Target response tester

[green]

This initial rapid fall-off you see on your slopes -- it's not skin-effect. When you excite your target, circulating currents can flow in any which direction they like. Those that go in tight loops, because they are constrained by a thin sheet, or a wires' edge-profile etc, decay quicker. What's left later on are the 'big' circulating loops, whose size is related to the larger dimensions of the target. It's nothing to do with skin effect.
Remember that pdf Eric linked to, about the test jig that measured long rods of metal decaying ? The maths of that showed the multiple decays, and then stated that the 'big' one is the one they want to measure and analyse. It involved Bessell Functions, which are called upon when things happen in two (or three) dimensions ... vibrating drumskins? heat transfer through a solid? Digital filtering of images? I've little exposure to their intricacies ...

Skin effect is something really relevant to continuous AC signals, like a VLF detector, or 60Hz/50Hz power line currents. The currents only flow in the outermost layer of the item. At microwave freqs, it's just the skin, microns thick that conducts, nothing at all goes on in the core of a wire, it could be a tube, with no difference. You don't see 'the effect of the skin' , it's always there, it doesn't 'show itself' under any particular given circumstance.

[ I hope all that is correct, I don't want to have to ask Carl to edit/delete it..]

Re: your turn-on / off measurement differences. What's different about the circuit? An I correct in saying the damping is different between the two states, one is damped by a resistor, typically 500 Ohms, the other will be a near-zero Ohm turned-on Mosfet ?

Another test. When I tested the 19mm length wires the other day I noticed orientation didn't seem to make much difference. Tested 3 wire sizes and a US nickel. Can get a lot different slopes with the nickel. Wondering if you see much change with 19mm of copper wire with the VLF.

[Ferric Toes]

Another test. When I tested the 19mm length wires the other day I noticed orientation didn't seem to make much difference. Tested 3 wire sizes and a US nickel. Can get a lot different slopes with the nickel. Wondering if you see much change with 19mm of copper wire with the VLF.

I measured 10 nickels on the conductivity meter ranging in dates from 1964 - 1997 and they all read 5.xx% IACS. The lowest is 5.22 and the highest 5.63%. Doing a quick test for TC the difference appears to account for about 1uS between the two. Further tests still to to be done on the quarter.

Eric.

[green]

I measured 10 nickels on the conductivity meter ranging in dates from 1964 - 1997 and they all read 5.xx% IACS. The lowest is 5.22 and the highest 5.63%. Doing a quick test for TC the difference appears to account for about 1uS between the two. Further tests still to to be done on the quarter.

Eric.

What is the peak Tx current and time when you test for decay? Wondering what we are doing different that I get a tau around 10us and you get around 8us for the nickel.

[green]

Another test. When I tested the 19mm length wires the other day I noticed orientation didn't seem to make much difference. Tested 3 wire sizes and a US nickel. Can get a lot different slopes with the nickel. Wondering if you see much change with 19mm of copper wire with the VLF.

Not correct. Looks like 2 slopes, target position effects where they chart. Thinking target current is circulating around the coin for the 10us TC, what would be the current path for the 1us TC slope?

[Skippy]

"what would be the current path for the 1us [edge-on nickel] TC slope?"
Edge-on, it's going to look much like the 19mm long copper wires, with a wire diameter of 1.6mm, and %IACS = 5.3, rather than 100-102.
Putting these figures into the 'wire equation' gives
TC = 0.029 x 5.3 x 1.6² = 0.39 microsecs.
It's longer than that because it's not a round wire, there is additional material beyond the nearest edge, so if you assumed it was 1.6mm x 3.5mm, for example, it might be plausible to calculate:
TC ~ 0.029 x 5.3 x (1.6 x 3.5) ~ 0.9 usec.
Very rough, it's likely that the extra 'depth' means that more of the '19mm' length is used for current flow, too.

[green]

Edge-on, it's going to look much like the 19mm long copper wires, with a wire diameter of 1.6mm, nickel thickness 1.95mm. Did you mean 1.95mm?

Trying to visualize the current loops. Another test with a .25x1x1 inch aluminum target on edge and flat. Thinking flat is one 1inch diameter loop. On edge is four .25 diameter inch loops side by side?

First part of decay is faster slope than either of straight line decays. Does that mean there are smaller faster loops somewhere?

Recorded 160us and 4ms constant rate Tx.

[Skippy]

"Did you mean 1.95mm?"
No, I meant 1.59mm, based on calculation (density/mass/diameter). The 'official' thickness is what you would get if you stuck vernier calipers on it, and it's probably the thickness of the edge, rather than any inner feature. I calculate the mean thickness of all my coins that I've tested, there's no other way to come up with anything accurate, even mic'ing various points across the surface will give figures in error.
"Thinking flat is one 1inch diameter loop."
Roughly, yes. I reckon that it's bigger, I'm pretty sure the currents do flow into the corners quite a bit, the most obvious effect of this is to give a stronger response. The time-constant doesn't vary that much, as, in simple L & R terms, the R maybe increases 15%, but the loop length of the 'inductor' goes up by a similar amount, so it's the subtleties of inductance varying with the ratio of (loop radius to wire radius), and some minor adjustments related to resistivity that change TC.
"On edge is four 0.25" diameter loops side by side?"
It seems reasonable, though I don't see why they couldn't overlap? And they'll probably be squashed circles, constrained by the 0.25" dimension.
"Does that mean there are smaller faster loops somewhere?"
Initially there could be countless billions of microscopic loops, but they'll presumably decay or combine in some fashion over time.

