Best Measurement for Graphite Shielding?

[6666]

so I cut a sheet of 2 mm craft foam into .5 inch wide strips

Thats an interesting experiment, one of the things I will be trying is a spiral wrap of 3-4mm polyethylene rope you get at a hardware shop.

[green]

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

Made a few measurements. Most were between 300 and 600 ohms A couple lower and higher. All less than 1000. The main purpose of the experiment was to determine if there was any signal loss. Need to try to get a more even layer of paint. Maybe the spray can would work better. A while back I tested for signal lose with an aluminum foil shield and didn't see any lose. Would be interested if anyone else has tested for attenuation and what they found. I added another layer of paint to the paper plates to see if more paint would attenuate the signal. Will try to test again soon. Maybe someone has a suggestion on how to improve the experiment.

[6666]

A while back I tested for signal loss with an aluminum foil shield and didn't see any loss.

NO LOSS ?, Did you do a complete wrap around the coil or was it insulated so foil did not touch on over lap ?

[baum7154]

Made a few measurements. Most were between 300 and 600 ohms A couple lower and higher. All less than 1000. The main purpose of the experiment was to determine if there was any signal loss. Need to try to get a more even layer of paint. Maybe the spray can would work better. A while back I tested for signal lose with an aluminum foil shield and didn't see any lose. Would be interested if anyone else has tested for attenuation and what they found. I added another layer of paint to the paper plates to see if more paint would attenuate the signal. Will try to test again soon. Maybe someone has a suggestion on how to improve the experiment.

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A nickle is a very large area and strong target. I would like to see the experiment done with a 3/8" square piece of aluminum can. I realize that attenuation is supposed to apply to all targets equally but the best indications of loss of sensitivity I have seen were on small/fast targets.

Thanks,
Dan

[green]

NO LOSS ?, Did you do a complete wrap around the coil or was it insulated so foil did not touch on over lap ?

I cut a piece of aluminum foil long enough to go around the coil. Tape 2 inch wide masking tape to the foil. Cut the tape foil wide enough to wrap around coil, cut pattern and glue to coil spacer. The foil wraps all the way around the larger coil with a small space near the ends. The foil on the smaller coil doesn't wrap all the way around but does help for measurements on the bench. Wasn't sure how to do a DD coil, so I was glad to see the EZ slide post by Silver Dollar, something I could get locally to try. I could paint the housing and not have to space each coil before adding the foil. I had read in some posts about attenuation with out stating a percent loss with aluminum foil and graphite paint, so I thought I would try to measure it. It's not instructions on how to do it. It's just how I did it with the materials I have. Spiral wrap would probably be better than craft foam, etc. Any other suggestions appreciated.

[green]

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A nickle is a very large area and strong target. I would like to see the experiment done with a 3/8" square piece of aluminum can. I realize that attenuation is supposed to apply to all targets equally but the best indications of loss of sensitivity I have seen were on small/fast targets.
Thanks,
Dan

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I'll try but I think getting the distance the same with the shielded and not shielded coil for a small target(shorter distances) might be a problem. The shield does lower the circuit resonance. To detect small targets you would like to get the delay as short as possible. If the delay is longer with the shielded coil the amplitude will be less. When trying to measure shield attenuation everything else needs to be the same. I am using a 5 usec delay with a 5 usec sample average for my measurements

[green]

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A nickle is a very large area and strong target. I would like to see the experiment done with a 3/8" square piece of aluminum can. I realize that attenuation is supposed to apply to all targets equally but the best indications of loss of sensitivity I have seen were on small/fast targets.

