Eric Foster... a coil question

[Anonymous]

Eric... how does a metal detector negate the wire and shielding present in the cable that attaches to the coil?
Seems to me that the coil would pick up the metal contained within the cable as it exists the coil.
Thanks...
Pete

 

[Anonymous]

Hi Pete,
You are right, in that a metal detector can detect its own wire coil and shielding unless certain precautions are taken. In a PI detector, the coil wire has to be thin enough for any eddy currents generated to die away considerably quicker than the shortest target signal you want to detect. If you want to use a thicker wire to carry a higher current, then stranded, or better still, Litz wire, will do the trick by breaking up the cross sectional eddy current path. Same with shielding, the conductivity has to be low enough to give a fast decay relative to the fastest target decay.
Eric.

 

[Anonymous]

That the larger the search coil diameter the less influence the seach coil wire would have in regards to the coil detecting it?
I'm not a PI user, but DFX user. I mostly search beaches here in Massachusetts. A PI machine is most interesting to me, but I think I would miss the decrimination part of it. While I dig most everything I usually don't dig what comes up iron.
From what I can understand regarding PI machines is that all signals sound the same therefore you must dig everything.
Pete

 

[Anonymous]

The fact that PI machine don't have discrimination is not quite true. It is true that they don't work like VLF discriminators but they can have very good sensitivity to gold jewelery and you can train your ear to hear the good stuff.
I have used the C.Scope CS6-PI on the beach and have found a similarity between the response of my Sovereign XS2 in the all-metal mode and a PI machine to rusted metal wire. Listen for the double blip and you can be almost sure it is a rusted iron target. On beaches in NJ, the old beach sand fences are made of twisted wire and wood slats and when they decay the beach is strewn with rusted iron shards of twisted wire.
When a target is detected, just cross the target left to right, up an down and at the two diagnals and listen for a double beep. If you hear it, then it is probably a rusted wire target. The double beep occurs at each end of the wire target. The farther apart the beeps are, the longer the rusted target is. If there is no double beep or the double beeps are very-very close, dig it! It may be a coin on end. Dig a few just to gain first hand experience. Cut a few different lengths of a coat hanger and take them to the beach and see what they sound like scanned in different directions (left-right, up-down, two-diagnals).
My PI machine audio signal has a more gradual ramp- up sound and more gradual ramp-down sound for rusted metal. Coins have a much more distinct or quick on and off sound when the coil passes over the target.
So even though PI machines don't have discrimination, you can train your ears to hear good targets about 75% of the time.
If you get a PI machine with variable pulse delay or frequency control, you can raise the frequency and if your target drops out, it is something of lower conductivity (like gold jewelery or nickels) to dig as rusted metal will be heard at both the higher and lower frequencies.
Keep looking at the PI detector forums to see new developments.
bbsailor

 

[Anonymous]

Hi Pete,
Often, larger coils are run at longer sample delays as you are primarily looking for big objects deeper. Coil/cable capacitance is then not so critical and neither is the wire cross section. However, if you were able to run at the same short delay as a small coil, then it would be.
Hope that makes sense.
Eric.

 

[Anonymous]

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