Graphite coil shielding for TGSL problems

[billr]

That is a good video.
That is also how I did a DD coil for my TGSL including the plastic null adjusting screw (this is really nice).
I did ensure a 'break' in the shielding on each coil and then insulated each coil with electrical tape before mounting in the coil shell.
DD coil works very well and has a little more distance than the Aluminum foil shielded DD I made.

Walt !!
Can you add a little info on how you went about doing the coils?
Like Resistance measurements at the start and finish of the paint. I was getting 1480 on average when checked either side of the break in paint. This dropped the more I moved the probe toward the staring point. Not sure if that was too high.
Also the wire around the coil pressed up against the paint. I tried that method but didnt notice any change in resistance drop as the guy on the video said he did it for. Did it anyway.
Other than that the coils were good for range 13.5" but boy where they noisy. How did you manage to calm them down. Mine sounded like an SOS signal regardless of how much I nulled.
Also agree graphite is better than the aluminum foil for range and doesn't need the disc pot turning to cancel it out.
Any info you can recall when making your coils would be so welcome.

[billr]

So anyway. Update.
Took Ricks advice and removed all paint on each coil. Messy job.
Re nulled and placed paper coated with the paint above and below the coils. At first without grounding wires attached to them and then with to compare findings.
Tried several sheets. Each with different resistance values to them.

It did remove a bunch of interference noise almost to complete silence but the GB pot still needed to be turned 1/8th from fully closed to avoid any disturbance when the coil was touched by hand.
The cost however was a loss of range to what I had before paint removal. Dropped from 13.5 to 12.25". Approx the same as my working aluminum coil.

So for me at least, painting the shell with graphite and not the coil is a choice of noise vs range.
Flip a coin

[waltr]

If I remember correctly, the graphite (polyurethane + graphite power) measure in the 500 to 2k Ohm range.

The coil shielding is NOT to reduce INTERFERENCE. It is to reduce to capacitance effect of triboelectric charges on objects, grass, etc.
You hand is such a source so good for testing.

The GB adjustment has NOTHING to do with shielding and triboelectric charge.

The GB is adjusted so when a piece of ferrite (minerialized ground) is waved across the coil there is no response AND Silver is still detected. If GB pot is turned too far TGSL will not respond to silver (coin).

I found that I had to very, very slightly tweak the GB pot in the field to eliminate the ground. Bob the coil up and down near the ground, if it beeps then increase the GB pot, if it doesn't and detects a silver coin then it is good.


Last: I can be Very difficult to evaluate inside a house, workshop, due to AC Mains EMI and EMI from many, many appliances, etc in and around the house. The EMI can cause the 'Morse code' sound (that sound like some piece of electronic/electric equipment in your house and when it sound is not there that piece of equipment is not emitting EMI. Been there trying to get my first detector working.
If the shield prevents this then it will also decease detection distance.

[rickb]

What is the sheet resistance of the shell shielding? measure from opposite edges for very rough approximation. Did you thin the paint and test each layer before adding more? Are you using the paper shields and shell shields at the same time? You only need one type.

[rickb]

Waltr.

Triboelectricity is a different issue. It relates to conductor movement inside a cable. It can be a problem with coil cables, but that's not the issue here, and shielding won't fix it.

[billr]

What is the sheet resistance of the shell shielding? measure from opposite edges for very rough approximation. Did you thin the paint and test each layer before adding more? Are you using the paper shields and shell shields at the same time? You only need one type.

Its a DD shell so measured from 12 o'clock to 6 o'clock at the widest section between 1500 and 2000 ohms. Brush painting isn't very constant.
From 9 to 3 o'clock roughly 200 ohms. That part is only wide enough for the coil plus a little space either side as you know.

Yes, I thinned to very water like consistency (dried fast after application). Only applied one coat to shell but could see a decrease in range and increase in noise suppression.

At first I cleaned the shell of any paint applied preciously and used paper shields to gauge how much paint to apply to shell. When using different thicknesses of paint on paper my results showed noise reduction and range.

After painting the shell with one coat I did try placing the thinnest coated paper on top to see what happens. Same as above unfortunately.

[Koala]

are you sure you haven't got a ground loop problem


I had a coil that you could just gently blow over the top and it would chatter away


I would temporarily jumper from the shield to Rx ground on the PCB and not connect the shield in the head

[billr]

Last: I can be Very difficult to evaluate inside a house, workshop, due to AC Mains EMI and EMI from many, many appliances, etc in and around the house. The EMI can cause the 'Morse code' sound (that sound like some piece of electronic/electric equipment in your house and when it sound is not there that piece of equipment is not emitting EMI. Been there trying to get my first detector working.
If the shield prevents this then it will also decease detection distance.

Thank you Waltr.
As always. Great info for leaning.
Yes I recall recently I posted some issue with another machine. PI having coil noise and it turned out to be underfloor piping or cables so I learnt from that to do all testing outside away from power cables etc.

Hmmm. I wonder if I have over or under shielded and its causing a conflict with itself being able to null silently? It's like it is fighting any correction made to it, resulting in the Morse code effect.
Just me thinking out load.

[billr]

[ATTACH]54375[/ATTACH]


are you sure you haven't got a ground loop problem


I had a coil that you could just gently blow over the top and it would chatter away


I would temporarily jumper from the shield to Rx ground on the PCB and not connect the shield in the head

What the heck. Anythings worth a try. Thank you. Will give it a go tomorrow.

[rickb]

billr

The sheet resistance is to low. It should be tens of thousands of ohms (1 to 5) at least to avoid signal lose. Need thinner paint. If you can't reach 10K or more at least shoot for a few K ohms.

[billr]

Really?
Well I guess I could add more coats and try in between each coat to see how it goes.
Think I will use some release agent I have for fiberglassing on the shell before I start. It's none conductive and water soluble. Removing cured graphite is a PITA even with acetone. MEK would do it real fast but prolly dissolve the plastic.

Keep ya posted on my findings.

[waltr]

Waltr.

Triboelectricity is a different issue. It relates to conductor movement inside a cable. It can be a problem with coil cables, but that's not the issue here, and shielding won't fix it.

https://en.wikipedia.org/wiki/Triboelectric_effect

[6666]

https://en.wikipedia.org/wiki/Triboelectric_effect

I have to try that on our cat.

[rickb]

Really?
Well I guess I could add more coats and try in between each coat to see how it goes.

Adding more paint make your problem even worse.

Maybe you're a troll, maybe not. In any case I'm done.

[billr]

billr

The sheet resistance is to low. It should be tens of thousands of ohms (1 to 5) at least to avoid signal lose. Need thinner paint. If you can't reach 10K or more at least shoot for a few K ohms.

Adding more paint make your problem even worse.

Is anyone else confused by this comment or is it just me?
Low ohms to me, means 5R, 10R as in a low ohm resisters.
High ohms would mean 10K upwards.
To get higher resistance value in the coating up to 10K the only way I know how to do that is by ADDING more paint making it thicker.
Thinner paint would be less over all and equal less resistance. Would it not?
Unless I am missing something. You cant have it both ways.