PI noise

[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]

Mutual inductance I suppose.

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

Eric.

[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.

[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.

[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]

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.

[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?

[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]

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?

[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.

Another test with thicker spacing(2.67mm and 4.85mm) craft foam + poster board spacer. Including chart(test 1) with .56mm spacing.

[Carl-NC]

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?

I cut long strips and fold them to get the layers I want. But I'm never trying to intentionally create space between the layers.

[green]

I cut long strips and fold them to get the layers I want. But I'm never trying to intentionally create space between the layers.

Thanks, I tried glue method reply #129 this morning with 1 thru 5 layers to see what it looked like. Charted TC vs number of layers and a line(TC of one layer*number of layers). Tried your method this afternoon with 5 layers to see if different. Same TC, I think your method is easier. 5 layer_2 is your method. Was looking at 5 layer_2, looks like I got some air trapped inside when I covered with the clear tape, could be more space than the glue method.

[green]

Thanks, I tried glue method reply #129 this morning with 1 thru 5 layers to see what it looked like. Charted TC vs number of layers and a line(TC of one layer*number of layers). Tried your method this afternoon with 5 layers to see if different. Same TC, I think your method is easier. 5 layer_2 is your method. Was looking at 5 layer_2, looks like I got some air trapped inside when I covered with the clear tape, could be more space than the glue method.

Made one with 16 layers, minimum trapped air. Thick enough that it doesn't chart a straight line on a lin log chart. With the right Tx waveform it might. TC depends on where the samples are taken, 10 and 20usec(TC=14.8usec) 20 and 40usec(TC=20usec). Not sure it matters, try each target to find which one reads the lowest(hole). Need to make the rest including some over 16 layers. 16 layers isn't enough for longer target delay times. Including chart with some data for the 5 and 16 layer targets. GEB(off, on)data, integrator out mv with targets at 200mm from coil.

[green]

Made more layered targets. Charted integrator out at three target delay times with GEB off and GEB on(aluminum foil_5) One and two layer(1.4 and 2.8usec TC's) are a lot better with the 6usec target delay. Eight layer(10usec TC) about the same as a US nickel is a little better at the lower target delays. TC chart,(aluminum foil_4)

[green]

A couple numbers to help use data reply#134. Bigfoot style coil Rx(two 8 inch round coils), Tx(oval coil surrounding Rx). Increasing the target distance by 30mm at 200mm start distance reduces the signal in half. Ball park(lose or gain about 30mm detection distance each time signal is reduced in half or doubled). GEB on doubles peak noise level with my circuit, lose about 30mm detection distance. Wondering if typical for GEB to double peak noise level. Lot of numbers to look at. Trying to make sense out of them. It would help to know what real targets would fall in the holes for the different timings(A) (B) (C).