Does coil size effect target decay time constant?

[green]

Haven't seem much if any with the comparisons I've done. Would a 1, 2, 4, 8 and 16 inch square piece of aluminum foil chart the same decay slopes with a 1.6 inch and a 8 inch diameter coil? Would a piece of aluminum foil 1 inch wide by 1, 2, 4, 8, and 16 inches long chart the same decay slopes with the two coils? Would expect each larger size to chart a TC twice the lower with the square target with the 8 inch coil. Wondering if the target being 10 times larger than the coil would have an effect.

[Carl-NC]

Yes, though I haven't figured out the math. Ideally, target tau depends on skin depth and the thickness of the target and not the physical size. But obviously, if you test 1, 2, 4, 8, and 16 inch squares of foil they will have different taus. This suggests there is an edge effect. You could test this by taking a 1.6" coil and measuring the tau of a 4x4 and 8x8 foil square. They should exhibit the same tau. But not so with the 8" coil.

[dbanner]

Here is an interesting video demonstrating eddy currents and thickness or number of layers. Although I'm not sure if anything can be inferred from this about taus, but it is interesting none the less.https://www.youtube.com/watch?v=fw_iYnzemMo

[green]

8 inch coil

[green]

8 inch coil with 1inch wide targets. More noise than I would like. Not much change in TC between the targets.

Will do the 1.6 inch coil tomorrow.

[Skippy]

Nice charts, Mr.Green. And exactly as I was expecting. The squares having a TC proportional to the edge length, as we've determined in the other previous modelling/testing threads . And the 1 inch strip having a TC dominated by the narrow width, as the eddy currents don't flow up and down the length of the strip well, mainly circles / squashed circles, with length similar to width.

[green]

Hi Skippy. any guess what the 1.6 inch solenoid coil will do? I've been wondering what the TC of salt water and wet salt beach is. Eric has posted target size with ground doesn't effect decay slope. Ground is the only target I've charted that doesn't change slope with size. Salt water is a large target compared to coil size. Was wondering if I could see a change with a 1.6in coil and a 16in target. Would coil size effect TC with salt water?

[green]

Test, 1.6 inch coil with squares. I'll do the 8 and 16 inch squares again to see if they repeat when I do the 1 inch wide strips. Looks like TC doubles when width doubles at least to target 5x coil diameter. Does show target doesn't go to infinity at least for this example.

[Skippy]

I'm expecting that you will have the test targets closer to the small coil, to get a more measureable signal ? This may increase the coupling between the coil and the target. The coil is damped, hence it will also damp the target eddy currents, slightly. So perhaps the measured TC values will be a little bit shorter?
Pure speculation, of course.

[green]

Test with 1 inch wide strips. Still need to check repeatability of the 8 and 16inch squares.

[Carl-NC]

That's about what I expected, although the target has to be bigger than I thought before the edge effect diminishes. The best place to look up the math for this is in eddy-based non-destructive testing.

[green]

tried the 8 and 16 inch squares at zero, 1inch and 2 inch distance from coil.

[dbanner]

Aren't the eddy currents supposed to follow the pattern of the excitation field, ie the monoloop.
How does a 2x2 show relative to a 1x4, and a 4x4 to a 1x16. It is clear that the more lateral and longitudanal surface area, ie a square, allows greater eddy current flow than a 1x.... merely because of the flux pattern of the monoloop. The mesh of the square being greater than the determinant sum of the meshes in the strip.
Speculation on my part, of course.

[green]

By the way while we are conversing. I been meaning to ask you if you have ever tested the TC of the foil emergency blankets? The one I have is made by Coleman. You can find them in the outdoor department in Walmart for $3 or $4. I cut two 24"x24" sheets and taped them to a piece of card board. Much like your foil squares test you posted in the past. I think it simulates saltwater. My non scientific test showed at 12 to 14uS it goes quite. The large surface area, I believe it simulate the beach better than a small piece of foil with the same tc.

from https://www.geotech1.com/forums/show...401#post264401

What started this thread. I tried charting the TC for a 24 in square piece of Coleman blanket using a 1.6in and a 8in diameter coil without success. Most of the time they chart the same TC but I hadn't tried a target a lot larger than the coil. Wasn't getting enough signal with the 8in coil so I tried the 1.6in coil, usually higher signal. The test in this thread shows TC might be good with target diameter not much more than 5 times coil diameter. 1.6in diameter coil can't chart TC for a 24in square. Charting the target TC requires a lot more signal than detecting it, maybe 10000 times? A 1us TC target decays to 1/10000 in 9.2us. Start amplitude and TC are two parameters that effect if the target cane be detected or charted. Noise being another, maybe more? Just trying to understand or learn something.

https://www.geotech1.com/forums/show...244#post237244 Does anyone know of more pictures showing salt decay? Can see a definite change at amplifier out at 10us, 15us and maybe 20us. Think it would mean TC higher than 1us?