

      Comment


      • #33
        PI detector shielding

        I should also mention that my detector is a Chance PI machine. Is the shielding requirement for this the same as for your TGSL? Tried to read the German forum info but could not find anything of significance on the Chance.

        Thanks,

        Dan

        Comment


        • #34
          Hi Baum
          Seen alot of positive feed back recently on the 2kohms per inch with pulse induction detectors in general, wont sit here and say yes it will be great with that particular project because ive not tryed and tested it.
          All facts ive stated on coil shielding/building between last year and this year is constructions ive built myself and proved by myself with months and months of experiments both in workshop and the real world.
          There were at least two main reasons of delays firstly you could make a mix of certain parts which on the offset looked successful, but later either fractured, flaked, under stress, or reaction of components with each other days/weeks after application.
          The goal I needed to reach was oviously good shielding, easy application, constant as possible, using easy to get and cheap components, and last but but not least long lasting and robust, which turned out a great success by me and now others, so well worth the effort.

          Why dont you go the extra mile and tryed it out and give your verdict, and if its a negative we take note of location where tested, and so forth, if everyones a negative on a particular MD design then back to the drawing board.
          I cant keep building projects to test coils its someone elses turn

          Regards

          Comment


          • #35
            procedure

            Hi
            Pictures speak a million words, so have a peak .
            Doing another 3 coils with Graphite one for the PI Minipulse 3 the other 2 for TGSL.
            Just came back from the workshop so there drying out, they will be ready for second coat this evening, so more pics to follow.

            Regards
            Attached Files

            Comment


            • #36
              graphite pencil on shell inner

              Recently deglazed the inside of a DD shell (georgi 27) using fine emery.

              Then spent a our rubbing the inside with a fat graphite art Pencil.

              If I use ally foil probes I can get about 20k over a square inch of the treatment.

              Maybe I try and get more on it.

              Will post pic if interest

              S

              Comment


              • #37
                I bought some Hagen Carbon for fish tank filters. Ground it to dust - sieved to get the fine dust.

                Mixed it 50 50 with plastic primer paint, brushed on. Seems ok, gives a rough finish tha the potting will hold.

                Im around 10k per square.


                Do you need to have this low ohmic treatment around four sides of winding ?

                Currently its only on shell - Not on the epoxy potting surface. So 3 sided only. Is this No Good?

                Do I add conductive laver to pott suface to form full 'box' in-section?
                S

                Comment


                • #38
                  Originally posted by golfnut View Post
                  I bought some Hagen Carbon for fish tank filters. Ground it to dust - sieved to get the fine dust.

                  Mixed it 50 50 with plastic primer paint, brushed on. Seems ok, gives a rough finish tha the potting will hold.

                  Im around 10k per square.


                  Do you need to have this low ohmic treatment around four sides of winding ?

                  Currently its only on shell - Not on the epoxy potting surface. So 3 sided only. Is this No Good?

                  Do I add conductive laver to pott suface to form full 'box' in-section?

                  S
                  Hi
                  Firstly with induction balance coils to be used in England you need maxium of 2k per sq inch although I personally found 1k or just over to be optium performance.
                  You need total coverage of the coil apart from the 5mm gap.
                  With DD coils its easer to graphite onto the coils following my suggestions as above before installing them into the shell, by doing that its alot less work and easer to check for constant over all continuity througout the coil surface.
                  Its as follows, make coil, lume, runny supper glue, 2 layers of insulation tape, hospital paper tape, 10mm copper stripes each end (5mm gap ) 2 coats of synthetic graphite leaving 24 hrs between each coat, then either insulation tape or self amalumateing tape, prefer the latter as its more robust, then its job done.
                  By doing it that way if you feel your a bit high or low on the resistance you adjust either by adding or light sanding.
                  Now when it comes to potting, do as I do, Firstly do nulling procedure as normal, whilst perfecting that spot glue the coils to housing using hot glue, its quick cheap and easy, when all said and done and your happy then run the hotglue along entire lenth of the coils to bond them to the shell.
                  By doing it that way you can now take your kit out for a good flogging in the real world and deside if its up to spec, if it is then go back to the workshop and poor your resin as final fix.
                  If your not happy with your coils just use hair dryer which melts the glue, and start over.
                  This procedure gives you a back door throughout the making of DIY coils.
                  If you do it it how you suggested you will not only have wasted time and money on coil making but also wasted a shell.
                  Cover your butt as they say
                  By the way note: im using yaht varnish now, dont use any thinners with it, just 4:1 , 1 part graphite 4 parts varnish, mix well and it goes on a dream with nice gunmetal gray finish.

