Crude coil sheilding paint

[Kev]

Hi Tinkerer,
I think the carbon rods in batteries are a very similar composition to motor brushes?

The coils are for fresh water nugget hunting so shield performance is vitally important with respects ground capacitive effects and minimal field disturbance.

Thanks for your input Tinkerer, I'm contemplating the epoxy imbedded shield idea.

Cheers
Kev.

[Kev]

hello all , thought i'd chip in with something,

a few months ago , i saw in my local diy store (focus in the uk) a tin of brushable blackboard paint , and it clearly stated on the tin "electricaly conductive" i wish i had purchased a tin now , the focus group here in the uk folded and got taken over by the b&Q group , and since then this magic electricaly conductive blackboard paint is not stocked.

Hi Dooley,
Don't worry, it probably was an anti-static type paint, which may have had a very high resistance, in the order of 100k to Meg Ohms to allow slow dissipation of of static charges without sparking. These can build on large surface area insulators, especially when rubbing out.

Cheers
Kev.

[simonbaker]

Hi Kev,

hey, you got me there. The truth is I don't know too much what all the differences are, without googling it. I usually sand down graphite brushes from starter motors etc., if I can not get graphite powder.
Graphite is very pure carbon that has a plate like structure. When you rub it, the platelets split in smaller platelets.
Carbon is more amorphous and less pure. Carbon black is often soot that contains a lot of impurities.
I don't know what the battery rods are made of, need to google it. Isn't Google a great resource?

I take it you are talking about salt water. This has it's own specific problems. Are your detectors for diving or the beach?
I used to build saltwater detectors 25 years when I was diving on old shipwrecks. I only wish I had had the knowledge and understanding about metal detectors then, that I have now.

For diving there is no need for a shield, since there is no EMI in Ocean water.
Tinkerer

I don't really know what shields actually do for metal detectors. EMI blocking doesn't make sense to me because it seems it would block the TX/RX signal as much as the EMI and not improve the signal to noise ratio. Also, it is not really shielding the coil loop as a whole so flux changes through the loop still get detected. The static discharge purpose seems to make some sense, but then why need such low resistance? Then there is something about ground capacitance???? I'd like to see some actual experiments with a variety of shields or some theory on what they do.

Certainly worth experimenting with and not accepting conventional wisdom, unless there is a ton of data out there we can look at.

-SB

[Kev]

Hi Simon,
Here's what the Master has to say on the subject.
http://www.findmall.com/read.php?34,...370#msg-134370

HH
Kev.

[maikl]

Here's the same Master a different way to shield http://www.findmall.com/read.php?34,1174380

[DOOLEY]

so , what is the "ideal" resistance per square inch for use on a PI then ??

so that us people with ocd / the want to be perfectionist people, can aim for , and then feel happy with knowing we have the perfect shield.

not sure about using ribbon cable as a shield though maikl , allthough i bet it works , not sure if it's ideal . with it being wrapped round will it not develope interferance ??

kev , the other thing about that diy blackboard paint i remember was that after painting on a non magnetic surface it enabled you to stick on those magnetic markers / fridge magnets etc on to it, so , by my rekoning it had to be a ferrous based conductive paint , which would be no good for any of use on here.

one thing for sure , anything has got to be better than kitchen foil , tried this , my home brew detector can "see" it , so currently run with no shield ,
which is a pain when setting detector up ,useual 50hz mains in the house and somewhere a 1hz clock signal !

[Tinkerer]

I don't really know what shields actually do for metal detectors. EMI blocking doesn't make sense to me because it seems it would block the TX/RX signal as much as the EMI and not improve the signal to noise ratio. Also, it is not really shielding the coil loop as a whole so flux changes through the loop still get detected. The static discharge purpose seems to make some sense, but then why need such low resistance? Then there is something about ground capacitance???? I'd like to see some actual experiments with a variety of shields or some theory on what they do.

Certainly worth experimenting with and not accepting conventional wisdom, unless there is a ton of data out there we can look at.

-SB

You said it: flux changes.

The TX and RX, are of magnetic nature. Changes in the magnetic flux (B field?) of the TX and RX coils.

We want to shield against EMI (E field?) that is generated outside the metal detector, often far away, although I found a good test for the shield to be the EMI that the hand emits.

I looked at the FFT of the EMI that my hand introduced into the un-shielded coil, at the output of the preamp. It was of a very wide spectrum, with the upper limit rolling off above the coil's self resonant frequency (300kHz).

