MattR and All

[Cody]

MattR, sorry to harp on this but we have seen the lines of force traced by iron filings on paper experiment. We have all seen the PR drawing and others of the flux from the coil which looks so nice and uniform.

I can see a uniform flux in free air but have often wondered what the pattern looks like in the soil. I frankly don't care what it looks like when refined iron enters the field. What I am attempting to do is get an idea of what the pattern looks like as we search due distortion by ground minerals. Given that ground minerals will never been the exact same for any instant in time it must be very difficult to predict how the search pattern varies. I guess it may come down to a simple statement of a dramatic change to very little.

I suspect this to be one reason why an air test, coins in a test garden, are useless and also why we seem to constantly need to find reason why we can hunt the same area and find good targets in a "worked out area" or even explain the notion that one machine is better than another. I have long thought that a major factor in the difference is in ground rejection effectiveness and a consistent penetration of the soil matrix. To the extent that one detector does this better than another and the user is suited to a particular machine then I can account for the different preferences.

I seem to do best with any of the top machines depending on which one I devote time to. My interest in the T2 and F75 leans itself to the idea that they may have better effectiveness when it comes to ground rejection, separation of targets, and not so much in discrimination. My personal opinion for years has basically been to use as little discrimination as we can and keep the others settings "UNDER CONTROL".

I think I am seeing February as a practical release date for the F75 which will make it about right for our weather. We are iced in this weekend and my old bones are not designed for cold weather with the diabetes acting up when it is really cold.

 

[MattR]

Domestic chores...Grand-children....etc.

Been stuck in front of the old computer for hours drawing diagrams.
Scrapping it all and starting again.. etc............Needed a break.....braved the weather...lived a little.

I thank you for being patient...Honestly, I'm working towards a reasonable reply......your's Matt

 

[Cody]

Thanks MattR, I thought you might have missed my post down the page.

 

[MattR]

Hi Cody! A hasty reply in acknowledgement to your post.
A few paragraphs of your thoughts covers a heck of a lot of subject matter, before any thing is discussed further, may I ask why you propose certain things.

'Air tests are useless'.

Sorry to disagree with you. That is incorrect.

How can you say that? (You are not alone in that assumption, for I've heared it said by many.)

[size=medium]Air testing is valid.[/size] What else can it but be?

The mistake people make is basically due to their miss-application of the results.

That in turn is due to a lack of understanding of matters.

There is no genius on this earth who can quantify the 'in ground' performance without he knows the 'in air' performance.

Your quest for comments on how the field is distorted when it passes through media other than/as well as, air is really a demanding task.

I know that you are well aware of that.

The difficulties in totally describing the process is too demanding in the context of a general forum debate.

I propose some simple statements. You chew them over and spit them back and lets see what we can make of things.

The classical flux pattern is distorted by:-

(1) The magnetic minerals in soil.
They deform the original shape by increasing the flux density and shunting the lines of force back to the search-head.
( this then is a screening effect )

[attachment 45819 Resizeofloopflux3.jpg]​

(2) The conductive ground minerals absorb power, creating an alternating current, which in turn generates an opposing flux to that which is causing it. (The ground is the primary target)
So this then also creates a further distortion to the applied field, but time delayed relatively to that original cause. (phase delay)

(3) Any conductive metals behave similarly to conductive ground but in a relatively more focused manner, and so cause additional field distortion which adds its vector component to that of the ground.

(4) The most dominant 'distortion' is caused by the variation in space-distance between search-head and reactive ground.

Your detector tries to reduce the effects of 'ground mineral distortion' by synchronous phase sampling of the signal.

The modulation imposed on the ground signal is attenuated by high pass filtering.

So there are a few thoughts for openers Cody....What do you think?

MattR.UK

 

[Cody]

I should have said air test are useless in that users think they are indicators for comparing one detector against the other in a PR sense to decide which one to buy. When used to compare ground performance then the constant should be free space as you indicate. You have to forgive me as I often speak in absolutes and then have a feast eating my words.

MattR, number 1 is what I think may be the major problem for designers. I have long suspected that most detectors do poorly in this area and when coupled with an attempt to reject trash targets of refined metals really don't do a very good job of it.

Number 2, I assume is your way of stating an application of Len

 

[Captain Kirk]

Matt,

In your diagram you show that there are more field lines in the ferrous soil than in the air. It is my understanding that lines of flux form closed loops. Each flux lines passing through the coil area must be continuous meaning that no new flux lines can originate in the space around the coil.

