Getting ready to build the MPP

[baum7154]

Hi Dan, yes, I kept the 1k resistor in place, & thx for pointing that out - I guess it makes sense when thinking about why I don't detect a piece of lead smaller than about .3 gram - the coil just isn't fast enough. I think one reason it's not faster is the wire resistance - I think I need to use a significantly larger wire (smaller AWG number) to get the resistance down (& impedance also) to well below 1 ohm. I think the SRF would go up a lot and the spike recovery would be at least 1 or 2 uS quicker. Any thoughts on an optimum wire size? - I'm running 26AWG stranded teflon coated now and it's too small/resistance is too high. I would like to get the right gauge wire and wind another coil exactly like this one (it's nice & tight). I'm able to adjust my MPP min main delay to match - it is capable of sub uS delay timing if I ever get a coil that fast...

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I've been using 24 AWG. Tried 26 AWG but did not like the result. Since each jump in wire gauge doubles the cross sectional area and lowers resistance perhaps you should give the 24 AWG a try. There is a trade off with increasing wire sizes in terms of greater eddy currents, inter-winding capacitance, weight, and physical wire bulk in the 3DSS coil form slots.

Regards,

Dan

[baum7154]

Hi Dan, yes, I kept the 1k resistor in place, & thx for pointing that out - I guess it makes sense when thinking about why I don't detect a piece of lead smaller than about .3 gram - the coil just isn't fast enough. I think one reason it's not faster is the wire resistance - I think I need to use a significantly larger wire (smaller AWG number) to get the resistance down (& impedance also) to well below 1 ohm. I think the SRF would go up a lot and the spike recovery would be at least 1 or 2 uS quicker. Any thoughts on an optimum wire size? - I'm running 26AWG stranded teflon coated now and it's too small/resistance is too high. I would like to get the right gauge wire and wind another coil exactly like this one (it's nice & tight). I'm able to adjust my MPP min main delay to match - it is capable of sub uS delay timing if I ever get a coil that fast...

--------------------------

Hi Kyle,

I know this has been discussed a lot before but the shape of your lead target and its orientation to the coil makes a big difference in detection distance. Is your lead target 'pure lead' or some unknown bullet/fishing sinker alloy? Is it a sphere , cylinder, or flat, thin shape?

I just took a piece of pure lead that was nipped off a block with a wire cutter weighing exactly 5.0 grains and measuring 4mm X 4mm X about 4mm thick with beveled sides from the cutting and trimming process. It is repeatably detected in air with my CHANCE PI at 1/2". Then pounded it into the following dimensions with the corresponding repeatable detection distances in air using my newest 4" X 12" 3DSS 328uh Racetrack coil with all targets presented flat to the coil:

5 X 5 X 1.5mm -- 19mm

6 X 6 X 1.0mm -- 35mm

7.5 X 7.5 X 0.7mm -- 42mm

The flat target was then nipped into a 6.5 X 6.2 X .7 mm shape weighing 3.98 grains or .257 gram with a detection distance of 19mm.


Note: The Racetrack coil is designed for fast/wide coverage of the ground and sacrifices some detection depth compared to a round coil.

Thanks,
Dan

[etwert]

Hello KPinAZ,
In SAMPLE INTEGRATOR (I think it's DIFFERENTIAL SAMPLE
INTEGRATOR) there accurate resistors, for example 1%. In SURF
MASTER, BARACUDA, HAMMERHEAD resistors determining
SAMPLE gain in both channels are accurate.

[etwert]

Hello KRinAZ,
Sorry for the mistake in your name. In SAMPLE INTEGRATOR (I think it's DIFFERENTIAL SAMPLE
INTEGRATOR) there accurate resistors, for example 1%? In SURF
MASTER, BARACUDA, HAMMERHEAD resistors determining
SAMPLE gain in both channels are accurate.

[KRinAZ]

--------------------------Hi Kyle,I know this has been discussed a lot before but the shape of your lead target and its orientation to the coil makes a big difference in detection distance. Is your lead target 'pure lead' or some unknown bullet/fishing sinker alloy? Is it a sphere , cylinder, or flat, thin shape?I just took a piece of pure lead that was nipped off a block with a wire cutter weighing exactly 5.0 grains and measuring 4mm X 4mm X about 4mm thick with beveled sides from the cutting and trimming process. It is repeatably detected in air with my CHANCE PI at 1/2". Then pounded it into the following dimensions with the corresponding repeatable detection distances in air using my newest 4" X 12" 3DSS 328uh Racetrack coil with all targets presented flat to the coil:5 X 5 X 1.5mm -- 19mm6 X 6 X 1.0mm -- 35mm7.5 X 7.5 X 0.7mm -- 42mmThe flat target was then nipped into a 6.5 X 6.2 X .7 mm shape weighing 3.98 grains or .257 gram with a detection distance of 19mm.Note: The Racetrack coil is designed for fast/wide coverage of the ground and sacrifices some detection depth compared to a round coil. Thanks,Dan

Hi Dan, nice job on creating the test targets & performing the measurements & on the creation of your new coil. I'm curious what field/detection pattern it has (e.g.cone shaped) and what delay (in uS) you're using, the ohms of your coil, and also the specs of the wire you used for your coil? I'm still thinking my resistance at 3.1 is too high, slowing my coil down (too long of a spike recovery time) - making it miss very short TC targets...we'll see when I build & compare to the next coil...next coil will be identical except with more optimum wire... I agree, the shape of the target & it's orientation to the coil (both air testing and in the ground) make a dramatic difference to detection depth - for all types of detector technologies I've used. So what I'm using are lead bullets & fragments that I recovered from the ground - with oxidization - that are deformed to similar shapes of the natural gold nuggets I have found. I only use soft lead bullets (ones I can easily scratch with my fingernail) - I don't use bullets that are jacketed with any other metal. This way I end up with test lead targets with as close as possible TC's to actual in the ground gold nuggets. As for orientation I always present the largest surface area of the target to the coil - the best case scenario for that target - for measurement purposes. Thanks - Kyle

[KRinAZ]

Hello KRinAZ,Sorry for the mistake in your name. In SAMPLE INTEGRATOR (I think it's DIFFERENTIAL SAMPLEINTEGRATOR) there accurate resistors, for example 1%? In SURFMASTER, BARACUDA, HAMMERHEAD resistors determiningSAMPLE gain in both channels are accurate.

