PI noise

[green]

Depends on what the detector is used for. If you're mainly looking for small nuggets you want to set it as high as you can.

On the spacing issue, you probably won't see much difference with only 2 layers and 0.28mm space. More layers with small spacing, or few layers with larger spacing will show a TC change. Try taking 2 pieces of foil and space them at 0.25, 0.5, 1, 2, and 5 mm. When I make my stacked target standards, I sandwich them pretty tightly in clear packing tape. There is no vertical current flow between the layers (during flat detection) so getting a low resistance contact isn't important, but minute gaps do matter as you get more layers.

Thanks for the reply. I've been trying to come up with the best way to keep the squares aligned when layering them. Thought about a thin layer of contact cement between 40x40mm layers glued to a 25x25mm poster board square and trimming foil after. I have folded the foil. 25x75mm folded three times for 25x25mm three layer as an example. I was having a hard enough time aligning the two squares when plotting the data. Any suggestions?

[Carl-NC]

Do you have a preference for which of the 16 targets match the hole?

Depends on what the detector is used for. If you're mainly looking for small nuggets you want to set it as high as you can.

On the spacing issue, you probably won't see much difference with only 2 layers and 0.28mm space. More layers with small spacing, or few layers with larger spacing will show a TC change. Try taking 2 pieces of foil and space them at 0.25, 0.5, 1, 2, and 5 mm. When I make my stacked target standards, I sandwich them pretty tightly in clear packing tape. There is no vertical current flow between the layers (during flat detection) so getting a low resistance contact isn't important, but minute gaps do matter as you get more layers.

[green]

This demonstrates skin effect. Thicker objects have higher apparent time constants. I use 25mm square household foil for target standards, and achieve a variety of taus by stacking multiple pieces. I have 1x - 16x thick, which lets me more accurately determine where a target hole might be.

Do you have a preference for which of the 16 targets match the hole?

[green]

If I understand correctly, the two squares of foil were in surface to surface contact (foil touching). The electrical contact between the the two would have been anything but good unless the two were mirror flat and pressed tightly together. The points of contact would be such that the resistance would be relatively high between the two with the result that the eddy current pattern would be exactly the same as if they were electrically separate (poster board touching). By contact resistance being relatively high we are talking tiny fractions of an ohm across very tiny inductances. The contact resistance would be large by comparison.

Eric

The squares were stacked with foil touching or poster board touching. Reply #120 you replied three coins stacked touching were the same as three coins stacked with cling film between them. Do you think foil twice as thick would be different than the two pieces touching? I wondered if a thicker space(.56mm) between would be different. It wasn't but maybe foil twice as thick would be.

[Ferric Toes]

Glued household aluminum foil to some poster board(.28mm thick). Cut two pieces 25x25mm. Charted one piece, two pieces(foil touching) and two pieces (poster board touching) all same location about 55mm away from center of coil. Expected to see a greater TC difference with poster board touching.

If I understand correctly, the two squares of foil were in surface to surface contact (foil touching). The electrical contact between the the two would have been anything but good unless the two were mirror flat and pressed tightly together. The points of contact would be such that the resistance would be relatively high between the two with the result that the eddy current pattern would be exactly the same as if they were electrically separate (poster board touching). By contact resistance being relatively high we are talking tiny fractions of an ohm across very tiny inductances. The contact resistance would be large by comparison.

Eric

[green]

Glued household aluminum foil to some poster board(.28mm thick). Cut two pieces 25x25mm. Charted one piece, two pieces(foil touching) and two pieces (poster board touching) all same location about 55mm away from center of coil. Expected to see a greater TC difference with poster board touching.

[Carl-NC]

I finally found some of the plots I did in 2013. First is a Nickel test that I did with 1 coin, then a second laid on top, and then a third. This demonstrated an interesting effect that three identical objects sitting on top of one another have a longer decay. This is true even if there is no electrical contact between them. Try it with cling film between coins and the result is the same. Linear time and amplitude followed by linear time/ log amplitude for this one shows a good single exponential fit.

