PI Info.

[Anonymous]

I collected the following online PI info that I thought might be helpful. I believe the first article was written by Ralph, and the rest were posted by Eric.
GROUND NOISES, GROUND BALANCING and HALO EFFECT
HOW DOES A METAL DETECTOR WORK?
To understand how a metal detector works, we first need to understand a bit about
magnetism and electricity. If we pass an electrical current through a wire, a magnetic
field is formed around the wire. Conversely, if a magnet is passed over a piece of wire,
it induces an electrical current into the wire. This is called an eddy current. In a
straight piece of wire, the induced magnetic field is very short-lived as the eddy current
has nowhere to go, dies out quickly, and consequently the magnetic field created is quite
weak. If the same eddy current is magnetically induced into a piece of wire with both ends
electrically joined (like a ring), these eddy currents effectively run round and round,
creating a stronger, more concentrated magnetic field which lasts longer.
The transmitter current of a metal detector is applied to the coil (of wire) and creates
a large, concentrated magnetic field around the coil. This magnetic field will induce eddy
currents into any metal targets in the ground and they will in turn create their own
magnetic field. This magnetic field around the target then induces a current back into the
detector coil. This is processed in the receiver and results in a sound from the detector.
(Or in the case of a detector with a threshold, a change in that threshold.)
Consequently, one of the

 

[Anonymous]

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[Anonymous]

Thank's for your super informative post Jeff. Would you shed some technical light on SD technology Vs GP Extreame technology? With its much higher capacity battery, how much more peak power output are the SD and Gps putting into the ground than say, the Infinium with its meager eight AA cells in series.Thank you in advance for any light that you may be able to shed on these subjects!
Regards Bob in New Mexico

 

[Anonymous]

Hi Bob,
The size and number of batteries is not necessarily an indication of the performance of a PI detector. The Goldquest SS also uses 8 AA batteries and draws only 85mA, yet its performance is up with the best. In fact on small gold, reports indicate that it cannot be bettered.
I shall be getting an Infinium soon, so I will then be able to report on its performance together with some general technology details.
Eric.

 

[Anonymous]

Good to hear from you Eric! I am usually up on Doc's forum, but there sure is some terrific technical Info. here.I am very anxious to learn of your findings, and read your thoughts on these matters Eric.My retired background included high power RF and pulse Mod. I was trying to understand the depth performance of modern P.I gold Prosp.detectors based on power and pulse width,but there is evidently alot more subtle stuff going on here!
Regards Bob in N.M

 

[Anonymous]

Glad to see your going to give the Infinium a try. One bit of news is that Garrett are supposed to be about to issue some new instructions for the machines use. It will be interesting to see what they come up with !
Brian

 

[Anonymous]

Hi Eric,
just a question about the findings on depth with the gold rings, just recently i was repairing and testing an sd2200 for a prospector here in Australia and was testing the unit after repairing it in a similar manner to the air tests in your report.
the test objects were gold nuggets ranging from 0.2g to 5g and a fine 9ct gold ring of about 1.5g which was broken at one point and the thing that i noticed in testing the ring was that with the ring joined together at the break, the detector was able to detect it at a reasonable depth (although less than in your tests due to the type of ring and detector used) which would concur with your findings but with the metal separated at the break slightly, hardly any signal could be heard even 2" from the ring and noticing the results that you achieved on the ring in your test i was just wondering if there is something more than just mass or surface area to consider with rings, as they seem to possess the properties of enhanced electromagnetic induction due to their shape which is a magnetic circuit much like the shading pole in a shaded pole motor etc especially when the ring is presented flat to the bottom face of a coil.
have you found this also Eric i would be interested to know.
Regards,tj

 

[Anonymous]

Hi,
If you go back and read the original post in this thread from Jeff K, it explains this very question.
Fred

 

[Anonymous]

Hi Fred,
thanks, for some reason i didn't read that part of the post but now that ties up that loose end for me.
Regards,tj

 

[Anonymous]

Hi,
I had never heard or read about that before so out of all that information, that one stuck with me the most.
Good luck,
FJ

 

[Anonymous]

