Some Information on Eddy Currents

[MattR]

[quote Cody]MattR, ....MattR I looked at several equations and "laws of physics" as to how far an electromagnetic field expands from a wire and you are correct.

I was wondering if there is any way that the halo and wet/damp soil increases the distance compared to free air.
Anyhow I sold my EX2 while taking care of mom during her terminal illness so will get an SE. I have started to order one for several days now and want to include the 8" coil for trashy sites. We are just getting into the good coin/relic season so I need to get with it.[/quote]

Hello again Cody.

Its Saturday 23 Sept., and a lovely sunny day here in the UK.

I did a search on your posts and was reading through them when I noticed a section of one which was intended for me.
Sorry I failed to read it sooner, but now i'd like to briefly reply to your query.

I believe that the enhancement obtained by detecting targets at depths that exceed their 'in air' range, is as follows.

Often you and I have conversed about 'the skin effect', whereby as the frequency of your search-flux is increased, the tendency of the current induced in the target, is to be forced to flow mainly in the surface layers of the object. If the boundary of the target is air then that current is confined to the obviously higher conductive target material. Air being a 'relatively' infinitely LOW conductivity.

When the target is in damp soil or indeed salt water, then there exists a relatively highly conductive boundary medium (compared to air). So the current induced in the target, being forced by the skin effect to expand its existence, does so to some limited degree and 'electrically' enlarges the target's size.

Your detector 'sees' this 'enlargement' of the basic target, and therefore is able to sense it at a deeper range.

As also stated in other posts, dampness and acidic conditions, cause a 'halo' of metallic 'salts' to be created around targets, through local galvanic action. I suspect ground minerals or other local metals near the target, behave as 'the other electrode' in this process.

Hope my thoughts on the subject are of interest.....MattR.UK.

 

[Cody]

Thanks MattR, great post and very helpful.

 

[Cody]

I am back and raring to talk about the new SE and other Explorer. I have an SE that should be here Monday so will start to test some of the features. I got side tracked over on the Minelab Forum and like a tech type had to follow the thread to the end but should have been over here. Hope all has been well for each of you and look forward to the exchanges.

I really like the Explorer series and some other detectors but do enjoy using the Explorer. I have long wanted VCO pinpointing on the Explorer. We have that and some other nice modest improvements which I really look forward to playing around with. By the way I don't think VCO pinpointing is a reason to get rid of your X-1 probe if you have one. I use a probe to find the targets in soil quicker no matter if I am using a machine with or without VCO pinpointing.

You will notice I talk about being effective and as the topics and threads progress it will become clear why. I am thinking we can have general threads, technical threads, and advanced technical threads so users can follow the ones that they are interested in.

Have good Sunday and glad to be back full time.



This is the time of year when we start getting enough raid to soften up the ground so this is the time to get ready.

 

[SnowCampRick]

Welcome back !!!

What do you make of this?

I was at a old house in town
I had read it was better to noise cancel
at max sen.
so i placed the coil on the ground and noise canceled at sen 32.
then I lowered the sen to 16 and tried to detect but the
falsing was very great. So I noise canceled again at sen 16
and all was quiet.

I did not get the result I expected Why?

 

[Captain Kirk]

MattR,

With your technical background I am sure that you understand the principle of "skin effect" very well. I would like to add some additional thougths to go along with your comments.

YOU STATED
as the frequency of your search-flux is increased, the tendency of the current induced in the target, is to be forced to flow mainly in the surface layers of the object.

MY THOUGHTS
When a conductor is subjected to a time changing magnetic field, then there is an induced eddy current. The induced eddy current flows in concentric rings around the axis of the magnetic field. The induced current generates an opposing magnetic field in such a manner that the resulting net eddy current is forced away from the axis of the magnetic field to the outer surface of the conductor.

If the conductor is a long cylinder (e.g. a wire) parallel to the axis of magnetic field, then the current does indeed get forced towards the outer ring or surface of the conductor.

If the conductor is a short cylinder (e.g. a coin as shown in your diagram) perpendictular to the magnetic field, then current is forced in the outer ring of the conductor with essentially no current flowing in the top and bottom surface area of the target. It appears as though your diagram shows significant current flowing in top and bottom surfaces of the targer.

At any rate, I completely agree with your feeling that a conductive target in a moderately conductive media will tend to make the target appear larger. This is especially true in the wet sand of a salt water beach.

