Tdi Pro Coil Comparison On The Goldfields

[Jim in Idaho]

7 hours ago, bobinyelm said:

Is it known what size/shape range it ignores? Or does it depend upon the coil used more than the machine itself?

Is it a function of the delay I wonder? It would be interesting if there were a way to "sweep" the delay, so that you could have it rapidly sweep +/-  20% above/below the selected delay setting. I've read that even the difference from 10us (the optimum for small low conductivity gold) to 13us can make a big deal in "seeing" things. Where I hunt in desert, 90% of "junk" is lead pellets or bullets, so stuff like tin foil or pop-tops wouldn't pose a bigger problem.

Obviously I am not a detector engineer-just a rhetorical curiosity.

Anyone ever detect an area at say 10us, and then again at 13 or 15us to see the differences in detecting ability in the field or a test garden I wonder?

More than 50 years ago I was an electronics engineer before changing careers, and back then it would not have been easy to accomplish that (analog and digital microcircuits were pretty new then), but today it would likely be relatively easy. If changing delay slightly would catch more targets close to the nominal ideal delay (without finding mostly junk) it would be interesting (but beyond my obsolete capabilities, unfortunately).

The problem can have many causes. Shape, porosity, alloys, etc. This problem is why ML's have so many adjustments, including variable timing. To find everything you may have to go over the same ground multiple times, with varied settings, but that beats leaving gold behind, though some will still not be indicated.

Jim

[bobinyelm]

On 3/22/2023 at 7:37 PM, Jim in Idaho said:

The problem can have many causes. Shape, porosity, alloys, etc. This problem is why ML's have so many adjustments, including variable timing. To find everything you may have to go over the same ground multiple times, with varied settings, but that beats leaving gold behind, though some will still not be indicated.

 

It would seem automatic variable timing/delay could be the easiest to incorporate (if that is a variable that improves more thorough detection), and would preclude multiple passes over the ground, so it makes me wonder why they have not done it.

Twisting a knob to select delay is kind of like having a knob like really old cars had on the top of the windshield to move the wiper blade. Now with microchips we have rain sensors that completely sense the rain, determine the intensity, select the speed of the wiper, and the speed/spacing of sweeps, and shuts off when the rain stops.

I mean the GPZ 7000 listed for $10,000 (now $8500 it seems). That's a LOT of money; for that price it should have wings and fly. I paid that for a PA18/150 Super Cub I had in Alaska not THAT long ago. You can practically buy/build a Super Computer for that kind of money https://www.asianscientist.com/2017/09/features/build-your-own-supercomputer-1000/ .

Software these days can work wonders.

The VLF Legend has multiple programs (There have been 11 software versions released, which are user downloadable, as user suggestions are adopted.) with virtually a huge list of user adjustable parameter, including multi simultaneous frequencies, or selectable single frequencies, all for $400 (machine with one coil or $600 with 2 coils, aux lithium battery, bluetooth headset and submersible to 10ft) , and it's not alone.

Most new VLFs are amazing, and I cannot believe that PIs couldn't benefit from this kind of magic.

Sorry for the rant, but I suspect that more is possible in PIs these days. Not enough competition, or large enough market.

I know there are detector designers/engineers that frequent the Forum. I'd love to hear their opinion on that.

[Cascade Steven]

I am not an Axiom owner but am curious if Garrett's Axiom suffers from the same "blind spot" issue and if so  this poses a similar question as to how to fix it other than cover the same area with different settings?  Thanks

[bobinyelm]

10 minutes ago, Cascade Steven said:

I am not an Axiom owner,  but am curious if Garrett's Axiom suffers from the same "blind spot" issue and if so  this poses a similar question as to how to fix it other than cover the same area with different settings?  Thanks

I do not own or have ever tried an Axiom, but in a video test by "Chris Ralph, Professional Prospector" of an SDC2300 and the Axiom, both seemed comparable on small gold, but the Axiom was MUCH more affected by hot rocks than he SDC.

'Blind spot' was not addressed in his comparison:  (Note he also demonstrates the GM1000 VLF on mineralized soil and hot rocks, but it's the Axiom vs SDC2300 ($600 Cheaper than Axiom) that is operative if his test is representative and ability to ignore hot rocks is important to the user.) 

 

[GB_Amateur]

2 hours ago, bobinyelm said:

...'Blind spot'...

Are you talking about "the hole" where certain gold nuggets don't sound off because of the ground balance point?  Steve H. has written about this a lot.  See this and this for starters.

2 hours ago, bobinyelm said:

...It's the Axiom vs SDC2300 ($600 Cheaper than Axiom) that is operative if [Chris Ralph's test shown on the linked video] is representative and ability to ignore hot rocks is important to the user.)

