gold nugget simulation?

[Qiaozhi]

As Carl has mentioned before:
Response conductivity is due to target metal type.
Response inductivity is due to target thickness.
Response strength is due to target surface area.

Which means that the inductance is mostly controlled by the skin effect (which depends on target thickness), and resistance is mainly controlled by the conductivity. Both target size and shape have an effect on the inductance and resistance. Therefore, although you only need to have the correct value of tau for modelling purposes, the problem is essentially a matter of figuring out what the tau needs to be. Which is why you'll not see any hard-and-fast numbers published anywhere.

[green]

Got some nuggets to chart. Charted time constant. Thinking of plotting amplitude vs distance with the 3-1 magnet wire coil and trying to find targets that match time constants and amplitude with the nuggets. Any other thoughts?

[Chet]

Got some nuggets to chart. Charted time constant. Thinking of plotting amplitude vs distance with the 3-1 magnet wire coil and trying to find targets that match time constants and amplitude with the nuggets. Any other thoughts?

Hi green,
Great chart,
The short time constants of the gold nuggets shows why a fast coil and early sampling is important to catch the signals before they decay below detectable voltage levels. Were the nuggets centered against the coil or some distance from it? I look forward to any coil tests.
Thank you for the information,

[baum7154]

It is a little surprising that the 18 grain nugget is a shorter TC than the 10 grain one. Is the thickness of the nuggets markedly different?
Then the metal composition of the nuggets is also unknown. Perhaps the 10 grain has some or a bit more silver/copper in its makeup. Do you know if these are all from the same geo area?

Regards,

Dan

[green]

It is a little surprising that the 18 grain nugget is a shorter TC than the 10 grain one. Is the thickness of the nuggets markedly different?
Then the metal composition of the nuggets is also unknown. Perhaps the 10 grain has some or a bit more silver/copper in its makeup. Do you know if these are all from the same geo area?

Regards,

Dan

Don't know about the geo area. I was surprised that the 18 grain nugget had a shorter TC also. It looks like it's area is more than twice the 10 grain nugget so it is probably a little thinner.

[Funfinder]

Practically seen there are 2 different things that counts:

What is the minimal (by alu faked) size of a single nugget the P.I. can detect at what coil-sweeping-speed
and:
what is the the middle size of a field with nuggets close to each other, but smaller one, so the whole metal field will add up enough.


If the detector with the small enough coil is unable to locate a single 2mm nugget made of alu, tin or whatever from 5cm distance (2'')
it is unpracticable under real conditions, because most gold-nuggets are not scattered around directly on the surface
with such a pretty large size, not even at the bottom of the bering-sea.

[homefire]

I use a #4 bird shot slightly flattened to simulate a nugget of the same size. Would be cool to see how one Charts out.

[green]

Used a aluminum roofing nail and lead shot to make targets with a shape and volume similar to the 10 grain nugget. Used a split shot to make a target close to a #4 bird shot. Plotted the targets plus the 10 grain nugget and a 9x9 mm piece cut from an aluminum can. Expected the nugget to have a longer time constant than the aluminum piece. It isn't maybe do to shape not being close enough. Going to plot amplitude vs target distance for the targets with a larger coil.

[Chet]

Used a aluminum roofing nail and lead shot to make targets with a shape and volume similar to the 10 grain nugget. Used a split shot to make a target close to a #4 bird shot. Plotted the targets plus the 10 grain nugget and a 9x9 mm piece cut from an aluminum can. Expected the nugget to have a longer time constant than the aluminum piece. It isn't maybe do to shape not being close enough. Going to plot amplitude vs target distance for the targets with a larger coil.

Hi green,
Very interesting comparison of different target materials that are used to simulate gold nuggets.


It looks like the damped flyback pulse is only limiting most target signals before 4us. This is a good example of what a fast coil with good damping can accomplish. With a slower coil the target signals would be much lower if the sampling was occurring after 10us or 15us on the chart.


Although somewhat degraded by the tail end of flyback damping, it appears that significant signals from the 9mm x 9mm piece of aluminum can was allowed 2us earlier than other targets because of its greater signal level.


It looks like the 9mm x 9mm piece of aluminum was limited at the end of its plot at a higher signal level than the other targets. Maybe the square shape close to the small coil is distorting the magnetic field in a way that causes this effect?


Again the chart shows how important a fast coil is to early sampling. Fast coils allow the highest signal levels to be sampled on gold nuggets. The short life of Eddy Currents in gold nuggets is shown as short Time Constants. It might be interesting for comparison to include a long Time Constant target in your charts.


Please check your PM.

[Funfinder]

Perhaps a horizontal (broad) small coil should be used so the
coil gets much longer time to receive the weak nuggets echo.

.._________//______
/------------// ---------\
\----------(O)----------/

25x10cm P.I.mono-coil
coil size: 25cm (10'') from left to right, 10cm (4'') back-forth
Even better are 2 coils inside a 25x10cm housing to pick up super-weak signals.

[green]

[Although somewhat degraded by the tail end of flyback damping, it appears that significant signals from the 9mm x 9mm piece of aluminum can was allowed 2us earlier than other targets because of its greater signal level.]

Each trace is a target recording minus a no target recording. I think the earlier 2 usec is do to amplifier saturation.


[It looks like the 9mm x 9mm piece of aluminum was limited at the end of its plot at a higher signal level than the other targets. Maybe the square shape close to the small coil is distorting the magnetic field in a way that causes this effect?]

The #4 shot and the 9x9 mm can were recorded at different scope settings. Scope resolution, the lowest level is the least significant A-D bit


[Again the chart shows how important a fast coil is to early sampling. Fast coils allow the highest signal levels to be sampled on gold nuggets. The short life of Eddy Currents in gold nuggets is shown as short Time Constants. It might be interesting for comparison to include a long Time Constant target in your charts.]


Didn't know what to expect for nugget time constants. The 3 to 6 usec is longer than some of the foil targets we were playing with.

[homefire]

Good Stuff from You Mr Green. Thank you.

?? Were are you pulling these samples? After the First Amp?

[green]

[Although somewhat degraded by the tail end of flyback damping, it appears that significant signals from the 9mm x 9mm piece of aluminum can was allowed 2us earlier than other targets because of its greater signal level.]
Each trace is a target recording minus a no target recording. I think it's because of amplifier saturation.

[It looks like the 9mm x 9mm piece of aluminum was limited at the end of its plot at a higher signal level than the other targets. Maybe the square shape close to the small coil is distorting the magnetic field in a way that causes this effect?]
The #4 shot and the 9x9 mm aluminum can were recorded at different scope settings. Scope resolution, the lowest level is the least significant A-D bit.


[Again the chart shows how important a fast coil is to early sampling. Fast coils allow the highest signal levels to be sampled on gold nuggets. The short life of Eddy Currents in gold nuggets is shown as short Time Constants]
Didn't know what to expect for nugget time constants. 3 to 6 usec is longer than some of the foil targets we were trying.

Sorry about the repeat. I wasn't getting an update when I tried to view the reply.

[green]

Test. Not seeing reply

[green]

Good Stuff from You Mr Green. Thank you.

?? Were are you pulling these samples? After the First Amp?

After the second stage. first stage gain 15, second stage 20