Iron Discrimination with a Pulse

[Tec]

Hi,

I find this works (attached), for anyone who is interested.

And it works like this:

Iron encourages the magnetic field, and allows better transfer of energy to the secondary coil.

Conductive targets steal energy with their eddy currents.

Experiment:

Rotate a bottle cap above;at some orientations, the Fe effect will dominate and increase power transfer, at other times the eddy current will dominate and decrease power transfer.




Tec

[TH'r]

I have been hunting relics for many years and have not observed this phenomena in the field. To the contrary, of iron "encouraging" the signal, iron gives an abrupt signal with a sharp cut off. It is often broken and usually won't center with the coil. The received signal seems to be distorted or interfered with in some way. Non-ferrous gives a smooth "sweet" sound that centers and what us oldtimers (that still sometimes hunt all metal by ear) like to hear. I will agree that this is related to magnetization of the target, and something that has not been quantified electronically. Maybe a new coil design would accentuate this difference. I remember that the old Garrett co-axial coil was especially good for this kind of hunting.

[Tec]

I have been hunting relics for many years and have not observed this phenomena in the field. To the contrary, of iron "encouraging" the signal, iron gives an abrupt signal with a sharp cut off. It is often broken and usually won't center with the coil. The received signal seems to be distorted or interfered with in some way. Non-ferrous gives a smooth "sweet" sound that centers and what us oldtimers (that still sometimes hunt all metal by ear) like to hear. I will agree that this is related to magnetization of the target, and something that has not been quantified electronically. Maybe a new coil design would accentuate this difference. I remember that the old Garrett co-axial coil was especially good for this kind of hunting.

You're right,
I am familiar with the broken sound of iron, and the double sound of a nail..
My explanation for this is that because iron has both an eddy current and a field effect, and the center of these is in different positions on the target.
And the effects are opposite.



So, sweep across the side-ways nail (as you know) you get: Mag field on the tip (no signal, because the detector is trying to detect eddy currents), then Eddy current normal signal in the middle, then abrupt no signal as the mag field at the end of the nail is more dominant.

A piece of wire is even more chaotic as different positions and orientations it is either an eddy current more, or a mag field more.

Something like that.

i.e if you have a perfectly vertical nail or tent peg, as you know, it sounds quite 'sweet', like a coin. because all the effects are lined up and there is no jagged fluctuations as you pass over it..

I am only familiar with using a PI detector..

The iron does encourage the field, but doesn't encourage the audio signal on the PI because it is setup to detect eddy currents.. the mag field works opposite and 'blanks the signal...

(The above is all conjecture, but I think it makes sense)

[Prospector_Al]

Hi,

I find this works (attached), for anyone who is interested.

And it works like this:

Iron encourages the magnetic field, and allows better transfer of energy to the secondary coil.

Conductive targets steal energy with their eddy currents.

Experiment:

Rotate a bottle cap above;at some orientations, the Fe effect will dominate and increase power transfer, at other times the eddy current will dominate and decrease power transfer.


[ATTACH]24832[/ATTACH]

Tec

Hi Tec,

Good observation. You mention being familiar with PI detectors. I have a White's VLF detector, and it behaves exactly the same way. Apparently, the shape of the coil excitation current is immaterial.

All the best,

Allan

[bbsailor]

Hi,

I find this works (attached), for anyone who is interested.

And it works like this:

Iron encourages the magnetic field, and allows better transfer of energy to the secondary coil.

Conductive targets steal energy with their eddy currents.

Experiment:

Rotate a bottle cap above;at some orientations, the Fe effect will dominate and increase power transfer, at other times the eddy current will dominate and decrease power transfer.


[ATTACH]24832[/ATTACH]

Tec

Tec and Al

When using a PI mono coil on the beach I noticed something interesting when detecting ferrous metal. By changing the sweep speed I noticed that ferrous metal targets produce a stronger signal when the sweep speed is increased conpared to a slower sweep speed. This is an accurate method most of the time espically when there is the classic double beep. The NJ beaches have dune fences made with twisted ferrous wire and wood slats. After a storm or when the dune fences decay, there are twisted shards of fence wire all over the beach. This sweep speed change tip works pretty well in these locations.

Joseph Rogowski

[Prospector_Al]

Hi Guys,

I have a theory: When a ferrous object stays in the ground for a long time, it becomes magnetized by the earth's field.

When you seep past a magnet like this, the signal amplitude is proportional to the sweep speed. Although the signal is not synchronous with the coil current, the magnitude may be such that it drives the input amplifier into saturation. That causes the intermittent sound...

If the target signal is capacitively coupled in some stage, that also makes the amplitude increase with sweep speed. This can be verified by sweeping slowly over a target. If there's an overshoot, the signal is capacitively coupled...

Al

[Tepco]

Most probably this is not the case. Earth field is week, like 50uT or just 0.5G, even with anomalies and cumulative processes hardly can magnetize anything much stronger. Compared to this, relatively week ordinary ferrite magnets are 2-3000G, detectors can respond to this, without TX applied, even to much weaker fields, say 50G, but this is still difference of two orders of magnitude. With TX running, field generated by detector is significantly stronger than Earth's field and can cause magnetization of metal object, completely overriding eventual small natural magnetism. Then, effect is combined eddy current and this magnetization component added, contrary to nonmagnetic material, when only eddy currents play. Considering motion detectors are AC coupled, this magnetization component will vary with sweep speed., eddy current component will not (apply for unipolar pulses, with bipolar pulsing situation can be different). Man made magnets are story for its self, magnetized magnet is almost in saturation, having very low permeability (unlike same demagnetized material), detector will see it almost like nonmetallic object, responding mostly to field. Experimental proof: turn off TX and see what happens. Not even strong magnet at very high (practical) sweep speed and normal distance can saturate input amplifier.


(Whit all rights reserved to be completely wrong with all this, just my opinion, not even theory)

[Mechanic]

Hi Tepco,

What you say is true, but only if a pi detector does not employ a method to cancel the earth's magnetic field or if it is not adjusted properly.
PI target signals are in the rang from 100khz to about 1khz and the sample windows are optimized for this.
The signal induced in a coil from a static magnet is equal to the sweep speed of the coil, which will range from .1hz to 15hz. This signal will not change much at all between the first and second sample, or over the whole timing cycle.

When I tune up a pi detector to cancel earth field effects, I use a ferrite magnet and adjust until the magnet is not picked up any more. However when the magnet is moved close to the windings there will be a response due to the ferrite the magnet is made from. If the adjustment is out just a little bit movement of the magnet will be picked up, if it is out a lot movement will be picked up from a good distance.

Cheers Mick

[Carl-NC]

I agree with Tepco's hypothesis. The TX impulse temporarily magnetizes an iron object, at which point sweep speed determines the strength of response, assuming the magnetic response dominates the eddy response. The transient magnetism may be significantly decayed before the EF cancelation pulse, unlike a static magnet or Earth field which are not transient. You could test for this by varying the EF pulse delay.

[Waikiki_Sweep]

Sure PI detectors are capable to magnetize ferric targets and even soil.
Magnetizing the target with the powerful impulse then checking it magnetic field probably will let to decide was it ferric or color metal.
Also demagnetizing target by opposite polarity impulses may give even more precise results.
There is some work for microprocessor driven PI detector to make impulses with different width and opposite polarities.