Cold and Hot Rocks (For New Users)

[Anonymous]

I read a post on the other forum where a new user was having problems with a Quatro or at least it was thought to be a problem. The assumed problem was you get a hit but when you go to pinpoint the hit it is not there.
If you run into this problem and you are new to the Quatro or Explorer or any VLF detector there are a couple of causes. One is that it is IRON and you hit the target with the edge of the detection electromagnetic field. <span style="background-color:#ffff00;">It is important to center a hit as iron in particular will become charged as you sweep over the hit several times and then not be detected in discrimination or if the iron mask is set to reject ferrous at that level. There will be a net charge of the iron or one can think of the iron as being magnetised.</span>
Cold rocks will do this also except you constantly get a hit no matter how many times you sweep the target as it does not become charged. However, in pinpoint they cannot be located. A cold rock is just a particular type of minerals in concentration in a rock that will respond this way. A hot rock will be hit when the coil is in motion and also when you go to pinpoint them. I keep a hot rock or two around just to have them for show. The soil is postive, neutral, or negative so if concentrated in a rock is cold or hot neutral.
A VLF responds different to targets in discrimination with motion of the coil than in all metal and no motion for pinpointing. Pinpoint is an all metal mode and can be of value to measure the size of a target, shape, and cold rocks. This is one reason why I do use pinpoint and if the audio is a little better when the coil is in motion then just remove a little soil from over the target and then pinpoint for a good sound. I use an old pair of boots and most of the time can just scrape the top soil away for a better hit. You can also do this to scrape away tiny bits of trash metals that can pop through. This will happen if DEEP is ON as it cannot tell a deep faint target from a faint target on the surface or near the surface. These targets however are very easy to ID by sound as they are more like little pecking sounds.
<STRONG>Always go for targets that are constantly there and can be repeated.</STRONG> Those target that want to crawl away or go away or not good hits if they are centered. If you work around the target sweeping and also in pinpoint you can get a solid indication if the target should be dug most of the time. The best lock on and ID is when the target is centered under the coil. I don't even think of what the target unless it is centered and then take a serious look at the display.
Have a good evening,
HH, Cody

 

[Anonymous]

Cody,
I wonder if you didnt see the post I made on the Quattro as it does have a slow pinpoint and one that many need to realize so they dont miss a good target. There is a slight delay as some will see it while others may not depending on how fast you pinpoint. If you get the signal and move the coil off to the side and push the pinpoint button and come right back to the target you may not get a pinpoint signal. If you get the signal and move the coil off the target and wait about 2 seconds before you hit the pinpoint there is no problem, or if you push the pinpoint right away and then wait about 2 or 3 seconds you will also have no problem.
I was having a problem with mine untill I realized what was happening,then compensate for it. I was hitting the pinpoint too soon and then I knew where the target was to be, but nothing there and if i held the coil on the target I could hear the threshold start to climb.
Never had this problem with the Explorer or the Sovereign, but see this on the Quattro. Like I say once you know it does this it is no problem, but the first few times a person was wondering what was happening.
What you are saying on Hot Rock I do see with any detectors at times as you know right away when you get a signal and in pinpoint there is nothing there, so you know you have a hot rock.
Rick

 

[Anonymous]

That is interesting and good information. I did not realize it had a slow pinpoint recovery. I need to get one in and check it to see how it compares to the operation of the Sovereign and Explorer.
Thanks for the information and good post.
HH, Cody

 

[Anonymous]

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[Anonymous]

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[Anonymous]

Nice info on the quattro as its probably a quirk of that particuliar model. I find pinpointing in Disc. works for me as pinpoint is basically an all metal mode and can be distracted by a non-discriminated target while doing in Disc. eliminates this problem Also changing from a flute tone to the steady low tone of the pnpoint distracts me. I guess its whatever works and luckily our area has little or no mineralized(hot or cold) rocks to contend with.Getting to know what works and pinpointing well is a must and have to agree with all other units except Minelab I use the pinpoint button.