Then I tinkered with the shielding, until I could not see any "hand noise" anymore. Good enough.

An exact definition of coil shielding is related to the sensitivity of the coil. A thumb rule would be, the detector should not pick up the shielding if presented to the coil.

Once I got the detector sensitive enough to detect gold leaf (1/250,000 of an inch thickness), at several inches distance, I found that the shielding with graphite compound was still working fine. Why?

The shield should not sustain eddy currents.

Tinkerer

[Comp]

I'm currently (!!) playing with this tape,........ Seems Ok, and a doddle to solder to!!


http://www.ebay.co.uk/itm/Copper-Foi...22097831220363


Cheers, Fred

[Kev]

You said it: flux changes.

Then I tinkered with the shielding, until I could not see any "hand noise" anymore. Good enough.

Tinkerer

Hi Tinkerer,
With Minelabs you can only shield to eliminate HF EMI and not LF or else the detector would become dumb, and no better than a home brew machine, throw away US$5K.

It is the capacitance of other objects especially the ground that must be completely eliminated. Eric says that if you do this properly then sensitivity is actually improved contrary to how we would think. Your hand would've been introducing a capacitive load to the coil and the EMI was using this to migrate to the coil.

Seems to me that a Faraday shield is specific to each detector and search head design and there are no set values other than rules that work for all. What is good for one design may dumb down another, hence the need to experiment with your own setup, as you cleverly did to arrive at the best compromise.

Cheers
Kev.

[Tinkerer]

Hi Tinkerer,
With Minelabs you can only shield to eliminate HF EMI and not LF or else the detector would become dumb, and no better than a home brew machine, throw away US$5K.

It is the capacitance of other objects especially the ground that must be completely eliminated. Eric says that if you do this properly then sensitivity is actually improved contrary to how we would think. Your hand would've been introducing a capacitive load to the coil and the EMI was using this to migrate to the coil.

Seems to me that a Faraday shield is specific to each detector and search head design and there are no set values other than rules that work for all. What is good for one design may dumb down another, hence the need to experiment with your own setup, as you cleverly did to arrive at the best compromise.

Cheers
Kev.

There is an important factor that I forgot to mention. The coil to shield capacitance. This can slow down a coil dramatically and make it useless for small targets.

A spacer of 4mm to 6mm between the coil winding and the shield helps a lot.

Tinkerer

[simonbaker]

You said it: flux changes.

The TX and RX, are of magnetic nature. Changes in the magnetic flux (B field?) of the TX and RX coils.

We want to shield against EMI (E field?) that is generated outside the metal detector, often far away, although I found a good test for the shield to be the EMI that the hand emits.

I looked at the FFT of the EMI that my hand introduced into the un-shielded coil, at the output of the preamp. It was of a very wide spectrum, with the upper limit rolling off above the coil's self resonant frequency (300kHz).

Then I tinkered with the shielding, until I could not see any "hand noise" anymore. Good enough.

An exact definition of coil shielding is related to the sensitivity of the coil. A thumb rule would be, the detector should not pick up the shielding if presented to the coil.

Once I got the detector sensitive enough to detect gold leaf (1/250,000 of an inch thickness), at several inches distance, I found that the shielding with graphite compound was still working fine. Why?

The shield should not sustain eddy currents.

Tinkerer

The physics of shielding is complicated to me, but your experiments indicate it works to reduce EMI (although for non-PI MDs the Synchronous Detector may effectively kill EMI more than a few hundred Hertz from the TX frequency because it is an extremely sharp filter), so I will believe they work, I'm just not sure to what extent and how.

In E&M theory, there really is no way to separate changing magnetic and electric fields -- one always goes with the other. You can't just have a changing magnetic field. However, there are some "near field" magnetic field components that are non-radiative and perhaps that is what we rely on with our MD coils, and so we shield against the radiative components that are small from our TX coil but large from EMI due to frequency differences. I think conductive shields work more effectively as the frequency increases -- to a point. After that point, they don't work well I think. So I would think maybe for non-PI MDs, our shields improve the S/N ratio for high frequency noise (> 20 kHz), but maybe they make it worse for low frequency noise (< 10 kHz, e.g. 60 Hz, 50 Hz) because they block our TX signal more effectively than the noise????

