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  • How to judge shield effectiveness?

    My question is - posted it on another thread with no answer - how best to judge shielding performance at bench level (before the coil is covered)? I mean I know how to see if the material is detectable but how to test shield effectiveness? Do they all work the same at this point or are some materials better than others?
    I've read where it is best to have a spacer between coil and shield. This is apparently not the case in cables.(?)
    There is talk about conductivity per inch. How does this relate to a good shield?
    We have several cans of conductive nickel spray at work - used for shielding plastic cases. So if I took this and sprayed the circumference of a coil, connecting ground one end, other end open, would it work well? Would a spacer improve it?
    I can take shielding off USB cables - it comes off in a nice tape-like strip. How to compare it vs other metalized mylar materials for our purpose?
    Lastly, the part about leaving a gap on a coil shield - isn't this rather ignored in the case of Minelab coils where the coil is encased in a graphite paper housing?

  • #2
    hello bklein ,

    just read your post and have wondered the same myself.

    asked similar question on here months ago , but allas no concrete answer.

    ive read that the gap between coil and shield reduces capacitance in the coil so making it "quicker" many times , so there is one rule to follow i supose.

    the shield with no gaps is a complete ring that developes eddy currents so is a no no , apparantly , as far as a total graphite shield goes , is that not a complete ring ?? therefor shouldn't work ??

    and yes , the whole "whats the best resistance" , just asked that on another thread about 10 mins ago , tinfoil and nickel would be near zero , and graphite would be quite high , so what's best , i think i'm wondering the same as you .

    regarding bench testing , would have to test a coil unrapped for decay time to set voltage after pre amp , then shield with various materials and test again to get an idea.

    this shield thing is all a bit of a black art isn't it , with no set answers ???

    Comment


    • #3
      Not exactly ”black art” but just necessity.

      My preferred shielding method is graphite, but coils are mostly flat spiral types so this is only option, no foils, tapes etc allowed.

      Shield conductivity: more conductive is better, for salt water operation conductivity must be lower than surrounding medium. Home made graphite concuptions work great (ordinary enamel lack + colloidal or fine powdered graphite used for any purpose), some experimentation is needed. Spray like Graphite 33 works too, but more expensive and not so conductive, almost entire bottle must be sacrificed for one coil, in few thick layers. Whit graphite shielding, no gap is needed, like whit foil, graphite is not conductive enough to sustain eddy currents, just spray it all over. For normal inland detecting, conductivity is not critical, even highly conductive layer will provide enough electrostatic shielding.

      Applying shield to any coil will add some capacitance, coil to shield, especially whit flat spiral design (larger surface area) and slow it down somewhat, different dumping resistor value is needed (lower) when shield is applied. For normal coils this is far less critical. Normal coils are slower (by design), im against using any foil etc, only reason why this foil shielding actually works is fact that foil itself has a very fast eddy current decay time (remember, putting more metal in search coil is bad idea) so this will not ruin completely operation of already slow coil.

      For bench testing, immerse coil in water (adjust salinity) and observe opamp waveform, (dumping R adjusted before), if not changes, shield is good. If not intended for water, just check any capacitive effect, laying hand over coil etc must not produce response or change in waveform.

      This is one well proven and very good coil for typical PI like surf or Bara, I can recommend this to anyone building those detectors. Coil is 25cm, made of 2x.25mm PVC insulated wire (like speaker type), 2mm insulation, 12.5cm ID, (DVD spindle cover) 25 cm OD, 30 turns, wires in parallel. Shielding is either lazy man’s way, spray everything whit graphite, or, better, use sandwich made of two 4-5mm layers, cardboard and epoxy will do, im using rubber like 4mm laminate floor substrate. This is to reduce shield to coil capacitance.

      IMPORTANT 1 Connection is: outer side of coil and shield to GND, inner side to coax center conductor. Reversed, will be much slower.

      IMPORTANT 2 Build 25cm coil this way, 28 cm is fine, same recepy, 35 turns 14cmID, 28cm OD, but DON’T make smaller one, 20cm coil will be way out of specs for typical detectors (this on picture is 20cm, but built for something completely different). 2:1 ratio seems to work best.

      Whit 25cm version, inductance will be around 270uH, optimum dumping R 310-350R for unshielded or sandwiched coil, around 270R for simply shielded, depending on shield and detector type 10uS or faster sampling can be achieved. Excellent pinpointing and “discrimination” by ears. If you don’t have casing, make just “ears” and cable holder, then use epoxy/fiberglass (boat repair kit), one layer will do, to enclose it. Below is scope pic of coil response, but I cant remember exact detector configuration used (pulse widht, transistor type etc), should look somehing like this anyway.
      Attached Files

      Comment


      • #4
        A very good question, the answer definitely differs between PI and VLF coils, and may well vary depending on the frequency of the VLF machine. For VLF, I trust the advice of George Payne, Graphite at 10 KOhm per square: (last paragraph)
        http://jb-ms.com/Baron/gb.htm
        Excessively low resistivity will presumably lower sensitivity to targets and EMI, but there must be a resistivity value below which EMI pickup is little-improved, but target sensitivity falls away.

        Comment


        • #5
          Rule of thumb for shield effectiveness for PI

          Pi detectors with a delay to the first sample over 20us, do not need any shield.

          If you are looking for targets with a TC of less than 10us you definitely need a shield.

          4 to 8mm distance between coil winding and shield makes for faster coils.

          Test your shield material with the detector. The detector should not "see" the shield material.

          Whatever shield you use, always leave a small gap somewhere on the circumference, 2mm is enough.

          Set your detector to maximum sensitivity and connect the coil without shield. Approach your hand to the coil. If the detector does not "see" your hand, you need no shield.

