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  1. #1
    IdahoJim's Avatar
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    TDI SL...how small can it detect?

    I've been doing some testing with my SL. Using both of the factory coils. Neither coil will detect an 8 grain ball of lead. Won't detect an 8 grain piece of nickel either. I see Nugget Bob's test in northern Nevada, and his 12" coil is seeing a 1 grain test piece of gold. Is there something wrong with my unit? It does see a silver dime at 10", and a little better with a nickel. I'm using GB off, gain at 8, PD at 10.
    Thanks,
    Jim

  2. #2
    tboykin's Avatar
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    Can you show me the test you're referring to? I have a hard time on targets that size with a TDI SL.

    I would guess the test gold is 1 gram, not 1 grain. In my tests anything less than a gram is pushing it for our pulse machines. That's when you need a GMT...

  3. #3
    IdahoJim's Avatar
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    Quote Originally Posted by tboykin View Post
    Can you show me the test you're referring to? I have a hard time on targets that size with a TDI SL.

    I would guess the test gold is 1 gram, not 1 grain. In my tests anything less than a gram is pushing it for our pulse machines. That's when you need a GMT...
    Here's a link to Digger Bob's video. Watch at the 3:50 mark
    https://www.youtube.com/watch?v=7rTA2CH_x8Y&t=409s

  4. #4
    IdahoJim's Avatar
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    Here's a link to a coil test, though they were using an Aussie TDI Pro, not the SL.
    http://www.findmall.com/read.php?73,2353162

  5. #5
    mntnflyr4fun's Avatar
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    My TDI PRO will detect a 4 grain nugget at about 1 1/2" with the stock 12" dual field, I have seen test results on different forums that indicate that with an SL, your 8 grainer may only be sensed 1/2" deep in a ground test and around an 1" in an air test.

    You don't say how you are testing either with target on the ground or in the air, and from what I understand the SL will do much better on depth with some sort of boosted power pack. (the main reason I bought the TDI PRO was the power).

    All that being said, it sounds like there is some sort of issue with your setup. Can you pump up the gain? I would do any testing with gain as hot as possible to get max results.
    My air testing with a 4 grain nugget on the 12" dual field coil only showed good results when crossing the middle of the coil so that both fields got a good look and it was not very sensitive (like an inch or thereabouts)

    From some published test results on other sites, I have seen that the SL might only test a target of similar size to that which you mention at and air test of 1" and in the ground at 1/2" and all the tests were performed with gain at max.
    Life is not a journey to the grave with the intention of arriving safely in
    a pretty and well preserved body, But rather to skid in broadside,
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  6. #6
    IdahoJim's Avatar
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    Quote Originally Posted by tboykin View Post
    Can you show me the test you're referring to? I have a hard time on targets that size with a TDI SL.

    I would guess the test gold is 1 gram, not 1 grain. In my tests anything less than a gram is pushing it for our pulse machines. That's when you need a GMT...
    Here's a link to Digger Bob's video. Watch at the 3:50 mark
    https://www.youtube.com/watch?v=7rTA2CH_x8Y&t=409s

  7. #7
    deolslyfox's Avatar
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    Thanks for sharing the video !!
    If there's anything I hate worse than diggin' pulltabs, its missin' a gold ring.

  8. #8
    IdahoJim's Avatar
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    Quote Originally Posted by mntnflyr4fun View Post
    My TDI PRO will detect a 4 grain nugget at about 1 1/2" with the stock 12" dual field, I have seen test results on different forums that indicate that with an SL, your 8 grainer may only be sensed 1/2" deep in a ground test and around an 1" in an air test.

    You don't say how you are testing either with target on the ground or in the air, and from what I understand the SL will do much better on depth with some sort of boosted power pack. (the main reason I bought the TDI PRO was the power).

