Once again about coil shielding

[Mrand]

Good shielding, is the base of correct work of the search head VLF metal detector. A most obvious method is the painting conductible graphite paint. Depending on paint and the manner of painting, the coat has a different resistance. Too small resistance causes losses of the signal. Too large resistance causes that shielding is ineffective. Somebody once said, that the contractual border of resistance was 10 kohm/inch. In praxis appear different values of the resistance.
Did somebody explore over the real efficiency of shielding, depending on the resistance the graphite coat?
Mrand

[Koala]

Nope

IDX works well from at least 1K-500K per square. No idea which is optimal.


However I have encountered problems with the vertical sides. Sometimes just the top and bottom is enough. Other times the outer vertical edge needs doing otherwise every blade of damp grass causes a false signal. Does appear that the thickness of the coil but sample size to small to be 100%

[Skippy]

Henrik did a bit of experimenting with this on his 'Deus small coil' thread, he ended up with quite a low value, 1 - 2 Kohms per square, I recall.

[Geo]

Hi Mrad.
I did not found problems with values between 1...5 Kohms.
I always sprinkles the coil house 2...3 times and it is OK.

[Mrand]

I use graphite paint (water type) which gives the resistance of the coat about 5-7 kohm. The problem is when I assemble the bottom cover. There appears the additional shift of the phase (several degrees). This results from the too low resistance of shielding layer. I want to enlarge the resistance to avoid this effect. The question is, as far as?
Geo, very thank for the C-scope schema. The detector already repaired. The friend is glad.
Mrand

[cacekspoko]

Bottom cover is covered by graphite too? Maybe there is a ground loop and bottom shield touch the top.

[rickb]

Mrand: Apply a thinned coat to the surfaces and check for proper shielding effect 1 layer at a time. Test using the ohms per square method on cardboard and keep thinning until you reach about 100 kohms per square. Paint the coil housing 1 layer at a time until proper shielding is achieved. I plan to use cardboard or thin plastic sheets attached to grounded wires for the shields.

http://four-point-probes.com/sheet-r...-applications/

[Koala]

I use graphite paint (water type) which gives the resistance of the coat about 5-7 kohm. The problem is when I assemble the bottom cover. There appears the additional shift of the phase (several degrees). This results from the too low resistance of shielding layer. I want to enlarge the resistance to avoid this effect. The question is, as far as?
Geo, very thank for the C-scope schema. The detector already repaired. The friend is glad.
Mrand

I find it also alters slightly.

However if null it first.

Then when I apply the top shield it moves one way but when I install the bottom it pulls it back to where it was.

Even if it didn't then its not enough to cause it not to ground balance correctly.