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Re: [TowerTalk] Weatherproof Sealant

To:, <>
Subject: Re: [TowerTalk] Weatherproof Sealant
From: Jim Lux <>
Date: Tue, 11 Jan 2005 09:48:08 -0800
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At 07:17 AM 1/11/2005, Keith Dutson wrote:
I need to weatherproof a relay box.  Under test conditions it accumulated an
inch of water in the bottom over a six month period.  What is a specific
brand of silicone sealant recommended?  I understand the kind with vinegar
smell (acetic acid) will corrode electronics components in a sealed
environment.  Also, is there a need to insert a desiccant in the box?

The box is NEMA industrial type, made of plastic with neoprene o-ring seal
on lid.  However, there are many holes drilled for stainless hardware used
to mount the relay and connect wires to the exterior.  The relay itself is
the open contact variety with self-cleaning contacts (hot switch 20 watts
RF), and supposedly impervious to damp conditions.

Thanks for any advice.

Keith NM5G

Weatherproof or waterproof?

Are you sure it's the silicone leaking?

How is the box vented? If the box is totally sealed, the pressure differential can get pretty high as temperature changes (to a first order, if it swings 0 to 40 C, about 1 -2 PSI). That kind of pressure can suck in a LOT of standing water, if there's a path.

An inch sounds like an awful lot to accumulate by condensation, but, you never know. If the box is set up in a way that accumulated rain water can be sucked in, and there's some form of trap ( imagine a tube from outside running up inside the box, and opening at the top. Water can suck in, up the tube, run down the outside of the tube into the bottom of the box, but then, when the pressure increases, it just blows air out the tube. Over time, it makes an effective pump.

All manner of rubber seals and/or silicone with tiny gaps and crevices can also act as a nice one-way valve. In combination with the pumping effect of temperature change, it can pump an amazing amount of water in.

If you don't need "waterproof sealed", then a vent hole at the bottom may do. Maybe what you really want is "rain-tight" (NEMA-3?)

As far as sealants go..

The "bathtub silicone" with the acetic acid will only outgas for a while. Eventually, it will be fairly inert.

There are two part RTVs and single part UV cured sealants available which are suitable for potting electronics. Check the Mouser, Digikey, or Newark catalogs. Or, if you have a local source for these things, call them up. Here in Los Angeles, I've used Sil-Pak in the past, but that's just because they were down the street from where I worked at the time.


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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