detection depth

[Skippy]

I was thinking of asking Tinkerer to go into more detail about his test proposals. The Coil-Area vs Target-Area one is interesting, though Target-Volume would also be worth checking.
Coinage has a tendency to increase in thickness as it increases in diameter. US coinage doesn't have many convenient 'series' of coins in ascending size/weight. We have quite a number of them here in the U.K. The Sterling Silver coinage covers 1 1/2d, 3d, 6d, 1s, 2s, 2 1/2s, 5s, covering a 40:1 weight range. Almost the same for the 50% silver series, and repeated in Cupronickel: 6d, 1s, 2s, 2 1/2s, 5s. And some shorter bronze series , 1/4d, 1/2d, 1d, and Decimal 1/2p, 1p, 2p. Also gold coins: 1/2, 1, 2, 5 Pounds. Some good data should be obtainable from that lot. Combine that with your theoretical/measured data for the target vs distance, and some useful results should be possible.

Some interesting data is coming from my comparison of real DD coil data with an equivalent mono-coil. I'll post up tomorrow.

[Tinkerer]

I was thinking of asking Tinkerer to go into more detail about his test proposals. The Coil-Area vs Target-Area one is interesting, though Target-Volume would also be worth checking.
Coinage has a tendency to increase in thickness as it increases in diameter. US coinage doesn't have many convenient 'series' of coins in ascending size/weight. We have quite a number of them here in the U.K. The Sterling Silver coinage covers 1 1/2d, 3d, 6d, 1s, 2s, 2 1/2s, 5s, covering a 40:1 weight range. Almost the same for the 50% silver series, and repeated in Cupronickel: 6d, 1s, 2s, 2 1/2s, 5s. And some shorter bronze series , 1/4d, 1/2d, 1d, and Decimal 1/2p, 1p, 2p. Also gold coins: 1/2, 1, 2, 5 Pounds. Some good data should be obtainable from that lot. Combine that with your theoretical/measured data for the target vs distance, and some useful results should be possible.

Some interesting data is coming from my comparison of real DD coil data with an equivalent mono-coil. I'll post up tomorrow.

The attached coil calculation pdf does not explain well, but it might be a starting point for further discussions.

[green]

The attached coil calculation pdf does not explain well, but it might be a starting point for further discussions.

[A 20mm ring has 1% of the surface area of a 200mm coil. A ring of 10mm diameter has 0.25% of the surface of the coil so we can expect about ¼ of the target response at the same distance.] from your pdf

ITMD states the signal is 1/8 not 1/4 for a target 1/2 the diameter. With a PI, target time constant effects signal strength. For thin targets not effected by skin effect, time constant is proportional to length and thickness for square targets. A 1x1inch square cut from aluminum foil would have twice the time constant of a.5x.5inch piece and equal time constant for a 1x.5inch piece folded once for .5x.5inches 2 layers. The 1x1inch piece should have 8 times the signal strength of the .5x1inch piece folded once. Did a test this morning, 200mm Rx. Integrator out with target at 2inches, 1x1inch(1140), .5x.5folded(136) and .5x.5(17). 8x136=1088 close to 1140. Don't know how target time constant effects a VLF detectors signal strength, or a formula to estimate signal change for thicker targets effected by skin effect.

The 1x1inch piece has a signal strength 67 times the .5x.5inch piece, not 4 or 8 times.

[green]

[A 20mm ring has 1% of the surface area of a 200mm coil. A ring of 10mm diameter has 0.25% of the surface of the coil so we can expect about ¼ of the target response at the same distance.] from your pdf

ITMD states the signal is 1/8 not 1/4 for a target 1/2 the diameter. With a PI, target time constant effects signal strength. For thin targets not effected by skin effect, time constant is proportional to length and thickness for square targets. A 1x1inch square cut from aluminum foil would have twice the time constant of a.5x.5inch piece and equal time constant for a 1x.5inch piece folded once for .5x.5inches 2 layers. The 1x1inch piece should have 8 times the signal strength of the .5x1inch piece folded once. Did a test this morning, 200mm Rx. Integrator out with target at 2inches, 1x1inch(1140), .5x.5folded(136) and .5x.5(17). 8x136=1088 close to 1140. Don't know how target time constant effects a VLF detectors signal strength, or a formula to estimate signal change for thicker targets effected by skin effect.

