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[TowerTalk] Non-corrosive RTV ; NEMA boxes

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Subject: [TowerTalk] Non-corrosive RTV ; NEMA boxes
From: "Kimo Chun" <>
Date: Tue, 11 Jan 2005 20:04:20 -1000
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FWIW, Dow Corning makes 748 Non-corrosive RTV sealant (White). 100% Silicone 
Rubber; Safe for food contact; Non-corrosive; Less objectionable odor; -67F to 
+350F (-55C to 177C).

Applications: Sealing corrosive-sensitive electrical and electronic equipment 
(electrical connections, wire entries, power and control cable connections, 
etc.); Bonding and sealing food processing equipments; Formed in-place gaskets 
for sensitive substrates.
I have the 3 U.S. Fluid Ounce ( 90 mL) tubes.

In general, NEMA-3R and 4X are the types of boxes you ought to use. However, 
the 3R may only retain its "rain-tight" rating when it is "upright". There's 
also no guarantee in a hurricane with horizontal wind driven rain, hi.

My personal rule, which I carry on to work whenever possible, is to never 
penetrate the roof (top of the box) unless it's the last resort. Walls can be 
penetrated with appropriate weather-tight connectors but even better if they're 
under an eave of a roof (or lip of a box - that's how the 3R boxes are when 
they are "upright"). Best to go from below whenever possible. It is also very 
true that it is difficult to make a box watertight and dry (except for 
specialized underwater boxes) especially here in the tropics (and as soon as 
you start making holes in it). Don't even try. Use drain holes. When mounting 
operating radio equipment (repeaters) permanently outdoors (at least here in 
Hawaii) I put the entire equipment cabinet in a larger wall mounted cabinet (3R 
or 4X) and use forced air ventilation to keep a stream of outdoor air 
circulating through to keep moisture build-up to a minimum. Of course, filters 
and deflectors are used. Compensation for your varied climate conditions must 
also be taken into account. We're lucky in that regard.

If you must mount things internally and through a wall, consider putting a 
little RTV on the screw before you drive it through the hole.
If you can find (and afford) specialized boxes some come with studs welded to 
the back of the box and have a floating plate / chassis that sits on these 
studs. This will allow you to mount some hardware without any holes in the 
outer box (not counting ingress and egress of cables, etc.) These are common on 
naval (brass) shipboard boxes but are very expensive. There are commercial 
boxes as well. Unfortunately, electrical boxes with any special qualities are 

Good luck.

Kimo KH7U

See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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