Target response tester

[Ferric Toes]

I did a modification to my tester by adding a switch to change the series resistor in the coil circuit. This gives me a choice of 250 or 750mA peak current in the coil. The attached plot uses the higher current and uplifts the amplitude at late times which will enable TC start measurements of 400 and 500uS. I did a plot to compare the differences in the decay of the clad and silver quarters. TX is 1mS long and the top of the trace is the amplifier saturation level. The silver quarter is obviously in saturation for longer, hence the shift at the top. I haven't yet done final TC measurements at these later times.



Eric.

[green]

Green : maths for exponential decay:

V = voltage; V₀ = starting voltage; t = time difference; TC = time-constant, Tau.

V = V₀ e ^{-(t / TC)}

=> ln (V / V₀) = -t / TC

and: ln (V₀ / V) = t / TC

You do have some dodgy 'scope ADC issues, I had noticed them before. I've forgotten ... your scope data ends up in an Excel table ? You could try the simple filter I posted in post #93.

Your plots of the 19mm straight wire length and the loops of the same gauge wire appear to show the same initial drop, which I would expect. There doesn't seem to be any evidence of a 'pause' at 22usec rate for any time, before the dominant full loop decay starts.

A simple one to try is: Filtered sample value = (0.25 x Previous sample value + 0.50 x Sample value + 0.25 x next sample value)

Tried filter. Added .04V, one division to original when charting so they wouldn't chart on top of each other.

[green]

Green : maths for exponential decay:

V = voltage; V₀ = starting voltage; t = time difference; TC = time-constant, Tau.

V = V₀ e ^{-(t / TC)}

=> ln (V / V₀) = -t / TC

and: ln (V₀ / V) = t / TC

You do have some dodgy 'scope ADC issues, I had noticed them before. I've forgotten ... your scope data ends up in an Excel table ? You could try the simple filter I posted in post #93.

Your plots of the 19mm straight wire length and the loops of the same gauge wire appear to show the same initial drop, which I would expect. There doesn't seem to be any evidence of a 'pause' at 22usec rate for any time, before the dominant full loop decay starts.

Thanks for the formula.

Tried changing scope scale to get more resolution(smoother trace) with a clad and silver quarter. Tried charting on the same chart but they were on top of each other. Charted on 2 charts, TC lines were added to right chart first then same slope lines were added to left chart. Right chart lot smoother. Negative:minimum input not as low, Positive: smoother and stays out of noise.

Looking at charts I see I labeled the Y axis scope scale zero's opposite.

[waltr]

Green : maths for exponential decay:

V = voltage; V₀ = starting voltage; t = time difference; TC = time-constant, Tau.

V = V₀ e ^{-(t / TC)}

=> ln (V / V₀) = -t / TC

and: ln (V₀ / V) = t / TC

Yep and finial equation is:
TC = t/(ln(Vo/V))

[Skippy]

Green : maths for exponential decay:

V = voltage; V₀ = starting voltage; t = time difference; TC = time-constant, Tau.

V = V₀ e ^{-(t / TC)}

=> ln (V / V₀) = -t / TC

and: ln (V₀ / V) = t / TC

You do have some dodgy 'scope ADC issues, I had noticed them before. I've forgotten ... your scope data ends up in an Excel table ? You could try the simple filter I posted in post #93.

Your plots of the 19mm straight wire length and the loops of the same gauge wire appear to show the same initial drop, which I would expect. There doesn't seem to be any evidence of a 'pause' at 22usec rate for any time, before the dominant full loop decay starts.

[green]

Hi Green, I have further optimised my measurements by expanding the portion of the waveform to be measured as far as possible, and setting the digital filtering to reduce noise as much as I can. I used a long 1mS TX time for all measurements, so that should not be an issue. My start points are 150, 250, and 350uS, These are not as you requested as I had already done them before I saw your last post. I did not go to later starts than 350uS as noise will cause errors. You can see a slight ripple in the tail after the measuring window of the 350uS plots. I tried to get as close as possible to the 37% of the start amplitude, which is considerably better than before, due to the improved scaling. I doubt there is anything more I can do to improve things on these targets.
The results for the clad quarter for TC are 127.7uS, 136.3uS, 140.1uS for the ascending order of start points. For the silver quarter the results are 160.0uS, 171.3uS, and 175.9uS.