          If the detector "sees" your hand without the shield connected, then it should not "see" your hand anymore after the shield is connected.

          If the shield is graphite paint, just add layers until the "hand test" is OK.

          The most important part of the shield, is to have a good connection to GND. A connection based on friction, (like aluminum foil not soldered) will make noise when the coil hits a rock or your shoe.

          Tinkerer

          Comment


          • #6
            Tepco great information thanks, one question
            you said 2x.25mm PVC insulated wire, but is it 2.5 mm wire ?
            in the picture the wire looks thicker than .25 mm
            cheers

            Comment


            • #7
              Wire is 2x .25mm sq (not dia) litz inside, having relatively high resistance ( 1.6R, both wires parallel, something like 20m ) but insulation is 2mm PVC, intentionally chosen to increase interwinding distance and reduce self-capacitance. Wire is “speaker type” with transparent insulation, like used in 5\1 or 7\1 home theatre wiring for low power speakers (but useless for actually connecting speakers), cheap and commonly available, just looks much thicker on images due to insulation.

              Comment


              • #8
                Hi Tepco


                I see .25mm sq thats about 23 AWG, 0.0668 ohm per metre

                thanks

                Comment


                • #9
                  I don't know of any litz wire that is "commonly available". Could you post a link to a source/spec? I see some on ebay and could order some from a wire specialist, but I don't know of a "common" variety anywhere. Is that clear insulation the same stuff that is on non-litz speaker wire?

                  Comment


                  • #10
                    Hi Tepco,

                    I like the idea about using that wire for coil (I agree with you that it's absolutely useless for speakers), so I will wind it tomorrow, but I have some questions, take a look on PM.
                    P.S. DUGA rocks !!!

                    Comment


                    • #11
                      Originally posted by bklein View Post
                      My question is - posted it on another thread with no answer - how best to judge shielding performance at bench level (before the coil is covered)? I mean I know how to see if the material is detectable but how to test shield effectiveness? Do they all work the same at this point or are some materials better than others?
                      I've read where it is best to have a spacer between coil and shield. This is apparently not the case in cables.(?)
                      There is talk about conductivity per inch. How does this relate to a good shield?
                      We have several cans of conductive nickel spray at work - used for shielding plastic cases. So if I took this and sprayed the circumference of a coil, connecting ground one end, other end open, would it work well? Would a spacer improve it?
                      I can take shielding off USB cables - it comes off in a nice tape-like strip. How to compare it vs other metalized mylar materials for our purpose?
                      Lastly, the part about leaving a gap on a coil shield - isn't this rather ignored in the case of Minelab coils where the coil is encased in a graphite paper housing?
                      Here is a very simple method that I have tested. See post #6:

                      http://www.geotech1.com/forums/showthread.php?t=18713

                      Comment


                      • #12
                        good thread

                        This is a good thread so I'll "subscribe".

                        I'd sure like to know more about what shields are supposed to do and not do. And I'd like to go in the field with two "identical" detectors, one with shielded coil, one with unshielded coil.

                        -SB

                        Comment


                        • #13
                          Tepco,

                          forgive me my ignorance, but what is the advantage of using 2 coils in parallel :
                          Coil is 25cm, made of 2x.25mm PVC insulated wire (like speaker type), 2mm insulation, 12.5cm ID, (DVD spindle cover) 25 cm OD, 30 turns, wires in parallel.
                          instead of a single coil of 15 turns -- cheaper -- less weight...

                          I can see the obvious advantages from your pictures, like it is easier to stick the windings together with some sort of super glue.

                          Any other advantages of using a "double" coil in parallel ?

                          Thanks for your information,
                          -- Bernard

                          Comment


                          • #14
                            You need certain number of turns (30) to get required inductance, but wire diameter is small (whit lot of insulation around) so both wires must be connected in parallel to keep resistance low enough.

                            You can have many advantages using this coil, two separate but magnetically strongly coupled coils, ideal for bipolar center taped drive, balanced, or two coils floated and driven with different voltage etc, just lovely for experiments.

                            (I just traveled thru Budjanovci today, consider changing your member name, this one is bit obsolete)

                            Comment


                            • #15
                              Originally posted by wallker View Post
                              Hi Tepco,

                              I like the idea about using that wire for coil (I agree with you that it's absolutely useless for speakers), so I will wind it tomorrow, but I have some questions, take a look on PM.
                              P.S. DUGA rocks !!!
                              I replay to PM publicly because this may be interested to someone else too.

                              For “sandwich” type coil, two sheets of material is placed above and below coil, shield is applied to this material from outer side in order do increase shield to coil spacing and reduce capacitance. I measured actual values some time ago, cant remember exactly, but almost 300pF is added if this “sandwich” is not used, graphite sprayed directly to the coil. Not dramatic, however, if coil is not shielded at all, may get like 8uS sampling, whit sandwich shield around 10, directly sprayed like 12-13, not lightning fast but good enough in most cases.

                              Whit typical detector design, MOSFET avalanche period and capacitance will limit sampling speed more than shield and coil self-capacitance itself, so there is no need to make something too complicated, you can try this coil completely unshielded, it is inherently very fast.

                              I never measured exact wire length needed, probably around 20m or so, obtain 25m, it is cheap anyway.

                              DUGA product is not the best and strongest resin, but just good enough for this, nice large and cheap package. Make some small-scale test first, resin is not too “energetic” in reaction, providing longer work time and very little heat during hardening period. (some other, more energetic types can screw up VLF coil balance due to excessive heat release, but not important for this)

                              In order to exploit full potential of this coil, detector must be properly adjusted, R dump value, and minimal sampling delay. Then this simple and cheap design can overperform about anything else, dual field included, in overall performance, i can even recommend this for factory made detector upgrade.

                              Comment

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