At 04:43 AM 9/08/2010, Kirk wrote:
http://parts.
would this be easier?
G'day Kirk
That is a great example of the type of box that after a while in a cold, damp, place magically gets water in it. Although it is IP65 rated, and _should_ be waterproof, as it thermally cycles, it "breathes", the air inside expands and pushes out the (dry) inside air, and when it is cold (and usually damp) again, it sucks in. The moisture is then condensed onto the internal faces of the box. Once the moisture is condensed and collected in the bottom of the box, you will always have 100% humidity and a prime environment for corrosion.
Using a box without considering the environment where it is being used may be worse than not using a box, if the installation is a normally dry environment but occasionally can get the right conditions to suck cold, damp, air in then the use of a box may be detrimental. People in sub-tropical climates probably don't need to do much, but here we're 40 degrees south and often have to.
This is the reason I started the thread, to get some discussion happening.
Regards
[Technik] James
From: James Massey <jcmassey@netspace.net.au>
To: electricboats@yahoogroups. com
Sent: Sat, August 7, 2010 11:04:08 PM
Subject: [Electric Boats] Protecting the electronics
G'day All
I just posted a discussion comment to Pitt, regarding choices of
materials, and realised that there is a thread of protecting the
electronics that really should be explored.
There are several methods of protecting the electronics, I'll discuss
the ones that I've used.
1) Pressure-sealed enclosures. This is where the box may get immersed
from time to time. It is very difficult to get a full pressure seal,
as air (and moisture) will go in along the strands of the wires
penetrating the box. I've achieved a level of seal that took months
to bleed down from 5PSI. To get this reqired welding using methods
used for pipe/pressure vessel welding, to make the box. The box then
got a flange all around the edge, about 6mm thick after machining
(welding imparts some distortion, the flange was then machined flat
again). The flange gets a 6mm (1/4") plate bolted to it, with 6mm
bolts about 25mm (1") apart. Where the wiring penetrated the box, I
used metal glands, with sealant on each part as it was assembled.
Each wire was bared off to expose the strands. Twisting opened the
strands, to take epoxy adhesive (soldering would also work), and
heatshrink tubing applied over it whilst still wet. The bundle of
wires were then wetted with hot-melt glue, and glue-lined heatshrinnk
applied over that. The gland is done up over the heatshrink once
cool. Where there was a remote box, I used airline fittings and
tubing to connect the boxes together, so the wires between were able
to be just poked through. This is time consuming to do, and rarely
necessary (I've done it twice in 20 years with electronics that lived
in pits that occasionally flooded)
2) Using an enclosure that comes with a rating such as "IP56". The
problem with these enclosures are the rating does not account for
pressure changes in the enclosure allowing it to push (warm) air out
when it expands, and sucking in (cold, usually damp) air when cooling
back down again. We have (very successfully) use the following method
many times. A section of insertion rubber as big as can be found
space for is used as a membrane to allow for exchange in and out of
the box whilst keeping seperation. Typically we use boxes that have
an equipment plate inside, set 10mm or so away from the back, so use
the majority of the back face area - just drill a couple of holes to
let the air in/out of the space behind the diaphragm. A flange is
made for the edge of the membrane, and sealant (Sikaflex or similar)
is applied around the edge of the membrane against the face of the
box. Push the membrane in a little before doing the flange bolts up,
so that it sits a little loose, free to move. As the box warms, the
membrane pushes back against the back of the box, as it cools it
moves forward agaist the plate, so it is important not to have any
sharp edges, ends of screws, etc where the membrane can move against
them. Electrical penetrations done with plastic glands are perfrctly
adequate, as there is no real pressure differential inside to
outside, so no impetus for any flows.
3) Lacquer. Either circuit board or electric motor lacquer, or one of
the proprietary "goos" that has either high build or used to 'pot'
(completely fill) the box. If you use high-build or fully potting
"goo", make sure it is a permanently-flexible, neutral (non-acid and
non-alkaline) type. Solid epoxies/polywhatevers will tear components
from boards over time, as they thermally cycle. Acidic or alkaline
"goos" will eat the parts. If the electronic "thing" lends itself to
be dis-assembled and lacquered, this can be successful in extending
the corrosion-resistance life many times over non-lacqured or
minimally lacquered parts. Any electronics designed for marine
application will (should) have this type of lacquering.
Heat. Heat is the enemy of electronics, so has to be able to be got rid of.
1) If the heat is being generated on a heat-sink that sticks out of
the object, or the object is designed to take an external heatsink
(such as a controller), it is possible to use the above box-membrane
method, with a hole cut through the side of the box for the heatsink
to stick through, sealed back against the box with sikaflex or similar.
2) If the box is used for something that blows air through itself,
for example many chargers and inverters, it is possible to use (or
make) a finned heatsink, that is inserted into the side of the box,
with fins both inside and out. I have cooled a box that is in a
wash-down room by using a fan inside the box, blowing air through the
inside part of the heatsink, and convection cooling the outside with
fins twice the size and no fan. Moving a lot of heat this way is a
challenge, if the part on the inside needs to be kept reasonably cool
but generates a lot of heat.
Moisture. Moisture is the catalyst for corrosion.
1) Once you have a sealed box with your "thing" inside it, the
easiest way of maintaining a dry condition is with silica gel. Silica
gel is a dessiccant, i.e. it absorbs moisture from the air, and works
great until it is fully absorbed, at which point you may as well use
a tea-bag. It is readily available in self-indicating form. where it
is (usually) blue when it is dry, and goes pink as it wets out. Once
it is wet you can easily re-activate it by holding it in an oven for
a time (a few hours, often) at just over 100 degrees Celcius. If you
get it too hot (over about 115C) you can kill it, though, so some
care is needed.
2) Warmth. As long as your electronic box is warmer than its'
surroundings, water will not condense in it. If your electronic
"thing" is always on, and only generates a little heat, an isulated
box may be all that is needed to look after it, but care is needed to
make sure that you are not letting it be too hot. (if you cannot hold
your hand on it, that is too hot).
3) Dry air/gas. If you have compressed air available, or another gas
that is "naturally" dry, it is possible to dry the air, or use the
gas, and bleed a very, very, small amount into a box - just enough to
overcome the natural push/suck cycle of the box, and ensure that the
box is on average bleeding dry air out, not moist air in. I can't
think of a scenario in a non-commercial marine environment where this
will apply, though.
That is unlikely to be a comprehensive list, but a good start. Over
to the list to discuss.
Regards
[Technik] James
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