    All that being said, it sounds like there is some sort of issue with your setup. Can you pump up the gain? I would do any testing with gain as hot as possible to get max results.
    My air testing with a 4 grain nugget on the 12" dual field coil only showed good results when crossing the middle of the coil so that both fields got a good look and it was not very sensitive (like an inch or thereabouts)

    From some published test results on other sites, I have seen that the SL might only test a target of similar size to that which you mention at and air test of 1" and in the ground at 1/2" and all the tests were performed with gain at max.
    Well, that's way better than my SL. I do have the 16.8 volt battery pack in mine. I haven't got an 8 grain nugget, but it won't detect an 8 grain hunk of lead, or nickel. I've been doing air tests outside. Tried both GB on and off. Also tried gain at max...no difference.
    Jim

  9. #9
    mntnflyr4fun's Avatar
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    Quote Originally Posted by IdahoJim View Post
    Well, that's way better than my SL. I do have the 16.8 volt battery pack in mine. I haven't got an 8 grain nugget, but it won't detect an 8 grain hunk of lead, or nickel. I've been doing air tests outside. Tried both GB on and off. Also tried gain at max...no difference.
    Jim
    sounds like both controller and coil might be worth a trip back to the factory for a checkup b4 the season kicks into high gear
    Life is not a journey to the grave with the intention of arriving safely in
    a pretty and well preserved body, But rather to skid in broadside,
    thoroughly used up, totally worn out, and loudly proclaiming
    WOW----WHAT----A RIDE!!!

  10. #10
    IdahoJim's Avatar
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    Yup...that's what I've been trying to decide. but, I didn't want to send it in without knowing if it was behaving normally, or not. I don't think the coil is the problem. it acts the same with either of the factory coils, or the Miner John 5 x 9 Folded Mono. Most of the guys I've heard from are saying the SL's are only good for stuff 1 gram, or larger. I guess I'll stick to my GMT for the smaller stuff, where it can be used.
    Jim

    Quote Originally Posted by mntnflyr4fun View Post
    sounds like both controller and coil might be worth a trip back to the factory for a checkup b4 the season kicks into high gear

  11. #11
    IdahoJim's Avatar
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    Yup, Tom, I think that's probably about right. Maybe Nugget Bob just got a really hot machine, or maybe he's mistaken about the size of the nugget he's using in the video.
    Jim
    Quote Originally Posted by tboykin View Post
    Can you show me the test you're referring to? I have a hard time on targets that size with a TDI SL.

    I would guess the test gold is 1 gram, not 1 grain. In my tests anything less than a gram is pushing it for our pulse machines. That's when you need a GMT...

  12. #12
    IdahoJim's Avatar
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    Well, I wanted more confirmation. I melted down a 9 grain, and a 6.68 grain piece of lead. The 9 grain at 1" was loud and clear in both White's coils. The 6.68 grain was easily readable, but soft. I also tested the 6.68 in the Miner John 5 x 9. Maybe, just maybe it might have a slight lead on the White's 7.5, but it wasn't much. So, in round numbers, it looks like 7 grains is where it's at on my TDI SL. That's less than half a gram, and I can easily live with that. That's a pretty small hunk of lead. Maybe someday I'll have a piece of gold that size and can run some more tests. Appreciate everybody's input.
    jim

  13. #13
    tboykin's Avatar
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    If you listen to what Digger Bob says at the 5 minute mark... all nuggets are different. The target he uses is flat placer gold held perpendicular to the coil, which is a larger surface area than something that hasn't been pounded into a disk.

    I think with the right combination of things detecting a 1-2 grain nugget with a TDI may be possible, but it would have to be an ideal situation... something rarely found while out prospecting!

  14. #14
    IdahoJim's Avatar
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    Good point. But, while the shape has an impact, the impact isn't as great as most people think. The magnetic susceptibility of a target is mostly based on the target volume. As you say, you might find a really small target that could be detected, but it would be pretty rare to do so.

    [pri
    Quote Originally Posted by tboykin View Post
    If you listen to what Digger Bob says at the 5 minute mark... all nuggets are different. The target he uses is flat placer gold held perpendicular to the coil, which is a larger surface area than something that hasn't been pounded into a disk.

