chance pi build

[baum7154]

Yes a modified version of that form but the same coil construction

[baum7154]

Still have not had the Chance in the gold fields but hope to soon. Only that will tell if it was worth the effort. I will keep you posted here on further development. As I mentioned a friend will lend me an 18 grain nugget he has so I can bury it and do in ground searches. I'm chomping at the bit to get out to the dredge piles and see what can be found. I am thinking of building another coil just for that purpose maybe 4" in diameter or a 4 X 6" coil to get into the smaller spaces. As we talked before my lawn is a disasterous place filled with an impossible array of targets. Back to you soon as I get more results.

Regards

[baum7154]

The self resonant frequency for my fast mono coil detailed in my post from 6-27-13 above is 1.0420 mhz including the twisted pair feed line. I believe this is quite fast and should allow the detector to see fairly small gold nuggets.

I did take the unit out to the old gold dredging piles for about two hours. The area has been heavily mined for over 150 years and was dredged in the 1930s. As a result it is littered with rusted cast iron flakes, corroded tin cans and the more modern aluminum cans.

I did get some flour gold as a result of panning some detected spots but I don't see how the detector was triggering on the flour gold. It appears that the detector was triggering on the large amount of black sand I got in each pan, typically 3 or 4 times what I would expect. There were also quite a few larger chunks of magnetite in each pan.

When I got home I separated 11 pieces of magnetite each the size of a pea and placed them within a 2" circle. The detector ignored them until the coil completely encircled them and then only gave an intermittent 2 increment signal strength on it's 16 division scale. The Discrimination (16 divisions scale) indicated on the first division that corresponds with GOLD. This is in keeping with the PI detector's ability to ignore some mineralization.

Then I took a volume of 1/5th of a cup of the fine black sand and the detector did trigger with about 3 divisions of signal strength from 5". It indicated on the Gold end of the disc scale. Moving the coil closer got the strength to 6 divisions and it still indicated Gold on the 1st division of the Disc scale. Once the signal strength got above 6 divisions the Discrimination moved to mid scale and then to the IRON end of the scale as the signal got above 7 divisions. This leads me to believe that the Discrimination is not operating or trustworthy below 6 Signal Strength units.

It should be noted that the coil I am using is a significant departure from the designer's recommended coil. My coil is 335uh and 2 ohms and the original design is 400uh and 1.7 ohms and the designer states that software was optimized for these values. Because of this it is entirely possible that my coil is causing some anomalies in the Discrimination. However my coil is much faster than the original.

I still have not gotten the loaner nugget to perform some in ground testing but will pass the info along as soon as it is available.

Does any one know why my detector triggers on the fine black sand but not on the magnetite?

Regards,

Dan

[baum7154]

To rephrase my question above :

Does any one know why my detector would trigger on the black sand which I believe is largely magnetite and not trigger on the big magnetite chunks in the tests above? I do know that the mass of fine black sand is much more than the mass of the magnetite chunks so maybe that is the reason.

Thanks

[baum7154]

Weighed the magnetite pebbles at 93 grains and weighed out 93 grains of black sand. Then tested them with the detector and the response was the same...virtually no response until the coil was encircling the target. Even then the response was intermittent at a maximum of 2 signal strength units. I did note intermittent jumps of discrimination from Gold indication about 80% of the time to very rapid and short excursions to the Iron end of discrimination scale.

The weight of the black sand I originally tested I estimate to have been 2600 grains which probably explains why it was detected and the magnetite chunks were not as they only weighed 93 grains.

[baum7154]

Was able to borrow a 128 grain gold nugget last night and the Chance PI with it's 8" coil was able to see the nugget at 9" or 22 cm. It solidly indicated as Gold at four signal strength increments. The 9' detection distance with an 8" coil would seem to indicate that a much bigger coil (12") might be good for larger nuggets

I am trying too borrow much smaller nuggets to perform more tests and will pass them along as they are performed.

Regards,

Dan

[baum7154]

I put the permanent BNC connector on my feed line and was able to operate the detector at a Guard Interval setting of 16.

At this setting the unit gave first indication detect detection on 0.001" thick aluminum foil at the following distances:

1 square inch - 10" (25 cm) distance, 1/2 square inch - 6" distance (15 cm), 1/4 square inch - 3" (7.5 cm) distance

On an 8.6 grain piece of dental gold it gave 6" (15 cm) first detection distance.

Again these are air tests in my basement and that is not the best environment for this testing but is the best I can do for now.

I will be working on a spiral wound coil as I believe it will allow even lower Guard Interval settings per TEPCO.

[baum7154]

Wow! what an improvement. I just optimized the critical damping resistor on my fast coil. The value I used is 1040 ohms and with that value the coil operates very well at a Guard Interval setting of 10. This change added some detection distance to some small gold targets. It appears the whole system operates much better! Can't wait to get it out on the gold fields.

[woodbob123]

Keep up the good work Dan I'm following your work with great interest building some "fast spider coils" to follow in your footsteps.

[baum7154]

Thanks Bob, it's a lot of fun to optimize this detector. Also thanks for starting this thread!

One note on the coil...I did not shield it and probably will not even though the option is still open. I found that the geometry of the spider wound coil does self-shield to a good degree only if you put the outermost winding end to system ground. I just tested this again in my yard this week and the difference was noticeable. Keep track of which wire is 'hot' and which is 'ground'. I did this by putting black & red heat shrink on the ends before I fiberglass encased the coil. I'm not doing further shielding because I don't want to lose any more signal or secondarily to slow the coil down with more capacitance. The twisted pair made up of the continuation of the teflon insulated #26 wire made a significant improvement in the speed and the total lack of any solder at the coil helps too. The twisted pair has not exhibited any detection of metal placed in close contact with it so I believe that it is an acceptable alternative to coax.

