Dead ferrites

[Anonymous]

Hi everyone,
Looking at Eric's waveforms at turn on and turn off, it appears that at turn on the ferrite rods get "kicked" but not at turn off. I saw the same thing with some ferrite rods, cores, emi chokes, I have around here.
So does this mean (must) that the amount of ferrite signal one gets in their PI receiver after turn off is a function of how long since turn on it has been, and not how long after turn off it has been (wait for ferrite signal to decay rapidly)? Thats the other thing, this signal is not decaying rapidly. Magnetic viscosity or whatever doesn't really matter, all we have to know is what it really does, not so much why, to build a good detector. Though its always good to know why.
Am I out in left field here, or does the time after turn off not really matter so much for most of these targets as how it has been since turn on?
JC

 

[Anonymous]

Once again based on what I'll call Eric's turn on waveforms, it appears that one could shorten the on time pulse from 350 us to say 25 us and sample with the transmitter off right after the short pulse, to avoid the induction balance issue. And still get the turn on information.
Then Long pulse the detector and get the information after things like ferrites have died away. Could this scheme be used for discrimination? <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
JC

 

[Anonymous]

In an induction balance it's pretty tough to see the magnetic viscosity of ferrite during transmitter-on, because it's buried in the permeability waveform, of which it's normally less than 1%. It's usually pretty hard to see even after flyback.
If you have a really robust "positive hot rock", that will improve your chances of seeing the viscosity signal, esp. if you're doing this with your kickstart transmitter.
If this is a Mystery of the Universe you really want to come face to face with, balance out the permeability of the positive hot rock with a piece of ferrite or magnetite located in a "backwards field" region of the loop arrangement, and you'll be left with the viscosity component along with a bunch of garbage you always see when a loop is nulled.
Then you can tell the Long Range Locator chaps, "Majick auras, that ain't nuthin', I've actually seen the magnetic time domain!" 'Spoze they can top that on less than four pints of barley juice?
(An aside to our LRL friends, yes, I have dowsed, successfully-- with coat hangers, not those fancy dowsing toys they sell for hundreds and thousands of bucks.)
--Dave J.

 

[Anonymous]

Hi JC,
The ferrite rod that I used in the test was one that I use to make a probe for PI detectors. You can wind a coil right on the rod and there is no detectable signal at switch off. The reason is, that although the rod has a high permiability, the magnetic relaxation (or viscosity) is extremely fast. The rod is magnetized during the on time, but as soon as the magnetizing current ceases, the domains revert very quickly to a random orientation. The large signal during the on time is simply the unbalance caused in the coil by the rod

 

[Anonymous]

Thanks Eric,
Guess my silly idea of sampling during the off time after short pulses isn't going to work. I was afraid this would be too simple. So guess we can't get away from some form of IB coil arrangement for measuring the ferrites in the soil. Oh well back to the drawing board.
JC

 

[Anonymous]

Thanks Dave,
Learn something new everyday on this forum.
Remember an add about canceling out iron/ferrites or something like that in a whites vlf detector, for 30 dollars. Never saw one in the flesh, but turned out it was some kinda rod of something that you put on top of the search coil.
You're explaination above explains how this thing might actually work (or at least do something {probably doesn't work as well as advertised).
JC

 

[Anonymous]

JC
I have a graph below showing how a passive signal canceler would work. Since we have been using X and R in some recent posts to refer to the reactive and resistive components of a signal I have labeled the axes of the graph X and R. And since you mentioned White's I oriented it the way White's does with ferrous signals on the left, good conductors on the right and poor conductors near the +R axis. A signal would be represented by a point on the graph or by a vector from the origin to the point. The length of the vector is the strength of the signal. The red line represents a ferrous signal. It could be canceled out by another signal that is 180 degrees out of phase with the signal you want to cancel and of equal strength.
But there are some practical problems with this. One problem is keeping the strengths about equal. If the red signal is an iron object then the strength will increase and decrease as you swing the coil over the object. If the canceler is at a fixed distance from the coil its signal strength will remain constant and will only cancel the iron signal when the coil is at one particular spot. This does not really accomplish anything.
If the red signal is the ground, then it might not vary too much as you swing the coil from side to side as long at you keep it at a constant height from the ground. So it might seem that this would have some use in canceling ground signals.
But the biggest problem is that the signals from materials all fall in the part of the graph above the X axis. Our green canceler signal falls below the X axis where the value of R is negative. It would have to have a negative resistance. There are not any simple materials that have this property.
You could build an active circuit that would put out a signal with this phase angle. But you would still have the problem of getting its strength to be equal to the strength of the signal you want to cancel.
Robert