    I think with the right combination of things detecting a 1-2 grain nugget with a TDI may be possible, but it would have to be an ideal situation... something rarely found while out prospecting!

  15. #15
    IdahoJim's Avatar
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    Tom brought up an interesting subject ie. the shape factor, and how it effects detecting smaller gold. So, being the curious guy I am, and in the interests of fairness, I decided to take one of my little lead nuggets...this one about 4 grains, and undetectable by the TDI Sl, and see what happens when I flatten it out to some extent. The nuggy was about a fat 1/8" as a sphere. The SL was unable to see it. I took it out to the shop and flattened it in the press between two blocks of tool steel. It ended up about 5/16" in diameter and about .021" thick. I had the Miner John 5 x 9 on the detector, so that's what I used. The flattened sphere was easily detectable at 1 1/2" from the coil! So, the shape factor plays a huge part, as Tom mentioned, in how small a target the SL can detect. I assume this goes for all of our detectors as well.
    On another note, John (auminesweeper) told me that having the Ground Balance "on" on the SL version of the TDI increases depth. I found this to not only be true, but it also increases the sensitivity to small targets, so Kudos to John for that bit of important info. This info flies in the face of what we usually hear about the ground balance of most detectors. If you are the praying sort, you might say some for John, who's having some health issues at this time. He's contributed a huge amount of info to the community.
    Jim

  16. #16
    IdahoJim's Avatar
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    I guess there's a limit on this flattening thing. I tried flattening a 2.2 grain nugget of lead. same diameter as the 4.1 grainer, but half as thick. Couldn't see it at all.
    Jim

  17. #17
    deolslyfox's Avatar
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    The thinner an object gets, the harder it is to see. Try it with foil. One piece 1" X 1". Then add another and see what happens. Then another, and another. As the "stack" thickens, the signal will improve.
    If there's anything I hate worse than diggin' pulltabs, its missin' a gold ring.

  18. #18
    IdahoJim's Avatar
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    Yup, Fox, there's definitely a limit to how thin you can go. That gets back to the "volume" thing in magnetic susceptibility. In the case of my test the difference was between .021" thick and .010". I could easily see the .021....couldn't see, at all, the .010.
    There's obviously a point there where you aren't generating a magnetic field in the target...it's almost like flipping a switch.
    Jim
    Last edited by IdahoJim; 04-12-2017 at 06:54 PM.

  19. #19
    deolslyfox's Avatar
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    Not enough material to produce the eddy currents necessary for a decent return. Thinner material is like smaller wire .... There are fewer electrons available ....

    While "size matters", all things being equal, "thickness matters more" ....

    When you get down to targets as small as you are talking about, it is my experience based on testing, (I haven't prospected), that you're better served with a higher frequency machine like the GMT. Also, a smaller coil which helps reduce ground return.

    Finally, I think (my own opinion) that finding really small targets is as much, or more, experience than anything else. I liken it to an experienced coin hunter pulling those "whisper" targets that others don't even hear. That little bump in the threshold that you "feel more than hear".

    But, like in shooting, "Aim small, hit small". I hope all your effort is wasted because your next hit turns out to be a two pound nugget !
    If there's anything I hate worse than diggin' pulltabs, its missin' a gold ring.

  20. #20
    IdahoJim's Avatar
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    Yup...I've got the GMT, and for small gold, it's the best. That said, there are places so mineralized you can't use a VLF detector. That's where the TDI comes into play. And, to have confidence in your machine is critical. Nugget hunting is tough, and if you don't know and understand the unit you're using, and its limits, you start questioning yourself about what you're doing. It's best to get that testing done before you ever set foot in the wilds.
    Jim

  21. #21
    tboykin's Avatar
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    Quote Originally Posted by IdahoJim View Post
    Yup...I've got the GMT, and for small gold, it's the best. That said, there are places so mineralized you can't use a VLF detector. That's where the TDI comes into play. And, to have confidence in your machine is critical. Nugget hunting is tough, and if you don't know and understand the unit you're using, and its limits, you start questioning yourself about what you're doing. It's best to get that testing done before you ever set foot in the wilds.
    Jim
    Jim, have you tried a smaller coil like the MJD 5 x 9 folded mono? Even the White's 7.5 DF should help on small gold. Really nice for getting into the bedrock creases you find when creek detecting.

