Shielding material?

[Russ_NY]

Hi Folks,
I know several of you good people have tried a variety of shilding techniques and materials. I have tried aluminum foil (kitchen variety), self adhesive aluminum and copper tape, Which is I am told too thick. I have seen the carbon based sprays and have used it for static disipation in the past. I hear about the nickel based paint, but have never seen a source for it. What about Aluminum paint? Has anyone actually tried it? I was planning on spraying a little test strip and seeing if it is conductive. From previous posts I gather that too heavy of a shield can generate it's own eddy currents masking the desired ones. Anybody have any idea what the actual resistivity should be? How do you actually measure it? Is there a standard size or shape? The entire subject of shielding materials seems very difficult to quantify. The folks who make detectors have somehow managed to come up with materials and methods which give them desirable consistant results. I personally have only seen the inside of one commercially produced coil, (Older Whites 4B)and it has no apparent shielding at all. Any responses will be greatly appreciated.
Thanks

Russ_NY

[Reg]

Re: Shielding material?

Russ,

I checked aluminum paint some time ago and it didn't seem to be conductive. I don't remember the brand right now. Anyway, I gave up on it.

There are several things one can use for shielding. Eric Foster uses lead foil tape. I recently saw some for sale on Ebay. See; http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2540941685&category=14 947

One would have to cut strips down a little since it is 1 1/2" wide, but I would think it would work since it is quite thin and it is fairly cheap for a roll of it. I just measured some and it appears to be about as thick as the type Eric Foster uses. The down side of this tape on Ebay is the adhesive has to be activated. I have not tried it yet so I am not sure how well Mek will activate it. If it doesn't activate it well , I will just wrap the foil on and then tape it in place. I have tried other lead foil with adhesive and it works fine on a mono coil.

Carbon based paints probably would work for the electrostatic shielding part just fine but most likely would not do much for reducing noise. Carbon based paints have been used for years on VLF coils and it seems to work fine on them. Again, I have not tried this yet but I am suspecting the shielding from ground capacitance will be fine but it will do little for noise.

Nickel Based paints are available. I saw some listed on the Lessemf website. See http://www.lessemf.com/paint.html

I don't know how well it works since I have not tried it.

Regular copper foil tape most likely will cause problems but the copper plated fabric tape I use works fine. I have both DD and mono coils working down less than 10 usec with no problems. I think Lessemf has some of the tape I use but I am 100% not sure it is the same.

I just purchased a roll of the 3m 1190-1 copper plated fabric tape I use from Contact East. I was informed a couple of months ago that purchases had to be a minimum quantitity of 8 or 9 rolls, but that seems to have changed.

I have also used 3M Semiconducting rubber tape and it seemed to minimize the capacitance effect but didn't seem to do that well on the noise factor. I had coils working down in the 12 usec with this.

Reg

[Don You g (Faux-Oro)]

Re: Shielding material?

In the Eric Foster PI forum It was mentioned that they used lead foil of some type on the PI detectors. I have a coil for an XLT, the small one that is for trashy areas, and it has the black powder sprayed inside of it. Looks like toner for a copy machine.
Don



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[alan c]

Re: Shielding material?

I believe the preffered thing is 'aqua-dag'.It's the black coating on the inside of telivision tubes...It's just painted on.
I have looked everywhere for a supplier and can't find one.If anybody can find a source of 'aqua-dag' please let me know.

[Reg]

Re: Shielding material?

Hi Alan

Check this link for graphite spray; http://www.action-electronics.com/gcelect.htm#Emi

I tried this a long time ago on a VLF coil and it seemed to work, but I have not tried it on a PI coil.

Reg

[Eric Foster]

Re: Shielding material?

Hi Russ,

In addition to the lead tape, I have used nickel paint for shielding, both the spray on and brush on types. Just measured a coil shell that I had used this on and it measures 1.5 ohms if you have the probes 1/2in apart. I fact with the probes anywhere and with any separation, the reading does not go above 2 ohms.

One word of caution with nickel paint (in addition to Sean's more serious one), nickel is a magnetic metal and with a PI, you do get a small amplitude long time constant decay curve. As it is a fixed signal, it normally zeroes out OK, but I'm not sure what effect it will have on a balanced coil arrangement.

Eric.