Consider the following experiment.
* Maintain a constant voltage waveform excitation of the coil. Therefore, the total number of flux lines passing through the coil area always remains constant regardless of the position of the coil
* The coil will always remain parallel to the ground.
* Start with the coil far above the ground. The flux lines will look as shown in your diagram for air.
* We now begin to lower the coil towards a relatively high permeability ground.
* As the coil is lowered towards the ground we begin to see some distortion of the flux lines with some of the flux lines being attracted to the low reluctance path of the ground, but most of the flux lines pass through the air surrounding the bottom coil.
* At some height above the ground the reluctance through the ground becomes much less than the reluctance through the air. The flux lines start to be noticeably distorted and the flux lines tend to get "sucked into the ground". It is not that the number of lines increases, but rather they see an easier path through the ground than through the air.

Does this make any sense?

Glenn

 

[Cody]

Kirk, do you assume a target in the soil sucks flux away from another target in the soil for the same reasons?

 

[Captain Kirk]

Cody,

Ferrous metals have high permeability and offer a low reluctance path to magnetic fields. Whenever there is a magnetic field present the magnetic field seeks out the lowest reluctance path. If there are multiple paths available, then the magnetic field divides between the paths according to the various reluctance paths (similar to current seeking the path of least resistance).

Consider the case in free space where there is a constant density of AC flux lines such that there are only a certain number of flux lines flowing in a given cross section of space.
* If a non-conductive ferrous object is inserted into the field, then the flux is "sucked" into the path of that low reluctance object. But, not all the flux lines get "sucked" in.
* If there is a non-ferrous conductive target inserted into the flux, then the eddy currents arise in the target to generate opposing magnetic fields with the net result being the original flux is forced around the conductor.
* If a object that is both conductive and ferrous is inserted into the field, then some combination of both effects results.
* If multiple targets are inserted into the field, then the flux seeks out the multiple paths according to how the reluctance is distributed and how the eddy currents effect the process. Mapping the flux paths is a very complex process that only nature can figure out very quickly.

Does that make sense?

Glenn

 

[Cody]

Yes, it does to me and you have stated it so nicely that I will print your post for my file. I mentioned that I often wonder why I have not thought these explanations through as well as you and MattR. When I think of the complexity of the soil matrix it is a wonder that our detectors work as well as they do.

 

[MattR]

[quote Captain Kirk]Matt,

In your diagram you show that there are more field lines in the ferrous soil than in the air. It is my understanding that lines of flux form closed loops. Each flux lines passing through the coil area must be continuous meaning that no new flux lines can originate in the space around the coil.

I understand what you are seeing in the simplistic diagram, but its simplicity is allowing you to probably miss interpret the picture.

The extra arrows are not made continuous for ease of drawing, but their continuity is implied (by your own statements, closed loops).

If you introduce a lower reluctance pathway to existing flux, then is not the result an increase in the total flux?....Matt

Consider the following experiment.

* Maintain a constant voltage waveform excitation of the coil.

But that doesn't necessarily dictate a constant current when circumstances change?......Matt

Therefore, the total number of flux lines passing through the coil area always remains constant regardless of the position of the coil

Only if circumstances surrounding the coil don't change....Matt

* The coil will always remain parallel to the ground.
* Start with the coil far above the ground. The flux lines will look as shown in your diagram for air.
* We now begin to lower the coil towards a relatively high permeability ground.
* As the coil is lowered towards the ground we begin to see some distortion of the flux lines with some of the flux lines being attracted to the low reluctance path of the ground, but most of the flux lines pass through the air surrounding the bottom coil.

But wont the total flux increase due to the incremental increase in permeable material in the flux's path?....Matt

* At some height above the ground the reluctance through the ground becomes much less than the reluctance through the air. The flux lines start to be noticeably distorted and the flux lines tend to get "sucked into the ground". It is not that the number of lines increases, but rather they see an easier path through the ground than through the air.

Don't think that proposition is correct....

If you introduce an 'easier' path for even only part of the total flux, does that not affect the total reluctance of the magnetic circuit..?

Does not the magnetic flux strengthen with that introduction?

Consider then how that is reflected back to the driving source, and what effect it has in the primary.........Matt

Does this make any sense?

Glen, do you agree that total flux is proportional to Uo*Ur*H(amp-turns)*area....?

So the ground's ferrous presence Ur increases the total flux...No?

Are we're getting close, or is there an error in our formulation of matters?....Matt

p.s.Off to bed Glen....it's 2am here..,,goodnight

Glenn[/quote]

 

[Jackpine Savage]

I agree with you Cody. Its a wonder we find anything at all with this technology. Certainly the engineers are aware of most all of the problems associated with the coil ground dynamic. I noticed when reading the T2 op manual that it mentions using 1 tone in "lumpy ground". Supposedly the continuous sampling helps even things out on the receiver end?? Anyway, when field hunting uneven ploughed ground some machines cut it and some don't. Toss man made iron into that picture and amd it quickly seperates the good design from the bad.

Tom