White's TDI DD coils

[AlexP]

I got my White's TDI a couple of weeks ago and had started to get desperate and was even thinking of selling it because of its lack of usable discrimination (I don't think that I will ever get used to the kind of discrimination it is supposed to have, as explained on some sites - by switching to high conductor and offsetting the GB). Plus, here in Bulgaria most coins are low conductor coins and if I switch to high conductor only, I'll lose the ability to detect them, not to say that when you offset the GB, the depth decreases also. But yesterday I went out to hunt for coins with a buddy who has a ML GPX-4500 and he was laughing at me how useless the TDI discrimination was and said that I should have bought a GPX instead. I was using the standard 12" concentric coil. At some point we decided to try the GPX 11" DD Commander coil on the TDI, and what a difference - with the target conductivity switch set to ALL the detector produced wavy (changing pitch) sound on every ferrous target we tried it with - various iron coins, small and medium sized nails and some others. And the signals from the non-ferrous objects were clear and stable and the pitch was not changing on them. The sensitivity to very small iron objects was reduced also, which should be a good thing for coin hunting. The detector wouldn't detect them. We didn't bury the test targets. Just put them on the ground and tried sweeping the the coil at different heights over them, so I believe it should be the same as if they were buried. I am going to get a DD coil for my TDI now.

Have anyone else tried using a DD coil on the TDI and what results did you get?

[Carl-NC]

You will probably have better luck asking this on the TDI forum:

http://tdi.invisionplus.net

[AlexP]

Yes, I decided to try the TDI forum first and registered yesterday, but I am still not allowed to post there (the administrator has to approve my profile).

[Reg]

Hi Alex,

Remember, you will have to send an email to Cougar Jim on the TDI tech forum before you will be able to post on the TDI tech forum. This was done to eliminate the spam. Here is a link to what he has to say about the issue.

http://tdi.invisionplus.net/?mforum=tdi&showtopic=328

Now, as I mentioned on the Find PI forum, using the DD coil on a TDI does display a form of discrimination on the TDI. A round coil does this a little better than an elliptical one. This disc feature is different from the specific disc features that can be obtained on the TDI by using the GB control. So, there are two methods that can be used on the TDI.

You mentioned the coins in Bulgaria indicate a low conductor. If that is the case, then you might want to select the low conductor mode and run at normal ground balance setting to eliminate most nails and other heavier ferrous junk.

What won't be eliminated will be thin ferrous metal, such as bottle caps, pieces of tin cans or other sheet metal. Fortunately, these objects generally have a much stronger signal and with practice much of this stuff can be determined. Keep in mind, this does take time and patience to learn. The stronger signal can generate a much harsher signal when the object is close to the surface.

Reg

[AlexP]

Hi Reg,

Thanks for the help. After some more testing of the Minelab GPX 11" DD coil on my TDI, I found out that the discrimination is not actually working so good, especially for deep objects. And it really loses some depth. So I won't be getting a DD coil. I will try to get the maximum depth possible off the TDI - with an 18" mono coil maybe. Will that coil also be less sensitive to very small trash objects, like small nails?

I will wait for a better discriminating PI - the next White's PI or the GPX-5000 maybe. Until then, I will carry one of my VLF detectors together with the TDI to check the shalow targets and thus minimise digging trash. That should be the best way to ensure that no good targets are missed.

[Reg]

Hi Alex,

I wouldn't give up on the DD coil or the TDI for that matter. To learn the best way to use both requires time and experience to obtain the best results. Remember, it does better on the shallow stuff in indicating ferrous objects than the mono and still works using the other TDI techniques on the deeper stuff just like the mono.

The inability to determine some ferrous objects are not limited to PI's. Most VLF's struggle to determine some nails, bottle caps, and certain other ferrous junk. So, this is not a new problem.

Reg

[Tinkerer]

Discriminating PI

Hi Alex,

I wouldn't give up on the DD coil or the TDI for that matter. To learn the best way to use both requires time and experience to obtain the best results. Remember, it does better on the shallow stuff in indicating ferrous objects than the mono and still works using the other TDI techniques on the deeper stuff just like the mono.

The inability to determine some ferrous objects are not limited to PI's. Most VLF's struggle to determine some nails, bottle caps, and certain other ferrous junk. So, this is not a new problem.