Kudos to TEPCO for his comments on setting the critical damping of the coil using a walk-down procedure to get to an operating Guard Interval setting of 10. I read somewhere a long time ago that the Chance must have the 390 ohm damping resistor because the programming was designed for it. NOT TRUE. In spite of that I did try to adjust damping using the BB Sailor method and my scope but could not get improvement. TEPCO greatly simplified the whole process, no scope, just step down to the next lowest GI setting and use the resistor network with a pot described by BB Sailor and adjust until the coil works at it's lowest Guard Interval setting. Hopefully the coil is fast enough to allow operation at the minimum setting of '10'. Then measure the resistor network resistance and put a resistor of that value in it's place. Alternatively, if you think your coil is fast enough just go straight to the '10 setting and adjust till it works. I'll try a spiral coil later because this spider coil now appears to work better than ever. The gold fields will be the final word.

Good Luck on your build! Keep me updated on your success.

Regards,

Dan

[baum7154]

AUDIO OUTPUT MODIFICATION

The audio output of the CHANCE is designed to feed a device floating off of system ground. I wanted the option to use headphones or the internal speaker. It is no secret that phones are much superior and I hear more nuance in the audio when using them. The problem is that the audio output should not be tied to system ground but nearly all headphone jacks have the sleeve at ground.

I found a TRIAD SP-42 miniature audio transformer nicely isolates the audio allowing it's 12 ohm secondary to be tied to ground on one side. It has a 50 mw power capability with a 150 ohm center tapped (75 ohm) primary and a 12 ohm secondary. The electronics wants to see 50 to 100 ohms on the primary so I used the 75 ohm center tap.

I installed a 1/8"/3mm switched stereo jack from Radio Shack so that when the phone plug is inserted the speaker is cut off. The headphones I use are 16 ohm mono/32 ohm stereo, switch selectable with independent left and right volume controls. There is plenty of volume with the CHANCE volume set at '1' and the independent headset volume controls really help with keeping it at the best level for me.

Dan

[baum7154]

The headphones I mention came from ACP DIRECT an educational supplier. They are LS3000 Labsonic Stereo / Mono Switchable Headphones
Prices start at $8.25. Very comfortable with replaceable ear cushions and a great value at under $10.

[baum7154]

Just posted this in COILS Forum, Chance PI Coil thread...Thought it would fit here as well

A wider and modified version of this coil is what I am using on my Chance detector. I used 32 turns of 26 AWG silver plated, stranded 600 volt teflon insulated wire on a 1.050 wide, .093" thick 8" OD. Lexan form with the slots machined .400" deep from each inner and outer edge. The feed line is a continuation of the coil wire 33" long and twisted at a rate of 3 twists per inch. This results in a 335 uh coil.

The system ground is connected to the wire end of outer winding and this results in what I believe is a degree of self shielding due to the geometry of this coil construction. I used 4 each lengths of 3/8" urethane foam 'Backer Rod' and 1/4" thick x 1/2" wide strips of urethane packing foam to fill out the cross sectional profile of the coil. The Backer Rod and foam strips were over wrapped with Curity self adhesive cloth hospital tape, using a 50% tape overlap on the inner part of the form. Consistent tape wrap pressure was used to avoid crushing the profile. Then at least 2 layers of fiberglass mat and resin were applied and a Lexan cross support with Lexan mounting ears were glassed in across the lower third of the assembly.

Initial critical damping of this coil performed with the coil suspended on the boom in the air required 1040 ohms in order to get the unit to operate down to a Guard Interval setting of '10'. While detecting a 1/2" SQUARE aluminum target on the ground I noticed that a damping resistance of 980 ohms gave the best signal strength. However the unit would not operate in open air at a '10' GI setting with that resistance value, it was just on the threashold of operation. I re-ran the damping adjustment in air last night and ended up with 1110 ohms and good overall sensitivity while achieving good idle state quietness of the detector i.e. just a few occasional beeps. Will test with the foil target on soil this evening to see what the impact to signal strength has been.

All of this leads up to my perception that bringing the earth/capacitance into the picture appears to lower the self resonant frequency of the coil and results in reducing the optimum value of the damping resistance. Has anyone addressed this issue in a different process for determining critical damping resistance? Otherwise I believe that setting resistance in open air does result in some over-damping of the coil when it is in use close to the earth. I do recognize that 'Earth' effect on a coil is highly variable so maybe damping in open air is the best we can do.

Regards,

Dan

[baum7154]

Tested detector with 1110 ohm coil damping on 1/2' target tonight over wet grass and saw no degradation in signal strength from last night's test over wet grass. Got the same 8 signal strength units at maximum proximity to target. Coil is 8" diameter.

Other targets were tested in air in the basement as follows:
TARGET DISTANCE
1 sq in x .001" thick aluminum foil 9.5"

1/2 sq in Aluminum foil 8"

1/4 sq in Alum foil 7"

1 cm sq Alum foil 4"

US Nickel 12"

US copper Penny 9"

US silver dime 7"

10.5 grain dental gold 6"

8.4 grain dental gold 4"

[baum7154]

CHANCE Sensitivity test

I performed another sensitivity test on CHANCE with the 8" fast coil today. Using a 4.5 grain piece of 14K jewelry gold (a 'head', made to hold a stone in a pierced earring but with no earring post) CHANCE gave first indication of detection in air at 1.750" or at 44.5mm. I don't know if this is 'good sensitivity' compared to some other detectors. Does anyone have comparable test results?

Regards,

Dan