Graphite powder as coil shielding

[satdaveuk]

[QUOTE=baum7154;192674]

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Dave are all Barracudas the same front end, motion vs non-motion? Can you point me to the latest schematic version or the specific one you are interested in?

Thanks,

Dan

Hi Dan
The one I am interested in is the original Barracuda (not the silverdog version) Its non motion , easy to find its hand drawn circuit together with hand written parts list it uses the tip127 in the front end.
If you have doubts will email you the necessary.
Thanks
Dave

[baum7154]

Satdaveuk, I will continue comment on the 2 stage amp mod at the TECH FORUM/chance pi build thread.

Dan

[baum7154]

[QUOTE=satdaveuk;192664]

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

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

Dave are all Barracudas the same front end, motion vs non-motion? Can you point me to the latest schematic version or the specific one you are interested in?

Thanks,

Dan

[baum7154]

In an effort to return this thread to its graphite shielding topic I have input information on the 2 stage amp modification in the CHANCE PI BUILD thread of the TECH FORUM. Questions and comments on the 2 stage amp are welcome there.

Thanks,

Dan

[baum7154]

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

[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]

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]

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]

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

[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]

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.

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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

[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]

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

[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

[Henrikras]

This Graphite 33 spray did I use... Easy and good quality.

http://m.rapidonline.com/ProductSearch/ProductDetails?ProductCode=87-0695&ModuleCode=30090