White's TDI DD coils

[Tinkerer]

Reg,

thanks for the answer.
I prefer lower speed myself so I will design for about 2 feet second, unless some good people on the forum come up with some advice.

Tinkerer

[Qiaozhi]

Reg,

thanks for the answer.
I prefer lower speed myself so I will design for about 2 feet second, unless some good people on the forum come up with some advice.

Tinkerer

From Whites Electronics website:
All modern detectors require some movement (sweep) of the search coil in order to respond to metals. If the search coil is swept too slowly, metals do not respond, or do not respond at as great of depths. Every model has an ideal search coil sweep speed, usually between two and four seconds per pass. Experimenting to find the ideal search coil sweep speed allows optimum detector performance. A first time user typically has to practice to find their comfortable search coil sweep technique. Seeing others with good search coil sweep habits is a big aid in learning. Practice makes perfect. The desire is to sweep the search coil evenly with the ground in smooth even swings. Overlap each pass by at least 50%, always keeping the search coil in motion. Recognizing where the beep is on each pass and shortening the passes to zero in on the location (pinpoint) takes some practice as well

[Tinkerer]

From Whites Electronics website:
All modern detectors require some movement (sweep) of the search coil in order to respond to metals. If the search coil is swept too slowly, metals do not respond, or do not respond at as great of depths. Every model has an ideal search coil sweep speed, usually between two and four seconds per pass. Experimenting to find the ideal search coil sweep speed allows optimum detector performance. A first time user typically has to practice to find their comfortable search coil sweep technique. Seeing others with good search coil sweep habits is a big aid in learning. Practice makes perfect. The desire is to sweep the search coil evenly with the ground in smooth even swings. Overlap each pass by at least 50%, always keeping the search coil in motion. Recognizing where the beep is on each pass and shortening the passes to zero in on the location (pinpoint) takes some practice as well

Qiaozhi,

thanks for the info.
Two to four seconds per pass seems to be about one to two feet per second. So I would be somewhere near "normal"
Now, about the pinpointing:
If the TINKERERS has a reasonably fast response, the audio should indicate the presence of a target with a precision of about one, maximum two inches, when in tracking mode. A coil with an open center, or if DD open sides give an advantage there.
For the TINKERERS_V1, I designed a tracking ground balance that switches automatically over to pinpoint mode with the presence of a target.
Well, at least, this is the idea. There are still some problems to be solved.
Problem #1, the pinpoint mode latches up at switch on. I need to add a reset button.
Problem #2, the threshold:
The signal is bipolar. It goes positive for gold and negative for iron. the threshold should be set at the edge of the noise level, in this case, there is a positive edge and a negative edge. At present I set them individually with R33 and R37 (see attached circuit), looking at the signal on the scope.
I need to find a better way to do that and it has to be easily field adjustable.

Any ideas are welcome.

Tinkerer

[Reg]

Hi Alex,

I have been thinking about your TDI discrimination and your coin conductivity.

You mentioned that most coins register as a low conductor. If that is the case, then you might simply select the low conductor mode, make sure you have the GB above 7 or so and most nails, bolts, and other thicker iron should be ignored or at least partially ignored even with the mono coil.

The typical normal ground balance setting is usually around 8 to 9 and at these settings, much of the thicker iron will clearly be distinguished as a high conductor signal or a broken signal where it is a high conductor much of the time. This is one clear giveaway that the metal is ferrous.

In other words, as a general rule, non ferrous metals will not change tones during a sweep if they are not on the surface. One the surface, they can generate an opposite signal, but raising the coil eliminates the dual tone on non ferrous.

Now, non ferrous objects on the surface can generate an opposite tone as the coil approaches the object or leaves the object. This characteristic is quite consistent in just where one will hear the different tones. Also, raising the coil a little will normally eliminate the approach or leaving signal, thus leaving a smooth audio response.

Ferrous junk will or can generate both a low tone and a high tone even at some depth because of how the coil "sees" the target. Since ferrous junk contains both magnetic properties as well as conductive properties, one can get different signals. Also, ferrous junk signals are normally much stronger signals when compared to non ferrous objects buried at the same depth. This translates into a more abrupt or harsher wider signal at depth on some ferrous objects, while it will or can become a multi tone or broken signal depending of the TDI is set for ALL or single conductor.

