Crude coil sheilding paint

[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 !

[maikl]

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

[Kev]

Hi Simon,
Here's what the Master has to say on the subject.

http://www.findmall.com/read.php?34,134370,134370#msg-134370

HH
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]

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.

[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.

[Tinkerer]

Hi Tinkerer,
I'm very interested in why you say to use graphite, is it just because of the resistance?

The dielectric constant of graphite can be up to 40 times greater than carbon, and since I'm building coils for underwater use, I'm already in a high absorption and field scattering medium so thought that carbon at least gives a little bit back over other shielding.

Cheers
Kev.

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

[Kev]

Plasti-kote # 281 Graphite Spray sucks for my application. Just saved everyone $10.
Sporatic (very) results even though the can says it is " ideal where lubrication and electrical characteristics are required"" and "for coating cathod ray tubes" "grid coatings"
and etc.
Seems that the graphite may sink to the bottom of the sprayed on area and the binder floats to the top and hardens. Thereby making it very difficult to attach a ground wire or read with a meter. After several coats I could only sometimes get a reading. And then only on non-porious items. I want/need it to work on cardboard/paper for some of my experiments.
Everyone elses ideas are much better than this spray.
Phil M

Hi Phil,
Did you paint over a connection/grounding wire, or are you trying to measure resistance on the surface of the paint?

Cheers
Kev.

[Kev]

it is important to use graphite, not carbon, although chemically nearly the same, the molecular structure is different and the graphite is a much better conductor.

Tinkerer

Hi Tinkerer,
I'm very interested in why you say to use graphite, is it just because of the resistance?

The dielectric constant of graphite can be up to 40 times greater than carbon, and since I'm building coils for underwater use, I'm already in a high absorption and field scattering medium so thought that carbon at least gives a little bit back over other shielding.

Cheers
Kev.

[turtlebowl]

Plasti-kote # 281 Graphite Spray sucks for my application. Just saved everyone $10.
Sporatic (very) results even though the can says it is " ideal where lubrication and electrical characteristics are required"" and "for coating cathod ray tubes" "grid coatings"
and etc.
Seems that the graphite may sink to the bottom of the sprayed on area and the binder floats to the top and hardens. Thereby making it very difficult to attach a ground wire or read with a meter. After several coats I could only sometimes get a reading. And then only on non-porious items. I want/need it to work on cardboard/paper for some of my experiments.
Everyone elses ideas are much better than this spray.
Phil M

[Tinkerer]

Hi Turtlebowl,
I'm using plasti-kote fast dry enamel. I started with about 30mls and finely ground 2 carbon rods from an old 6V heavy duty carbon battery. That gave me about 1-2k Ohm. I ground up a 3rd rod and that gave about 500 to 600 Ohms per inch, I added a 4th rod and arrived at the 80 - 100 Ohms per inch that I required.

Gorge Payne recommends using between 1 - 10K Ohms for a VLF, but my Minelab needs much less than that to ground balance properly, it seems, unless it's just this coil configuration in particular.

For the ground wire I solder the middle of a two meter length of tin plated wire about the thickness of this | (maybe slightly thinner) to the shield side of the coax and wrap each arm spirally around the coil about an inch apart towards where the the shield gap will be, which sits exactly opposite the hole in the shield where the coax enters. The two wires don't meet, but are terminated before reaching the gap, then begin painting over the whole lot.

Painting a shell is a bit harder, I use a piece of flexible mylar pcb strip from inside an old hard drive. Strip the top layer of mylar off the copper, and solder a fine connecting wire perpendicular to the traces, then glue the strip to the shell with the copper side facing up, then paint over this, you need surprisingly little copper surface area to get a good connection. If you then cover this with a small amount of low shrink epoxy it will last the distance.

HH
Kev.

Your method is about as good as it goes. For the graphite compound, it is important to use graphite, not carbon, although chemically nearly the same, the molecular structure is different and the graphite is a much better conductor.
since you already use epoxy, you might as well use epoxy as the binder for the graphite paint. About 1% resin, 98% graphite and 1% fumed silica (Aerosil) First mix the resin, then add enough solvent or thinner so that when you add the graphite and fumed silica it is easily paint-able. The silica prevents the paint from running off. (tixotropic) The silica also adds some coherence.

Adding more epoxy, reduces the conductivity and increases the strength.

The paint will harden (cure) once the solvent has evaporated and make a hard solid coat. Brushing the hard paint increases the conductivity.

To measure such shielding a method "measure by square" is used. It can be 1" square or one centimeter square, but it is always between 2 wires embedded in the shield. Ohmmeter probes directly on the shield do not give consistent results.

If the resistance is too high, (after thorough cure), a second coat can be added.

If the resistance is too low, some of the coat can be sanded off.

For casting epoxy resin in the coil mold, I recommend adding glass micro balloons. It makes the mixture much lighter and is totally inert.

Tinkerer

[DOOLEY]

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.

[simonbaker]

Kev, have you any powder to paint ratios jotted down and their respective resistance Opis to use as a guide? What paint base did you settle on and are you spraying or brushing?
I tryed some stuff with rubber cement last night and was dissapointed. To gooie, even diluted.
I like Mikle's post about the elmers becuase it is so easy but I wonder about anything water based that is used outdoors becuase of condensation??
Anyone have any knowledge or thoughts on condensation VS water based sheilding? Maybe there wouldn't be condensation if both inside and outside temp of enclosure were equal?
I'm going to the hardware store for graphite and screw around some more with this. I am looking for Opis about 600- 1K.
Phil M

I tried elmers type glue for binding a set of TX/RX coils and it seemed that after I put the glue on, the coils would hardly get any depth. I also noticed that the glue kind of got stiff and crumbly after a year or two, so I would probably avoid it. But I like it because non-toxic.

Has anyone tried bathtub caulk? Have no idea if it would work, but it shouldn't get brittle.

-SB

[Kev]

Hi Turtlebowl,
I'm using plasti-kote fast dry enamel. I started with about 30mls and finely ground 2 carbon rods from an old 6V heavy duty carbon battery. That gave me about 1-2k Ohm. I ground up a 3rd rod and that gave about 500 to 600 Ohms per inch, I added a 4th rod and arrived at the 80 - 100 Ohms per inch that I required.

Gorge Payne recommends using between 1 - 10K Ohms for a VLF, but my Minelab needs much less than that to ground balance properly, it seems, unless it's just this coil configuration in particular.

For the ground wire I solder the middle of a two meter length of tin plated wire about the thickness of this | (maybe slightly thinner) to the shield side of the coax and wrap each arm spirally around the coil about an inch apart towards where the the shield gap will be, which sits exactly opposite the hole in the shield where the coax enters. The two wires don't meet, but are terminated before reaching the gap, then begin painting over the whole lot.

Painting a shell is a bit harder, I use a piece of flexible mylar pcb strip from inside an old hard drive. Strip the top layer of mylar off the copper, and solder a fine connecting wire perpendicular to the traces, then glue the strip to the shell with the copper side facing up, then paint over this, you need surprisingly little copper surface area to get a good connection. If you then cover this with a small amount of low shrink epoxy it will last the distance.

HH
Kev.

[turtlebowl]

Do tell.
I'd like to see the recipe for carbon comp resisters. The binder would be handy and especially the method of bonding to wire.

I ordered a can of plasti-kote #281 grapghite spray today and will get it tomarrow to try for shielding. $10 per big spray can at the car parts store.