Ground Balance Theory

[Ferric Toes]

Hi Eric,

thanks for the interesting results.

I think, that the term "viscosity" needs a clear clarification right now.
How would you define or declare the term "viscosity"?

Cheers,
Aziz

PS: there is an interesting non-linearity < 20µs, which should be followed as well.

Hi Aziz,
You raise an interesting point. At the moment, viscosity has no units but it is equivalent to measurements done for frequency dependence of magnetic susceptibility. The de facto method used for some years is to measure a soil sample at 0.47kHz and 4.4kHz using a Bartington MS2B susceptibility meter. The difference in the two results is equated to the magnetic lag, or viscosity, that the sample displays and the higher the difference the more severe the effect on a PI metal detector. The Red Hill soil susceptibilities are 869 and 729 x 10^-5 SI units respectively, giving a difference of 140. Knowing this, I calibrate the viscosity meter to a value of 140 at 30uS delay. Both sets of measurements are done at 20degC. I have chosen RH Soil as a calibration sample as it is not an exotic material such as Oz rock, or Tiva Canyon Tuff, and it has a mid range susceptibility with good frequency differential. Having calibrated the viscosity meter in this way, I can then measure other soils and rocks and get a figure for the respective relative "viscosity". As the VM is a PI instrument the results relate directly to the severity or otherwise that a PI metal detector would experience and have to GB out. Comparing a table of frequency difference measurements for different materials on the MS2B does not always equate to the same reading on the VM, so there are differences presumably in the magnetic mineral makeup of the various samples. More research is needed, but the VM was primarily designed so that a direct relationship with PI existed, and could be done in one measurement. You cannot backtrack though and deduce a figure for the susceptibility from the VM reading, at least not for the moment.

The small deviation for times <20uS will be investigated. The VM figure is always a bit higher than expected at these shorter delays, as though there is a faster decay superimposed on the t^-1. Conductivity? I doubt it, but something is there particularly in the Oz samples.

Eric.

[deemon]

Hi Aziz,
You raise an interesting point. At the moment, viscosity has no units but it is equivalent to measurements done for frequency dependence of magnetic susceptibility. The de facto method used for some years is to measure a soil sample at 0.47kHz and 4.4kHz using a Bartington MS2B susceptibility meter. The difference in the two results is equated to the magnetic lag, or viscosity, that the sample displays and the higher the difference the more severe the effect on a PI metal detector. The Red Hill soil susceptibilities are 869 and 729 x 10^-5 SI units respectively, giving a difference of 140. Knowing this, I calibrate the viscosity meter to a value of 140 at 30uS delay. Both sets of measurements are done at 20degC. I have chosen RH Soil as a calibration sample as it is not an exotic material such as Oz rock, or Tiva Canyon Tuff, and it has a mid range susceptibility with good frequency differential. Having calibrated the viscosity meter in this way, I can then measure other soils and rocks and get a figure for the respective relative "viscosity". As the VM is a PI instrument the results relate directly to the severity or otherwise that a PI metal detector would experience and have to GB out. Comparing a table of frequency difference measurements for different materials on the MS2B does not always equate to the same reading on the VM, so there are differences presumably in the magnetic mineral makeup of the various samples. More research is needed, but the VM was primarily designed so that a direct relationship with PI existed, and could be done in one measurement. You cannot backtrack though and deduce a figure for the susceptibility from the VM reading, at least not for the moment.

The small deviation for times <20uS will be investigated. The VM figure is always a bit higher than expected at these shorter delays, as though there is a faster decay superimposed on the t^-1. Conductivity? I doubt it, but something is there particularly in the Oz samples.

Eric.

Thank you , Eric , for your very interesting observation . But here is one strange thing that I can't understand . As you wrote in this topic before -

I have just done some viscosity measurements on a piece of ferrite rod of a grade that is used for radio antennas. This has been suggested in the past as a possible substitute for magnetically viscous ground. Compared to the Red Hill soil, which I tend to use as a medium viscosity standard, the ferrite gives only 27% signal. i.e. a spot reading of 40 instead of 140. The problem though is that the slope is different from any natural soil or rock that I have measured; -1.27 compared to a range of -0.99 to -1.05 for the latter. Red Hill soil is -1.03.