                  Regards

                  Dave

                  Comment


                  • #39
                    Potting and shielding

                    I don't know what type of detector you are using but if it is a Pulse Induction machine I would not pot it in resin. Resin potting increases coil capacitance and slows down coil decay. It is outlined on this forum that PI coils should be potted in urethane foam. I have done this and used the graphite shielding Satvedeuk outlines and the coil response has remained fast. Critical to the graphite process is light sanding and measurement of resistance after each coat. Initially the sanding will appear to lower resistance and continued sanding will begin to raise resistance. If you have not sanded the surface of your graphite shield at all I think you will be surprised how very light sanding will lower the resistance measured compared to the unsanded values. I would use very fine sandpaper initially in the range of 240 to 400 grit. I set my VOM probes at a 1" separation using a block taped between the handles to keep a consistent spacing during ohmic readings. My shielding ended up at a resistance in the range of 1300 to 1500 ohms per running inch all over the coilexcept for the 5mm gap. Good luck!

                    Comment


                    • #40
                      Originally posted by baum7154 View Post
                      I don't know what type of detector you are using but if it is a Pulse Induction machine I would not pot it in resin. Resin potting increases coil capacitance and slows down coil decay. It is outlined on this forum that PI coils should be potted in urethane foam. I have done this and used the graphite shielding Satvedeuk outlines and the coil response has remained fast. Critical to the graphite process is light sanding and measurement of resistance after each coat. Initially the sanding will appear to lower resistance and continued sanding will begin to raise resistance. If you have not sanded the surface of your graphite shield at all I think you will be surprised how very light sanding will lower the resistance measured compared to the unsanded values. I would use very fine sandpaper initially in the range of 240 to 400 grit. I set my VOM probes at a 1" separation using a block taped between the handles to keep a consistent spacing during ohmic readings. My shielding ended up at a resistance in the range of 1300 to 1500 ohms per running inch all over the coilexcept for the 5mm gap. Good luck!
                      Hi Baum
                      Just thinking about you and wondered how you got on, So the same formular works for both IB and PI, far as shielding, well thats great because since your posting I also made two PI coils using the same formular and my findings marry up to yours.
                      You will find that when you were lightly sanding the graphite mix you were in effect taking the surface of the varnish which settles at the top uppermost of the mix when dry, so when checking for resistance you were getting better contact with the graphite rather than going through the insulation of the varnish, hence why you were getting lower ohms readings after light sanding, very good point through and glad you brought it up for others here.
                      Also a good point about potting coils, I presumed Golfnut was talking about IB format, where as potting is required due to the crytical setting that are needed to be maintained to keep the two coils balanced.
                      Cant see the point of potting a PI format coil, whats the point anyway in adding further poundage to a often heavy already coil thats not crytical.
                      Was thinking of using that foam that comes of a can which expands, very light and alot cheaper, plus quicker of course.
                      Picture is my 6 inch coil PI with no shell, water proof and solid as rock, the lugs and cable jonny is mounted directly to the coil, doesnt look pro but certainly does the job.

                      Regards


                      Originally posted by golfnut View Post
                      I bought some Hagen Carbon for fish tank filters. Ground it to dust - sieved to get the fine dust.

                      Mixed it 50 50 with plastic primer paint, brushed on. Seems ok, gives a rough finish tha the potting will hold.

                      Im around 10k per square.


                      Do you need to have this low ohmic treatment around four sides of winding ?

                      Currently its only on shell - Not on the epoxy potting surface. So 3 sided only. Is this No Good?

                      Do I add conductive laver to pott suface to form full 'box' in-section?

                      S
                      Hi
                      Firstly with induction balance coils to be used in England you need maxium of 2k per sq inch although I personally found 1k or just over to be optium performance.
                      You need total coverage of the coil apart from the 5mm gap.
                      With DD coils its easer to graphite onto the coils following my sugestions as above before installing them into the shell, by doing that its alot less work and easer to check for constant over all continuity througout the coil surface.
                      Its as follows, make coil, lume, runny supper glue, 2 layers of insulation tape, hospital paper tape, 10mm copper stripes each end (5mm gap ) 2 coats of synthetic graphite leaving 24 hrs between each coat, then either insulation tape or self amalumateing tape, i perfer the latter as its more robust, job done.
                      By doing it that way if you feel your a bit high or low on the resistance you adjust either by adding or light sanding.
                      Now when it comes to potting, do as I do, Firstly do nulling procedure as normal, whilst perfecting that spot glue the coils to housing using hot glue, its quick cheap and easy, when all said and done and your happy then run the hotglue along entire lenth of the coils to bond them to the shell.
                      By doing it that way you can now take your kit out for a good flogging and deside if its up to spec, if it is then go back to the workshop and poor your resin as final fix.
                      If your not happy with your coils just use hair dryer which mets the glue, and start over.
                      This procedure gives you a back door throughout the making of DIY coils.
                      If you do it it how you suggested you will not only have wasted time and money on coil making but also wasted a shell.
                      Cover your butt as they say
                      Attached Files

                      Comment


                      • #41
                        PI coil construction

                        Great to hear from you again Satdaveuk. I'm still finishing up on 2 Chance PI detectors using the boards from Silverdog. One is using a spider wound coil and the other is using the original planar basket weave coil from the Russian designer. I do use the urethane foam in the spray can for potting and then shape the foam to near final dimensions, wrap that with the paper hospital tape and begin the graphite shielding process. Once that is complete I wrap the graphite with a thin cloth hospital tape and apply a fiberglass mat and resin coating over the tape to protect all from rocks etc. My goal is to achieve minimum capacitance in the coil, coax, and detector electronics. This allows the coil to convey very short responses from small gold targets. Soon my detectors will be completed and I'll have them on the gold fields to see what the real world results may be. I'm using RG62 coax to keep capacitance down in that area. A 40" length of this coax measures 42pf on my capacitance meter.