Reg

Hi Reg,

I have been spending time trying to design an IB-PI that discriminates FE.
The discrimination works quite well, but some FE targets are not well discriminated, just as you describe above.
I do not have a discriminating VLF to compare with, so I would much appreciate if you could describe in some detail how the targets that defy the discrimination on the VLF behave.
Example a bottle cap. With my IB-PI, if presented flat to the coil, it's response is about like that of a quarter (US 25c coin) When presented on edge it responds very clearly as FE. In any angle it responds as FE except when flat, so that twisting the coil at an angle, makes it show as FE.

Many doubtful targets show their "true colors" that way.

In general, the magnetic targets give a very weak response with the IB-PI.

How far is this behavior similar to a VLF?
Does my IB-PI with this quality of discrimination have some validity?

Tinkerer

[AlexP]

Yes, the DD coils do provide an easier way to discriminate shallow iron objects than the standard TDI coil, but I wouldn't rely too much on it as I know that I will miss good targets. I don't mind to carry one of my VLF detectors too. I have one that is very light and has dependable discrimination. And it doesn't interfere too badly with the TDI. Both detectors work reasonably stable when they are 4 or more meters apart.

In general DD coils have less depth than mono coils. Can it be assumed that a 18" DD will have about the same depth as the standard TDI coil? If someone who have DD coils from their Minelab SD/GP/GPX and the TDI would be so kind to test them on the TDI, that will be of great help to me and others.

[AlexP]

I have another question. If I build a 0.5m x 0.5m square shaped frame coil, will the TDI become deaf for very small objects like nails and bolts? And will the detection depth be improved a bit also? I won't care about the discrimination in that case.

[Reg]

Hi Tinkerer,

Both PI's and VLF's seem to have difficulty with thin flat metal trash such as a bottle cap. Also, both seem to have trouble with a nail if positioned just right such that it is in the ground with the head or point upward. At least, this is what I have experienced.

I can't tell you if your discrimination design has validity or not, but I would continue to pursue it until I was satisfied one way or another.

Personally, I feel it is going to be difficult to develop a perfect discriminating detector. As you pointed out, some ferrous items can display signals much like a non ferrous object if positioned correctly.

Also, like you mentioned, I have noticed that turning the coil at different angles to the object can cause some ferrous items generate a better ferrous response. The problem with this is turning a coil perpendicular will cause a depth loss so deeper objects still can be a problem.

Reg

[Reg]

Hi Alex,

I doubt you will ignore much iron with a .5m by .5m coil. Iron junk generates a very strong response in compared to a non ferrous items of the same size, so ignoring iron is more difficult to do.

Here is one thing to think about when building a large coil for the TDI. The TDI, like the GS 5 uses a form of autotune that is used to help keep the threshold constant. This is done similar to the autotune on the Hammerhead for those who want to know how it is done.

Now, on any autotune circuit, a larger coil can actually have the potential to lose depth simply because the coil signal is slow rising as one sweeps the coil. So, if the coil is moved too slowly, it can lack depth.

On the plus side, one can move faster with a larger coil with less fear of missing some targets.

So, what I have found is every coil size has an ideal sweep speed for maximum depth. Now, one also has to keep in mind that internal filtering can alter this feature considerably since it can counteract the autotune feature if the filtering is too slow.

Reg

[Tinkerer]

Hi Alex,

I doubt you will ignore much iron with a .5m by .5m coil. Iron junk generates a very strong response in compared to a non ferrous items of the same size, so ignoring iron is more difficult to do.

Here is one thing to think about when building a large coil for the TDI. The TDI, like the GS 5 uses a form of autotune that is used to help keep the threshold constant. This is done similar to the autotune on the Hammerhead for those who want to know how it is done.

Now, on any autotune circuit, a larger coil can actually have the potential to lose depth simply because the coil signal is slow rising as one sweeps the coil. So, if the coil is moved too slowly, it can lack depth.

On the plus side, one can move faster with a larger coil with less fear of missing some targets.

So, what I have found is every coil size has an ideal sweep speed for maximum depth. Now, one also has to keep in mind that internal filtering can alter this feature considerably since it can counteract the autotune feature if the filtering is too slow.

Reg

Hi Reg,

thanks for the info about the FE discrimination. I am making the TINKERERS V1 "open source" so that "any tinkerer" has a chance to try and experiment with the IB-PI FE discrimination. From your description of the VLF FE discrimination, it seems to be equivalent.
I know the IB-PI can still be improved a lot, this will be done with
TINKERERS V2. Hopefully by that time I will have some feedback from field tests done with the TINKERERS V1.

Another matter that I would like to tap your knowledge and experience on, is the sweep speed.
The DD coil has a much narrower area of high sensitivity than an equal diameter mono coil. The response therefore has to be much faster.

