Sealing PCB into a weatherproof enclosure?

[cds333]

Lets say I am starting with a PCB that comes out of a device that is intended to be mounted outdoors. A wifi bridge for example. Due to the cheap plastic construction I lost the original case, so now I need to make a new one.

I am thinking sch40 PVC pipe with some waterproof cable glands for the cat6. It will be plenty sturdy, and waterproof of course.


I have no problem engineering the enclosure itself, but rather what to do about the changes in atmosphere.


What would be a better strategy- sealing it up as air tight as possible, and adding a bag of dry desiccant inside- or leaving a waterproof vent to allow pressure, temperature, and humidity to equalize? (I can't imagine the original case was completely air tight)

I notice certain vandal-proof cameras that are weather-sealed do include a small bag of desiccant, while other outdoor-rated devices are nowhere close to air tight and seem to rely only on the circuitry not getting wet, and hoping it will not be bothered by humidity.

What would the pros and cons of both be?

Thanks!

[IDEngineer]

Temperature swing is the big question. Where I live we go from subzero in the winter to 100F+ in the summer, with occasional single days seeing 40-50F swings from night to day. That's begging for repeated condensation cycles inside a sealed enclosure, which is probably why you're seeing dessicant packs... the hope is that the pack will absorb the ambient moisture when the unit is sealed and then retain it, leaving a dry atmosphere that won't see condensation.

Another good rule of thumb is that sealed things seldom stay that way - and once moisture/etc. gets in, all that sealing instantly becomes a liability instead of an asset because it fights to prevent things from getting back OUT. This is a classic quandry in engine compartments; you want to seal pulleys and belts against water intrusion, but if (when!) water gets inside that enclosure it can't easily escape again.

Can you pot the PCB? That would solve most of your challenges. No airspace means no condensation at the PCB/component level. If you need future access for programming/etc. just use flying leads coming out of the potting compound.

[NiHaoMike]

Easy way is to have the cables enter from the bottom, with holes drilled for a snug fit but not perfectly sealed.

[floobydust]

I've seen many products fail trying to seal it up tight. Water got in, despite o-rings and glue etc. and baffled the mechanical designers.
It seems to be pressure changes. For example, after/during rain it cools an enclosure and creates a small vacuum that sucks in a few drops of water that are on the enclosure and at the seams. Or the cable glands.
Car headlights for example use a Gore vent because the pressure changes are large due to heating.

If you can put a small vent somewhere guaranteed dry, that works well. An enclosure that "breathes" works well.

[georges80]

Conformal coating (sprays) are easily available and you can just spray it over all the PCB 'stuff'.

There's also PTFE membrane material that can be put over a 'vent hole' to allow pressure equalization while preventing moisture/humidity etc from getting into the otherwise sealed case.

Google for Gore membrane/vent etc or PTFE membrane/vent etc. Such products are commonly used for outside devices - e.g. outdoor wifi routers, etc etc.

cheers,
george.

[ogden]

Buy IP67-rated outdoor (plastic) enclosure of right size, add GORE PolyVent valve which is designed exactly for such purpose. Never ever try to make outdoor electrical enclosures/cabinets air-tight.

[Swake]

For what my own experience is worth: if you cannot get hold of a specific membrane, make sure to use some mesh fine enough to avoid insects getting in. Any opening must be on the bottom AND protected from direct ingress ie. avoid direct contact with rain or wind, there must be something equivalent to a labyrint.
If the device will be installed near sea or a similar harsh environment you have to protect the electronics against corrosion. There are sprays for this as already suggested.
The PVC housing might not be very resistant to UV from direct sun exposure. Avoid direct exposure and/or paint it with UV protective paint. you will have to renew the paint every year or two.

I have several cheap PTZ indoor ip-cameras installed under a patio since about 5 years and they still work as new. Obviously these are not exposed to rain but temperature goes from freezing to too-hot-for-a-human and the electronics doesnt care.

[cds333]

Excellent advice guys; thank you!

Those damn gore vents require you to fill out an application to buy them?!? What's the deal with that?

However if they are just a PTFE membrane inside a threaded vent housing I'm sure I could print one pretty easily.

[nctnico]

As others wrote: water tight doesn't exist.
Putting a conformal coating on the board and have a drain hole at the bottom of the casing is a good solution.

[SmallCog]

A method I’ve seen is to use a bag half full of air with some desiccant inside it which is plumbed to the sealed device. Changes in atmospheric pressure will have the bag inflating and deflating. The device remains at the same pressure as outside but there’s no outside air moving in or out the device.

Alternatively the device can breathe through a desiccant breather such as: https://in-situ.com/en/outboard-desiccant-for-stripped-and-tinned-cables

Both are potentially overkill for your example

I’ve been known to drill a hole in a project box and glue in a syringe filter as a bug/dust proof vent.

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[jpanhalt]

"Bubble point" filters have been used for decades to allow air to pass while stopping the flow of a liquid.  In my experience, water and water solutions were the liquids involved.  I don't recall the pore size off hand.  As I recall, the pore size for filling chambers was on the order of a micron or less.  We used Millipore filters (now Sigma-Aldrich).  Here are a couple of references that might help. 

https://www.sigmaaldrich.com/US/en/products/filtration/laboratory-filter-membranes/millipore-filter-membranes
https://hydronixwater.com/bubble-point-definition/

I think you could print the holder but would need to buy the filter membrane.

[strawberry]

proper link to: 20Pcs/lot 13mm 0.22 (um) Nylon Syringe Filters Disposable Microporous
https://www.aliexpress.com/item/1005002484311010.html