Big Coil Problems on a PI


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Grubstake,

Any chance of posting a snapshot of the "pull-tab mod"? Sounds pretty amazing to me. :o

Reg,

Could something similar potentially be done with the monocoils? :o

Regards,

Rex

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Hi Rex,

Mono coils do not lend themselves to adjusting as easily as DD coil. On a mono coil, no matter where you place a piece of metal, the same results will happen. Now, if by chance a coil isn't in spec then it depends just which way it is out as to whether a piece of metal will help. So, a small piece of metal can be used to "tune" a coil if the the adjustment needed is of the correct type signal.

BTW, normally, the metal used to make the adjustment is brass, lead, or possibly aluminum. Iron products will generally cause too much of a change, plus they will or can distort the coil field.

So, generally speaking, the same trick doesn't generally work well on a mono.

As I mentioned before, the reason it works well on a DD type is because you can get different results by just changing the position of the tab. Place the tab inside one of the two windings but not in the overlap zone and you get one response. Place the same tab in the overlap zone and you will get a different response.

Now, Grubstake indicated he placed his first tab on the left side. If his Coiltek coil is wound the same as the one I took apart, then the left side is the transmit side. Remember, on a ML where you can use a DD coil as a mono, you can tell easily which is the transmit side by simply switching to mono and see which side detects the metal.

Once again, it doesn't hurt to try various experiments to see just what will happen. As long as everything is done on the outside, the worse that will happen is the coil will lose sensitivity. The best is the coil will become hypersenstive and will detect gold better, especially the small stuff.

Reg

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Hi Eldorado,

Here is a link to the info posted by Eric on the Geotech forum. Jeez, I didn't realize it was that long ago. It was posted in June of last year and has the heading low noise front end for PI.

http://thunting.com/geotech/forums/showthr...highlight=front

Now, as for how you should set your PI up, low power or high power, etc, what are you planning on using it for?

My initial suggestion is to set the unit up for low power for several reasons. You can normally get very decent depth at low power. The ground response is much less so it is easier to use DD coils or ground balance for that matter. Battery fluctuation isn't as severe. It is easier to get the sampling delay down to lower values. This makes up for much of the power loss. Smaller batteries can be used so a lighter detector can be built. As an example, one of my units weighs in at 3 lbs and that is with batteries. The beauty is, this little unit is extremely sensitive.

Simply stated, it is easier to get things to work well with lower power. Now, pulse length is another story. If you are planning on looking for gold, don't be afraid of using a short pulse time. I have used and still do use a pulse width of about 20 to 25 usec and still get very decent depth.

Now, once a person is satisifed with the design, then they can increase the power and try to compensate for the changes.

Reg

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Hi Reg,

interesting thread that one at Geotech.

My detector is to be used to find gold nuggets. personally I prefer to dig many shallow holes but it seems that many people want "the deep one" ( specially novices) The first question is always "how deep does it go"

Since I dont go much out to the goldfields myself, i have to build a detector for the young ones who have the stamina to go and dig.

I agree, it is easier to build low power. Question is what do other people want???? Is there maybe a forum where we could put the question and get sensible answers?

ELDORADO.VE

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HI Eldorado,

Your question about what people want can be asked on this forum as well as others. I don't think you will find a truly objective forum anywhere, so this one should work.

Ok, lets talk depth and nuggets in general. Now, once you get a basic unit built, I think you will find that most nuggets found in the US are smaller than 1/4 oz, so the target should be to build a unit that is light and does extremely well on the most common size.

Now, remember that the ability to detect smaller gold does not mean it doesn't do well on the larger sizes. By simply reducing the sampling time on a low powered unit you signficantly increase the signal available which compensates for much of the loss you might encounter when using brute force. Also, noise is less so there is an increase there also. So, all gold signals are increased.

One more little tidbit, I think you will find that a short pulse on time, maybe less than 50 usec will do just as well as a longer pulse in most cases. It is only on the extremely large gold that the longer pulse might be of value and here I am talking about in the multi ounce range. So, keeping the pulse on time and current low, you reduce the need for very large batteries. After all, even if you use a ML coil, and pulse it with maximum current possible, you will only have a pulse current of about 1/2 amp at 50 usec simply because of the time constants involved. Ironically, the 50 usec is the basic time frame used by ML for their small gold channel.

Now, your design with the resistors will still allow more current than the time constant will allow, so you will not see any improvement to speak of by removing them, if you use a short pulse on time.

Lower power allows for an easier reduction of the ground signal which plays a very important part in the detection of gold. In fact, on my low powered unit, I find even with a pulse on time of less than 25 usec, the depth is comparable to that when the pulse is twice the length on the typical nuggets I use for testing. One might be able to run a very controlled test and see a difference, but the general testing doesn't indicate much change.