 

[Anonymous]

Cody,
We have discussed this before and I still do not buy into the idea of a "charge" on iron. With an AC excitation signal, I see no way that any target can accumulate either an electrical or magnetic bias that does not get cleared out with the next pass over the target with the coil.
Maybe you could clarify that again for me.
HH,
Glenn

 

[Anonymous]

When eddy currents generate an electromagnetic field then energy is stored in that field, self inductance, and has an instantaneous magnitude. That magnitude depends on the history of the applied electromagnetic field that generates the eddy currents. When the electromagnetic field of the transmitter first induces eddy currents into the iron the stored energy is minimal. As we sweep the target the instantaneous magnitude will increase and at that time maximum energy is stored in the self induced magnetic field. I am not sure where the point is were we have the best time constant to id the metal as iron but is at some % of maximum the iron is rejected. I have not thought about this a lot but if we disagree then I can look at this in greater depth.
I am getting older and may well mix terms in a way that is not as exact as an engineer would prefer. I take liberties also trying to think of how to explain the way different metals respond without getting more technical than necessary and not getting anywhere with the explanation for new users.
It was a nice day here so got out for a couple hours and found three nice mini balls. It was great to fire up the Explorer and here that lovely sound of a good target. In IM-14 with the digital display the iron nails would read in the 28 ferrous and 30 conductivity range so when the tone is high it was a nail every time. When the ferrous dropped down to the teens and the conductivity was in the twenties it would be a nice mini ball. I really like Iron Mask with the Digital display as it really helps to relate the tones to the conductivity and ferrous content. I could not help but wish I had some of the new users out there to show what a great detector the Explorer is and how easy to use when setup to ones taste.
HH, Cody

 

[Anonymous]

Cody,
Eddy currents will decay with an L/R time constant. The L relates to the permeability of the medium around the target and the R relates to the resistivity of the target. I think that it is safe to say that these time constants are less than a mS. Therefore, one can not sweep the coil back over the target in less than a mS. Therefore, any induced eddy currents will decay from one sweep to the next. Also keep in mind that during the less than 1mS period for the coil excitation the eddy currents will reverse direction several times because of the reversing polarity of the coil excitation. The same thing can be said with respect any residual flux induced into a ferrous object.
Therefore, my conclusion is still the same. I see no mechanism for stored energy in the eddy currents and any residual magnetism will be wiped out with each successive pass of the coil. Maybe I am missing something here.
HH,
Glenn

 

[Anonymous]

I don't think it is how fast you sweep the coil but how much flux cuts the target. <span style="background-color:#ffff00;">The frequency of the TX is such that the targe appear to stand still no matter how fast we sweep the coil.</span> <STRONG>It was not my intentions at all to suggest it has anything to do with mechanical sweep speed.</STRONG>
However, I see no problem with the stored energy in the self induced electromagnetic field of the target as being critical to the generation of a TC as that energy attempts to maintain eddy current flow. It appears to me that the electrical properties of the target, self inducatnace, and decay of eddy currents is how the metal is identified.
I took a quick look at this area in the patents and think the explanation are in line with my posts. My explanation are in reference to patent 5,506,506 page 7 and 8. I would post the explanations but you can look it up and see what you think and help me to understand how self inducatance is not included. Anyhow, as an engineer you can help me to understand this and I guess some others may be interested.
HH, Cody

 

[Anonymous]

