Re: [EVDL] Weatherproof boxes (Was: Juicebox)

[Chris Tromley via EV]

On Fri, Jun 15, 2018 at 11:46 PM Bill Dube via EV  wrote:

> One of the mistakes folks make it to attempt to make the box completely
> sealed. You really can't seal a box effectively without going to extreme
> measures. It gets expensive to do this. Because of changes in
> temperature and atmospheric pressure, air will find some way to come it
> and out, dragging humidity and perhaps water with it. It is, as you say,
> best to accept the fact that a box will breathe and to deal effectively
> with that breathing.
>

​

All true, which was an even bigger issue decades ago when I was packaging
aircraft electronics.  Throw big pressure changes due to altitude into the
mix and a difficult problem gets much worse.

But then a simple and effective solution became available and the whole
issue disappeared. Gore, Donaldson and probably others make vent membranes
from expanded Teflon (Goretex) that allow water vapor to pass but not
liquid water.  Donaldson has a version that does the same for oils.  These
vents are packaged in a variety of forms and sizes, including bare membrane
dots with an adhesive ring.  They're not widely available in onsey-twosy
quantities, but they're out there.

So just build your box, make it water-tight (not too big a deal), drill a
hole somewhere and cover it with a vent membrane.  You just solved the
breathing problem, while keeping your electronical innards moisture-free.
Temperature changes can still cause internal vapor to condense, but if
there's even a little temp rise inside the box that generally goes away.
Conformal coating is still a good idea.  With that and an enclosure vent,
reliability of out-in-the-wind electronic equipment is vastly improved.

Chris
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Re: [EVDL] Weatherproof boxes (Was: Juicebox)

[Matt Awesome via EV]

> One of the mistakes folks make it to attempt to make the box completely
> sealed. You really can't seal a box effectively without going to extreme
> measures.

Here's a really interesting video dealing with trying to seal bicycle
hub motors.

https://youtu.be/gwlbAJLzI_w?t=1242

It's a long video, that link should skip you to 20:42 into it. (I
usually watch at 2x speed, takes a few seconds to get used to).

The guy giving the talk is the guy who designed the CycleAnalyst, he
more or less founded the E-bike scene in Canada 15 years ago. At this
point he'd been running his factory/shop/service center for 7 years,
and he's sharing his experience.

In summary:

1 - When motors/controllers/etc get hot, the air inside expands. Some
air finds a way out to equalize pressure. When it cools it contracts,
air finds a way back in. It brings water and humidity in with it.

2 - You can seal around the wires really really well. That's nice,
water still gets in. How?

3 - Inside the wire, in the "air gap" between the wire metal and the
wire insulation, the suction is enough to suck water up it like a
straw. Fine, so seal around that as well on both ends. That's nice,
water still gets in. How?

4 - By wicking inside the stranded wire itself. Circular hexagonal
packing/tessellation is only 90% dense. Water, umm, finds a way.

5 - Gradually in wet areas (Vancouver, rainy winters, similar to
Seattle), this works like a pump. Ride bike, heat air, stop bike, suck
in water. Eventually the whole motor fills with water and rusts solid.

6 - Give up, paint the motor internals with waterproof enamel, drill
some holes in the motor case for the inevitable water to drain out,
problem solved.

Mirrors much of what Bill said.
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[EVDL] Weatherproof boxes (Was: Juicebox)

[Bill Dube via EV]

One of the mistakes folks make it to attempt to make the box completely 
sealed. You really can't seal a box effectively without going to extreme 
measures. It gets expensive to do this. Because of changes in 
temperature and atmospheric pressure, air will find some way to come it 
and out, dragging humidity and perhaps water with it. It is, as you say, 
best to accept the fact that a box will breathe and to deal effectively 
with that breathing.


It is best to place the vent in a place that will naturally be sheltered 
from drips or will not draw drips into the enclosure. Your tube that ran 
up the back of the enclosure is a good example. Next, it is important to 
make the "exhale" of the box expel any water somehow has that found its 
way into the box. Your vents in the bottom of the enclosure are a good 
example of that, (but they would also tend draw in drips from the 
corners outside.)


A good design might be to put a small rail around the bottom edge of the 
box, and put the vents on up on the inside of that rail. Adding a 
desiccant in-line with the vent can really keep the inside dry, but you 
have to service/change the desiccant regularly, which most users won't 
do. What does really help is to keep the interior dead volume to a 
minimum so it will breathe as little as possible. If the power is 
available, a small heater makes a huge difference by keeping the 
interior above the dew point, stopping condensation. But that can be an 
unacceptable loss of energy.


Potting the components to completely eliminate the dead volume around 
the parts is a very effective approach. It makes the device 
unserviceable, however, and it sometimes makes the heat build up 
unmanageable. Conformal coating is probably the most cost effective method.


Bill D.
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