Graphite powder as coil shielding

[sdesic]

Hi guys

I had trouble finding graphite in spray.
Can I use this one ?

https://www.masters.com.au/product/9...?bmUID=kqlNDP_

it is Graphite powder ... I was thinking of spraying paint and then applying coating of powder.

Thoughts ?

[Thomas]

Yes, that graphite will work fine, I would make a mix of the graphite and or paint, epoxy, alcohol, naphthalene, kerosene,
what ever you can get for a carrier as long as it won't damage the wire insulation and brush it on, embed a piece of copper in it
for a tab to connect a wire to, also don't forget the gap between the two halves of the coil. Don (dfbowers) did a PDF on the subject.

[sdesic]

Any advice on amount of graphite ?
I have 50g.
Paint will be fine as it is sticky enough.

I can also use it to shield plastic box for detector.
I must find Don's document on shielding. I have on on fast coils for PI.

[Thomas]

Shielding isn't as important for PI detectors, some people don't even use shielding for PI.
But.. use as much as you can to get close to saturation, with out making it paste, it should flow smooth.
If the paint is water based add a little water to thin, if oil base enamel use mineral spirits to thin.

[Henrikras]

This Graphite 33 spray did I use... Easy and good quality.
http://m.rapidonline.com/ProductSear...duleCode=30090

[baum7154]

If you are shielding for a PI detector I would keep the resistance per running inch of surface to about 2000 ohms. I did one coil at 1300 to 1500 / inch and noticed some loss of sensitivity at that resistance. PI detectors mostly need shielding if you can detect your hand on the coil at the shortest delay you expect to detect at, typically for small gold in earth.

Good Luck

Dan

[baum7154]

If you want the full story and procedure for graphite shielding look up the Synthetic Graphite thread by Satdaveuk. He is 'Da Man' on this topic. It is worth reading the procedure he uses to get to a target resistance with a durable shield.

Dan

[sinclairuser]

after playing with silverdogs unshielded coil on a standard pi, i'm no longer convinced that it is required at all, if you are above 20us dont bother, just concentrate your efforts in other areas of your design.
i'm currently waiting to try a shielded version, as the detector i'm using is capable of 12us and a shielded coil may bring that back, a good coil that matches the specs of the pi, is more important than shielding it.
a bad coil is just that, shielding wont help it on a standard pi, being well constructed and on spec is what matters.
that said, baum7154 is correct if you must have a shield, satdaveuk's recipe works great, and you can substitute acrylic lacquer for polyurathane varnish, but cellulose lacquer attacked my coil shell, so avoid that.

[baum7154]

Shielding isn't as important for PI detectors, some people don't even use shielding for PI.
But.. use as much as you can to get close to saturation, with out making it paste, it should flow smooth.
If the paint is water based add a little water to thin, if oil base enamel use mineral spirits to thin.

--------------------------

You are correct that some people don't use shielding at all for PI. They are the ones who are not using the shortest sampling delays because the targets they seek do not require it. I sample at 8us in searching for small gold, but I don't add a shield to my coils any more. I tried graphite shielding on one of my coils and in my opinion it is the best material for shielding that we have, yet it degraded sensitivity a bit. Is this shielding project intended for sub 10us sampling in searching for small gold in the range of 1/2 gram or less? If so I would only apply enough graphite to get a 2K ohm / inch of resistance, and use a very thin undetectable wire as a contact and keep all solder joints very slight, preferably no solder in the coil at all.

Just my 2 cents worth.
Regards,

Dan

[Qiaozhi]

I sample at 8us in searching for small gold, but I don't add a shield to my coils any more.

Has the signal at the pre-amp output settled close to 0V when your sample pulse starts, or does it start when the signal amplitude is higher than this? Depending on the circuitry after the sampling gates, different designs can tolerate a larger offset voltage at the input of the sample gates than others. So often it is misleading to simply say "I sample at 8us". It might actually be the case that the pre-amp output signal doesn't settle close to 0V until much later than 8us. In which case an electrostatic shield may not be that useful.

[baum7154]

Without going into great detail and run the risk of high-jacking this thread my 2 stage output is at or very near zero in 7us. My sample delay opens at 8us and remains open for 12.5us. My properly damped coil settles in about 6us. When I say that I don't add shielding to my coils anymore it is because the geometry of my coils allows them to be both very fast and to self shield allowing them to retain their speed.

PS. I should add that the 2 stage amp is a modification I did precisely because the single stage was not coming out of saturation soon enough for the 8us sample.

Dan

[satdaveuk]

Without going into great detail and run the risk of high-jacking this thread my 2 stage output is at or very near zero in 7us. My sample delay opens at 8us and remains open for 12.5us. My properly damped coil settles in about 6us. When I say that I don't add shielding to my coils anymore it is because the geometry of my coils allows them to be both very fast and to self shield allowing them to retain their speed.

PS. I should add that the 2 stage amp is a modification I did precisely because the single stage was not coming out of saturation soon enough for the 8us sample.

Dan

Hi Dan
In your opinion could the 2 stage amp be implemented into other designs on here for experiments.
Regards

[baum7154]

Hi Dave, Yes a 2 Stage could be added in most cases where faster sampling is needed. An appropriate design for the specific detector would need to be implemented. In my case it was added by interfacing through the single stage IC socket so no significant mother board changes were necessary.

Regards,

Dan

[satdaveuk]

[QUOTE=baum7154;192658]Hi Dave, Yes a 2 Stage could be added in most cases where faster sampling is needed. An appropriate design for the specific detector would need to be implemented. In my case it was added by interfacing through the single stage IC socket so no significant mother board changes were necessary.

Regards
Dan
Hi Dan
When or if you get some spare time have a ponder over the Barracuda and let me know in your opinion if anything along those lines could be done there for added improvement .

Regards
Dave

[baum7154]

I will take look at it and let you know Dave. I think it must be possible though.