For de-mining, the standard sweep speed is 1m/sec. I find this a fairly fast sweep speed.
In your experienced opinion, what would be an acceptable sweep speed for a 10 -12 inch coil?
For larger coils the sweep speed would be slower and for smaller coils it would be faster, but what would be acceptable limits?

Thanks for your help

Tinkerer

[Reg]

Hi Tinkerer,

It is hard to determine just what would be an ideal sweep speed because there are so many variables involved. It is just not the coil size but all the other filtering that compounds the issue as well as the actual overlap area.

The 1 meter per second might be a little fast for the best results when using a DD, but, again, the other factors have a dramatic effect.

Keep in mind the target response will not be the same as the sweep speed. On a mono coil, the target response is closer to the sweep speed, but on a DD, the target response is much faster since the actual effective detection zone is so much smaller, so all other secondary filtering has a greater or more critical effect. This is why it is generally recommended one operates a DD at a slower sweep rate.

So, your question doesn't really have an ideal answer but does pose some interesting questions once a person thinks about the issue.

Reg

[Tinkerer]

Hi Tinkerer,

It is hard to determine just what would be an ideal sweep speed because there are so many variables involved. It is just not the coil size but all the other filtering that compounds the issue as well as the actual overlap area.

The 1 meter per second might be a little fast for the best results when using a DD, but, again, the other factors have a dramatic effect.

Keep in mind the target response will not be the same as the sweep speed. On a mono coil, the target response is closer to the sweep speed, but on a DD, the target response is much faster since the actual effective detection zone is so much smaller, so all other secondary filtering has a greater or more critical effect. This is why it is generally recommended one operates a DD at a slower sweep rate.

So, your question doesn't really have an ideal answer but does pose some interesting questions once a person thinks about the issue.

Reg

Thanks for taking the time to answer.
Let me reformulate the question:
I know that you have a lot of field experience and a very thorough understanding how metal detectors work.
This is why I ask you how fast a sweep speed to design.
Or, how fast would you like to swing a coil of a certain size if I design the detector to your wishes?
A 8" mono coil?
A 12" mono coil?
A 16" mono coil?
DD of the same sizes?
Let's say how many inches/second?

This should help me getting an idea of how fast the response should be, hopefully.

Tinkerer

[Reg]

Hi Tinkerer,

Your question reminds me of a situation I encountered many years ago. Although it is unrelated to metal detectors, the perception is, so I will relate the incident and why it relates.

Many years ago, several of us from work decided to stop for a drink at a popular dance hall/bar. While there one of the guys and I had a conversation and during that conversation, he would stop to utter something like "WOW, would you look at that". I would turn around and not see anything I thought was spectacular so I would go back to the conversation at hand. After the third time, I questioned the guy, thinking maybe my eyesight was failing or something.

Well, he quickly pointed out what he thought was a real beauty and casually mentioned he liked his women big. Well, what he was oohing and awhing about was someone who was by any standards huge, about twice the size of a normal woman across the back end.

The point to this story is simple, we all have specific preferences and in many cases, they will not agree with someone else. This will hold true for sweep speed also.

Those old timers like myself who owned some of the older discriminating models that required a fast sweep speed have a tendency to sweep faster than someone who began with one of the very slow motion units.

So, there is, in my opinion, no perfect or best sweep speed or technique. To compound this situation, one really has to adapt to the design of the detector they are using for the best results. Since most people including my self do not redesign an entire detector, we adapt and modify what we have. So, we change to meet many of the detector's characteristics.

Now, to be honest, I woudn't want to even try to figure out what would be the ideal sweep speed that would meet the needs of the general public. I have a difficult time even trying to meet my needs since I have a tendency to change tactics depending upon what I happen to be searching for.

So, I am not the one to answer your question with a specific answer. Instead, my recommendation is to design what you feel is correct and then let others try it. I think what you will find is there will be no absolute consensus.

Sorry, I coudn't be more specific, but right now, I really do not have any specific sweep speed I prefer for all occasions. I will say, I don't expect that speed to vary much across different coil sizes, but may vary more across coil designs, such as different rates for a DD than for a mono. Even then, I wouldn't want to try to pin down any particular speed for either one of these because even the environment may have an influence. As an example, one can't swing the same speed easily over a very rocky terrain as they would over a nice level grass field.

So, I am not trying to avoid the question, but rather to point out the difficulty in trying to find an answer.

Reg