Hi Bob,
About a year ago I did some testing using an SD 2100 to see what was being done in regards to the pulse signal out of the coil. The test was done using a coil of wire, a loading resistor across the coil of wire and a scope to observe the actual pulse signal. With this technique one can basically tell what the pulse duration is and also the pulse intensity. I have not done a similar test on the Extreme so I do not know what it looks like. However, I do not expect to see any dramatic differences.
Unfortunately, the pics I took of the pulses are lost somewhere on my computer. In a nutshell, what I saw was similar to what ML describes in their patent, a pulse of a longer duration followed by a series of shorter pulses. The sequence repeats continuously
Now, I have also done a whole lot of experimenting with a PI including building coils, altering delays, building ground balance circuitry, etc.
Based upon the coil signals I observed and the experimenting I have done, I can guess what ML is doing and why. One has to remember that this is a guess.
It is my guess that the two pulse lengths are there for a couple of reasons. First, the short pulses are there to detect the smaller gold and the longer pulses for the larger deeper gold. Both are needed if one is to have ground balancing incorporated in the design.
When mono coil is used, ground balancing is done by taking a later sample and subtracting it from the primary sample. When a short pulse is used, the subtraction technique to eliminate the ground response also causes larger gold to produce little response or even a negative response.
Now, when a longer pulse is used, the ground signal subtracts at a different setting which allows larger gold to be detected with a strong signal. However, with the longer pulse, the smaller gold generates little or no response.
So, when you combine the two pulse durations, the whole range of different sized nuggets can be detected while eliminating the ground signals.
The upside of this design is the ground response can be eliminated or at least greatly reduced, depending upon the ground conditions. The downside of this design is, it is much more suseptable to noise interference, both external and internal.
As such, it is quite common the hear noise complaints from SD users in areas where another PI, such as my pseudo Goldquest, is quiet as a church mouse.
Now, as to battery size and power consumed, I have found my Goldquest clone to be as sensitive, or in some cases, more sensitve to smaller gold. this is due to both the shorter delay and the reduce noise level.
One also has to take into account that the subtraction process used in ground balancing also reduces the gold signal strength on larger nuggets having a time constant greater than the time when the ground subtract signal is taken. In other words, when you subtract the signal from the ground, some nuggets will also produce a weaker signal due to the subtraction process.
I do not know how ML alters this ground reduction process to minimize noise either. In other words, they may shift the time when the subtract signal is taken. If such is the case, then it is quite possible that two similar SD's might produce different signal strengths from a particular nugget buried at a particular location. That is one of the reasons, I do not get excited when I hear of a particular incident where one detector appeared to be much better than another.
Well, I have taken up too much space already. Hope this helps.
Reg

 

[Anonymous]

Reg... I once read an article that said the shorter pulses were used to eliminate ground noise. As I recall it had to do with the decay time of the surrounding ground vs. the gold decay time. I sent Eric a link to that article several years ago, so he may have saved it.

 

[Anonymous]

Hi Jeff,
Very interesting and informative information in your post.
I would like to mention something about the ground signals as they pertain to metal detectors. First, the ground minerals having magnet properties do contribute significantly to the "ground" response but I am not so sure that there are not other factors or conditions such as clay that can also contribute significantly to the ground signal.
There has been a lot written about the conductivity of clay and how it affects a detector. However, most of this is written in articles regarding geophysical research. In this case, it is the conductive properties of the clay that can have an influence.
Over my 35 or so years of using and experimenting with metal detectors, I have tried to confirm some of the things written such as the halo affect and never really been able to prove it exists, at least to the affect that it will increase depth.
I have a couple of key test coins I buried about 12 to 13 years ago and so far I see little difference in the level of detection. I do see a greater influence due to the moisture content in the ground however.
Instead, I have found strange quirks that can happen to lead a person to believe in the "halo" affect.
Holes, however, do stranges things to a target signal. When one digs a hole and places a target at the bottom of the hole, a VLF will normally detect the object better than when the object is covered. However, I have found the opposite to be true with a PI.
Let me give you an example. My last trip to AZ, I wanted to see just how heavy magnetic black sand influenced depth of detection so I dug a hole in the bottom of a drywash ladened with the black stuff. With the hole open, I couldn't detect a nickel sitting at the bottom of a hole about a foot deep. However, when I covered the nickel by filling the hole, I could easily detect the same coin with a nice strong signal.
I have conducted similar tests using holes that already existed and obtained similar results. However, I also found ground conditions do seem to have an influence on the test results. Also, the key to observing this strange condition is to make sure the object is sitting near the depth of maximum detection.
In all tests, I used a smaller lower conductive object such as a lead bullet, a gold nugget, a nickel, etc. The reason being such lower conductive targets are harder to detect thus should be more likely to show such variations.
I like Eric's posting of typical depths of detection of gold nuggets. One should also take into account the other factors that Eric has mentioned in the past, including the actual composition, size, shape, etc when trying to determine just how deep one might be able to detect a certain size nugget.
Example, I have a 5 grain and a 6 grain nugget I normally use for testing. I can detect the 5 grain better than the 6 grain most of the time. Right now, I am not sure if this is because of the composition, shape or what.
When I experiment with my ground balance on my PI, I find that when I ground balance the detector with the detector set at minimum delay, a very solid 1/4 oz nugget disappears right along with the ground signal, yet I can readily detect one of my 3/4 oz nuggets. A different 3/4 oz nugget disappears just like the 1/4 oz one does.
If I increase the delay, which also increases the pulse width, then I can readily detect all the larger gold nuggets.
I guess what I am trying to say with this long winded post is, I have found metal detectors generally follow the theories involved. However, there are a lot of factors that can readily influence the results of various tests, and many of those factors have not been documented in detail. So, when a particular test doesn't follow suit, it can lead to frustration and or confusion. Also, normally the first thing that comes to mind is how do I fix the "problem" when in fact, no problem exists.
Reg