Thanks,
Dan

Another test. Removed the graphite paint and layer of masking tape. Spiral wrapped a second layer of 2 mm craft foam for 4 mm spacer. Added ground wire and recorded data. Added graphite paint. Circuit resonance: no paint 1.67 Mhz, single layer foam with paint 1.27 Mhz, 2 layers of foam with paint 1.24 Mhz. Thought resonance would be higher with 2 layers of foam. Spiral wrapped the ground wire about three times(same piece of wire used for single foam test) for the two layer foam instead around the outer diameter for the single layer foam. Maybe that or the lower resistance readings is the reason. Most resistance readings from ground wire to painted surface between 200 and 400 ohms. Recorded data. The chart shows a small decrease in signal, real or measurement error I don't know. I added a line that I call distance constant(the distance increase to cause a 63 percent decrease in signal. I mentioned distance constant in a thread awhile back but didn't get any response, so I'll try again. If time constant is the time increase to cause a 63 percent decrease in signal and distance constant is the distance increase to cause a 63 percent decrease in signal then time constant and distance constant are the same. A one time constant increase in sample delay causes a one distance constant loss of signal. For the 9x9 mm aluminum can with a TC of 2 usec a 2 usec increase in delay would cause a 1.37 inch decrease in distance for the test coil. Aziz plotted some curves in thread(the relationship between coil diameter and detect distance in PI detectors, reply#54). Using the data at radius and diameter the distance constant for a 10 inch diameter coil is 1.7 inches, a 20 inch diameter coil is 3.3 inches.

[green]

Another difference between the 2 mm foam and 4 mm foam I forgot to mention. The 2 mm foam had a small space not painted , the 4 mm foam there was no space, the whole coil was painted.

[baum7154]

Good work Green, very interesting results! Is there a way you can try this coil in the field to see how it behaves in both detection and noise immunity? It appears that the low shield resistance has no effect on sensitivity so it should do well.

Regards,

Dan

[green]

Good work Green, very interesting results! Is there a way you can try this coil in the field to see how it behaves in both detection and noise immunity? It appears that the low shield resistance has no effect on sensitivity so it should do well.

Regards,

Dan

All I have is the coil driver, amplifier, integrator, and control. Each on different boards connected together. I've been wanting to do ground balance and discrimination, so I haven't built a complete detector let.

[6666]

3/8" square piece of aluminum can

What target does that simulate ?

[baum7154]

A 3/8" square aluminum pop can target is just a good challenging target. However it does weigh about 0.3 grains and that is the volumetric equivalent of 2.136 grains of gold.

hope this helps

Dan

[Tinkerer]

A 3/8" square aluminum pop can target is just a good challenging target. However it does weigh about 0.3 grains and that is the volumetric equivalent of 2.136 grains of gold.

hope this helps

Dan

The surface area of the target, as presented to the magnetic field lines, accounts for the amplitude of the response.

The volume or thickness of the target accounts for the TC of the target.

Of course the target material conductivity as well as its magnetic or non-magnetic characteristics play an important role too.

[green]

What target does that simulate ?

[A 0.47 gram nugget (7.25 grains) can be emulated with a 9mm x 9mm aluminium square from the side wall of a Cola can. Your 10mm x 10mm square of aluminium foil is much thinner, so I doubt you'll be able to detect it.] from mini pulse plus #343 by Qiaozhi

3/8 inches = 9.5 mm, so it should make a good target to emulate a gold nugget

[baum7154]

[A 0.47 gram nugget (7.25 grains) can be emulated with a 9mm x 9mm aluminium square from the side wall of a Cola can. Your 10mm x 10mm square of aluminium foil is much thinner, so I doubt you'll be able to detect it.] from mini pulse plus #343 by Qiaozhi

3/8 inches = 9.5 mm, so it should make a good target to emulate a gold nugget

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I agree.

However I'm not clear on how a 9 mm square piece of Al can .004" thick is truly equivalent to a 7.25 grain nugget. It will weigh even less than the .375" square and consequently have a volumetric equivalent of only about 2 grains of gold. This is why I have been trying to get a variety of actual nuggets in the sub-gram range to correlate Al can targets to the real thing. (See the discussion in the Gold Nugget Simulation thread) The ratio of weight for gold vs aluminum of equal volume is 7.12:1 derived from specific gravity.


The toughest AL can target I use is 1/4" square and it is detected with CHANCE PI with 8" dia. coil at about three inches. In volume it is equivalent to about 1 grain gold however I need to get a micrometer on some 1 grain samples to get an average thickness and see how that compares to the .004" thick AL can material.

One gold target I use is about 4.5 grains of 14kt but not flat or square. It is more of a hollow tapered cylinder and is detected at about 3" as well.

The toughest gold target is a small collection of gold flakes. The largest of them has a surface of about 3 stickpin heads and this is detected at about 2". I'll try to weigh this one and let you know if it is detected by itself.

Dan