The 1x1inch piece has a signal strength 67 times the .5x.5inch piece, not 4 or 8 times.

Another test including 3 different targets. The aluminum cylinders are some test pieces I had, odd diameters same thickness. The copper pieces were cut from copper tubing that was cut open and flattened.
First question: why should signal strength change as cube of target diameter instead of square of target diameter?
Second question: target decay(target material, target thickness, Tx shape and on time, probably others effect decay)and GB on off might have a greater effect on signal than target diameter with a PI detector, are there things that cause the signal to not change as the cube or square of target diameter with a VLF detector?

GB was adjusted to cancel ground when on.

[Dutchie]

Possibly the difference between PI results and VLF results may be based on the following.
One major part of the Rx signal for PI is caused by Eddy currents - https://en.wikipedia.org/wiki/Eddy_current
For a round subject the signal is LOWER than for a ring of the same diameter, material and thickness.
A ring behaves like a secundary coil. A round subject more a lot of very very small coils... "less organised" and a much lower magnetic field. Eddy currents on a thin surface layer seem to be responsible for most of the effect. All other forms than a ring are much less efficient.
In subjects with a long time constant - say more than 100 microseconds behaviour may be more similar. Main problem after 200-500 signal is mostly to weak (compared to noise).

Hope someone can adjust or complete this reply

[Tinkerer]

[A 20mm ring has 1% of the surface area of a 200mm coil. A ring of 10mm diameter has 0.25% of the surface of the coil so we can expect about ¼ of the target response at the same distance.] from your pdf

ITMD states the signal is 1/8 not 1/4 for a target 1/2 the diameter. With a PI, target time constant effects signal strength. For thin targets not effected by skin effect, time constant is proportional to length and thickness for square targets. A 1x1inch square cut from aluminum foil would have twice the time constant of a.5x.5inch piece and equal time constant for a 1x.5inch piece folded once for .5x.5inches 2 layers. The 1x1inch piece should have 8 times the signal strength of the .5x1inch piece folded once. Did a test this morning, 200mm Rx. Integrator out with target at 2inches, 1x1inch(1140), .5x.5folded(136) and .5x.5(17). 8x136=1088 close to 1140. Don't know how target time constant effects a VLF detectors signal strength, or a formula to estimate signal change for thicker targets effected by skin effect.

The 1x1inch piece has a signal strength 67 times the .5x.5inch piece, not 4 or 8 times.

I think you are right about the signal strength of the ring. I forgot that the diminishing factor is "cubed"
About the foil, it behaves in a very "non-linear" way, this is why I do not like to use it anymore for comparative purposes. I suspect that when a target is thinner than the skin effect probably all targets will behave this way. When I talk about skin effect, I mean the the depth of immediate eddy current penetration. This will be dependent of the frequency of the TX pulse. For VLF, very low, for PI somewhere in the mega Hz range.
In thicker targets, the eddy currents expand deeper into the target. This expansion is slow, at the speed of the time constant.
Multi layer targets are supposed to attenuate the eddy currents from one layer to the next according to the theory of the old laminated iron cores in transformers. However, if we fold foil or stack coins it does not seem to happen that way.

[Tinkerer]

skin effect

In the attached skin effect table we see that the skin depth of lead is nearly 3 times that of aluminum. Will we see the same "non-linearity" as with aluminium foil for a target that is 3 times thicker?

[green]

Would like to see more discussion replies #49, #50 and whatever.

Another reason for making the distance vs amplitude charts.

[you will be well worked digging everything...especially iron...way more junk than most people realize...18 to 24 inches on quarters is not unreasonable...take a good shovel!!!] statement from another forum discussing GPX4500. Searched GPX4500, has a 11inch mono. 18 to 24 inches is a fair spread and quarters is plural so I don't know if it could detect one quarter at 24inches.

