Best Measurement for Graphite Shielding?

[green]

-----------

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.


Dan

Qiaozhi referenced a nugget size to a square cut from an aluminum cola can. That is the only time I've seen a simulation target referenced to a nugget size with two decimal places. I've seen lead shot and aluminum mentioned for simulation. I don't know why there aren't some time constant charts published for different size gold nuggets. Been thinking about it but sure what size makes sense and gold isn't cheap. For PI detectors I would think time constant is the most important parameter to measure. Measuring all nuggets at a fixed distance to compare amplitude would be nice, but the detector power supply, delay time, gains,etc would need to be the same for the amplitude comparison.

[6666]

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.
[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.

Thanks guys, have taken note of this, just did a quick test on untuned but working MPP with 6 inch test coil, and the 10mm 3/8 sq piece was no go, but 15mm 5/8 was good for about 50mm - 2 inches .

[baum7154]

Thanks guys, have taken note of this, just did a quick test on untuned but working MPP with 6 inch test coil, and the 10mm 3/8 sq piece was no go, but 15mm 5/8 was good for about 50mm - 2 inches .

~~~~~~~~~~~~~~~~~~~~~~~~~~

Do you know how fast you are sampling with the MPP? Just ran the delay to find the threshold where the targets just begin to trigger my CHANCE PI for the 1/4 X 1/4" target and the 3/8" square target. The 1/4" square just appears flush with the coil face at a sample delay of 13.4us and the 3/8" target just appears at a sample delay of 18.8us. Note this test was run with the spider wound bigfoot coil just wound on Friday. At a sample delay of 8us this coil sees the 3/8" square Al target at 5" distance. Any way I think those are the delay thresholds necessary to begin to see these targets, at least on the CHANCE PI.

Cheers,

Dan

[green]

For reference, the time constant for target cut from an aluminum coke can, .25 x .25 inch 1.5 usec, 9 x 9 mm 2 usec, .5 x .5 inch 2.7 usec, 1 x 1 inch 5.3 usec.

[6666]

~~~~~~~~~~~~~~~~~~~~~~~~~~

Do you know how fast you are sampling with the MPP? Just ran the delay to find the threshold where the targets just begin to trigger my CHANCE PI for the 1/4 X 1/4" target and the 3/8" square target. The 1/4" square just appears flush with the coil face at a sample delay of 13.4us and the 3/8" target just appears at a sample delay of 18.8us. Note this test was run with the spider wound bigfoot coil just wound on Friday. At a sample delay of 8us this coil sees the 3/8" square Al target at 5" distance. Any way I think those are the delay thresholds necessary to begin to see these targets, at least on the CHANCE PI.

Cheers,

Dan

Thanks for posting the above info.

Do you know how fast you are sampling with the MPP?

I will have to get the CRO out and have a look, my MPP is totally untuned as I had to replace a voltage reg after some investigative surgery and have just put it back together to the point where it detects metals, but I need to check all the timings again.

[6666]

Thanks Green for the TC's

[green]

Was looking at some of the charts I posted in the past to see if I could learn something. Comparing the 2 x 2 inch gold simulation and the 1 x 1 inch gold simulation, with no folds ,2 folds and 4 folds the time constant doubled with 4 times the area or volume. The 2x(x) chart the time constant doubled with 2 times the area. The length stayed the same where the length and width both changed in the above comparison. The 2 x 1.25 in foil_6,5,4,3,2,1 chart the time constant stayed the same, total area changed. Each section though connected stayed the same.

[baum7154]

Intuitively the result in the '654321' test makes sense to me since the strips were all consistently slotted causing discreet areas for eddy current boundaries and the removal of successive strips made no difference.

The anomaly you point out in the 2xx chart points out the thickness effect but also the elongation effect that we have discussed in this forum before. As I recall there was an example of a 38% difference in the tc when the target was allowed to be elongated compared to keeping it square. I think the targets in that case were cut from pop can.

Dan

[green]

Intuitively the result in the '654321' test makes sense to me since the strips were all consistently slotted causing discreet areas for eddy current boundaries and the removal of successive strips made no difference.

The anomaly you point out in the 2xx chart points out the thickness effect but also the elongation effect that we have discussed in this forum before. As I recall there was an example of a 38% difference in the tc when the target was allowed to be elongated compared to keeping it square. I think the targets in that case were cut from pop can.

Dan

aluminum cans from earlier post. The 1 x 1 divided by the .5 x 1 is 38 percent, the .5 x 1 divided by the .25 x 1 is 100 percent. One thing I've learned is I don't know how to predict what the time constant will be yet. Would like to have plots of some small gold nuggets to compare with the aluminum or lead shot if the aluminum targets don't match time constant and amplitude. I think they both need to match for a good simulation.

[baum7154]

I agree. I think aluminum can is a very good and readily available material worldwide that is easily fabricated but I also think it is a bit thinner than a typical 5 grain gold nugget. That is partly why I tried to figure the volumetric approach between Al and Au. It is entirely possible if not probable that a 5 grain nugget is 3 or 4 times thicker than Al can material. If that proves out to be true then Al can material could be folded to get closer to the thickness requirement and the weight of the piece could be correlated to the gold equivalent from the 7.12:1 ratio of SG Au to SG of Al.

Dan

[Tinkerer]

Intuitively, a multi layer target behaves like a multi layer iron core from a transformer. The eddy currents are mostly confined to each layer.

However, the eddy currents of each layer have their own magnetic field and the magnetic fields add up somewhat like the multi turns of a coil. It is actually the influence of these fields, within the coil field, within the Earth's field that we measure.

An interesting test target is a sphere of lead, or rather several identical spheres of lead like fishing lead sinkers, that can be hammered into different thickness. This way we can easily observe the difference in response of targets of the same mass, but different shape.

Looking at the different response of these targets, gives insight into the effect of the target area presented to the coil field x the diameter of the coil.

Another interesting effect is to look at the response caused by the skin effect that becomes specially significant when the targets are very thin.

[Davor]

Just a thought, graphite compared to metals is a poor conductor even in bulk form, and as such its signal should subside fast. Poorer conductors have thicker "skin", so any layer of graphite should work fine with a coil.

[baum7154]

I wonder though if a thick layer of graphite in a fully surrounding Faraday shield with a proper gap has an attenuating effect on the transmit signal more pronounced than a thin layer? Any thoughts or tests?