Eric Foster

Hi Reg and All, Getting signals off dry grass is something I have not experienced. I have seen signals generated by wet grass and seaweed on the beach though, and these are eliminated by adequate shielding, so I guess that dry grass signals would be cured by the same means. I have always used the maximum amount of shielding that the detector design will tolerate. With a PI you can tell when you have reached this as you start to see the decay curve of the shielding coming in just ahead of the sampling point. As has been pointed out, to keep costs down, many manufacturers use graphite or nickel loaded paint as shielding either on the inside of the shell, or on a circular piece of stiff paper. Much better is a metallic tape wound over the coil itself, such as a lead tape or a woven copper braid tape. Normal aluminium or copper tape is no good as it is too conductive and the decay curve swamps the signal of the targets you want. Static can be a problem in dry environments and it is the sudden discharge from the coil shell in the form of a spark that can cause the audio to beep. I was once involved in a metal detector system for a factory that produced polythene bottles and static was a big problem. Sometime you could draw 1/2in sparks off the conveyor belt if you put a finger too close. Sparks often occurred inside the coil shell by electrostatic induction and these would give a strong false alarm, particularly if the spark was to or from the live end of the coil where it joined the core wire of the coax cable. If the spark was to the outer coax braid then there was not much effect provided the braid was properly grounded at the electronics end. The problem with hand held detectors is that nothing is properly grounded, except to itself and the whole detector plus operator can build up a charge which is released when the coil's shell touches something that is even slightly conductive. Making sure that the join between the coil wire and the coax core is well sleeved with heatshrink, will greatly reduce sparking at this point, but this is not something the user can do unless he makes his own coils. I have tried antistatic sprays which are effective for a while until they wear off, only a few seconds work to respray though. As you say Reg, it is imperative that the shield does not move relative to the coil. That can generate severe noise as the coil is swept and bumped over the ground. The coil then acts like a crude microphone. Eric.

Hi Wyndam, Firing clay pots or bricks in a kiln does two things. It converts magnetite into maghemite which enhances the signal that PI's detect, and also as the fired clay cools, it "freezes in" a remanent magnetisation that is aligned with the earth's field. This latter is used for magnetic dating provided you have recorded the orientation of the bricks or pot before they are disturbed. Same with some rocks, as you say. A PI detector, if properly designed and set up, should be immune to remanent magnetisation, but it will still respond to magnetic lag, or viscosity, caused by superparamagnetic grains of maghemite. That is why in many areas, a method of ground cancellation is needed to remove this signal. Eric.

Hi Steve, I know the Coiltek probe well - I designed the sensor. The ferrite core used in that one is 2.75in long, but to get the ranges I was thinking of you would need to have a core around 6in long. It's like a bar magnet, the longer it is, the further the magnetic field extends. The Coiltek probe was designed for the SD's and would probably work OK on the GS5, although maybe not optimally. I still have some wound cores and will give it a try and see what ranges I get. Eric.

Hi Steve, For the type of searching you do, would a long probe be a useful accessory? The first GS5's had a small probe clipped to the side as a pinpointing aid, but this was dropped in later versions. What I am thinking of though, is a long probe,say 30in, which can be used in less accessible places instead of a conventional coil. I have one here that I made about 10 years back for a different detector, but one could quite easily make such a probe for the GS5 type machine. Most of the shaft would be handle, but sensors can be made that have quite a respectable detection range. For example. I would expect to be able to detect a US nickel at 6 - 8in, maybe more. The outer diameter of the one I have is 1in and it is made of 1/10in thick fibreglass tube, fully waterproof and immensely strong. All the facilities on the detector are still fully functional including the all important ground balance, for mineralised areas. Probes are strange things, some detectorists can't do without them, and others never use them. However, there is no getting away from the fact that for searching between rocks and in dense undergrowth, they are very useful. The one described above can even double as a walking stick. Eric.

Yes, nuggets do it too. I found a 4gm nugget that I can tune out with the GB control on the GS5B. This was quite a different point on the control to where the ground balances out, but this should not affect the characteristics of the notch. Adjusting the control carefully for no signal, the nugget was then put into a cup of boiling water. The nugget then gave a wee response. After 10 minutes in the freeze compartment of the fridge, it now gave a woo. The changes did not seem as great as for the lead, but it was very definite. I have no idea what the temperature coefficient of metal conductivity is, and whether it is different for different metals, but the important thing is that temperature change can make a difference in detectability in an instrument that is notching out the ground response. One other important point that I have not yet mentioned, is that the tests, both lead and gold, were done with the target being brought down vertically on the coil axis. If you pass the target from side to side, different things happen. The latter of course is what would occur in the normal course of searching. Because the magnetic field from the coil gets more horizontal as you approach the coil edges, and the targets have a certain thickness to diameter ratio, initially you get a wee, followed by a null when the target is on axis. You get another wee as it passes the other side of the coil. So in practice you do not get a situation where the nugget completely disappears. It is still audible but as two responses. You would not get this if the target was spherical as the eddy current decay is the same whatever the orientation to the coil field. If the target's major response is not notched out, then the on-axis signal is dominant and you would hardly notice the coil edge signals. Eric.