- ferrite also shows this magnetic viscosity behaviour , and because of this some people used it like a "viscosity equivalent" . OK , but the strange thing is that I cannot see it in my experiments . Of course , knowing your experience I never doubt a little in your measurements , I believe that you made it quite precisely .... but what is the cause of my results that I obtain here ? When I bring a piece of ferrite to my search coil , I see nothing ... the only thing that I see on my scope ( setting 100 uS on full screen , like your graphs ) is a minor change of a coil current ( pulse amplitude ) , but nothing even similar to the signal that decays with a 1/T law . When I bring a piece of metal - I see a good target response , I can measure its TC , and so on - but ferrite gives nothing . The circuit of my device is the same that I published here , in this topic - http://www.geotech1.com/forums/showt...y-recuperation

But the most funny thing is that I use a search coil that wound on the ferrite core ! And most of my experimental coils ( I have 5 coils ) had been wound on many glued together ferrite rods , used in radio antennas . And the only one bad thing that I noticed since I use them - is a quite moderate energy loss during a flyback pulse , I loose about 5% of my coil current after current reverse - of course , it's not a problem - but when flyback is finished I see only a pure horizontal "plato" , without any decaying curves on it . You see , if I see the curve like been shown on your graphs , without any metals near the coil - I'd immediately reject all this ferrite coil idea . And I specially tried to bring to this coil another pieces of ferrite with different sizes and magnetic properties and see nothing , as I told before . And I'd never seen a reaction from any kind of stones that I can find here , in our North Caucasus region . What is interesting , some of that stones does contain an iron oxide ( magnet attracts them ) , but reaction is zero . But today I read you message here and thought - how can it be ? You write that in your measurements ferrite rod is only a 3 times less "viscous" that a typical "medium" soil ... but on the other hand , I have not one rod in the coil , but 12 of them , and seeing nothing of this kind , but why ??? Can you explain this ? I am thinking about it all day , but haven't an answer ... may it be because I don't shut up the current in the coil , but reverse it ? Can this "constant current interval" after flyback pulse in my device somehow prevent the ferrite target of "demagnetizing" and from this signal decay also ? Of course , it may be quite interesting to try a kind of those Australian stones , but we haven't them here .

[Davor]

You simply have a better ferrite.

[Ferric Toes]

You simply have a better ferrite.

Davor is right; you have a better ferrite for this application. There are many different grades and finding a suitable one is not easy, and trying to understand the relevant specs on a data sheet is almost impossible. The rod that I tested for viscosity is one of a batch that I bought years ago and are useless for anything to do with PI. Other rods I have, like yours, I can wind coils on them and they act like a pinpointer probe with only the weakest viscosity reading (about 3 on my VM meter). If you PM me, I can send you a few "viscous rods" as I have no use for them. I kept them thinking that I could use them as an artificial viscosity target, but I have enough natural soils and rocks for this purpose. If the rods give no signal on your pulse reversing rig, then you have got something worth exploiting.

I have always run into problems when joining ferrites. Had one today where I was building a coil whose sole purpose is to detect nothing. In other words I wound a coil on a full pot core to stop any external field. By the way, this is a grade that only has a small viscosity. Can't see it on the VM but just see it on scope when the coil is in the core. All is OK until I bolt the two halves together, when I get a sustained ringing. If I put a layer of double sided Sellotape between the coil halves then bolt up, the ringing is gone. I have seen this before when joining rods or rings end to end - always need a very thin mechanical damper between sections. Another necessity, also seen today is to ground the rods, or pot core, because otherwise you pick up huge amounts of rf em.

Eric.

[Davor]

You can also wrap them in foil. Transformer shielding was a must with microphone transformers.

[Ferric Toes]

You can also wrap them in foil. Transformer shielding was a must with microphone transformers.

I used to wrap 0.1mm lead foil over a coil on a ferrite rod to shield it. Aluminium or copper foil was too conductive and gave a huge eddy current signal. Lead was also good as you could solder a ground wire to it - if you were quick. Lead now banned in EU so alternatives have to be found. I still have a stock of woven copper fabric tape, but that will run out soon. Just grounding the rods or cores with a drain wire helps as the ferrite is usually partially conductive.

Eric.

