Magnetic decay of gold verse lead

[Newbie]

Hi all, long story short I have built surf PI with the help of reading mountains of posts. Thank you all. The detector works a treat but I have only a mans gold wedding band for testing. I have built several coils with mixed results. All coils detect the ring so I want something smaller. lead behaves in a very similar manner when using a sluice and I was wondering if the lead also behaved the same way when it comes to magnetic decay. I have seen posts where foil was used but my limited understanding is the magnetic decay is to slow for accurate imitation of gold.

[baum7154]

Aluminum acts much like gold to a PI detector. One of the best test targets I use is a 1/4" X 1/4" piece of aluminum can side wall. This target seems to be about equivalent to a 5 grain piece of 14 kt jewelry gold that I also use for tests.

Regards,

Dan

[baum7154]

If your Surf PI does not see the 1/4' X 1/4' piece of aluminum soda can you can make the target larger, perhaps 1cm X 1cm. The TARGET TIME CONSTANTS thread has a lot of info on this. GREEN has measured T/C of many standardized targets. Al can is .003" to .004" thick so it is 3-4 times slower to decay than foil which is typically .001" thick. Thinner foil makes a faster decay. Thickness is the main determinant of T/C but shape has an effect too. Keeping the targets square minimizes the shape effect of T/C but making long thin rectangles has a significant effect on T/C. I have seen T/C change by 30% or more due to elongated shape so I keep my targets square.

Lead does simulate gold well at all in terms of decay. In my experience aluminum is best and nickle is next. However a post type of pierced earring in 14Kt gold is a good target especially with the post cut off. One made to hold a 4-5 mm stone should be about 5 grains without the post and it is a pretty challenging target.

Good Luck!

Dan

[baum7154]

Lead does simulate gold well at all in terms of decay. In my experience aluminum is best and nickle is next. However a post type of pierced earring in 14Kt gold is a good target especially with the post cut off. One made to hold a 4-5 mm stone should be about 5 grains without the post and it is a pretty challenging target.

Good Luck!

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Lead does NOT simulate gold well at all in terms of decay. Sorry I missed typing that in the quote above. Lead has a conductivity that is 9 times less than gold and that is a major factor in the time constant of a metal. Aluminum is much closer to gold in conductivity and therefore more closely approximates gold for a given size, shape, and volume. Many times I wish it did not when I am digging out small pieces of aluminum foil & pop tabs that the detector said could be gold nuggets.

Dan

[Newbie]

Thankyou Baum7154, this is good news. To date the surf is having no trouble with ring pulls so at least that is in the right direction. I have no ,shortage of aluminiam cans so i will get to cutting.

Thanks Michael

[Newbie]

Hi Dan, I have tested my coil on the foil but it does not see it. I see in other posts you have made spider coil. How do you work out the number of turns for a given wire. Do you use a standard coil calculator or is there a special calculator/calculation. I like the idea of the coil being self shielding. Thanks in advance
Michael

[baum7154]

Hi Dan, I have tested my coil on the foil but it does not see it. I see in other posts you have made spider coil. How do you work out the number of turns for a given wire. Do you use a standard coil calculator or is there a special calculator/calculation. I like the idea of the coil being self shielding. Thanks in advance
Michael

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Michael, did you try to detect aluminum foil or aluminum can? Did you start with 1/4" X 1/4" square of the aluminum? If so you should probably start with a 1" X 1" square of the can and work your way down in smaller size squares until you can no longer see them. If you cannot see a 1" X 1' piece of can then try a whole aluminum can. Foil is 3 to 4 times faster and more difficult to see so don't start with the foil.

The number of turns was determined in my coil by winding a coil of more inductance than needed and measured with an inductance meter. Then I took one turn at a time off of the coil testing small gold/aluminum targets as I went and found the that the best performance for my detector and coil was 32 turns. This measures 335uh with the meter. Generally though, about 300uh is used in searching for small gold. That would be about 30 turns on an 8" coil. You can use the coil calculator that QUIOZI has posted as a sticky at the top of this forum to determine how many turns will equal 300uh.

If you want to build the spider coil I use I have documented it extensively in the CHANCE PI COIL thread of the COILS forum. You should read the whole thread if you have not already done so. The exact recipe is there along with photos of the build, test results, and CHANCE PI detector info. That coil should work well with any PI detector and is quite fast. The most difficult part is making the Lexan coil form but it can be done with hand tools and careful attention to fabrication. The use of 600 volt PTFE/Teflon insulated stranded 26awg wire is required for best results. This coil built with enameled solid wire will result in disappointment, trust me I have done it.

