gold nugget simulation?

[green]

I've been searching for gold nugget time constants. Found one reply suggesting 5 usec, didn't give size. Another metal detector site states, can detect one grain nugget at 1 to 2 inches, a match head size at 3 to 5 inches. Aluminum foil has been suggested as a target material. Size and shape are a couple variables effecting time constant. I plotted a 2x2 and a 1x1 inch piece not folded and folded. If I calculated correctly and they were made of gold they would weigh about 1.3 grams and .32 grams. What size nuggets should I expect to find with a PI detector with a 8 inch coil? I don't have any nuggets to measure TC's. I was wondering if someone with a PI detector and gold nuggets could compare nuggets vs the folded aluminum targets. Maybe there is to much variation in shape to give a good comparison or TC number for a given nugget weight. Folding the foil around a needle worked best for me.

[baum7154]

Interesting tests and results. In my mind the two things that are required for gold detection are the ability to sample fast enough and the surface area of the target. In your fold tests surface gets less as the T/C gets longer. It seems to me that the 1" square folded 6 times to a 1/8" x 1/8" near cube makes a pretty good/tough target. I'll have to try it and see how it works.

I use some pieces of dental gold as targets, one 8.4 grains @ 3" and the other 10.5 grains @ 4.5" with 8" coil. I have also detected a 128 grain nugget at 9" with my 8" coil, piece of cake, when they are about as large as a 9mm bullet, but a bit flatter. My most difficult test target is a collection of gold placer flakes in a glass vial equivalent in volume to a match head from a book of matches. In my testing last night on the 2 stage amp retrofit of CHANCE PI it did detect this at about 1". I also really like a 1/4" X 1/4" piece of aluminum pop can for a realistic target. CHANCE PI sees this at about 2". This target represents a 1/2" X 1/2" .001" thick piece of foil folded twice.


Dan

[baum7154]

I just ran the aluminum foil .001" X 2" folded 6 times and CHANCE PI sees this at 4" with an 8" coil. Also ran the 1" X .001 foil folded 6 times and CHANCE PI sees this at 2.7" to 3", easier than I thought it would be. The 1/4" X 1/4" x .004" piece of soda can is still a tougher target as CHANCE PI only sees it at 1.75", not 2" as I stated earlier.

Dan

[green]

Thanks for the replies.

[green]

I just ran the aluminum foil .001" X 2" folded 6 times and CHANCE PI sees this at 4" with an 8" coil. Also ran the 1" X .001 foil folded 6 times and CHANCE PI sees this at 2.7" to 3", easier than I thought it would be. The 1/4" X 1/4" x .004" piece of soda can is still a tougher target as CHANCE PI only sees it at 1.75", not 2" as I stated earlier.

Dan

Plotted the .25x.25in aluminum can vs 1x1in aluminum foil to see why the can is harder to see. The data was plotted using a small coil, targets were in the same location. Sample time determines which target is the highest amplitude. Not sure if using a smaller coil has any effect.

[baum7154]

I think it is due to the longer T/C of the foil being a slower decay is easier to detect. The surface areas are about the same in reality so it seems the biggest difference is in the T/Cs. A small fast coil should have the advantage of a more concentrated field necessary to see the targets. The foil target is effectively 16 times thicker than the soda can is.

Dan

[baum7154]

I've been searching for gold nugget time constants. Found one reply suggesting 5 usec, didn't give size. Another metal detector site states, can detect one grain nugget at 1 to 2 inches, a match head size at 3 to 5 inches. Aluminum foil has been suggested as a target material. Size and shape are a couple variables effecting time constant. I plotted a 2x2 and a 1x1 inch piece not folded and folded. If I calculated correctly and they were made of gold they would weigh about 1.3 grams and .32 grams. What size nuggets should I expect to find with a PI detector with a 8 inch coil? I don't have any nuggets to measure TC's. I was wondering if someone with a PI detector and gold nuggets could compare nuggets vs the folded aluminum targets. Maybe there is to much variation in shape to give a good comparison or TC number for a given nugget weight. Folding the foil around a needle worked best for me.

In addition to size and shape the metallic alloy composition of gold nuggets is not consistent. For example the presence of other metals such as copper, silver, & lead in varying percentages will be 'slower' than pure gold for a given size and shape. You are right that there is no standard nugget. However if you can detect the 1/4" piece of AL can at 3" I'd say that you have pretty good chance of seeing a match head size nugget at 3" or more so it is probably a pretty good test target. More important is to bury the test target in soil of your search area to test if you can still see it. By the way I have read that the 'typical gold nugget' is about 22K gold on average...not sure if this is entirely true but pure gold nuggets must be harder to detect than the 22k alloy nuggets.

