Chance PI coil...

[Chet]

Hi guys kellogs hear i am interested in building the chance coil i have viewed all of the different threads about it just wondering if it suitable for my surf pi would appreciate your help what size wire and how many turns is it the same as what has been posted in the threads Regards kellogs

Hi kellogs, I can’t find what inductance is used for the Surf PI coil. It will probably work ok with around 300uh-350uh. Since you don’t have an oscilloscope I have laid out a procedure that will get you to the best coil inductance.


If you can find some 22awg—26awg tin plated (Teflon insulated if possible) avoid solid copper or silver plated wire and use the template from post #88 of;

http://www.geotech1.com/forums/showthread.php?21931-Getting-ready-to-build-the-MPP/page4

Practice 3 or 4 turns around the form with string, cord or scrap wire to get winding procedure correct. Caution; it is easy to get confused and start winding backwards half way through.


The basket wound coil needs more turns than a tightly wound coil. So it will require approximately 32-35 turns on the 8” coil form. It should take less than 30m of wire.


Without test equipment I recommend that you cut the slots a little deeper to allow 40 turns to fit in the coil form. Then run the following tests;
1. Make some small test targets of varies sizes from an aluminum can and some small lead targets. Don’t worry about coins at this time, they are easier to detect than small aluminum and lead.
2. Test the targets with your existing coil setting on the ground or on a plastic waste basket. Make sure there is no metal nearby. Wave each target over the coil and record the detection heights.
3. Now place the new 40 turn coil in the same position with the coil cord or twisted pair that connects the coil to the detector. Connected in a manner that you can easily disconnect the outer wire to unwind it one turn at a time. This may require cutting and soldering each time. The final solder joint will not affect the coil performance.
4. Wave each test target over the new coil and compare the height to the previous height recorded. It may not detect the smallest test targets until some turns are unwound.
5. Unwind one turn laying the extra wire length out into a wide loop away from the coil and solder it to the end of the coil cord to repeat the test again.
6. Repeat measuring and unwinding coil turns until you pass through the best height measurements.
7. Add a turn or turns back if you pass up the best measurements.
8. Cut and remove the long arc of extra wire.
9. Solder and insulate the final connection.
10. Save the targets and the final test measurements for use with future coil builds.

[kellogs]

Hi guys kellogs hear i am interested in building the chance coil i have viewed all of the different threads about it just wondering if it suitable for my surf pi would appreciate your help what size wire and how many turns is it the same as what has been posted in the threads Regards kellogs

[Lima41]

COIL PI

COIL PI.

[baum7154]

Your hand acts as a capacitor plate. Since the coil's capacitance is already low your hand won't add enough to get the coil into the unstable region. Coils with larger capacitance will be affected by your hand. It's not "shielding", it's just low capacitance.

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If what you say is true then why does it detect my hand when the system ground is put on the end connected to the innermost windings?

Regards,

Dan

[Teleno]

Yes, this is the coil with some self shielding. My 334uh 8" coil in the pictures posted earlier does not see my hand fully encircling the coil with the detector set at 8us sample delay and 'wide open' in all other settings.

Your hand acts as a capacitor plate. Since the coil's capacitance is already low your hand won't add enough to get the coil into the unstable region. Coils with larger capacitance will be affected by your hand. It's not "shielding", it's just low capacitance.

[Silver Dollar]

No the Litz is double nylon 60/44 which is equivalent to awg 30.
It was short money on Ebay so I thought I'd give it a try. I need to
get a transmitter going to see how fast it is. I have a hammerhead,
chance, Delta Pulse and minipulse but none running yet. It seems I
am missing parts and then order them but have gone off to other things.

I've been building all kinds of coils lately but getting those PI detectors going
is next on my list...

[green]

Thanks for the information.

[baum7154]

I got my Litz wire in and wound up a Big Foot Bubble Gum Coil;
[ATTACH]31130[/ATTACH]

With 100 feet of Litz I was able to make a second test coil, the Top Hat coil;
[ATTACH]31131[/ATTACH]

The Bubble gum is 320uh and the Top Hat is 440uh. Now I just need to program my Chance board!

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Good job!
Is your Litz wire also teflon/PTFE insulated? What gauge is it? Have you done SRF measurements on the coils yet? It will be good to get your CHANCE going and see how the new coils work.

Good Luck!

Dan

[baum7154]

Are the dimensions the mean, inside holes or outside holes? If I calculate correctly the circumference is the same as a 9.4 inch round. Larger than your 8 inch round. Are you happy with the size? Would you try something different? Thinking about making one.

``````````````````````````````

Actually the 4 X 12.5 size is measured at the midpoint between the windings edges and is visible on the pattern I used in the photos. It is the center point of the coil form and it is the point of greatest winding density. It seemed that the winding centers were the best way to define this. I actually calculated the included area of this coil to be equivalent to a 7.7" diameter round coil. I very much like the sharp 'blade' search field of this coil which is about 8" long and appears pretty consistent through it's length. It should allow much faster coverage of an area than the cone shaped field of the round coils. I also like the pinpointing ability of the rounded ends. The physical size gets into smaller spaces than the round configuration. All this said, I still need to revert to the 530kHz filters in my detector to regain the very small gold detection before I can truly judge performance of this coil.
I'll post results as soon as I have them.

Regards,

Dan

[Silver Dollar]

I got my Litz wire in and wound up a Big Foot Bubble Gum Coil;


With 100 feet of Litz I was able to make a second test coil, the Top Hat coil;


The Bubble gum is 320uh and the Top Hat is 440uh. Now I just need to program my Chance board!

[green]

I have built a 4" X 12.5" oval coil with 8.5" parallel sides in accordance with Eric's 3:1 profile recommendation. This coil is a fast spider coil using the instructions I posted in post #126 using only simple tools. The construction pictures are posted below. I will post test results for this self shielding coil in the next few days.[ATTACH]31039[/ATTACH][ATTACH]31035[/ATTACH][ATTACH]31036[/ATTACH][ATTACH]31038[/ATTACH]

Are the dimensions the mean, inside holes or outside holes? If I calculate correctly the circumference is the same as a 9.4 inch round. Larger than your 8 inch round. Are you happy with the size? Would you try something different? Thinking about making one.

[baum7154]

Tested SRF of this coil with its 53" twisted pair feed line attached and it measures 1.097mHz and 328uh inductance. This is a bit more inductance than I'd like in the final coil but it compares well with the 334uh inductance of my fast round coil for test comparisons. Unfortunately I modified the preamp to have three stages of 425kHz 1st order LP filters and it seems to degrade detection distance of really small gold flakes in a vial by about 50%. Will do some more testing and maybe revert to the previous 530kHz filters before resuming testing on this coil.

Dan

[baum7154]

Thanks, I just checked the self shielding on it and like all the others I can grasp the coil without the detector seeing it with the sample delay set @ 8us.

Regards,

Dan

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Built this coil using no machine tools to show that it is easily done following the procedure of Post #126 of this thread, and to get a larger 3:1 prototype for my tests in the ''Elliptical Coil Question' thread.

Forgot to mention that reversing the leads from the coil at the detector, i.e. putting system ground on the innermost windings, the detector immediately sees my hand.

Dan

[6666]

Good work Dan.

[baum7154]

Thanks, I just checked the self shielding on it and like all the others I can grasp the coil without the detector seeing it with the sample delay set @ 8us.

Regards,

Dan