                        Like you I have many plans for other PI coils. The next one will use Teflon insulated solid copper 23 awg wire in a spider wound 6" X 4" coil of elliptical configuration. My most recent coil used 26 awg enameled wire. It is hoped that the Teflon insulated wire will give considerably lower capacitance than the enameled wire does.

                        When all is assembled I'll post some pix. Like you say they are worth a thousand words. Great job on your coils!

                        Happy Hunting!

                        Dan

                        Comment


                        • #42
                          Originally posted by satdaveuk View Post
                          Was thinking of using that foam that comes of a can which expands, very light and alot cheaper, plus quicker of course.
                          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.

                          Comment


                          • #43
                            Spray Foam

                            I used Great Stuff on my last coil. The canned foam likes to have humidity/moisture to accelerate the cure. Since I don't use a molded enclosure for my coils I used a plastic bucket that was a close fit to my coil and form. I drilled a dozen 1/8" relief holes 1/2" in from the inside perimeter and lightly misted the bottom corner of the bucket perimeter with water. Then I put down a thin layer of the foam in the perimeter corner and placed the coil and form on top of the foam and appropriate spacers to get the foam thickness needed, and weighted it down. Excess foam pushed out of the drilled holes and the foam came out just the thickness needed. The same process was repeated for the other face of the coil.

                            If I were to try this foam in a prefab coil housing I would drill relief holes in the top half, foam the lower and upper halves lightly, and sandwich the coil between them letting the foam expand into the coil and vent the excess out the top. In a more humid environment than Colorado I might let the foam exposed to the air for a minute or two before sandwiching them.

                            Regards,

                            Dan

                            Comment


                            • #44
                              Faraday shield geometry

                              Satdaveuk what do you think of placing a single copper foil contact band at the shield midpoint just opposite the 5mm shield gap? This would mean less copper near the coil due to only one foil contact band instead of one on each side of the gap. Just a thought.

                              Comment


                              • #45
                                Originally posted by baum7154 View Post
                                Satdaveuk what do you think of placing a single copper foil contact band at the shield midpoint just opposite the 5mm shield gap? This would mean less copper near the coil due to only one foil contact band instead of one on each side of the gap. Just a thought.
                                Hi Baun
                                Ive had a good think on that one and the way its all turned out I dont think we really need that end strip at all now, its purpose was mainly to check for accurate over all continuity during the early days, so it served a purpose at the time but now not needed. So single 10mm copper strip at start or finish not both and thats it.
                                Dont know if it will give any noticable improvement, but certainly wont degrade it, and the less metal within range of the coil has to be a plus, so good thinking.
                                Regards


                                Originally posted by golfnut View Post
                                I bought some Hagen Carbon for fish tank filters. Ground it to dust - sieved to get the fine dust.

                                Mixed it 50 50 with plastic primer paint, brushed on. Seems ok, gives a rough finish tha the potting will hold.

                                Im around 10k per square.


                                Do you need to have this low ohmic treatment around four sides of winding ?

                                Currently its only on shell - Not on the epoxy potting surface. So 3 sided only. Is this No Good?

                                Do I add conductive laver to pott suface to form full 'box' in-section?

                                S
                                Hi
                                Firstly with induction balance coils to be used in England you need maxium of 2k per sq inch although I personally found 1k or just over to be optium performance.
                                You need total coverage of the coil apart from the 5mm gap.
                                With DD coils its easer to graphite onto the coils following my sugestions as above before installing them into the shell, by doing that its alot less work and easer to check for constant over all continuity througout the coil surface.
                                Its as follows, make coil, lume, runny supper glue, 2 layers of insulation tape, hospital paper tape, 10mm copper stripes each end (5mm gap ) 2 coats of synthetic graphite leaving 24 hrs between each coat, then either insulation tape or self amalumateing tape, i perfer the latter as its more robust, job done.
                                By doing it that way if you feel your a bit high or low on the resistance you adjust either by adding or light sanding.
                                Now when it comes to potting, do as I do, Firstly do nulling procedure as normal, whilst perfecting that spot glue the coils to housing using hot glue, its quick cheap and easy, when all said and done and your happy then run the hotglue along entire lenth of the coils to bond them to the shell.
                                By doing it that way you can now take your kit out for a good flogging and deside if its up to spec, if it is then go back to the workshop and poor your resin as final fix.
                                If your not happy with your coils just use hair dryer which mets the glue, and start over.
                                This procedure gives you a back door throughout the making of DIY coils.
                                If you do it it how you suggested you will not only have wasted time and money on coil making but also wasted a shell.
                                Cover your butt as they say

                                Comment

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