Clad plots [ATTACH]45363[/ATTACH] [ATTACH]45364[/ATTACH] [ATTACH]45365[/ATTACH]

Silver plots [ATTACH]45366[/ATTACH] [ATTACH]45367[/ATTACH] [ATTACH]45368[/ATTACH]

Eric.

Thanks, My question on calculating tau at 50 or 100us times between samples relates to sampling integrator out instead of amplifier out. The integrator gives some filtering that should help. Was wondering if it could be a better way for measuring tau after two time constants. Was wondering what the formula was to calculate tau from two samples without trial and error.

Your decay looks real smooth. Is the Pico scope smoothing the decay? Read your reply again, missed the digital filtering the first time?

[Ferric Toes]

I'm on a learning curve also. Increased chart size to see better what is happening. Appears straight line decay after approximately two target time constants. Wondering why two target time constants, and if target related. Maybe some targets take more or less than two time constants? Noticed I wasn't getting the same decay with clad quarters. Charted two with the largest difference of four coins I tried along with a silver quarter. Charted coin close to coil and spaced for near full scale signal at beginning. Clad and silver coins same location. Clad and silver chart same the first 200usec.

Eric, how much difference is the time constant if you take the first amplitude value at 300, 325, 350us with the method reply#204?

Hi Green, I have further optimised my measurements by expanding the portion of the waveform to be measured as far as possible, and setting the digital filtering to reduce noise as much as I can. I used a long 1mS TX time for all measurements, so that should not be an issue. My start points are 150, 250, and 350uS, These are not as you requested as I had already done them before I saw your last post. I did not go to later starts than 350uS as noise will cause errors. You can see a slight ripple in the tail after the measuring window of the 350uS plots. I tried to get as close as possible to the 37% of the start amplitude, which is considerably better than before, due to the improved scaling. I doubt there is anything more I can do to improve things on these targets.
The results for the clad quarter for TC are 127.7uS, 136.3uS, 140.1uS for the ascending order of start points. For the silver quarter the results are 160.0uS, 171.3uS, and 175.9uS.

Clad plots

Silver plots

Eric.

[green]

Tried a couple things yesterday. Exercised the brain without much success.

First: tried to calculate tau with input from Eric's chart http://www.geotech1.com/forums/attac...1&d=1549399390. (Vo=119@300us, Vt=77@350usec)What is the formula for calculating tau? Found a formula that worked with trial and error.

Second: tried to see if I could prove not straight line decay in the beginning isn't caused by skin effect. https://www.powerstream.com/Wire_Size.htm gives a frequency for 100% skin depth for different size wires. After some calculations, not sure if skin effect could be the cause. Maybe someone could try with the wire charts included. Recorded each target two times to check for repeatability. Both recordings the same until lower amplitude where noise caused difference. Looks like my scope has some error. A step about 1.25 divisions below .4 volt line on all the recordings as an example. Started with copper wire for targets. Thinking different thickness copper plates maybe 45mm square for the next try. Any thoughts appreciated.

[green]

I'm still on the learning curve with the Picoscope and tried some other features today. The y axis scaling feature was useful as this enables the part of the waveform of interest to be magnified and the cursors set more accurately. I have also shifted the y zero to the bottom line. Today's results for clad and silver quarters as as attached. I measured both with a TX width of 800uS and at 300uS start point. My workshop temperature is also 19 - 20C.

Eric.

[ATTACH]45342[/ATTACH] [ATTACH]45343[/ATTACH]

I'm on a learning curve also. Increased chart size to see better what is happening. Appears straight line decay after approximately two target time constants. Wondering why two target time constants, and if target related. Maybe some targets take more or less than two time constants? Noticed I wasn't getting the same decay with clad quarters. Charted two with the largest difference of four coins I tried along with a silver quarter. Charted coin close to coil and spaced for near full scale signal at beginning. Clad and silver coins same location. Clad and silver chart same the first 200usec.