[deemon]

Davor is right; you have a better ferrite for this application. There are many different grades and finding a suitable one is not easy, and trying to understand the relevant specs on a data sheet is almost impossible. The rod that I tested for viscosity is one of a batch that I bought years ago and are useless for anything to do with PI. Other rods I have, like yours, I can wind coils on them and they act like a pinpointer probe with only the weakest viscosity reading (about 3 on my VM meter). If you PM me, I can send you a few "viscous rods" as I have no use for them. I kept them thinking that I could use them as an artificial viscosity target, but I have enough natural soils and rocks for this purpose. If the rods give no signal on your pulse reversing rig, then you have got something worth exploiting.

I have always run into problems when joining ferrites. Had one today where I was building a coil whose sole purpose is to detect nothing. In other words I wound a coil on a full pot core to stop any external field. By the way, this is a grade that only has a small viscosity. Can't see it on the VM but just see it on scope when the coil is in the core. All is OK until I bolt the two halves together, when I get a sustained ringing. If I put a layer of double sided Sellotape between the coil halves then bolt up, the ringing is gone. I have seen this before when joining rods or rings end to end - always need a very thin mechanical damper between sections. Another necessity, also seen today is to ground the rods, or pot core, because otherwise you pick up huge amounts of rf em.

Eric.

Yes , it can be an explanation .... but I am not quite sure You see , I made not a one coil - for the first time I wound a coil on 12 parallel square-shaped ferrite rods ( glued in stack 3*4 ) . Such rods were used in old transistor pocket radios . The second coil was made of the same rods , but 18 pieces , glued 6*3 in the length - it is a long coil . After this I tried another rods , round and long ones ( used in a big transistor radios , not a pocket type ) - I glued them 7 in parallel . All those ferrites has mu=600 . Then I tried to make the core from about 20 pot cores , glued one on another like a "tower" - this ferrite has mu=2000 . These were all Russian-made ferrites . And on the final I found a several heavy and thick Epcos ferrite "bricks" , for a powerful switching-mode welding machine transformers ( mu=2700 ) - I glued 3 of them and made a long ferrite "stick" . Comparison of all that coils was predictable - Epcos win , because of its higher permeability and overall quality . Although this coil has the best sensitivity , comparing to previous ones , so I used it in the final device , I decided that it's not enough ( big air core coil still has a longer search distance - maybe a long amorphous core with ultra-high permeability will beat it , sometimes an experiment will show ) .... anyhow , as we speaking here , I "ate the dog" with all this ferrite stuff And what is interesting - some ferrites were better , some worse .... but the difference was only in the energy loss during a flyback , and overall sensitivity that correlated with ferrite permeability and the total core dimensions . But what I never had seen - these viscous properties , that you found in your cores . Of course , it might be interesting to test them and compare with what I have here . I wanna make my contribution to this "ground-balance story" and try to find the optimal solution of the task , but the main problem is that I cannot find the ground that is "bad enough" In our region , as I can see , my device is completely ground-insensitive .....

[Midas]

Here's my analysis of Eric's soil data:

I got very similar results for the exponents, though not identical for some reason. First graph is the data (blue) with predicted curve (orange) overlaid. Second is the log/log plot. But what I found most interesting is in the third graph which looks at the slope between each sequential set of points. Both samples seem to have a definite downward trend somewhat in violation of a the t^-1ish rule in which it should stay the same throughout. Its a small effect admittedly but the question remains is it measurement error or true non-perfect t^-a decay?

[Ferric Toes]

Here's my analysis of Eric's soil data:
I got very similar results for the exponents, though not identical for some reason. First graph is the data (blue) with predicted curve (orange) overlaid. Second is the log/log plot. But what I found most interesting is in the third graph which looks at the slope between each sequential set of points. Both samples seem to have a definite downward trend somewhat in violation of a the t^-1ish rule in which it should stay the same throughout. Its a small effect admittedly but the question remains is it measurement error or true non-perfect t^-a decay?

Brilliant! That is just what I was hoping someone would do. My graphing does a best fit, so the exponent is averaged over the whole curve, but the fit is pretty good. I always noticed that the 10uS sample was a bit higher in amplitude than it should be and wondered if it was an instrumental problem, but I could not see what was going on further down i.e. change of slope. Now with your results and what I have measured over the past two days, I see that it is not an instrument problem, and I will explain why.