Best of luck,

Dan

[baum7154]

Hi Dan, I have tested my coil on the foil but it does not see it.
Michael

Do you know what sample delay you are running? If you are not seeing the Aluminum pull tabs from a can maybe you are not running a short enough sample delay. A whole aluminum can is 42us T/C according to GREEN and I expect you have no problem seeing it. A 1/4" X 1/4" piece of can is 1.5us T/C. If you can detect the whole can but not a 1" X 1" piece of can then perhaps your sample delay is too long and the target response is being missed by not sampling quickly enough.

Hope this helps

Dan

[geoscash1]

Do you know what sample delay you are running? If you are not seeing the Aluminum pull tabs from a can maybe you are not running a short enough sample delay. A whole aluminum can is 42us T/C according to GREEN and I expect you have no problem seeing it. A 1/4" X 1/4" piece of can is 1.5us T/C. If you can detect the whole can but not a 1" X 1" piece of can then perhaps your sample delay is too long and the target response is being missed by not sampling quickly enough.

Hope this helps

Dan

Hi Dan,

with the above quote, which or what is the best way for adjusting the sampling delay, using the trim pot on the board and testing the sample with each turn for best result or is there a different way to make this happen. My surf does exactly the same as Michaels, although it will detect a 1x1 piece of pepsi can (barely), it will only detect it approx 1 inch above the coil face, it wont detect anything smaller as far as AL pepsi can test pieces. I will assume the only way is using the trim pot on the board to get you into the best or closest proximity, then use the external delay to fine tune for best result.

EDIT: Am reading the Time Constant Thread now, didnt catch that earlier before I posted.E

[baum7154]

You are probably right. I am not familiar enough with the Surf PI to advise on this but I'm sure there are many out there who can. It sure does sound like the delay is backed off on the detector probably in order to operate it on the salty beaches. The problem is that too short a delay in the range of 10us will cause a lot of falsing in the salt water environment.

Dan

[baum7154]

Hi Dan,

My surf does exactly the same as Michaels, although it will detect a 1x1 piece of pepsi can (barely), it will only detect it approx 1 inch above the coil face, it wont detect anything smaller as far as AL pepsi can test pieces.

EDIT: Am reading the Time Constant Thread now, didnt catch that earlier before I posted.E

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A little more information...That 1" X 1" piece of aluminum can has a T/C of 5us per GREEN's tests. Three T/C of 5us is 97% of the charge loss in 15us. Since you are barely seeing the sample at 1" it looks like your sampling is delayed about 15us or greater if you are actually catching the smallest target decay in the last two T/C of it's response. Full target decay is generally considered to take 5 T/Cs. That 1" X 1" aluminum can is a big bright target for a detector set in the range of 5 to 10US and should be detected at 9" or more in my testing with an 8"coil.

Regards,

Dan

[geoscash1]

Thanks Dan for that last post, that makes sense so I'll do some messing with things until I can get some better results too.. I'd be happy if it detected it at 6 inches...lol

[green]

[Three T/C of 5us is 97% of the charge loss in 15us.]

Maybe wrong. Not a big deal but I thought 3 TC was 95%.

[baum7154]

You are correct sir!

[bbsailor]

baum 7154, and all,

The TC thing works like this. One TC is 63.2 percent of maximum current as defined by the coil's inductance divided by the total DCR of the coil circuit resistance including the MOSFET on-resistance, coil wire and lead wire and the total applied voltage to the coil circuit during the pulse. That is why they use a very large capacitor near the coil to prevent the voltage from drooping too much below the battery voltage. The second TC is 63.2 percent of the remaining current times 63.2 percent that equals 86.5 percent and again 63.2 percent of the remaining current to maximum is 95 percent (sorry my previous 97 percent was a little off). The point is that three TCs is about as long as you want to wait to fully charge a coin size target's mass. Some PI designs only go up to about 2 TCs worth of TX pulse time. That is based on the overall PI design philosophy and other tradeoffs that need to be accomodated.

Some larger targets that receive very short TX pulses (relative to their size) only receive a surface charge that does not penetrate to the center of the metal mass, therefore the eddy currents do not last as long as if there was a longer TX pulse to fully penetrate the metal. The optimum length of the TX pulse is governed by the metal size, type, shape and internal resistance which determines that target's time constant. US nickels have a shorter TC than copper pennies. Some large underwater detector coils looking for large masses of metal use TX pulses in the milliseconds range.

Joseph Rogowski