 

[Anonymous]

Hi JC,
I checked again this afternoon by setting up the PPD1 and looking at the signal from a tray of Australian laterite. I wasn't quite correct in my earlier description. The on time signal looks just the same as for the non-viscous ferrite rod i.e. no modification to the front edge is apparent. The off time viscous signal is apparent but rather less than for the pliers. The off time viscous signal gives a strong response on a straight PI. so you can imagine the difficulty that the on time permiability signal is going to give, unless there is a way of cancelling it. As I mentioned before, I used to put the coil on the ground and let the electronics re-balance the coil; then draw the coil back over the object. A DD coil would be much better as you get + and - signals from the surface mineralisation which largely cancels out.
Eric.

 

[Anonymous]

Hi Eric,
Thank for the input and the patience. I reread your description with the waveforms where you had explained the rise time of the current (a question I asked later) and the response from the ferrite rods. Guess I got so excited looking at the waveforms I didn't read the description so well.
I will be looking at these effects in a bit more detail (different pulse widths, materials, etc., now that I have a set up to do it with, thanks to Thomas B.
As you've explained before the DD coils have the advantage of canceling the ground some (if it is uniform under the coil), I've seen posts where the DD is less sensitive (on PIs) and wonder if this is just because for same size search head the effective area of the DD is smaller than the monoloop or is it something else. In other words the sensitivity of the DD could be made up by making it bigger, though more awkward, and heavy.
Thanks once more
JC

 

[Anonymous]

Hi Robert,
The lastest Kellyco catalog I have is Spring of 2000, and in it is a thing called "Iron Eliminator". A friend of mine, asked me about it and I wasn't sure how it worked or if it worked.
I read some post about it awhile back but don't remember the outcome. It is for Whites Eagle, Spectrum XLT, or Quantum, except Silver Eagle.
Not sure if they still sell it, and you may have already heard of it. It says:
Eliminate up to 100% of small iron, mineral and hot rocks in soil, iron eliminator is a tuned circuit which changes the phase response of iron 180 degrees out of phase. In this mode, iron is eliminated from being detected. Small size 5/8 inch dia 5 inches long, no battery required, water proof .... $27.95
An interesting patent along these lines if you haven't already checked it out is: US 5247257 Electronic metal detector return signal phase changer. think it is on Carl's site.
Thanks for the response. Good information as usual.
JC

 

[Anonymous]

Hi Dave,
Here are some links I collected on magnetic viscosity and some experimental approaches to ground cancelling and visualizing the viscosity signal. I don

 

[Anonymous]

Thanks, Thomas, for the nice of references.
MAGNETIC TAPE
Because magnetic tape is supposed to be reasonably permanent, the maghemite particles in it are manufactured to carefully controlled specifications to produce single-domain particles of a size distribution which will include very little of the smaller superparamagnetic material (the primary cause of magnetic viscosity in most soils), and very little multidomain material either (too easily demagnetized).
Magnetic viscosity in magnetic tape results in "print-through" from one layer of tape to the next.
SOIL
You can take the soil, mix it in water, and let it settle. Most of the clay will be in the top layer, and in most cases, that's where most of the magnetic viscosity will also be. You can further classify the clay hydraulically, magnetically, or both, separating certain size and composition fractions from others. With some diligence you can isolate some material which has a pretty high loss angle. (I haven't actually done this, but hope to some day.)
SOURCES OF MAGHEMITE AND RELATED MATERIALS
Iron oxides are widely used as pigments, both in paints, and in ceramics. Ceramics supply houses often carry crude red clays and oxide earths some of which contain quite a bit of maghemite.
Steel wool, such as you can buy in a hardware store, can be rusted in water to produce maghemite rust. You may have to do a little experimentation with the process in order to get a reasonably good yield in a short period of time.
--Dave J.