gold nugget simulation?

[Ferric Toes]

Here is an interesting paper that I have referred to from time to time. Although written with conductive mineral prospecting in mind, the same principles apply to small conductive targets, such as nuggets, coins etc. It also deals with 'diffusion' and why the fundamental TC only occurs in 'late time'. It could be that an oblate spheroid is a useful shape to model a nugget.

Eric.

http://www.geonics.com/pdfs/technicalnotes/tn7.pdf

[green]

Charted the nuggets. Looks like most of the decay TC's between 1 and 2 usec. Nuggets look a little larger than I was expecting, probably couldn't chart much smaller nuggets with my tester.

Rx_(2)1.5in fig8,

[Ferric Toes]

Having a few lengths of 12mm brass rod which I had previously measured an IACS of 25%, I decided to make some test pieces of various sizes and plot the decay curves. Both ends of a 100mm long rod were checked for IACS and also one end was machined along its length for about 20mm to give a flat surface 12mm wide, on which another conductivity reading was taken. Again a reading of 25% was recorded. All samples subsequently machined from this rod will then have that same value of IACS%, even though they may be too small to register on the Hocking instrument. The exception was the 10mm disc which still gave the correct figure when accurately centred on the Hocking probe. The 25% figure for the brass rod is less than the 30% from a 10gm nugget from Southeast Australia, but higher than for a 1/2 sovereign at 16.8% or a 1oz Double Eagle at 15.3%.

The picture shows the test pieces made thus far which all have a thickness of 2mm. The discs range in mm from 10, 6, 4, and finally 3mm diameter, with rings all 10mm diameter but with 1.0, 0.75, and 0.5mm band thickness. A US Dime is for size comparison. They are sitting on top of my solenoid coil system, with a separate wound solenoid to show what is inside the box, The coil is 45mm in diameter and the test piece sits on a spacer in the centre of the solenoid such the it is in a uniform and uni-directional field. Unlike the apparatus in the Bean and Nesbitt report, the coil is both TX and RX. The winding is 0.25mm solid Kynar insulated hookup wire and is wound on a layer of woven copper shielding material to prevent capacitance effects with the object being tested; which is usually mineralised soil.



The plots are done as a linear display on both axes as this represents what is seen at the output of the front end amplifier of most PI detectors. The fastest decay is the 3mm disc followed by the 4mm which appears from the graph to have a TC of 5.75uS. the 6mm disc is not on this plot, the top one being the 10mm diameter.



The initial amplitude is always set at 1000 on the instrument's meter scale, but as you can see, the best fit exponential curve does not always meet it at the 10uS line. This indicates a distortion, either due to instrument non-linearity, TX switch off shape or time, or due to early time currents in the target. I have a test for that which I will cover in another post, together with the responses for the rings. I now measure the starting point for the true TC from the 15uS line.

Eric.

[green]

Found a formula for sphere time constant that I had written down from another thread. Used it to calculate time constant for lead shot. My sons friend loads shot shells and is bringing some shot when he visits this spring. Will see how close my measurements come to calculated then.

[green]

Having a few lengths of 12mm brass rod which I had previously measured an IACS of 25%, I decided to make some test pieces of various sizes and plot the decay curves. Both ends of a 100mm long rod were checked for IACS and also one end was machined along its length for about 20mm to give a flat surface 12mm wide, on which another conductivity reading was taken. Again a reading of 25% was recorded. All samples subsequently machined from this rod will then have that same value of IACS%, even though they may be too small to register on the Hocking instrument. The exception was the 10mm disc which still gave the correct figure when accurately centred on the Hocking probe. The 25% figure for the brass rod is less than the 30% from a 10gm nugget from Southeast Australia, but higher than for a 1/2 sovereign at 16.8% or a 1oz Double Eagle at 15.3%.

The picture shows the test pieces made thus far which all have a thickness of 2mm. The discs range in mm from 10, 6, 4, and finally 3mm diameter, with rings all 10mm diameter but with 1.0, 0.75, and 0.5mm band thickness. A US Dime is for size comparison. They are sitting on top of my solenoid coil system, with a separate wound solenoid to show what is inside the box, The coil is 45mm in diameter and the test piece sits on a spacer in the centre of the solenoid such the it is in a uniform and uni-directional field. Unlike the apparatus in the Bean and Nesbitt report, the coil is both TX and RX. The winding is 0.25mm solid Kynar insulated hookup wire and is wound on a layer of woven copper shielding material to prevent capacitance effects with the object being tested; which is usually mineralised soil.