[green]

"Did you mean 1.95mm?"
No, I meant 1.59mm, based on calculation (density/mass/diameter). The 'official' thickness is what you would get if you stuck vernier calipers on it, and it's probably the thickness of the edge, rather than any inner feature. I calculate the mean thickness of all my coins that I've tested, there's no other way to come up with anything accurate, even mic'ing various points across the surface will give figures in error.
"Thinking flat is one 1inch diameter loop."
Roughly, yes. I reckon that it's bigger, I'm pretty sure the currents do flow into the corners quite a bit, the most obvious effect of this is to give a stronger response. The time-constant doesn't vary that much, as, in simple L & R terms, the R maybe increases 15%, but the loop length of the 'inductor' goes up by a similar amount, so it's the subtleties of inductance varying with the ratio of (loop radius to wire radius), and some minor adjustments related to resistivity that change TC.
"On edge is four 0.25" diameter loops side by side?"
It seems reasonable, though I don't see why they couldn't overlap? And they'll probably be squashed circles, constrained by the 0.25" dimension.
"Does that mean there are smaller faster loops somewhere?"
Initially there could be countless billions of microscopic loops, but they'll presumably decay or combine in some fashion over time.

Thanks

[Ferric Toes]

Hi Green, Here is the graph for a US quarter that I promised. The figures were taken off my later MK2 instrument and I plotted them on a log/lin scale which is what you use. 30uS is the earliest start time although this could be reduced by mods to the clock generator. From 30uS until 100uS you can see the departure from the single exponential due to the 'diffusion' process.



Eric.

[green]

Hi Green, Here is the graph for a US quarter that I promised. The figures were taken off my later MK2 instrument and I plotted them on a log/lin scale which is what you use. 30uS is the earliest start time although this could be reduced by mods to the clock generator. From 30uS until 100uS you can see the departure from the single exponential due to the 'diffusion' process.

[ATTACH]45311[/ATTACH]

Eric.

Hi Eric, thanks for charting the US clad quarter. I see we don't get the same decay, wonder why. Including the chart I posted reply#164. I had 137us vs 115us TC you got? My data is higher than straight line projection in the beginning where your data is lower?

[Skippy]

Green: Do you think it's possible your O-8 coil is responsible for the discrepancies? You don't really have a 'central' position, where fields are symmetrical. Above the centre of one half of your 8-RX, you are not in the middle of the O-TX .

[green]

Green: Do you think it's possible your O-8 coil is responsible for the discrepancies? You don't really have a 'central' position, where fields are symmetrical. Above the centre of one half of your 8-RX, you are not in the middle of the O-TX .

I've looked at some recordings I made awhile back that I think were made with a mono coil and different amplifier. Recordings similar to what I have now. Probably have to try the mono coil and different amplifier again without the log amplifier to be sure.

[Ferric Toes]

Hi Eric, thanks for charting the US clad quarter. I see we don't get the same decay, wonder why. Including the chart I posted reply#164. I had 137us vs 115us TC you got? My data is higher than straight line projection in the beginning where your data is lower?

I thought it would be interesting to measure the TC on the raw signal out of the preamp. According to this we are both out in our instruments. The cursor measurement gives 122.9uS for the TC. It is not easy to set the cursors to the nearest mV on the y scale I used, so there will still be a very small error. I started the measurement at 100uS as this seems to be the start point for the straight line decay. My TX width at 100uS delay is 300uS which appears to be a bit short. TX for 99% target energisation should be 5 x TC, or > 600uS say. If I took a starting measurement at 200uS delay, then I would have a 600uS TX width. I can begin to see the change in amplitude of the RX waveform for a US quarter when I reduce the TX width to 570uS. My TX width is always 3 x the sample delay, e.g. at 500uS delay the TX is 1500uS. The repetition rate drops in proportion and current draw stays the same. TX current flat tops at 250mA.



Eric.

[Ferric Toes]

By the same method, a nickel has a TC of 10.4uS

Eric.

[green]

I thought it would be interesting to measure the TC on the raw signal out of the preamp. According to this we are both out in our instruments. The cursor measurement gives 122.9uS for the TC. It is not easy to set the cursors to the nearest mV on the y scale I used, so there will still be a very small error. I started the measurement at 100uS as this seems to be the start point for the straight line decay. My TX width at 100uS delay is 300uS which appears to be a bit short. TX for 99% target energisation should be 5 x TC, or > 600uS say. If I took a starting measurement at 200uS delay, then I would have a 600uS TX width. I can begin to see the change in amplitude of the RX waveform for a US quarter when I reduce the TX width to 570uS. My TX width is always 3 x the sample delay, e.g. at 500uS delay the TX is 1500uS. The repetition rate drops in proportion and current draw stays the same. TX current flat tops at 250mA.

[ATTACH]45314[/ATTACH]

Eric.

Added two lines to chart posted above through 100 and 200us. Got a time constant closer to yours. Before I had my TRT I did what you did plus. Two recordings, no target and target. Subtracted no target from target recording(cancels no target offset in the beginning that I couldn't zero out with a mono coil) and charted difference linear-log with Excel. I'll try the old way with the quarter hopefully today. Didn't get straight line decay until closer to 200us delay with my TRT, included chart. Do you have a way to subtract and chart two recordings, target and no target? Wonder if a 600us Tx with your tester(first sample at 200us) wouldn't give a time constant closer to the 137us I got.

10.4us for the nickel looks good, wonder why your other method gives a different answer.