So, focus on noise reduction, a decent ground elimination system and a good audio system and you should have a winner that will be very competitive with the big boys.

Reg

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Hi Reg,

thanks for the explanation. This gives me enough info to make my decision. I agree that most of the nuggets are small and if somebody hits a spot with large nuggets deep down it is worthwile for him to get another machine for that purpose.

I have looked at the gain of the preamp and it has no more surprises after I found the source of the noise that produced the "sweet spot".

I have to finish the coil now and see where the first sampling falls then.

What do you think of my idea to use a S+H amplifier to average the sample and eliminate most of the noise and then amplify the now DC current?

I can use a sample time of 5us or even shorter for that.

ELDORADO.VE

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Here you Go Rexb., heres pictures of what I did with the pull tabs, just used scotch tape to put them on. When I put the left one on, not as much depth or as stable, but after adding the second one on the rigth, the threshold got very, very smooth and lots ore depth.No kidding. Just play around with it, it blows my mind what it did. So try it and see for yourself. Grubstake

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Hi Eldorado,

The use of sample hold amps is a good idea. The differential integrator is a variation of a sample hold but does decay between samples. This will reduce the gain somewhat.

Just don't forget to sum your main sample with a later sample to eliminate the earth field effect. Otherwise, you will get some strange signals including signals from magnetite.

Reg

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Hi Grubstake,

It appears you have placed the tab very close to being right on the winding.

Now, I think you will find there will be a variation in how much of a difference a tab makes if you move the tab around on the side.

For example, if you were to move the tab about half way to the center, I think you will see the tab doesn't do much, or as much. So, it does make a difference.

As for what is happening, you might want to go back a page and look at the three pics I posted near the end of that page. The first looks like a pan with a handle, the second is more like a cut away side view of a gold pan.

Now, if you were to have the second view as the norm of the coil, and you were to add the tab where you did, you would cause the second pic to shift to look more like pic 1.

Ok, if you had signal that looked like pic 1 and added the tabs, then the signal would shift to look more like pic 3, which is the one with the overshoot.

Ok, just to confuse things even more, if you were to put the tabs along the centerline which is in the overlap zone, the signals would reverse. By this, I mean, if you had pic 1, it would look like pic 2 with the tab in place. If you had pic 3, then you could end up with pic 1.

Remember, pic 1 is the normal pic. So, on a DD coil, it is quite possible to take a coil that is not working or working well and make it work.

Reg

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Hi Reg & Eldorado,

Hi Guy's, I'm new to PI & detecting in general, but very lucky :rolleyes: I was hoping you guy's could answer some of my Questions? I'm using GP 3500, and Coiltec Platypus DD Pro coil, and detecting in creek, underwater - noticed that targets are very muffled, LAMAR, answered that to me. I tried Detac amp it was to noisy, then I've been reading about Manual Tune, played with it and found a sharper response on targets underwater, Settings are DD, Deep, fixed, and run threshold 1ok, Manual Tune from center-counter clockwise 31/2 turns. Now the smallest piece 0.8 dwt, and before the changes my smallest pice was 1.2 dwt. and feel I'm getting more depth? I've read that Manual tune changes Timing, and Deep mode only audio, like amp?

Also, what would be the best way to waterproof a regular coil, since not to many all terain options, and if water got in coil could it hurt my GP35?

Thanks,

Ed

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Hi! REG, I have tried moving the pull tabs, but right where you see them in the pictures, I get the most stable threshold and peak responce to the smaller gold. Makes the coil hyper sens. and has great depth, and stable and smooth on the threshold. Thanks, I'm still playing with this and may find out some more interesting stuff. Grubstake

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Hi Big Ed,

Sorry, but I don't have the specific information you want on the 3500. Hopefully, Rob or one of the other guys will jump in and answer your questions.

I can give you a generic answer to some of your question, as certain things are common to all PI's.

As an example, PI's by nature are very susceptible to external man made noise. Generally, such noise is rhythmic in nature, meaning it repeats. In such a case, if the pulse/sample rate is sort of in sync with it, you will have a lot of noise to contend with. This is because the sampling will occur at the same time as the noise, much of the time. The result is a target signal plus the noise signal.

By changing the pulse rate just a little, the sampling or testing for a target will no longer occur at the same time but will only occur at a much less frequent nature. As such, the target signal will be much more distinct.

So, most manufactuers have a control that will allow the pulse rate to be changed so this problem can be minimized.

The key giveaway to this noise problem is the general threshold. If it is choppy or unstable, the target response will be unstable also and will be choppy and unstable. This can result in a very serious depth loss. The amount of loss is dependent upon the noise level, but could exceed 25% or more. Any depth loss will also result in the loss of sensitivity to smaller gold since the signal is already weak and short. As such, it can easily get lost in the noise signal.