Firstly there are no timing errors throughout the range of delays. At the shortest delay (10uS) the clock is running at 100kHz and measured today on a precision frequency meter with an oven controlled reference, is accurate to 1Hz. Likewise, the TX and sample widths are all as accurate. This being so if measurements are taken at 10uS, 20uS, 40uS and 80uS there should be a factor of 2 between each succeeding pair for a slope of -1. e.g. say 500/10uS, 250/20uS, 125/40uS, and 62.5 at 80uS. I also did the intermediate values 15 - 30, 25 - 50uS etc. Having done this I also plotted a downward trend in exponent as you have for the Oz Rock and Red Hill soil. Basically, I took the shorter delay reading, halved it, and divided it by the longer delay reading. e.g. (1000/2)/494 = 1.0121 for the first reading of the Tuff discussed below.

Now, it could be argued that there is some, as yet, undiscovered error in the instrument - but for the following. I plotted the results for Tiva Canyon Tuff, which although derived from volcanic ash has some interesting properties. The main one being a very high frequency dependence of susceptibility, >20% resulting in a high viscosity reading relative to its dc susceptibility. With this material the trend of the exponent goes the other way i.e. it increases at late times. To my thinking, this rules out an instrumental error.

To improve the S/N figure for the Red Hill and Tiva Canyon material I used 20gm samples instead of 10gm. This almost doubles the signal throughout the range. Theoretically it should double exactly but the bigger pot is poking slightly out of the sensor coil where the flux density will be a bit less. I repeated the measurements a few times and it is consistent.

As you say, we are dealing with very small changes here. The Tuff goes from a slope of -1.0121 between 10&20uS to -1.055 between 50&100uS. Plotting out the full results for the Tuff and doing a best fit curve, it almost looks as though the exponent has its own power law of +0.026.

Eric.

[Aziz]

Hi all,

this is what I get. It seems, that the "extrapolated" exponents (with least error to the measurement) doesn't correlate to each sample timings so it is very likely a normal and noisy measurement. I don't see a significant decrease of the exponents (see below).


Please, feel free to experiment with the Excel table (see below).
Decay_Eric01.zip

Cheers,
Aziz

[Ferric Toes]

Hi all,

this is what I get. It seems, that the "extrapolated" exponents (with least error to the measurement) doesn't correlate to each sample timings so it is very likely a normal and noisy measurement. I don't see a significant decrease of the exponents.

Cheers,
Aziz

Here is the one that bucks the trend and gives an increase in the exponent with time. Noise not a problem even at late times and integration time longer than would be for a PI detector.

Eric.

[Aziz]

Hi Eric,

ok, I have checked the point, that you fit the exponent between two readings (Amplitude(t) = A1, Amplidute(2*t) = A2). Is this correct? Or how do you calculate the exponents in the last picture?

If I have understood it correct,
(1) B*t^b = A1
(2) B*(2*t)^b = A2

solved 1+2 for b:
b = log2(A2/A1) = log(A2/A1) / log(2)

But if I put the parameters into the formula however, I get different values.
Aziz

[Dave J.]

Eric, for reasons I am not at liberty to disclose, I regard your interpretation of the Tiva Tuff data as correct. (However, this is a poorly understood area of magnetic physics and I may be wrong.)

--Dave J.

[Dave J.]

I have always run into problems when joining ferrites. Had one today where I was building a coil whose sole purpose is to detect nothing. In other words I wound a coil on a full pot core to stop any external field. By the way, this is a grade that only has a small viscosity. Can't see it on the VM but just see it on scope when the coil is in the core. All is OK until I bolt the two halves together, when I get a sustained ringing. If I put a layer of double sided Sellotape between the coil halves then bolt up, the ringing is gone. I have seen this before when joining rods or rings end to end - always need a very thin mechanical damper between sections.

Eric.

Magnetostriction.

--Dave J.

[Dave J.]

I have just done some viscosity measurements on a piece of ferrite rod of a grade that is used for radio antennas. This has been suggested in the past as a possible substitute for magnetically viscous ground. Compared to the Red Hill soil, which I tend to use as a medium viscosity standard, the ferrite gives only 27% signal. i.e. a spot reading of 40 instead of 140. The problem though is that the slope is different from any natural soil or rock that I have measured; -1.27 compared to a range of -0.99 to -1.05 for the latter. Red Hill soil is -1.03.

Eric.

Electrically conductive ferrite material.

--Dave J.