  22. #22
    IdahoJim's Avatar
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    Tom, I own both of those. While they're great for getting into the tight places, I haven't found either one to be much more sensitive to small targets than the 12" dualfield. That small center coil on the dualfield does a good job on small stuff, though you have to be sure and get a good overlap to take advantage of it. I've tested the MJ 5 x 9 FM against the 7.5 dualfield, and if it has an advantage, the advantage isn't great. The MJ might see a bit smaller targets, but the 7.5DF is a little deeper. Overall, it's a toss-up.
    Jim

  23. #23
    IdahoJim's Avatar
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    Well, guys...I took deliver today of a 1 grain gold bar. 3mm x 6mm. tested it on the GMT with the 6 x 10, and it easily saw it at 3"....faint, but clear. Not surprising. Then, later on, I thought I should try it on the TDI SL...not expecting much after my previous tests using lead and nickel. The SL could easily see that bar at 1 1/2" with the Miner John 5 x 9! But, both Whites dualfield coils could see it at 1 3/8"! That made my day! I couldn't be happier. Obviously, the TDI is designed for gold, and gives the best test results with that metal. I'm really looking forward to the summer prospecting season.
    Jim

  24. #24
    mntnflyr4fun's Avatar
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    Now that's a nice small picker size for a test. I have a 4 grain nugget I am going to do some testing with on my TDI PRO.
    Life is not a journey to the grave with the intention of arriving safely in
    a pretty and well preserved body, But rather to skid in broadside,
    thoroughly used up, totally worn out, and loudly proclaiming
    WOW----WHAT----A RIDE!!!

  25. #25
    IdahoJim's Avatar
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    Reg Sniff told me the difference in sensitivity to gold is a result of the pulse width. The TDI series pulse is optimized for gold. For silver, a wider (or longer) pulse would be helpful. It's interesting that the pulse width could be optimized for the metal sought.
    Jim

  26. #26
    Where does one get yhis 16.8 volt pack? My battery says 12 volt 2900 mah on it?

  27. #27
    IdahoJim's Avatar
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    A guy that goes by "reidman" sells them. Do a 'net search...should turn up.
    For more on the small gold sensitivity look at my tests with the different battery voltages. With the 16.8v battery pack, the 12" DF factory coil is seeing that 1 grain bar at 3" in air.
    Jim

  28. #28
    Quote Originally Posted by IdahoJim View Post
    A guy that goes by "reidman" sells them. Do a 'net search...should turn up.
    For more on the small gold sensitivity look at my tests with the different battery voltages. With the 16.8v battery pack, the 12" DF factory coil is seeing that 1 grain bar at 3" in air.
    Jim
    Hey Jim I got hold of him but he said the 16.8 is only for the Pro not the SL. Did I misread your post or are you using ot anyways?

  29. #29
    IdahoJim's Avatar
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    Nope...I have no idea what he's talking about, unless he quit making the pack for the SL, or he misunderstood what you asked. The packs are completely different in shape. Also, I was under the impression the original TDI and Pro ran on 5 cell packs, which nominally would be 18+ volts, with a max of about 21v. Wish I could help, but I don't have a clue. One guy, on another forum, asked me about the SL packs, and said he found them on EBay. You might do a search there.
    Jim

  30. #30
    IdahoJim's Avatar
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    At one time, he went by a seller on Ebay. I just checked, and did not find the SL pack on EBay.
    Jim
    Last edited by deolslyfox; 07-26-2017 at 09:08 AM. Reason: Removed Seller's Name

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