Hi Reg and Steve, Yes, I worked out a way of measuring the "hole" in the response of the GS5B. It required finding a metal that I could file down easily in 0.1 - 0.2gm stages and that had enough weight to measure in small quantities on electronic scales. I tried aluminium but the final sizes were going under 0.1gm, which is the minimum I can weigh. Gold would be best, but I was not about to start filing down gold nuggets, so I settled for lead. Lead is considerably less conductive than gold, so a greater mass is required to give the same decay curve. What I was looking for was the point where the low tone changed to a high, and the metal became almost undetectable. Another advantage of lead is that it came be built up again with solder, so that the experiment came be repeated several times. The results were surprising. The low point of the notch turned out to be 6.0gm but 0.1gm either way and we were back to 50% detection range and either a low (6.1gm) or high tone (5.9gm). You still get 80% range for a 5.7gm and a 6.3gm. This indicates how narrow the notch is, which also gave rise to another surprising effect. I would file the lead say, to a point where the response disappeared. If I left the lead sitting on the bench for a while, the low tone and detection range would come back. I then found that if I held the lead in my fingers and waved it over the coil, the low tone and range would reduce, and then come back as a high tone. This is obviously a temperature effect which was proved by cooling the lead to 0degC in a fridge, measuring it, and then heating it in a cup of boiling water. The otherwise undetectable piece went back to either a low tone or a high tone and full detection range. Physics books tell you that when a metal is heated, its electrical resistance increases, and the opposite when it is cooled. I didn't expect this would show up on a metal detector. With the GS5B, the notch moves if you alter the pulse delay and re-ground balance. Changing the delay to 15uS (about 1/2 rotation) moves the bottom of the notch to 6.3gm. For gold the equivalent target would be smaller because of gold's higher conductivity. Pure gold is 70, relative to copper at 100, and some Australian nuggets that were recently measured on a conductivity meter, ranged from 20 - 50 This reading is independent of size. USA nuggets are generally thought to be lower conductivity. Lead is about 9 for comparison. So it looks as though for a gold nugget of average conductivity 35, somewhere about 1.6gm would be in the notch. This raises all sorts of interesting questions as to what happens in the field. Searching on a cold day, compared to a hot day? Not only the ground signal changes (verified in an earlier experiment) but if nugget signals change as well? Hmmm...... Eric.

Hi Reg and All, I agree with Reg’s explanation, although it is a puzzle as to why Minelab PI’s are so critical on coil parameters. One of the basic features of the PI principle is that, because the coils are untuned and resistively damped, a PI detector should be able to cope with a fairly wide variation in these parameters. Some years ago I bought a SD2200 for test purposes, and actually ended up manufacturing aftermarket coils for it. These ranged from 3in, 8in and 11in monos, to 8 x 24in and 10 x 30in rectangular coils for skid or trolley mounting. The reason for making the 8in and 11in monos, was that some customers wanted fully waterproof coils for searching beaches down to, and into the water. My open centre coils were fully potted, and being an open ring, had little water resistance when submerged, and were not bouyant. The larger rectangular coils were as a result of considerable correspondence with Chris Hake, who trolley mounted them and trundled across the salt lakes in WA looking for nuggets. The thing I found was, that even small departures from 300uH inductance and 0.4 ohms resistance resulted in a coil that was either noisy or simply didn’t work. Coil and coil/shield capacitance was also a factor, but did not seem quite so critical as the first two. The way I managed to make a good coil was as follows. I looked at the response of a Minelab 11in mono when connected to one of my Deepstar PI detectors. The transmitter was adjusted to pulse a similar amplitude of current into the coil and the initial damping resistor was the same value as the SD2200 appeared to have. The scope was connected to the first receiver amplifier output so that the low level ringing and/or overshoot and recovery time could be observed. Any size coil that I made had its response tailored by the addition or subtraction of resistive damping, and also by some novel inductive damping to cancel out any capacitive overshoot. Once I got the responses matched, the coil always worked on the SD. As Reg says, as a PI design strives for ever shorter sample delays, the coils will become more critical, particularly in terms of inductance and capacitance. Damping needs to be more finely adjusted too, so that almost all vestiges of ringing and overshoot have vanished. In my SD2200 tests, it did not appear that signal sampling was taking place at a “short†delay. The small object sensitivity and range, fairly well matched that of the Deepstar, which samples at 15uS. It did appear to me at the time, as though something additional had been built in to the SD circuitry, so that the detector would not perform satisfactorily, unless you used a ML coil. However, that may not be the case, and it might be just a side effect of the circuit topology used. The addition of a variable delay control overcomes a lot of problems. It may only mean shifting back a couple of uS to get the detector working again, as Reg explains. Usually, big coils are used for hunting for larger and deeper nuggets anyway, and these nuggets will have reasonably long decay curves, so moving back a few microseconds is not going to make much difference to the received signal. Regarding the Goldscan 5, it was quite a challenge to make a detector that would operate not only on its own coils, but also those of ML, Coiltek and Nuggetfinder. This is largely because of the big difference in winding resistance, and differences in the way the shielding is done, which influences the circuit capacitance. However, it has been achieved, and from tests that have already been done, many of these alternative coils will still work properly even at the shortest delay on the GS5. While in Australia recently, a prospector produced a coil about 30in diameter and the GS5 accepted that OK. Eric.