metal wingnut

[stanley]

this may sound a like stupid question but if you look at some old detectors IE garret groundhog-arado 120b-some feildmasters
the attachment screw and wingnut are made of metal
why does the detector not see this as a target
was the coil built with the nut in place and then (tuned) nulled out
(i cant get my head round this) maybe someone out there can tell me
as the old saying goes "if in doubt ask"

[Max]

this may sound a like stupid question but if you look at some old detectors IE garret groundhog-arado 120b-some feildmasters
the attachment screw and wingnut are made of metal
why does the detector not see this as a target
was the coil built with the nut in place and then (tuned) nulled out
(i cant get my head round this) maybe someone out there can tell me
as the old saying goes "if in doubt ask"

Hi,
yes, you are right. There were many things like that expecially in the past.

Some screws and wingnuts are made of metal or have metal parts inside e.g. plastic body.

Some are made of special metals/alloy (e.g. some bounty hunter models as far I remember) to reduce impact of metallic/ferrous content of screw-wingnut to the magnetic field.

Very often zinc is used as plating to prevent corrosion in such stuff.

Why they aren't detected ?
Is the coil(s) made to ignore screw/wingnut ?

Easy.

If a small metal object is in a fixed position respect to the coil(s) and doesn't introduce too much perturbations in the magnetic circuit it will not be detected by the MD circuit.

In detectors having such things you can notice a beep or sound anytime you move the coil around the axis of screw or any time you e.g. hit a stone or the ground, cause of different vibrations propagation along the coil/stem/pole... in some just sweeping will make they sound...thus it happens everytime there is a relative movement of screw/wingnut respect to the coil(s).

So, that old detectors aren't "tuned" for nulling out the metallic wingnut/screw ... it's just ignored cause its position is fixed respect to the coil... and untill it stay fixed respect to coil(s) it's almost "invisible".

Actual trend for VLF is to avoid totally metallic stuff near coils, where possible, and that is possible also cause of the reduced weight of parts respect to the past. That means many less disturbing false due to movements of coil e.g. during sweep in "good" detectors.

Mainly cheap series detectors (e.g. made in China but not only) mount now this kind of metallic parts.

Endly there are also cases in which having a metal part e.g. a washer of brass or something similar could give you better performances: Eric Foster wrote about this in his forum... to avoid some PI problems related to switchoff (spot overvoltage) cause the presence of that metallic part give additional balance to special PI fast coils with overshot phenomenons.

Best regards,
Max

[Max]

Hi,
sorry I forget a thing... and seems a mistake now :

when I say

"So, that old detectors aren't "tuned" for nulling out the metallic wingnut/screw ... it's just ignored cause its position is fixed respect to the coil... and untill it stay fixed respect to coil(s) it's almost "invisible". "

I meant coils of detectors, but also...

Actually this is true for e.g. motion detectors: you don't need any special tune on circuit cause object is in fixed position respect to the coil (and it's small too).

In non-motion detectors things are a bit different cause you actually null the effect of screw-wingnut by detector controls, I mean by GEB or by some other circuit that controls output behaviour.

By GEB a detector e.g. VLF/IB/TR can rotate phase coordinate system to compensate the small deviation due to the screw-wingnut: result is detector is ground balanced and "screw-balanced" at same time!

By other controls I mean e.g. in old BFO... you change the local oscillator frequency to get a fixed known tone with the screw-wingnut in place. Same thing for off-resonance, but could be done also adjusting e.g. filters to give same kind of effect.

In other kind of VLFs, non-motion this is often made by controlling the gain of an op. amp. or the threshold of a trigger and the like: result is the same, circuit is tuned for stable operations/null-signal with screw-wingnut on coil assembly.

Problems related are always as described. Hits and movements give false signals.

Best regards,
Max

[stanley]

it all makes sense

MAX Thanks for the info
it makes sense now
good these forums
thanks
stan

[robby_h]

Hi Max,
I posted this on the ProspectinginOz forum and you can follow the thread here if you wish.

http://groups.yahoo.com/group/Prospe...z/message/3244

Remove the ferrite core from a TV line output transformer, ground balance a ML pi to typical ground using an 8" mono or an 11" DD and normal settings, fix the GB and raise the coil away from anything that can influence it and pass the core across the coil. You will get a response which should be relatively mild and will depend on the model.
Note the response and then place a ~1 gm nugget on the center of the coil, wait until the threshold stabilizes and then pass the core over the nugget. The nugget should then respond as though it is moving relative to the coil. Place a small coin off to one side at a distance where it would be normally detected then you will get a response when the ferrite is passed over the coin.

You can do the same tests with a VLF motion detector but balance out the ferrite first.

This probably doesn't concern those with very mild ground but it can cause problems on the gold fields because some rocks have the same properties as a ferrite core. If the field is disturbed sufficiently then any metal fixed to the coil will respond as if moving relative to the coil. Bruce Candy's litz wire patent helps address this problem.
Rob.