Crude coil sheilding paint

[multieagle]

A big Yes use Yacht varnish, proved beyond no shadow of a doubt, 12 months down the line no deterioration at all, easy to apply and cheap, works wonders also good for shielding styrene housings.

This may be a foolish question, but guess a newbie is allowed that latitude. How is this grounded? Do you run a naked ground wire through the varnish/graphite/carbon? Are the coil wire/cable connections within, below, or above the epoxy or varnish shell?

[WM6]

A big Yes use Yacht varnish, proved beyond no shadow of a doubt, 12 months down the line no deterioration at all, easy to apply and cheap, works wonders also good for shielding styrene housings.

You mean as electrostatic shielding - mixed with graphite? Which brand/product did you use?

[satdaveuk]

Can't we use any good quality paint (that will stick to the coil shell or coil covering) and add the same ratio of graphite powder?

A big Yes use Yacht varnish, proved beyond no shadow of a doubt, 12 months down the line no deterioration at all, easy to apply and cheap, works wonders also good for shielding styrene housings.

[WM6]

Take your AVO meter to your store and ask for measure conductivity (resistance) of those artist carbon paper. If it show conductivity, then only question how to connect ground wire to those paper remain.

[multieagle]

Somewhere I recall reading that carbon paper, the kind used by artists to transfer drawings, images to art paper, could be used as shield material. It would have to be pasted onto thin cardboard. I do not wish to spend the $25 locally to test it out before finding out if it can indeed be used. Anyone have an opinion? It certainly looks similar, except for thickness (tracing paper), to carbon coated paper I've seen removed from a Minelab coil , in a YouTube video http://www.youtube.com/watch?v=BY7WFyCVO_A.

[Tinkerer]

Good graph Tinkerer,

500 pf including including the cable capacitance if a good yard stick for the Minelabs.

Have you opened up a ML coil to see that the winding is level with the bottom of the foam in order to place the windings as close as possible to the target.......but in doing so the ML winding is hard against the Shielding thus adding maximum c. Their testing must of told them that it is more important to gain 4mm of depth in relation to coil to target proximity than to reduce the coil capacitance and to optimize DV/DT.

Thanks for the feedback.

The matter of the coil capacitance is quite complex. For example, only about half of the winding to shield capacitance actually does "show up" when we calculate the capacitance by the self resonant frequency of the coil. What happens to the rest of the capacitance?

We have the inter-wire capacitance and the winding to shield capacitance and the cable capacitances, maybe some of these capacitances add up in series and so get reduced?

It seems that for every answer to a question we find, so many more questions turn up.

Ah, by the way, the graph above shows the di/dt, the current, not the voltage, since it is the coil current that generates the magnetic field.

Tinkerer

[detectormods]

Good graph Tinkerer,

500 pf including including the cable capacitance if a good yard stick for the Minelabs.

Have you opened up a ML coil to see that the winding is level with the bottom of the foam in order to place the windings as close as possible to the target.......but in doing so the ML winding is hard against the Shielding thus adding maximum c. Their testing must of told them that it is more important to gain 4mm of depth in relation to coil to target proximity than to reduce the coil capacitance and to optimize DV/DT.

[Tinkerer]

Coil to shielding capacitance

Now, how does it look with 500pF capacitance?

Take note of the time it takes for the coil current to reach near zero A.

Also look at the coil current decay slope. See how it drops first and then goes up again. What this means is that the current generates eddy currents of one polarity in the target and then as the current direction reverses, it nulls these eddy currents and only when the current is on the final slope it starts generating the eddy currents that give us the target response.

Tinkerer

[Tinkerer]

coil to shielding capacitance

100pF capacitance

[Tinkerer]

Shielding capacitance

Why paint the coil shell? Paint the coil core as this will reduce noise as the coil is locked to its Faradat shield. Aim for 500pf or less shield to coil capacitance.

A spacer between the coil winding and the shielding greatly reduces the coil to shield capacitance.

What influence does the coil capacitance have?

Below is the coil current of a traditional PI, critically damped, with 100pF capacitance.

Take note of the coil current decay and the time it takes to reach near zero A.

Tinkerer

[petravka]

Whether the original MINELAB TS1000 graphite to the coil shell.

[detectormods]

Why paint the coil shell? Paint the coil core as this will reduce noise as the coil is locked to its Faradat shield. Aim for 500pf or less shield to coil capacitance.

[petravka]

Whether this is good, and so on. I'm interested in how to apply the final layer of graphite?

[detectormods]

When shielding your coils for Minelab detectors try to aim for 80 ohms per inch, this is the standard Minelab and coiltek values. If you look at the newer Nugget finder coils the resistance is close to 30 ohms per inch. The new range of commander coils are 700 ohms per inch. I suspect that it may be so high in order to get the fine gold mode working correctly on the gpx5000. The Q is fairly high on the NF 12" at 5 while the others range between 3.2 and 4.6

[mikebg]

Graphite shielding

Graphite shielding
In the 1940s started the use of conductive water-based graphite dispersions for shielding. Minelab places inside search heads paper sheets covered with aquadag for coil shielding.
Search the WEB for "aquadag"

http://www.semicro.org/Aquadag%20E%20TDS.pdf