Best Measurement for Graphite Shielding?

[Silver Dollar]

I'm modding an old Radio Shack MD to work with a SMT TGSL board I recently got and
used the Tractor Supply Graphite Paint for shielding. One side reads 5K per inch and the
other 3.5k. Is this OK or should I add more paint?



Paint Link
http://www.tractorsupply.com/en/stor...-12-oz-aerosol


I found these quotes in the forums;

If you are shielding for a PI detector I would keep the resistance per running inch of
surface to about 2000 ohms. I did one coil at 1300 to 1500 / inch and noticed some
loss of sensitivity at that resistance. PI detectors mostly need shielding if you can
detect your hand on the coil at the shortest delay you expect to detect at, typically for
small gold in earth.


Try to keep the shielding to 1K or lower.


Graphite is much more resistive and hence it's tau is below 1us.
Because shield is fighting electric fields, and there you have high impedance's, the
shield conductivity is irrelevant, hence graphite is a better shield.

[Chet]

I have a disassembled Minelab Goldsearch double D VLF coil that I am rebuilding for the Minipulse kit that I am building. It has a paper shield top and bottom. The graphite coating appears to be very thin and evenly sprayed. Measurements taken 1" apart anywhere on the surfaces are 1200 to 1400 ohms. The measurements at 2" apart run about the same. Measurements at 6" apart run 1400 to 1600 ohms. Measured at less than 1" they vary considerably.
I hope this is useful.
Chet

[Silver Dollar]

My Deep Search coil had 10K on one side and 5K on the other so I'm guessing 5K or less will work!

The Minelabs are know to be a bit critical with their shielding Tesoro's not so much..

[Chet]

Some more graphite shield measurements. I just opened a COILTEK 10" x 5" mono coil to repair it for a friend. It has paper shield on top, bottom and around the outer edge with a heavy uniform coating of graphite paint. Measurements at 1" separation averages 95 ohms. Measurements at 2" separation averages 100 ohms. At 6" separation the average is 160 ohms. These readings seem very low but do not seem to affect its sensitivity. He uses it for nugget hunting only and has found many 5-10 gram nuggets.

[baum7154]

Is it correct that the detector used with the 5 X 10 monocoil is not a PI machine?

Dan

[Chet]

Is it correct that the detector used with the 5 X 10 monocoil is not a PI machine?

Dan

Hi Dan
The 5 x 10 coil is used on a Minelab GPX 5000. The 8" GoldSearch Double D coil is from some kind of VLF machine.

[baum7154]

Impressive that Minelab works with that low of shield resistance. Perhaps it has something to do with the shielding only covering three sides of the coil i.e. top, bottom, and outer edge instead of totally covering the windings except for the mandatory loop gap. My experience is that the shielding should be about 2000 ohms/inch for my PI. Actually the first quote in Silver Dollars first post in this thread is from me.

Regards,

Dan

[Chet]

There is approximately 1/8 of foam between the coil and the paper shield which may help.

[baum7154]

I usually use about 1/4" of spacing coil to shield and that helps with keeping capacitance lower.

Dan

[green]

I did an experiment to measure attenuation with EZ slide graphite paint from tractor supply. Don't have any housings, so I used paper plates. Taped a couple strands of 32 awg bare copper wire to the each plate in a few spots. Painted the wire and plates with EZ slide. Measure typically 300 to 600 ohms from wire to different spots on the plate. Coil 7.5 inch ID 8 inch OD, 290 uh. Placed coil inside bottom plate, covered with second plate. Using two plates not painted, circuit resonance (R damping not connected) 1.67 Mhz. With the painted plates, circuit resonance 1.49 Mhz. Measured integrator volts out using a nickel at 3 and 5 inches with the painted and not painted plates. Measured almost the same, the painted measured a couple percent higher probably due to plates not being identical. I was expecting to see some attenuation but didn't. Should I expect attenuation or is my experiment faulty?

[baum7154]

I think having two separate plates leaves a circumferential gap fully around the plane of the coil and does not simulate full coil coverage typically used. That is not to say that the coil is not being effectively shielded but I don't have any experience with the two separate plates shielding effectiveness. I think the detection was aided by the circumferential gap and would be less signal with full Faraday shielding. Just my thoughts.

Dan

[Chet]

Hi Green
What a great experiment. Both configurations 1.67 MHz vs 1.49 MHz (R damping not connected) have the characteristics of fast coils. You didn't state whether the coil was closely wired to the circuit board or though a cable that would add capacitance and slow down the coil response. Measuring the minimum delay time available for early target sampling between the two configurations might be another way to compare them.
I noticed your posts in the Minipulse Plus forum. I have just completed assembling the rev C kit. I am only interested in using it for nugget hunting. Most of the nuggets found with detectors here in California are below 10 grams. So the goal is to focus on improving on the early detection period as close to discharge curve as possible. The first changes will be to raise the pulse rate and reduce the width of the sample gate. With these discussions on shielding and the many other discussions in the coil forums on fast coils I hope to build a coil that is up to the task.

[green]

[You didn't state whether the coil was closely wired to the circuit board or though a cable that would add capacitance and slow down the coil response.]

Twisted pair: 28 awg stranded, teflon coated, 12 inches long. Coil resonance was 2 Mhz without twisted pair.

[green]

Another experiment. Don't have any spiral wrap, so I cut a sheet of 2 mm craft foam into .5 inch wide strips. Glued the ends together and spiral wrapped the coil with one layer. Then wrapped with a layer of masking tape. Taped 3 strands of 32 awg bare copper wire around coil in a few spots for ground wire. Painted with EZ slide. The circuit resonance dropped from 1.67 Mhz without the shield to 1.27 Mhz with the painted shield. Didn't see any attenuation at 3 and 5 inches with the nickel.

[Chet]

Hi Green
I like your experiments, was the shield resistance similar to the 300-600 ohms that you had on the plates?
Thank you,