Cody,
You stated:
<span style="background-color:#ffff00;">I don't think it is how fast you sweep the coil but how much flux cuts the target. The frequency of the TX is such that the targe appear to stand still no matter how fast we sweep the coil.</span>
#1 It is not a matter of how much flux cuts the target. It is a matter of the time rate of change of the flux that causes response. The faster you sweep the coil, then the greater the effect on the targer. But, sweeping too fast may reduce the detectors ability to respond.
You stated:
<span style="background-color:#ffff00;">However, I see no problem with the stored energy in the self induced electromagnetic field of the target as being critical to the generation of a TC as that energy attempts to maintain eddy current flow.</span>
#2 The transmitted signal from the coil will induce eddy currents in a conductive portion of the target. But, the polarity of those eddy currents reverses each time the coil voltage changes polarity.
#2.1 Any eddy current that has been induced will decay to essentially zero in less than about 1mS after the coil is moved away from the target.
#2.2 If the coil passes again over, then there is no residual effect of the previous passing of the coil over the target.
#3 For the ferrous component of the target, there will be a concentration of the magnetic field in the target. The polarity of this magnetic field will also reverse with each polarity change of the transmitting coil.
#3.1 After the coil is moved away from the coil, then any magnetic field in the target will be reduced to the residual flux value for the target. This is probably very small since the coil is moving very slowly relative to the rate of change of the polarity. This will essentially degauss the target.
#3.2 When the coil is next passed over the target, then the behavior will essentially be the same as if there had been no previous passing of the coil over the target.
CONCLUSION:
#4.1 There is no charge or stored energy in the target that will effect the way the detector responds in successive passes.
#4.2 If one is searching in the Semi Auto mode or in the pinpoint mode, then there may be some noticable difference in the way the detector responds to the target. This is not because of the way that the target responds (because of stored energy or any other reason), but rather how the electronics of the detector responds.
You stated:
<span style="background-color:#ffff00;">It appears to me that the electrical properties of the target, self inducatnace, and decay of eddy currents is how the metal is identified. </span>
#5 I agree with that completely. But, those effects die out very quickly (in the mS time range) when the coil completes its pass over the target. Therefore, any induced eddy currents will be long gone before it is physically possible for the hunter to pass the coil over the target again.
You stated:
<span style="background-color:#ffff00;">I took a quick look at this area in the patents and think the explanation are in line with my posts. My explanation are in reference to patent 5,506,506 page 7 and 8. I would post the explanations but you can look it up and see what you think and help me to understand how self inducatance is not included.</span>
#6 I do not have a copy of that patent, but I do not think that I need to review it to answer you question. <STRONG>Self inductance of the target is an important aspect of the behavior of the Explorer II</STRONG>. But, any effects of self inductance are gone before the hunter can sweep the coil over the target in two or more successive passes.
These are my opinions, but I could be "blowing smoke". I hope this helps. If we need more discussion, then email may be the way to continue the discussion.
HH,
Glenn

 

[Anonymous]

That is a good explanation and I am very interested in this as I pretty much just said what is in the patent I referenced. I don't think you are blowing smoke at all. I communicate with several engineers and very much enjoy the learning process. I have absolutely no problem saying I was wrong, did not understand correctly, and then correct my thinking and go from there. I very much appreciate your thoughts experience in these areas.
I don't think that the iron has stored enegry after the flux is removed. Some magnetic domains remained in alignment after the flux is removed even if the flux is AC. I don't believe they all return to a random state. I have looked at this and knowt that we applied AC bias to tape recorders to put the domains in a high state of flux so when we applied a signal it was easier establish a bit on the tape.
What I am not sure of in one area is how elastic the domains are. I have read various opinion on this and some well informed writer clain that a piece of iron can hold the magnetism for years. I frankly doubt that. I do howere mainteain that in the sense of stored charge in the self inductance the metal does become charged and has residual magnetism depening on the electrical and magnetic properties of the metal.
However, we agree on the primary method of identificaion of the metal so I will take some time to look at the areas where we differ.
I don't need to tell you how much I respect and appreciate your opinions. I was hoping to see you post and help new users to get more out of the Explorer. We all gain from the exchanges. I often thought that I don't get much from a discussion where everyone agrees. It is the knowledge gains when we disagree that produces a lot of good benefits.
HH, Cody

 

[Anonymous]

Glenn and All, do you think the flux from the detector can cause the magnetic domains to be in a high state of flux such as biasing the head on a tape recorder. If so would subsequent passes or the wiggle be an advantage as the metals magnetic domains become more easily aligned. (I understand permeability and retention in different types of meatals.)
I know that ferrite has magnetic domain that align with the magnetic lines of force by quickly revert to a random patters. However, I think iron depends on the retention of some of those magnetic domains in alignment. I the metal is induced with a strong enough magnetizing field the domains will stay in alignment such as when we create a permanent magnetic with a DC current produced electromagnetic field. I know the magnetic domain in ferrite respond differently than those in iron, I think.
HH, Cody

 

[Anonymous]

Cody,
Thanks for your kind thoughts.
I would just like to say something about residual magnetism and minor hysteresis loops.
#1 When a ferrous object is placed in a magnetic field, then the magnetic domains tend to line up with the field. As the field intensity is increased, then more magnet domains are aligned until the point of saturtion is reached when essentially all of the magnetic domains are in alignment. A further increase in the magnetic field intensity has little effect on aligning further domains.
#2 When the magnetic field is removed, then the "elasticity" as you referred to it, will cause the domains to become less aligned and the flux density in the ferrous object returns to the residual flux density. Using this technique we can magnetize a screw driver.
#2.1 In order to reduce the magnetic flux density in the ferrous object to zero requires the application of a reverse magnetic excitation. This is referred to as the coercive force.
#3 About minor hysteresis loops.
When a ferrous object is excited with a sinusoidal magnetic field with a constant amplitude, then the magnetic domains will alternate back in forth in an orderly manner around a minor hysteresis loop of the BH curve. This loop will be around the origin of the BH curve. It is a minor hysteresis loop if the ferrous object is not driven into saturation in either direction.
#3.1 If the applied magnetic field is gradually reduced in intensity, then the hysteresis loop will gradually reduce in size around the origin. When the field is finally reduced to zero, then the hysteresis loop also become nearly zero and the residual flux is reduced to zero. That is how degaussing is accomplised on a magnetic tape or other media.
Hope this helps,
Glenn