The way a conductive material shields against electromagnetic waves requires precise currents in the shields. I guess by "eddy currents" you mean extra currents that slop over that are not precisely the ones that cancel the field. So yes, we don't want eddy currents, but we do want some currents if we want to cancel EMI. If we just want to dissipate static charge, I would think we don't need much conductivity.

I'm just thinking out loud, I don't have a good model in my head for how shields work, especially ones that wrap around the wire bundle as opposed to a "box". So I'll rely on experiments such as yours for guidance for now. Also, I tend to think in terms of non-PI MDs, and I think you're focusing on shielding PI coils, so maybe some differences there.

Regards,

-SB

[turtlebowl]

Found this at Home Depoe. A product made by Blaster and cost about $7 per can. It is quick, uniform and works really nice. I found it in the key making area.
http://www.blasterchemical.com/graphite-dry-lube.html

Ohm value very much depends on how thick the application sprayed on. Single medium coat reads about 2-3K, thin coats up to about 8K per square inch. 3 coats about 600 ohm.
I did not buy a can but happened to have a ohm meter in my pocket and the rep let me try the spray in the store. I sprayed a paper sack and brought it home for more testing.
I think one would need to spray a sealer over everything when finished as the graphite will rub off with medium abrasion.
hope this helps someone. Please let us know of any further experimenting with this product.
Phil M

[turtlebowl]

I am relatively new to this Coil section of the forum so have not read much of what is contained here. So forgive if this has been discussed to death already but it sounds like
there is continuing discussion of the actual purpose of shielding??
I was told 25 years ago by an engineer in Forest Grove that helped with designing my pinpointer coil that the purpose of the sprayed on shield was to dissipate Static Electrity generated in the plastic from rubbing it against things. The use of Graphite prevented eddy currents from forming in the shield. At least that was the intent of the shields from the 1980's.
I suspect you guys are talking of something else though huh?
Phil M

[simonbaker]

I am relatively new to this Coil section of the forum so have not read much of what is contained here. So forgive if this has been discussed to death already but it sounds like
there is continuing discussion of the actual purpose of shielding??
I was told 25 years ago by an engineer in Forest Grove that helped with designing my pinpointer coil that the purpose of the sprayed on shield was to dissipate Static Electrity generated in the plastic from rubbing it against things. The use of Graphite prevented eddy currents from forming in the shield. At least that was the intent of the shields from the 1980's.
I suspect you guys are talking of something else though huh?
Phil M

I know I don't fully understand the effects of our shields... especially since there seem to be quite different types of shields, from the high-conductive foils to the lower conductive sprays and graphite mixtures. So anything that sheds light is interesting.

Of course, even engineers may not really know all that a shield is doing -- the bottom line is someone noticed that things go better with a shield. I'd like to see actual situations (different soils, EMI environments, humidity, soil surfaces, etc.) demonstrating an unshielded coil vs a shielded coil, for each type of shield material.

And then there are other variations -- such as just shielding the RX coil and just shielding the TX coil -- how would that compare too?

Also variations in how you ground the shield and coil leads -- dfbowers noted some interesting variations with the TGSL coils.

All these variations are part of shielding.

Regards,

-SB

[Comp]

I know I don't fully understand the effects of our shields... especially since there seem to be quite different types of shields, from the high-conductive foils to the lower conductive sprays and graphite mixtures. So anything that sheds light is interesting.

Of course, even engineers may not really know all that a shield is doing -- the bottom line is someone noticed that things go better with a shield. I'd like to see actual situations (different soils, EMI environments, humidity, soil surfaces, etc.) demonstrating an unshielded coil vs a shielded coil, for each type of shield material.

And then there are other variations -- such as just shielding the RX coil and just shielding the TX coil -- how would that compare too?

Also variations in how you ground the shield and coil leads -- dfbowers noted some interesting variations with the TGSL coils.

All these variations are part of shielding.

Regards,

-SB

My take is the purpose of the shield is twofold,........ A, to screen the coil from random electronic signals, (Radio waves, static noise, E.T. {LOL} ) which could cause a 'false
' signal, and B, to place an 'earthed screen', in between the coil/winding, and ground,... because as you swing the search coil near the deck, variations in ground capacitance will again, cause false signalling,...... {I personally suspect this is less of a problem with PI,.. but dont bet on it, I'm a newbie!!!}

Of course, if the Guru's can enlighten me otherwise with their wisdom, I will happily stand corrected.......

Cheers, Fred