Detecting lead shot

[green]

I have read some detectors can detect sub grain nuggets. GPX 6000 being one of them. Thinking #9 lead shot might be about as difficult to detect, I sent some targets to someone with a GPX6000. He couldn't detect #8 or #9 lead shot. Asked in another forum if anyone has tried detecting #8 or #9 lead shot. reply
#8 1 grain lead shot air tested on my 14" DD coil at 1" in Cancel/Normal.
#9 .75 grain lead shot was touching the left edge of the 14" DD in Cancel/Normal.
#8 1 grain lead shot air tested on my 11" mono coil at 1.5" in Normal.
#9 .75 grain lead shot air tested on my 11" mono coil at .5" in Normal.
Another reply was something similar but less distance.
The targets I sent included craft sticks with 1 piece of shot and sticks with 4 pieces of shot. He didn't detect the #8 or #9 targets with 4 pieces either, which should have 4 times the signal strength with the same time constant.
Possible reasons, higher EMI or faulty detector. Possible other reasons?

Don't know if being able to detect #9 lead shot is important. Maybe learn something trying. Using a smaller coil makes it a lot easier. Wouldn't be a good coil to hunt with, maybe a backup coil if a spot was found with many small nuggets?

[Carl-NC]

#9 shot is roughly equivalent to a 0.5 grain nugget which is worth $1.50-$2.00. Some prospectors don't want to chase pieces that small, some do.
I know on the SDC-2300 the #9 is right on the edge, sometimes I can detect it by rubbing the coil, sometimes not. I don't know why.

[waltr]

Multiple pieces are not to same as a larger piece of same mass.
This is why chains are hard to detect, many small pieces.

[green]

Multiple pieces are not to same as a larger piece of same mass.
This is why chains are hard to detect, many small pieces.

I didn't suggest 4 pieces were the same as a larger piece of the same mass. I stated multiple pieces add signal with tau remaining the same if pieces aren't stacked. Shot was side by side not stacked. Signal should be 4 times higher with 4 pieces.

Maybe your reply was a statement and not referencing the 4 #9 shot target in my post #1. My error?

[green]

A couple charts attached in another thread and a decay chart.

I can detect #9 lead shot with the 133mm fig8, maybe a little less than 1inch (4usec delay). About 1/2 the diameter the GPX 6000 11inch mono. Would need to increase signal about 8 times to detect it with a 11inch coil.
some possible ways.
decrease delay to 2.5usec
increase peak Tx ampere turns 8 times with delay still 4usec
increase Tx rate (improves S/N from EMI, does it improve S/N for ground signal?) how many times would Tx rate need to be increased?
other ways to increase signal 8 times?

Reducing coil size to detect #9 lead shot farther than 1 or 2 inches probably doesn't make sense.

[Altra]

Hi Green, how are you measuring a delay of "4uS" and "2.5us". Are thes figures measured at the turn off of the TX transistor or some other standard? If these are actual figures it would be great to see a scope shot of the pre-amp output with a overlay of the first sample logic signal.

Another good test is the Coke can squares. A 10mm square vs a 5mm.

Thanks

[green]

Hi Green, how are you measuring a delay of "4uS" and "2.5us". Are thes figures measured at the turn off of the TX transistor or some other standard? If these are actual figures it would be great to see a scope shot of the pre-amp output with a overlay of the first sample logic signal.

Another good test is the Coke can squares. A 10mm square vs a 5mm.

Thanks

The 2.5 usec delay is what I would need to get 8 times increase in signal for the #9 lead shot with a 4 usec delay. Maybe I could decrease the delay to 3.5 usec giving a 2 times increase in signal. Trying to figure out what I'm missing if the GPX6000 can detect a #9 lead shot at .5 inches with a 11 inch mono.

Recorded a 20mm, 10mm and a 5mm square cut from a Coke can side. Scope triggers on Mosfet gate driver. Not much signal at amplifier out for the 5mm square, easily seen at integrator out.

Integrator out with 4us delay and 6us sample to compare with gold targets chart for 5 and 10mm Coke can squares. 5mm square (61mV), 10mm square (1000mV). 5mm square about the same as the 4grain nugget, more signal than any of the nuggets a thru g.

[Carl-NC]

Keep in mind that tiny targets are often only detectable as they pass directly under the windings. The #9 shot will never register at the middle of the SDC coil, only when you rub it around the perimeter of the coil. Probably the same is true with the GPX-6000.

[Altra]

The 2.5 usec delay is what I would need to get 8 times increase in signal for the #9 lead shot with a 4 usec delay. Maybe I could decrease the delay to 3.5 usec giving a 2 times increase in signal. Trying to figure out what I'm missing if the GPX6000 can detect a #9 lead shot at .5 inches with a 11 inch mono.

Recorded a 20mm, 10mm and a 5mm square cut from a Coke can side. Scope triggers on Mosfet gate driver. Not much signal at amplifier out for the 5mm square, easily seen at integrator out.

Integrator out with 4us delay and 6us sample to compare with gold targets chart for 5 and 10mm Coke can squares. 5mm square (61mV), 10mm square (1000mV). 5mm square about the same as the 4grain nugget, more signal than any of the nuggets a thru g.

Thanks, The signal drop off from 10mm to 5mm is dramatic. I've seen it my own tests and your graphs quantify it.

[EL NINO]

had a note .. it is definitely good to compare a group of 4 or more .. lead shot number 8 or number ... against a weight equivalent.


This will determine the extent to which the detector is able to detect a group of such small targets.

[green]

Keep in mind that tiny targets are often only detectable as they pass directly under the windings. The #9 shot will never register at the middle of the SDC coil, only when you rub it around the perimeter of the coil. Probably the same is true with the GPX-6000.

Charted signal amplitude for my 133mm fig8 coil at lower distances. Don't know how a mono coil would chart.

133mm is inside dimension for one of the Rx coils.

[Carl-NC]

As I expected for 0mm. It's interesting to look at the results for a constant 10mm, 20mm, and 30mm distance, or for a constant 100mV response.

[green]

Charted the coil at longer distances. Used a piece of coke can side 20mm square for target, more signal for longer distance.