 

[Anonymous]

Cody,
I answered some of your questions in the post below.
Regarding ferrite and other magnetic materials
#1 Ferrite is a magnetic material designed to have very very low conductivity in order to reduce core losses when operated at higher frequencies.
#2 There is a huge variety of different ferrite materials for different applications. Some ferrites have very low residual flux density while others can be quite large (greater than 85% of the saturation flux density).
#3 There is a family of magnetic materials (e.g. Metglass) that has almost a rectangular hysteresis loop and very very low conductivity. These materials are used in magnetic amplifiers for switch mode power supplies.
HH,
Glenn

 

[Anonymous]

Thanks and as I said I greatly appreciate your comments and great posts.
Patent 5, 506,506,

 

[Anonymous]

Cody,
You said:
<span style="background-color:#ffff00;">This is because the eddy current produced magnetic field has it own associated stored energy (in other words self inductance) <STRONG>which in turn tends to conserve the eddy current</STRONG>.

 

[Anonymous]

- Always go for targets that are constantly there and can be repeated -
Along those lines, I've found that the big hot rocks (I'm thinking fist size and bigger) with a few inches of depth fade to nothing after a couple sweeps in discrimination mode, if you use "auto" rather than "manual" mode. And if you lift the coil a bit it will fade to nothing right away. I think the Explorer sees the big rock as part of the ground then adjusts to it. If you stick it out in "manual" the tone will always be there.

 

[Anonymous]

Yes I agree, and that is not my statement but the design engineer of the Explorer. I posted that and indicated that this was taken directly from the patents becasue you said you did not have the patent.
My implications are that the permability of the iron changes with the history of the applied electromagnetic field from the coil. The magnetic and electrical properties of the iron are variables in the induction and decay of eddy currents. Also, the permability change with time, the the number of lines of force, and the state of the iron after the electromagnetic TX field is removed. I don't think it behaves like ferrite in that the electrons return to a random state but that there is residual magnetism at some level. the residual magnetism then is a factor to consier.
I don't disagree with you but will step out and state what I think if going on to spark a discussion. I don't mine making a mistake if it gets a good discussion going and we learn.
I think we have to consider the amount of flux that penetrates the target, if it is a pluse of energy or sine wave, frequencies, and the magnetic and electrical properties. I believe a primary different between iron and other metals is that iron can be magnetised. That to me is explains the way the electrons spin and the atoms respond to the TX electromagnetic field.
I understad LR circuits and I thought these for years in AC Circuits analysis and also in complex circuits. I am a little lazy about going back and looking at areas I am rusty in but can do so if needed. I also understand the different between first, second, and third order approximations of a cirucit or electronic component. By that I mean that LR being a pure inductance and resistance and such as a diode being analyzed as a short in one direcion and and open in the other instead of taking into consideration the DC resistance of the diode and leadkage currents.
This is really an attempt to explain why I see iron respond in digital form and as far as tones are concerned. It interest me why iron will be detected but then when we center the coil over the iron for good induction it will be rejected. I have dug holes where those sounds "went away" and found iron and not a false signal. I can move the coil around on iron and go from complete acceptance to complete rejection. That is what fasinates me although it is easy to deal with.
Thanks agian,
HH, Cody

 

[Anonymous]

That is one more way to not have to dig them up. I guess we all look for the best ways to find the most targets with less digging. I think it is really good to know how in an iron infested site where you are sure there are goodies mixed in with the iron. I ran into one yesterday where I dug one nail after the other and then could see where the digital reading hit in conjunction with the tones. After about an hour I was sure I was rejecting those nails that are such a problem to new users. I will dig them at a new site and also more than normal after laying off during the winter to warm up and remaind me of the tones and visual indications.
HH, Cody