PI Vs VLF Air Tests

The thing with a PI machine is that you have a very slow pulse rate, for example a machine with a 1m * 1m coil runs at about 90 PPS(90hz), have you heard cars go by at a distance with loud music? and all you heard was the base drum? well the frequency is low and therefor travel much further, the same goes with the PI machine but the problem is that non ferrous metals have a very fast decay time and especially gold, this is where the VLF machine has an advantage with it's frequency of lets say 18Khz making it 200 times faster than the PI machine. By having this fast frequency the non ferrous metal gets charged with eddy currents many more times per second and this gives the receiver coil a chance to see the eddy currents coming from the non ferrous metal. A problem with a PI machine is the amount of magnetic field you are sending into the ground at each pulse, this is why we have an off time to let the field dissipate it's self into the ground otherwise you would get the field bounce into the receiver coil and your detector would be screaming at you all the time. So should you want the PI machine to run at a faster PPS you will need to reduce the amount of power, this will allow you to reduce the off time and the eddy currents to be picked up by the receiver coil, another thing to keep in mind is that the object in the ground gets a much higher charge from the PI machine so you end up with a stronger eddy current. You need to remember that a VLF machine has a coil that has been null'd and the eddy currents make it's null change but the PI machine simply waits for the returned eddy currents to come to the coil. If you test a machine PI or VLF in air you will not get the distance that the machine will see the object in the ground because of the halo effect that you get with the ground and also the humidity, iron rusts and dissipates it's self into the ground making it easy for the detector to pic it up and other metals like copper oxidize. Gold dose nothing making it hard to detect so the faster the PPS of frequency the better and I should think that an air test would be very close to when you find it in the ground.              

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Xavier

Ah, Xavier, my Grasshopper, you have learned well.

The other reason is that soil is irrelevant to a PI machine since the particulate size of oxides and leached metals is so small. With a VLF/TR machine those bits actually attenuate the signal - they absorb part of it. Those masses of particles can also completely hide a larger target below it. But the PI shines in that kind of environment. The fact that the eddy currents only travel on the surface of an object means they have to travel on the surface of those particles and they simply cannot. Not enough mass. The VLF/TR signal is different and during the on/off time the signal is transmitted directly into that mass of particles and doesn't even try to magnetize them. They are far too small but they DO transmit the signal from particle to particle making an effective mask which we cannot see through. That's in the all metal mode.

In discriminate mode, they can penetrate the mass of oxides to a limited degree but the signal that gets through is much weaker. Remember, the receiver is working harder, not the transmitter. The receiver is the reason that discriminate works because the return signal is filtered out by the circuitry inside. That filtering also has the effect of making the item appear to have a weaker return and this can cause your machine to miss an object or two.

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		Quote:			Posted by Lord Dracon
	Interesting topic, I wonder if underwater makes any difference .... you can see through that   for your tests. Water is denser than air and the ground that we sometimes dig is damp. just a thought

Like soil, water is mostly invisible to a PI machine including salt water. A VLF/TR on the other hand will have its signal pretty mangled in salt water mostly due to mineralization in the water. Salt is very corrosive and metal bits are universally spread throughout the water in suspension.

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		Quote:			Posted by GoldDigger1950
	Ah, Xavier, my Grasshopper, you have learned well. The other reason is that soil is irrelevant to a PI machine since the particulate size of oxides and leached metals is so small. With a VLF/TR machine those bits actually attenuate the signal - they absorb part of it. Those masses of particles can also completely hide a larger target below it. But the PI shines in that kind of environment. The fact that the eddy currents only travel on the surface of an object means they have to travel on the surface of those particles and they simply cannot. Not enough mass. The VLF/TR signal is different and during the on/off time the signal is transmitted directly into that mass of particles and doesn't even try to magnetize them. They are far too small but they DO transmit the signal from particle to particle making an effective mask which we cannot see through. That's in the all metal mode. In discriminate mode, they can penetrate the mass of oxides to a limited degree but the signal that gets through is much weaker. Remember, the receiver is working harder, not the transmitter. The receiver is the reason that discriminate works because the return signal is filtered out by the circuitry inside. That filtering also has the effect of making the item appear to have a weaker return and this can cause your machine to miss an object or two.

So in Normal type soils which would be deeper because if the PI Air Tests A Coin at 16 inches and the VLF See's it At 22 inches, In Mild Ground would the VLF see deeper??

Posted on: August 13, 2015, 07:06:43 am

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		Quote:			Posted by Lord Dracon
	:-  :-  :-  OK RR now I'm getting confused, if wet ground can upset a PI machine, why are they supposedly better for beaches? is it the salt water or is it the mineralisation the PI ignores to an extent? (Sorry I'm assuming alot of questions recently, as homie shook up what I thought to be correct hence doing my research   ) Posted on: August 13, 2015, 07:00:08 am thanks golddigger you answered my question

This is One thing that I have doubts in Because A lot of people on Another forum Claim that Their PI's Go Nuts in Wet Clay type ground and that the signal bounces Back causing the machine to make false signals, I thought that they should ignore it too.

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		Quote:			Posted by homefire
	Some Clay has High Mineral / Matalic content.  When Wet all those Ions, Particals are seen as ONE item.

Thanks Homie, Yes that was the issues I was talking about, and because it see's it as one lump it signals like it has found something, In OZ even when the ground is Dry most places out in the Bush will give a Ground reading as high as the meter Goes, yet to can still ground balance it out the Nuggets have to be almost A Couple of Grains in weight for a VLF to be to see them, But I still am not convinced about that, because if the Machine can Balance out the Ground then it should not have an effect,t which is why we Ground Balance the machine in the first place.

If a VLF Is Ground Balanced so the ground is supposed to be Invisible yet if they cant see very deep even though it has been GB'd then something else is going on that needs to be eliminated,  which seems to be missing on VLF machines,

Thanks Mate,, RR

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