This demonstrates skin effect. Thicker objects have higher apparent time constants. I use 25mm square household foil for target standards, and achieve a variety of taus by stacking multiple pieces. I have 1x - 16x thick, which lets me more accurately determine where a target hole might be.

[Ferric Toes]

Mutual inductance I suppose.

Correct. The three coins in the same plane have the same decay as one, but the amplitude is 3X.

Eric.

[Teleno]

This demonstrated an interesting effect that three identical objects sitting on top of one another have a longer decay. This is true even if there is no electrical contact between them.

Mutual inductance I suppose.

[Ferric Toes]

I finally found some of the plots I did in 2013. First is a Nickel test that I did with 1 coin, then a second laid on top, and then a third. This demonstrated an interesting effect that three identical objects sitting on top of one another have a longer decay. This is true even if there is no electrical contact between them. Try it with cling film between coins and the result is the same. Linear time and amplitude followed by linear time/ log amplitude for this one shows a good single exponential fit.


Next is a non-conductive volcanic ash, so the signal is purely a viscous magnetic decay. This is plotted on linear time/amplitude and log time/log amplitude scales and has a slope close to the theoretical t^-1.


Any other viscous material from fired brick, volcanic basalt, to Australian ironstone, has a similar decay and when plotted will run substantially parallel to the Tiva Canyon material. Soil from Chico in California or Red Hill Virginia, is the same. The only major difference is the amplitude, and somewhere I have a plot of six or more samples from these different sources on the same graph. I will post this shortly.

Eric.

[green]

Decay charts for the Ca ground, US nickel, US quarter and the 1 troy oz 99.9% copper coin. Tx=160usec constant rate(6250 amps/sec). The nickel decayed close to a straight line on a lin-log chart. The others close to straight on a log-log chart. Charted target recording-no target recording.

[green]

Still don't know how to calculate ground slope using the times in (target delay_1). For a ground slope of -1, I think the formula for GEB sample time is(GEB sample time=GEB sample start time/target sample start time*target sample time). (21.2/6*10.2=36.04), (29.4/10*14.4=42.34), (36.6/14*17.6=46). Needed 100 for my bench setup. Modified the controller so I could control which switches turned on mainly to see how much target signal there is when taking the EF sample. The EF signal is a little high because I didn't see a easy way to turn both EF switches on(just the 100usec time)should be GEB sample time-target sample time. Didn't expect the ferrite bead(1in O.D. 5/8in I.D. 1/2in high)readings. The targets weren't at the same distance from the coil in (GEB_on_off_2), just shows relative readings for each target.

[Ferric Toes]

Can someone calculate the ground decay slope using the delay and sample times that cancelled ground in the test (target delay_1)?

The best thing I can do at the moment is to find my decay plots for various coins and also iron mineralised ground from various places, including Ca and Va.

Iron mineralised material always has a similar decay, so if you take some ground from California, Virginia, England, France, Italy, Cambodia, Australia, or indeed any country, and plot its decay, it is always about the same. Large or small quantity makes no difference. What does change dramatically is the amplitude, and the ground in parts of Australia can be more that 10x that of bad ground in California. Fired earthenware pottery and housebricks have the same decay

Metal targets are quite different and the decays depends on the conductivity, surface area, and mass.

I will post some plots shortly.

Eric.

[green]

Can someone calculate the ground decay slope using the delay and sample times that cancelled ground in the test (target delay_1)?

[green]

Ground is a large noise source. A test comparing some different ground and ground simulation targets. Ca. ground from California gold field, Al. ground from my back yard. Each in a quart zip lock bag. Hot rock from California, Brick I was using for another project. Charted change in integrator out(no target vs target placed on one of the figure eight Rx coils). The two grounds were close, GEB reduced the hot rock and brick but didn't cancel. Are test results what I should expect?