So, it is a good idea to get to know all the controls on your detector and what they can do for you. Don't be afraid to run with the sensitivity at a lower setting. Sometimes, less is more and works better.

Now, as for trying to waterproof a coil, my recommendation is to simply buy the proper coil if possible. You might be able to get a regular coil waterproofed but then you may not and water may leak in in small amounts. If that happens, the water could damage the shielding and render the coil useless. In may cases, a simple piece of coated paper is used for the shielding.

As for trying, you need to try to seal any places where the plastic may have an opening. This would include the area around the strain relief for the wire as well as any place where the housing has been glued. Silicone seal works about the best, but I would recommend you use the type II kind. This is the silicone seal that doesn't have the strong odor. The original silicone seal has an acid in its composition that can eat copper and other metals and cause serious problems. This is why it can destroy oxygen sensors on autos.

As for water in a coil hurting your detector, only ML can say for sure. My initial guess is it will not if the water is fresh water. Saltwater is another matter. The exception would be in a stream that is heavily polluted. In that case, there may be enough pollution that the water is highly conductive.

My guess is the 3500's have regulation built in and if that is the case, then the current will be regulated to a maximum amount. This could protect the unit, but I wouldn't bet on it as a certainty. Instead, I would operate the unit under the assumption that any external problem such as water leaking into a coil housing could cause damage, just to be safe.

Here is a little tidbit of information not related to your questions. Normally, areas where water is running on a regular basis will have very little or no ground problem signals. Such areas will be extremely quiet on a detector that has no ground balance at all. Most ground noise is associated with clay based grounds that have various levels of iron oxides that are causing the signals. In active stream beds, this clay is generally washed away from the surface or near surface so the offending signals are also gone. Unfortunately, the water does not wash away any external generated noises so they still have to be dealt with.

Reg

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Well, Isn’t science great when it works for you? Reg - do you mean to say, if the tabs are placed on the edges or rims of the DD coil they will even further increase sensitivity?! Maybe I’ve misunderstood, which is easy for me to do. Nice photos Grubstake, thanks once again, maybe I’ll get around to a little experimentation out here. The views for this thread are pretty high, looks like someone’s paying attention.

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Hi Rexb,

Sorry if I have misstated what can or will happen when experimenting with coils. To answer your question, no, just placing tabs along the edge will not always cause an increase in sensitivity.

PI coils are strange to begin with and as many people have learned, are hard to build that will work on a ML. It is easier on some of the other PI's, but that is a different story.

Now, when building a coil, one shoots for a good stable coil that works whether it is hot or cold. This means, the coil isn't erratic and provides decent depth.

Now, what happens when a coil is built is the coil can be perfect and meet the criteria desired. When it does, it works great. However, if that same coil just happens to be a little off, then it is possible that the coil will be hypersenstive, or it could be just plain dead with little sensitivity. It all depends upon just how far off from ideal it is as well as which way it is off.

Now, if you take an good working coil such as a DD and add something like a tab, it can "upset" or shift the coil somewhat. This can cause a coil to get worse and hardly detect anything, or it can shift things slightly into the hypersenstive zone. If it shifts too far, then it will go past the hypersensitive zone and into the dead zone.

So, depending upon just how the coil is made, it can be operating in the dead zone or it can shift a little and be in the hypersenstive zone, or be normal and be in the normal sensitive zone. The difference between the three really isn't much.

If the coil is in the dead zone, then the threshold is very stable but the coil has very little sensitivity. Usually, when in the hypersensitive zone, the coil is "touchy" meaning it may generate false signals more than normal and react more to the ground signals. Now, if by chance the coil is in the dead zone but right on the very edge, then the threshold could be nice and quiet, but when a small target is sensed, the coil shifts to the hypersensitive zone and the target is suddenly heard with a very strong signal. This particular setting would be ideal if it were possible to build into a coil on a regular basis. Unfortunately, it isn't that easy and has to be done on a coil by coil basis and matched perfectly to a particular detector. Also, things can change with temperaturem ir be very sensitive to any vibration, which could cause false signals. So, to minimize such problems, coils are built to operate in the normal sensitive zone where there are few surprises. This way, coils are more tolerant of different detectors and will work on most of them for which it is designed. In other words, if the coil is built to work on a ML, then it will work on most ML's.

Moving a tab or any other piece of metal around on the coil will alter just how much of a change occurs. Some places will cause more of a change than others. What isn't known until it is tried is whether the change is good or bad. One just has to try it and see.

So, to answer your question, no, just placing a tab above the windings will not necessarily cause an increase in sensitivity. It all depends upon the coil involved. On some coils, it may help, on others, it may hurt.

Reg

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