Eric, how much difference is the time constant if you take the first amplitude value at 300, 325, 350us with the method reply#204?

[Ferric Toes]

I'm still on the learning curve with the Picoscope and tried some other features today. The y axis scaling feature was useful as this enables the part of the waveform of interest to be magnified and the cursors set more accurately. I have also shifted the y zero to the bottom line. Today's results for clad and silver quarters as as attached. I measured both with a TX width of 800uS and at 300uS start point. My workshop temperature is also 19 - 20C.

Eric.

[green]

Thanks for the testing. Something to compare against. Thought we were getting similar results. Wasn't getting near 183us with my silver quarter. Have been using constant rate instead of constant current. Some tests with a silver quarter constant current and constant rate. With amplifier not over scaling(top chart)all decays about 150us TC after 180us delay. With coin closer amplifier over scales, TC increased to 157usec all traces after 350usec delay. Constant current, constant rate and Tx time didn't seem to have much effect on time constant after a certain delay time.

Changed the 150us slope on top chart to 157us same as bottom chart. Looked good, drawing straight line decay after two target time constants might be best. Thinking decay the same on both charts, just don't see it out to 300us on top chart with 1/6 the amplitude? Want to check calibration again today.

[green]

I have a 1964 silver quarter which I am told is 90% silver and 10% copper. IACS measures 88%. It requires a TX width of 800uS to fully charge and at 200uS start point the TC is 165uS. Increasing to 400uS start point the TC is 183uS.

Also, by the same method, I measured a 4.4gm gold nugget from the Victorian goldfields, Australia. The TC is 23.8uS.

Eric.

Thanks for the testing. Something to compare against. Thought we were getting similar results. Wasn't getting near 183us with my silver quarter. Have been using constant rate instead of constant current. Some tests with a silver quarter constant current and constant rate. With amplifier not over scaling(top chart)all decays about 150us TC after 180us delay. With coin closer amplifier over scales, TC increased to 157usec all traces after 350usec delay. Constant current, constant rate and Tx time didn't seem to have much effect on time constant after a certain delay time.

[Ferric Toes]

Quarters look the same up to 120usec. Silver quarter, slower decay after 150usec.

4msec Tx cr

I have a 1964 silver quarter which I am told is 90% silver and 10% copper. IACS measures 88%. It requires a TX width of 800uS to fully charge and at 200uS start point the TC is 165uS. Increasing to 400uS start point the TC is 183uS.

Also, by the same method, I measured a 4.4gm gold nugget from the Victorian goldfields, Australia. The TC is 23.8uS.

Eric.

[Ferric Toes]

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.

First I plotted with a TX width of 300uS and a delay of 100uS to start of plot which gave a TC of 122uS. Increasing the TX width in 100uS steps up to 600uS gave an improvement of TC to 123uS. Increasing TX width beyond this point gave the same TC reading. For a clad quarter, 600uS TX width is the optimum. Using this width and increasing the delay to 200uS for the start point gives a TC of 134.6uS, which is close to your figure.

The first plot is triggered from the start of the TX (red trace) to confirm the TX width.

The second plot is triggered from TX end, as have previous plots of this type.

In the first plot, even though the TX pulse was long enough to fully saturate the coin (late time at turn on), waiting until 200uS after turn off is necessary to observe the single invariant true TC. of over 134uS.

To compensate for any variations in the zero base line, the target is removed after each measurement and a button is pressed to auto-zero the final dc amplifier stage. Obviously this does not apply to the scope'd plots I am currently doing.

Eric.

[green]

Shouldn't you be using a proper silver 25c coin, not a clad one? It seem that it would be a worthwhile comparison, anyway, having data for both. There's quite a pronounced difference in conductivity between the (copper) core and the (CuNi) cladding. It may show up what is a target characteristic, what is test-jig related.

Quarters look the same up to 120usec. Silver quarter, slower decay after 150usec.

4msec Tx cr