[ATTACH]45043[/ATTACH]

The plots are done as a linear display on both axes as this represents what is seen at the output of the front end amplifier of most PI detectors. The fastest decay is the 3mm disc followed by the 4mm which appears from the graph to have a TC of 5.75uS. the 6mm disc is not on this plot, the top one being the 10mm diameter.

[ATTACH]45044[/ATTACH]

The initial amplitude is always set at 1000 on the instrument's meter scale, but as you can see, the best fit exponential curve does not always meet it at the 10uS line. This indicates a distortion, either due to instrument non-linearity, TX switch off shape or time, or due to early time currents in the target. I have a test for that which I will cover in another post, together with the responses for the rings. I now measure the starting point for the true TC from the 15uS line.

Eric.

Hi Eric
Have you tried charting your data linear log? Sometime my decay curve doesn't straighten out until after one or two target time constants. Not sure it's always real or something with my tester.

How long is each target sample? Do you chart the start, mean or end of sample time?

[Skippy]

I tried analysing Eric's latest data, but there's no clear pattern, especially when you try and add in the ( few ) other figures we have for thicker targets. Must finish off my faux nuggets.
The figures I pulled from Erics decay curves were:
10mm diam. has TC = 18 usec
4mm diam has TC = 5.9 usec
3mm diam has TC = 3.53 usec.

[belax2018]

Hi everybody.

I will say in advance sorry for my laziness.
But anyway.
Could please someone make a table with columns?:
1. Nugget size WxHxD
2. Wieght
3. Time constant

Also the same table for different aluminum pieces - better from Cola cans - as these cans are mostly standardized over the countries.

The only data I have found was from Mr.Green for his 4,10,18 grain nuggets.

Thank you.

[green]

Found a formula for sphere time constant that I had written down from another thread. Used it to calculate time constant for lead shot. My sons friend loads shot shells and is bringing some shot when he visits this spring. Will see how close my measurements come to calculated then.

Son brought a #4, #6, and a #8 shot shell. Glued 10 shot not touching of each size to craft sticks. Charted TC, measures a little less than calculated. Maybe shot size is a little off?

[Skippy]

How did you calculate TC, when you don't know what alloy the shot are made from? Each size may be a different alloy too, based on the discussion earlier in this thread.
I'm not being nit-picky, it's simply that electrical properties of alloys are/can be VERY sensitive to metal percentages. Getting a reasonable weight of a particular shot size together and casting it / machining it with some degree of precision, and comparing this with physically identical targets of known metals ( obviously pure lead in this case) may be worthwhile, though difficult.
I'm still very busy with 'life' at the moment, and am away from home. When I get back home, I'll try and see if I have any lead alloy data filed away on my PC.

[waltr]

I was also thinking that the exact alloy is needed for calculation.
Check this list:
http://eddy-current.com/conductivity...y-resistivity/

Note different conductivity for slightly different alloys.
Also heat treating to either anneal or harden will change the conductivity.

I think that your measurements are very close to calculation so all good.

[green]

How did you calculate TC, when you don't know what alloy the shot are made from? Each size may be a different alloy too, based on the discussion earlier in this thread.
I'm not being nit-picky, it's simply that electrical properties of alloys are/can be VERY sensitive to metal percentages. Getting a reasonable weight of a particular shot size together and casting it / machining it with some degree of precision, and comparing this with physically identical targets of known metals ( obviously pure lead in this case) may be worthwhile, though difficult.
I'm still very busy with 'life' at the moment, and am away from home. When I get back home, I'll try and see if I have any lead alloy data filed away on my PC.

TC sphere=4E-7*4.872E-6/pi*(diameter m/2)^2 for lead.

Not knowing the exact alloy is maybe a good reason to use square pieces of regular strength aluminum foil cut for desired TC for reference targets.