Lets say I have a 200mm coil that can detect a coin at 400mm and I would like to detect the coin at 600mm. Using the chart, signal level about 2.5 defines minimum signal. Increasing the S/N by 10 with the same coil would be one option. 10 times the coil current(10 times the ground signal so may not be an option), any others? Using a 500mm diameter coil would be an option if noise didn't increase, is it possible to shield coil so noise doesn't increase? It's a challenge, what would be the best way to increase the detection distance the coin can be detected from 400 to 600mm(maybe just detecting in the air not ground?

More discussion #51 and#52 also. Tinkerer replied while I was typing.

[Tinkerer]

https://www.eddyfi.com/ndt/basics-skin-depth/

[Tinkerer]

Would like to see more discussion replies #49, #50 and whatever.

Another reason for making the distance vs amplitude charts.

[you will be well worked digging everything...especially iron...way more junk than most people realize...18 to 24 inches on quarters is not unreasonable...take a good shovel!!!] statement from another forum discussing GPX4500. Searched GPX4500, has a 11inch mono. 18 to 24 inches is a fair spread and quarters is plural so I don't know if it could detect one quarter at 24inches.

Lets say I have a 200mm coil that can detect a coin at 400mm and I would like to detect the coin at 600mm. Using the chart, signal level about 2.5 defines minimum signal. Increasing the S/N by 10 with the same coil would be one option. 10 times the coil current(10 times the ground signal so may not be an option), any others? Using a 500mm diameter coil would be an option if noise didn't increase, is it possible to shield coil so noise doesn't increase? It's a challenge, what would be the best way to increase the detection distance the coin can be detected from 400 to 600mm(maybe just detecting in the air not ground?

More discussion #51 and#52 also. Tinkerer replied while I was typing.

https://www.aliexpress.com/item/1x-1...460.0.0.lRmHZz
This shielding tape seems to give quite good results. It would be interesting to see how much it attenuates the noise on the table that you posted in post 41. (use spacer)

[green]

I think you are right about the signal strength of the ring. I forgot that the diminishing factor is "cubed"
About the foil, it behaves in a very "non-linear" way, this is why I do not like to use it anymore for comparative purposes. I suspect that when a target is thinner than the skin effect probably all targets will behave this way. When I talk about skin effect, I mean the the depth of immediate eddy current penetration. This will be dependent of the frequency of the TX pulse. For VLF, very low, for PI somewhere in the mega Hz range.
In thicker targets, the eddy currents expand deeper into the target. This expansion is slow, at the speed of the time constant.
Multi layer targets are supposed to attenuate the eddy currents from one layer to the next according to the theory of the old laminated iron cores in transformers. However, if we fold foil or stack coins it does not seem to happen that way.

I like thin targets because they decay in a straight line on linear time log amplitude charts. Makes it easy to predict what happens with a simulation program or with spice(parallel RL)for target simulation. Trying to simulate a specific thicker target is a lot more difficult.

[6666]

Hi Green I think you said somewhere that you use a multimeter on the output of the integrator for your measurements, what type of coupling do you use ?

[green]

Hi Green I think you said somewhere that you use a multimeter on the output of the integrator for your measurements, what type of coupling do you use ?

Voltmeter(DC volts, 100uv resolution) is connected to the output of the sampling integrator #2(resistor values same ratio but different value than schematic)used picture that was already stored, integrator #1 not used. Differential amplifier connected to coil, all stages are direct coupled except the last amplifier stage is capacitor coupled, not shown on the schematic.

[6666]

Voltmeter(DC volts, 100uv resolution) is connected to the output of the sampling integrator #2(resistor values same ratio but different value than schematic)used picture that was already stored, integrator #1 not used. Differential amplifier connected to coil, all stages are direct coupled except the last amplifier stage is capacitor coupled, not shown on the schematic.

thanks , so your multimeter is simply direct connected. are you using any load resistors
cheers

[green]

[QUOTE=6666;237787]thanks , so your multimeter is simply direct connected. are you using any load resistors
cheers[/QUOTE

Yes direct coupled and no added load resistors, that stage direct couples to the next stage, 10k resistor to inverted input of next stage.