Regards,

Dan

[baum7154]

I have been experimenting with small pieces of 1/8" aluminum rod used for TIG welding to simulate gold nuggets in 1/3 gram/5 grain, 2/3 gram/10 grains, and 1 gram/15 grain increments. Doing the calculation on the difference between aluminum and gold specific gravities it turns out that .7 grain of aluminum is about equal to 5 grains of gold in volume. A .280 piece of 1/8" aluminum rod weighs about 2.1 grains and is the equivalent volume of a 1 gram piece of gold. These are compact targets and have less surface area than most flat types of nuggets I have seen.

Tests in air using my CHANCE PI detector with 8" fast coil give the following results:

15 grain simulated gold target - 3 3/4"

10 grain simulated gold target - 3 1/4"

5 grain simulated gold target - 2 5/8"

[baum7154]

To clarify:

Tests in air using my CHANCE PI detector with 8" fast coil give the following results:

2.1 grain aluminum simulating a 15 grain gold target - 3 3/4"

1.4 grain aluminum simulating a 10 grain gold target - 3 1/4"

0.7 grain aluminum simulating a 5 grain gold target - 2 5/8"


Regards,

Dan

[baum7154]

Regarding the targets made of 1/8" diameter slugs of aluminum or any aluminum it is more accurate to say that every .7 grain of any kind or aluminum is equal in volume to 5 grains or 1/3 gram of gold. All you need is an accurate scale to measure in grains. This is derived from the specific gravity of the two metals, gold being 19.32 grams/cu. cm and aluminum being 2.712 grams/ cu.cm a 7.12:1 ratio. 5 grains gold / 7.12 = 0.701 grains of aluminum. My 1/4" X 1/4" piece of .004" aluminum can weighs just 0.16 grain or about the volumetric equivalent of 1.141 grains of gold. Note that this is for pure gold not nugget gold alloy.

By the way CHANCE PI detector with 8" fast coil sees a 0.7 grain piece of .090" X 1/8" aluminum rod at 4" for first detection and at 2.5" for a pretty solid tone always indicated solidly as gold/aluminum on it's VDI. The 1/4" X 1/4" X 004" piece of aluminum can is also detected at about 2.5" probably because of it's broad area.
Regards,

Dan

[baum7154]

Did a little comparison between the .7 grain, .090 X .125" aluminum slug (simulator for a 5 grain piece of gold) and an actual 4.5 grain piece of 14KT jewelry gold and the detection distance is virtually the same.

Also ran the 1.4 grain, .180 X .125" aluminum slug ( simulator for a 10 grain piece of gold) and a 10.8 grain piece of dental gold and they also were the same detection distance.

Dan

[green]

Charted some more data. Don't have any round aluminum, going to get some. Cut a square 5 x 5 mm x 1.65 mm out of a piece of aluminum. Recorded a fishing lead split shot, 3/0 I think. The time scale was to large to plot the aluminum foil and plot the other targets. I had ball parked the aluminum foil at .001 inches in the past. This time I weighed a 200 mm x 200 mm piece on the reloading scale, .1 grain resolution. If I did the math correctly the thickness is .00063 not .001 inches. The aluminum piece is equivalent to a 1 gram gold in volume. The lead shot a 1.35 gram gold. Want to record smaller pieces of aluminum. Think the square aluminum or Dan's slugs would be a better target for simulating gold nuggets. How small a nugget should we try to simulate for a PI detector? Don't think the lead shot would do. Was surprised all the targets plotted close to the same amplitude at the same distance, and that there wasn't a bigger difference between the lead shot as is, pinched shut ( not smashed), and cut in half through split. Coil on time was 85 usec. Probably have to record some actual gold nuggets to verify simulation and get an idea how much effect shape has.

[green]

Plotted two more aluminum targets. 5.7 x 2.77 x 1.65 mm aluminum and 4 x 3.66 x 1.65 aluminum. Both targets plotted almost the same with a time constant of 11.3 usec and a peak amplitude about half of the 5 x 5 x 1.65 aluminum target at 5 usec. The volume is about half of the 5 x 5,so they simulate a .5 gram target. Expected to see a difference do to shape. If they do simulate gold the time constant is higher than I was thinking. Will chart the aluminum targets when done with recording all of them.

[Tinkerer]

The split in the lead target probably generates 2 different sets of eddy currents, not the same as a full round lead target.
You could try just one half of the target as half sphere and flatten the other half down to about 0.5mm thickness, to see the difference in response.

Nugget hunters claim that the conductivity of gold nuggets is much less than pure worked gold.

[green]

[Nugget hunters claim that the conductivity of gold nuggets is much less than pure worked gold.]

I worked in measurement all my life. Trying to get a good TC number for a target is fun for me. Sampling sooner than 10 usec will find more lead shot and aluminum foil. I wish I new if it would help find small gold chain or nuggets. Are small chain and nuggets hard to detect because of low TC's, low signal amplitude or both? Looking at photo's of nuggets I can see there could be a large difference in TC for a given weight nugget. Chain is in a loop which causes a higher TC, but could still be low do to size. I still wonder why the numbers aren't published, maybe not as important as I think they are. Maybe more important with PI than VLF?