Thanks for you thoughts on the subject.
Glenn

 

[Cody]

Sorry to be so late on this but almost forgot after reading your post as so much is going on. Always noise cancel after changing the sensitivity. I have read all kinds of posts of how to do this or that with the Explorer and have found that the best information is right in the Owners Guide.

 

[MattR]

YOU STATED (MattR.UK.)
as the frequency of your search-flux is increased, the tendency of the current induced in the target, is to be forced to flow mainly in the surface layers of the object.

MY THOUGHTS (Captain KIRK)
When a conductor is subjected to a time changing magnetic field, then there is an induced eddy current. The induced eddy current flows in concentric rings around the axis of the magnetic field. The induced current generates an opposing magnetic field in such a manner that the resulting net eddy current is forced away from the axis of the magnetic field to the outer surface of the conductor.

If the conductor is a long cylinder (e.g. a wire) parallel to the axis of magnetic field, then the current does indeed get forced towards the outer ring or surface of the conductor.

If the conductor is a short cylinder (e.g. a coin as shown in your diagram) perpendicular to the magnetic field, then current is forced in the outer ring of the conductor with essentially no current flowing in the top and bottom surface area of the target. It appears as though your diagram shows significant current flowing in top and bottom surfaces of the target.

At any rate, I completely agree with your feeling that a conductive target in a moderately conductive media will tend to make the target appear larger. This is especially true in the wet sand of a salt water beach.

Thanks for you thoughts on the subject.
Glenn

******************************************************

Dear Glen. Thanks for your comments and thoughts on the subject of

 

[Steve(MS)]

with your picture. I have to admit that I am not as well versed in the technical aspect as others but it agrees with the what I have seen over on the Geotech forum as how a coin reacts as a signal from the generated signal from the coil to a coin. I also agree with in general with you as to the coil size limits the depth on smaller objects,IE moderate sized coins and other objects the same size. The variable is the soil and its composition as to how deep an object will be detected. It seems to me (mineralization not included)that the more compact the soil is like a dense clay type, less depth will be realized but if there is a mixture of sand to the soil then the depth of detection will increase. For instance, in sand that has a low mineral content then greater depth of detection can be had. Then there is the wild card of how the soil reacts on an object to cause corrosion, maybe less detection depth, or the halo effect for greater detection depth. I know some do not believe in the halo effect on semi and precious metals but there has to be some permutation in the immediate surrounding soil.

 

[Captain Kirk]

Matt,

What a great presentation. Your more comprehensive diagram quickly opened my eyes without even having to read the text. I was previously thinking only about the "case B" field distribution.

I would like to better understand the significance of the "case A" field distribution and its effects on the performance of the detector. Perhaps this explains why a rusty nail sounds like a silver coin when either side of the outer rim of the Explorer is passed over the nail. I have also noted the following phenomenon for a deep nail. In this case the nail clearly show up as a nail when the center strip of the coil is directly over the nail ("case B field applies). But, when the coil is moved progressively off of dead center (and the "case A" field comes more into play), then the ferrous component of the target seems to diminish and the target looks more and more like a silver coin. My basic thought on this behavior is that a horizontal nail presents a much bigger cross sectional area (reducing the permeability of the field matrix) to the "case B" field than to the "case A" field.

Thanks for you help,
Glenn

 

[Cody]

MattR, can you draw the same but replace the rectangular A and B with a circle, coin A and B.

Great explanations and very nice.

 

[Steve(MS)]

Check this out on the link and click on Induction Basics, I think it is a interesting read. geotech.thunting.com/cgi-bin/pages/common/index.pl?page=metdet&file=info.dat

The part that is missing is thunting.---
Hmmm, I guess no links are allowed.

 

[Captain Kirk]

Steve.

That is a pretty good article. It also gives the basics of why the eddy currents tend to flow in the outer rim of the coin. Note that if the target were a perfect conductor, then essentially all the current would tend flow in the outer ring and essentially no changing magnetic field can be present in the interior region of the coin (a bit over over-simplification here).

The net result is that most of the imposed magnetic field lines will be distorted around the coin instead of through it. Just the opposite effect occurs in a ferrous target with low conductivity (so that only low magnitude eddy currents are produced), since the magnetic field tend to be "sucked" through the target.

HH,
Glenn

 

[Cody]

Consider the illustration and I apologize for the poor drawing. If our discussion continues I will get a program to make more acceptable drawings. Induced eddy current and skin effect interest me in that there is skin effect when a voltage causes current to be forced outward from the axis of the conductor. Eddy current that flow on the surface of a coin flow due to an external electromagnetic force. I am simply considering the possibility that those eddy currents are on the surface of the coin as shown in the illustration. There may be more eddy current swirls around the rim of a coin than in the center but I suspect there are eddy current all over the surface of a coin.