In the case of the Axiom, @Steve Herschbach likely has more experience than anyone, anywhere with the Garrett Axiom given he was a major tester of it months before it was even announced.  He's also written a lot about it here and produced videos for Garrett (available on YouTube), including how to eliminate hot rock signals.  I'm out-of-my comfort zone saying more than that, and I haven't watched the video, not that it would help since I'm ignorant at that level, not being interested in the Axiom due to rarely getting to nugget hunt.  But something doesn't sound right about the SDC2300 outperforming the Axiom unless it was for a very narrow choice of ground conditions, targets, etc.

[bobinyelm]

One commenter to the test on YouTube by Chris Ralph mentioned the Axiom has 2 settings for balancing ground for mineralization and he used only one, so best to wait for Steve to weigh in, or to specifically post in a Garrett Axiom Forum at this site or others?

I know when I spoke to Miner John, he was enthusiastic about the Axion, and he was a contributor to Whites for their design and testing, so should be knowledgeable. He suggested the Axiom as a worthy upgrade for my no longer state of the art TDI.

I certainly do not want to in any way denigrate the detector given I have not seen or used one. I am completely agnostic, and just trying to learn myself.

[GB_Amateur]

25 minutes ago, bobinyelm said:

...Best to wait for Steve to weigh in, or to specifically post in a Garrett Axiom Forum at this site or others?

Steve will see the attention notice and respond.  Starting a new thread at this point is more likely to lead to noise than signal.  ?  If Steve feels this topic needs its own thread he'll take it from there.

[Steve Herschbach]

5 hours ago, Cascade Steven said:

I am not an Axiom owner but am curious if Garrett's Axiom suffers from the same "blind spot" issue and if so  this poses a similar question as to how to fix it other than cover the same area with different settings?  Thanks

The PI “hole” is a characteristic of single channel ground balance systems as used on the TDI and its predecessor's made by Eric Foster, and any other single channel systems out there, like the QED. Minelab pioneered dual channel ground balance as a way to fill the hole. The SD 2000, SD 2100 and SD 2200 actually gave you the ability to independently balance each channel. This went away on later models with automatic ground balancing. Garrett came up with their own version starting with the Infinium, then the ATX, and the latest with Axiom. The best explanation I’ve seen of it online was by Garrett engineer Brent Weaver at the link below.

In addition, the Axiom has a proprietary “ground balance window” feature which is basically a hot rock notch setting. I’m sure this is what Chris was referring to, and why a SDC seems better if it’s tuning out rocks the Axiom is not by not using the control. The SDC is very aggressive and so it really knocks out problem rocks, but it’s not without a cost in missed gold.

[Steve Herschbach]

All ground balance systems tune out and miss gold nuggets. Gold responses and ground responses overlap. Tuning out any particular ground range or hot rock tunes out the gold that reads the same. Simply put ground balance really is just another type of discrimination system with similar issues with masking as you see with standard discrimination. Try to notch out aluminum, you will also notch out gold.

Dual channel systems reduce this ground balance “hole” issue significantly but do not eliminate it, as in the GPX 6000 example given. If you are tuning out ground, you can lose gold. That’s why applying the minimal amount of ground balance needed and digging a few hot rocks is not a bad thing. Running multiple machines over ground will almost always reveal a few nuggets missed by other machines, or taking a detector like a GPX 5000 and running multiple times with multiple settings tends to find a few missed nuggets. The GPZ 7000 probably gets more gold in a single pass than anything else due to ZVT technology, but rest assured it still misses gold that reads just like the ground it is tuning out. If you don’t tune out the ground, you can’t find the gold either since now the ground also sounds off.

Another example is tuning out saltwater. Gold and salt signal overlap. If you do not tune out the salt, wet salt ground signals continuously. Tune out the salt, now the gold that reads like salt goes away also. It actually is an issue that cannot be fixed using conductivity or signal constants as your basis of operation. Such blind spots are inherent in the technology.

• Steve’s Guide to Why Detecting Thin Gold Chains Is Difficult
• Steve's Guide to Detecting Tiny Gold Jewelry In Saltwater
• Steve's Guide to Testing Weak Gold Targets & Ground or Salt Settings

All this was a long winded way of saying that the PI hole is severe with single channel machines like the TDI, where adjusting the ground balance can actually eliminate good targets just like a disc control, and people have used it was such. Dual channels lessen the problem but do not eliminate it. So while an Axiom would be far better than the TDI in that regard, it’s not perfect, nor is any detector that rejects anything. In fact, engaging the Axiom “ground balance window” feature engages a ground balance notch with a huge range that can be set to knock out almost anything, just like with the TDI, and needs huge caution when used for the same reason

The bottom line is simple with metal detectors. Everything, everything is a trade off of sorts. Eliminating anything via filters has a dark side, one that is rarely mentioned. We focus on what we find, and we almost never know about what we are missing.