The optimum graphite shield resistance

[soverey]

I have been reading multiple threads on the resistance of graphite coil shielding, but I was unable to find a clear answer to what is the required resistance of graphite shielding in order to be effective, without having negative impact on the sensitivity of the metal detector. Some say it is around 2000 ohms per square inch, others 5000, or 10000 ohms. Maybe some members that work in the metal detectors industry could shed some light on this topic.
Thank you.

[Koala]

I would be interested in what original manufactures use.

[Skippy]

Surface resistivity is measured in Ohms per Square. Not "Ohms per square inch".
It's not easy working out what commercial coils measure, as there are no handy straight-line sections to probe on. And of course some use graphite-loaded plastic shells, not painted insulating shells.
From Henriks experiments on his Deus small coil project, it would seem that 500 Ohms per square would be a good figure to aim for. Detector designer George Payne suggests higher figures, and says there can be problems going below 1000 Ohms per square.
I have prodded an XP GMAXX coil, it looked like it's surface resistivity value was in the 500 Ohms area.