I have to take a trip as we own a home in another town and I am the administrator of my mother

 

[Captain Kirk]

Cody,

You stated:

Eddy current that flow on the surface of a coin flow due to an external electromagnetic force. I am simply considering the possibility that those eddy currents are on the surface of the coin as shown in the illustration. There may be more eddy current swirls around the rim of a coin than in the center but I suspect there are eddy current all over the surface of a coin.

If you go to the link that Steve(MS) suggested you will see a very good PDF file on the subject of "induction". This shows that all the eddy current loops are flowing in the same direction at any given time. This tends to force most of the eddy current to the outer rim, especially as the excitation frequency increases and the conductivity increases.

As MattR described in his response to my comments, in the "case B" situation, there is essentially no current flowing on the horizontal surfaces of the coin.

HH, and I hope your trip is successful.
Glenn

 

[BarnacleBill]

It's all about electron migration towards the the outer surface as frequency increases. At audio frequencies the only individuals that purport this are snake oil audio interconnect salesman and the audio fringe lunatics that consider themselves "audiophiles". No greater truism for that crowd, "a fool and his money are soon departed".

In practical terms I believe that if you took a Silver ring(jewelry) of equal diameter and mass to a Silver US Quarter, that detection depth would be similar. It would also be interesting to orient both targets vertically, on edge, and note the depth response. Anecdotally I believe they will be similar, therefore negating the theory advanced here that the "flat surface" of the coin is where the action is, coupling or otherwise.

I believe that the rim of the coin is where the action is, and a ring is a coin without it's innards. Much of this could be explored by using a, well somewhat high frequency(2KHz-100KHz) clamp-on ammeter over the transmitter coil feed wire to discover coupling efficiency of a coin versus ring. Do I have the time? No, but it would be an interesting exercise for someone retired with time on their hands.

HH
BarnacleBill

 

[Jackpine Savage]

I recently bought a new multi function printer and they wanted half the cost of the printer for a USB cable. Yeah Right. Same with audio interconects what a ripoff.

As for the "skin effect" I don't know squat about the technical aspects, but you are right on about coins vs rings. A quick air test will show that even a 14K gold ring with a diameter close to that of a quarter will air test the same distance as long as the ring is of decent sized cross sectional dimensions ie; nice an chunky.

Tom

 

[Steve(MS)]

Actually, according to the link that is provided above, the author states that a ring of similar conductance and size has a little stronger signal than a similar sized coin. I haven't verified the accuracy of the statement but it is interesting non the less. before I get in over my head on this one, I welcome others more knowlegeable put in their opinions for us to learn. I do know that the 100 KHZ unit I have gets a stronger signal on a nickel than it does on a dime, haven't figured that one out yet. Is that what you mean by that it only becomes a factor @ 100KHZ?

 

[BarnacleBill]

a conductor of electricity. It is a well understood and researched phenomenon. The effect really only begins to be troublesome at over 100KHz. So my point is, that at say 10KHz, a middle of the road metal detector frequency, the current is not travelling along the outer skin(flat portion) of the coin, it is also travelling along the very inner core.

Consider a clad quarter, the point the other posters seem to be pointing towards is that the current being induced by the detector is only travelling along the Silver outer portions. Whereas I am proposing that at the ~10KHz it is travelling through the Copper center also, so that "Skin Effect" has no part in the way the coin is being detected.

I think the rim is where it's at, that's my story and I'm stickin' with it unless someone can show me empirically I'm wrong. And then....I "might" begrudgingly concede.

[attachment 36466 SkinEff.gif]

HH
BarnacleBill

 

[Steve(MS)]

Have you had a chance to read the article that I alluded to,it's on the geotech.hunting.---. Click on metal detectors, then info, then induction basics. I think it is a interesting read by Carl M.
Take care,HH,
Steve(MS)

 

[Jackpine Savage]

When you refer to skin effect you have to take into consideration the magnetic permeability (conductance of the target and the frequency, this adds another dimension to skin effect and its called "skin depth". So are all the eddy currents (little circles) always on the surface of the target, NO and the detectors don't seem to care thanks to some very good engineering and math used by the engineers. There I rode the fence on that one and my apologies to you tech heads. LOL

Tom