What this means is when analyzing a buried object, one can sweep the target from a couple of different angles and if it changes tones or when you are in the single tone mode, the target stutters signal wise, then most likely it is iron. So, thicker ferrous junk can have a tendency to have a narrow abrupt and/or broken signal, while a non ferrous low conductor will normally create a narrow but smoother consistent response. This may not make much sense but will once a person listens to target signals. The more one becomes familiar, the easier it is to distinguish the signals.

The type of ferrous junk that is harder to separate is the thin metal such as that from tin cans. Now, this metal has a tendency to be at the surface and will have a wider abrasive or harsh response that is much stronger and wider than a typical non ferrous piece of metal the same size.

I hope this helps. Remember, it does take a while to fully learn the subtle differences between metals on the TDI. Now, once it is learned, then one can minimize the amount of ferrous junk dug. The design isn't perfect but it does work quite well when compared to other PI forms of discrimination.

Reg

[AlexP]

Hi Reg,

I am beginning to get very pleased with my White's TDI. I found a gold ring on the beach a couple of days ago. I am new at this hobby (been metal detecting for about 1 year), but since I started, this is my first gold found.

And I started to get used to the sounds the machine produces. I can tell with 60-70% confidence whether the object detected is iron or is a non-ferrours object. But I don't think that I should rely 100% on this. Unless I get lazy, I'll dig everything.

Most of our small coins which are low conductor coins have mainly nickel content, and I have noticed that some of them produce a bit wavy sounds when they are near the surface. Perhaps this happens when they are not sitting flat in the ground/sand.

Do you think that with a certain type and size of coil it will be possible to get a little better depth than with the 12" standard coil and at the same time lose the ability to detect very small objects?

Regards,
Alex

[Reg]

Hi Alex,

I really haven't done much testing of larger coils and whether they will allow you to ignore smaller objects, but I would think that some of the smaller stuff will be at least reduced in signal by going to a conventional 12" to maybe a 14" round mono coil and not the dual field type.

The dual field design enhances some of the smaller targets. Since the TDI was primarily designed for nugget hunting and small nuggets are far more common, then it only made sense to build a coil that wouldn't miss as many of the smaller ones.

Now, a regular mono doesn't have that increased sensitivity to the smaller stuff so it should help a little. The problem will be trying to ignore small iron which has such a strong signal to begin with. This is where the ground balance control would help.

Now, depending upon just what you are wanting to find, you can try adjusting the delay some or the ground balance (GB) or both to see if there is a combination that works best. Also, keep in mind the sensitivity control enhances all signals, so reducing the sensitivity level may make it easier to separate out objects simply by the signal itself.

So, there are lots of ideas to try and techniques that might help. The key is to be willing to try them to see what happens. I wouldn't dismiss something because the initial testing didn't work out that well. The more you learn the features of the TDI, the more it will make sense to try things a little differently that just may work well.

So, now it is a matter of more experience and practice.

Your indication that your low conductive coins may give a "wavy" signal when they are near the surface is not that uncommon. A trick I use is to raise the coil a little and see what happens to that same object's signal. Sometimes, there is the edge effect, meaning if the coil is less than maybe 3" to 4" from the object, it can be detected by the edge where the windings are. By raising the coil above that range, then the signal is normally that coming from the main coil field. If this is the case, then it is highly unlikely a coin, ring or other non ferrous object will give you a double blip signal.

Now, I am not sure about your coins, but gold, silver, copper, lead, and most other non ferrous objects will give you a single response even on a long thin object, while a ferrous item like a nail will give you a double blip type signal. A good test for this is to use a brass or copper screw that is a little long, or maybe a piece of brass brazing rod, and a similar size piece of ferrous material such as a longer screw or nail. When using a mono coil, you should only get a single signal from the brass but can get a double blip or wavy signal from the ferrous item is it is oriented correctly.

This is something people should try when they have the right objects to test with.

BTW, it is good to hear you found your first gold ring. Hopefully, more will come soon.

Reg

[AlexP]

Hi Reg,

Thanks for the tips. I got overburden with work and don't have much time to go out metal detecting lately.

Am I right in thinking that with a mono coil it will be harder to distinguish the ferrous from the non-ferrous objects than with the standard concentric coil? Will it with a mono coil be almost like there is no discrimination at all?

Alex