 

[Anonymous]

Hi Jeff,
When I built my first PI kit, I found that magnetite hotrocks would produce a very strong response. This particular PI kit did not have any form of earth magnetic field subtract signal, and had a pulse length of about 250 usec.
When I reduced the pulse duration on this particular detector to about 80 usec or so, the "hotrock" signal all but disappeared. Since this detector would respond to a magnetite hotrock, it would also respond to black sand in a similar fashion. So, with this detector, yes, reducing the pulse length would reduce the ground response that was due to the black sand.
On PI detectors with a design like Eric uses, the second sample used to eliminate the earth field effects seems to eliminate the magnetite signal even when using a longer pulse time.
Reg

 

[Anonymous]

Hi Reg,
Just an additional point that may be relevant to your experience. I have found that a large proportion of hot rocks are magnetised. If you pass one over the coil of a PI detector without the earth's field cancellation pulse, it will give a response due to its remanent magnetisation inducing a voltage in the coil. It will do this even if you switch the transmitter off.
With a second sample pulse to give earth's field cancellation, it will also cancel the signal from magnetised objects, as you get a similar amplitude on each of the sample pulses.
Eric.

 

[Anonymous]

Hi Eric, long time no post (for me anyway)! (I have been flat out working on the usual stuff. <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT="">
I saw this post re the Infinium tests you proposed to do and I'd just like to throw 2 cents of my experience with it for what it's worth. :-D
I did 3 months field testing with the Infinium last year.
imo The Detector is nothing like what us Aussies are "used to" re the Minelab gear in how it operates on very fine gold at present, however it has benefits and features which are tantalizingly useful given the ground conditions and areas that inhabit gold here ....even in this Detectors early stages.
Some points I observed-
Out of the box it wouldn't compete with an Xtreme or most SD's for tiny gold. (.1 grams etc)
In extremely hot ground I found it would eat just about anything on the market I've tried at present for gold over 3-4 grams, as there is next to no ground noise or hotrocks when used in adverse ground conditions.
I only came across 2 slight hotrocks in all the areas I tested over 3 months and they cancelled out on the second swing back. (Kalgoorlie- Laverton West Aussie)
The supplied 14inch DD coil would appear to be a tad limited in how fast you can swing it (compared to Minelab gear) when targeting very small nuggets, however it compares to or outperforms nicely with an SD2200/14"DD in extremely hot ground in the tests I did.
This is most likely due to the lower pulse freqs used.
A big plus is the Infiniums discriminator, the reverse discrimination system they have really does work well.. I found a 21.6 grammer in a trashy test area near a minesite with no probs in amongst the tin etc. (There's no way I would take my 2200/2000mod in there for all the trash calls)
All round I think the Infinium has great potential for future evolvement and in targeting rubbish areas that are beyond the patience of most professionals and also in locating gold targets that would be mistaken for hot rocks or hot ground in most of our mineralised ground.
Considering that the Infinium is competing against a "main player" in Aussie that has had nearly 8 years R&D done on it.. I recon as it stands or with a few more peripheral features like a range of coils and power supply options etc, the unit fills a potentially profitable gold finding gap in the market.
I've already found good gold with it and no doubt some further R&D will improve on that too. <IMG SRC="/forums/images/wink.gif" BORDER=0 ALT="">
I'm keen to hear of your experiences with it.
Cheers
-gaz