Hi etwert, I'm just using standard 5% resistors - my gain seems to be good, I modded my MPP to add a Gain/Sensitivity control so I can reduce gain as needed - and it's rare that I have my gain set higher than 50% - since the ground iron mineralization is so strong where I'm detecting - the only time I have gain turned all the way to most sensitive is when I'm measuring detection depths of test targets - & I usually do those tests on more mild ground. Thanks - KRinAZ

[KRinAZ]

-------------------------------------------------------------------------I've been using 24 AWG. Tried 26 AWG but did not like the result. Since each jump in wire gauge doubles the cross sectional area and lowers resistance perhaps you should give the 24 AWG a try. There is a trade off with increasing wire sizes in terms of greater eddy currents, inter-winding capacitance, weight, and physical wire bulk in the 3DSS coil form slots.Regards,Dan

Hmmm, ok thanks Dan. I'm curious if you tried other larger sizes & found a size that was as large as practical? That would establish a ball park range of sizes - 24AWG being the minimum to ???AWG being the maximum - that would be very good to know! It would be quite interesting to wind identical coils with each AWG size in the range & document all the performance specs - SRF, inductance, ohms, damping resistance, spike recovery time, etc.Thanks - Kyle

[6666]

Thinking of building & measuring fast coils - I think I'm going to get another MPP board & just build the power sources [needed] and just the TX circuit - to purely observe the coil with no RX circuit influence - I'll do that before the next coil build. Also thinking of fast coil sampling - anyone read the KB article about the MPF technology on Minelab's new SDC 2300? It's linked to on the product page and quite an interesting read!

Hi KR if you are going to just build the PS and TX and maybe preamp, BBS made a post about uping the plus and minus 5 volt supply on the preamp to knock off a uS or 2 from the preamp recovery time, I simed it with about 8-10 volts on the preamp and on the sim the higher voltage did knock off 1-2 uS, just might be worth looking at.

[KRinAZ]

Hi KR if you are going to just build the PS and TX and maybe preamp, BBS made a post about uping the plus and minus 5 volt supply on the preamp to knock off a uS or 2 from the preamp recovery time, I simed it with about 8-10 volts on the preamp and on the sim the higher voltage did knock off 1-2 uS, just might be worth looking at.

Many thanks 6666, very interesting, I'll look into that.

[KRinAZ]

I recently received a .22 gram flat gold nugget as a thank you, & tried it out as a test target on my MPP - got some interesting and different results than expected:

Air tests with delay at shortest, sensitivity at most sensitive, SAT about at project spec -

1) target oriented parallel to coil, repeatable target signal in center of coil - distance of about 2.5" 6.35cm
2) target oriented parallel to coil, repeatable target signal when right over coil winding at edge of coil - distance of about .75" 1.14cm

1) target oriented perpendicular to coil, repeatable target signal in center of coil - distance of about .25" .635cm
2) target oriented perpendicular to coil, repeatable target signal when right over coil winding at edge of coil - distance of about 1" 2.54cm

I repeated the above with a test lead target I made to be reasonably close to the gold target in physical dimensions and got about the same results.

The good thing is that - this is the smallest gold & shortest TC target I have had a target response from.

It's interesting how the target response is reversed based on the orientation - and that the response is stronger right over the coil windings for the perpendicular target. When scanning the ground this would have the effect of two target signals.

The down side is that when I adjusted my MPP to be reasonably stable while scanning over mineralized ground (much lowered sensitivity & much faster SAT) I could not detect the little nugget.

So I guess I'm back to wanting GEB on my MPP, I'll reread Davor's & other's posts on GEB & think on it for a bit & see what it will take to try to add GEB to my MPP...

Each target is next to a US nickel (coin)

[Qiaozhi]

If you imagine looking at a cross-section of the coil windings, with the magnetic field lines circling around the cross-section, you can see why the parallel target is strongest at the coil center, and weakest at the edge. And vice-versa when the target is perpendicular to the coil.

What are the diameters of the gold and lead test targets?

By the way, do you have the names of the Gold and Lead images swapped over? The lead target looks gold in color, and the gold looks lead colored.

[Chet]

Hi KRinAZ

That is good results for a such a small target.

Qiaozhi my view shows gold color for the nugget. The lead has a dark blue color with the nickel a blue hue. Must be different browsers. I am on Goggle Chrome.

Have a good day,
Chet

[6666]

gold color for the nugget. The lead has a dark blue color with the nickel a blue hue.

Me to.

[Achillion]

KRinAZ, you have made some mods to the standard MPP. I guess you have tuned the damping resistor. Do you know what value it is.
I'm testing at the moment and although I'm able to sample at 7-8us, I find the value quite low (between 350 -370 ohm).

Jacob

[Qiaozhi]

Qiaozhi my view shows gold color for the nugget. The lead has a dark blue color with the nickel a blue hue. Must be different browsers. I am on Goggle Chrome.

Have a good day,
Chet

What?
I'm sure they were the other way round, and I checked it more than once. Must have had too much wine.