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Condensation inside a close box

by Dalit
Tags: condensation, humidity
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Dalit
#1
May21-09, 06:56 AM
P: 36
I have an out-door cabinet (with PCB) that over time is being damaged due to condensation (humidity).
I would like to minimize these damages over time (corrosion, etc...)
The cabinet is closed but not 100% sealed. Cabinet size is 80X20X20cm.

How do I avoid the damages due to condensation of air inside the cabinet?
Changes I can implement are:
1. Changing the external geometry of the cabinet.
What would be the preferable geometry?
2. I can change the coating of the internal components (cabinet included).
Any recommendations?
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FredGarvin
#2
May21-09, 08:14 AM
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There is nothing you can do geometry wise to solve this problem. There are specific electrical enclosures and PCB meant for this kind of usage. You would be better off to get a PCB design that includes some kind of coating finish to protect it from condensation or getting a proper enclosure.

The only other thing you can do is to treat the air in the cabinet to ensure a constant atmosphere inside the cabinet.
Dalit
#3
May21-09, 09:49 AM
P: 36
I can not ensure the atmospere inside a cabinet. I will have to use cooling system all year long. That would be very expensive.
I would like to chanel the water drops, after being condensed, in a pre-designed chanels.
I am sure electric cabinet, installedout-door, have the same problem, or should have...

Bob S
#4
May21-09, 02:42 PM
P: 4,663
Condensation inside a close box

You could use a desiccant like silica-gel in bulk, and regularly recharge it. Or you could use a heater inside the cabinet to always keep the temperature inside the cabinet above the ambient temperature.
MATLABdude
#5
May21-09, 03:06 PM
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Is it possible to weather seal the box? (Compressed) adhesive weather stripping, caulking, gaskets, etc.? If you have components that get hot, you have to be a little more careful, but you can heat sink these to the outside of the enclosure.
Dalit
#6
May24-09, 01:25 AM
P: 36
MATLABdude, what do you mean by weather seal?
The cabinet can not be sealed, but the problem is with the air trapped inside the "box".
Hot/cold weather will cause air to condense inside, and I am trying to minimize the problem causes by the water drops accumulating inside the box.
I am not concern with the getting hot but getting wet.
Dalit
#7
May24-09, 01:28 AM
P: 36
Bob, Silica-gel is not a long term solution for me, since I will have to change it in a regular bases, it is not very easy to open the cabinet.
It's like an out-door electric cabinet. I wonder what is their solution for humidity inside the cabinet.
MATLABdude
#8
May24-09, 03:26 AM
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By weather seal, I probably mean environmental seal: things like gaskets and o-rings for things that open/close or fit together, and welding or silicone (or other) sealant for seams. Basically, measures which will keep moisture out of your box (save for when you're opening /closing the box), and allow your can / bag of desiccant to go that much further (get the kind that colour changes so you know when to replace / regenerate the desiccant). A not-as-good alternative may be to use the weather stripping (closed cell only!) that you buy in hardware stores, which will keep copious amounts of moisture (rain, snow, etc.) from seeping in if applied properly.

Outdoor electronics (that can't be completely sealed in) often have parts that *are* sealed in, with weatherproof connectors for electrical connections, and parts which can't be sealed are, as Fred Garvin says, potted or conformal coated. Then they're tested to ensure that they don't (have unacceptable) rates of failure. For more on these types of coatings, here's a handy app-guide from MG Chemicals (they make various types of electronic coatings):
http://www.mgchemicals.com/downloads...pguide0405.pdf

Given that your box is tough to crack open, you should consider a case redesign / reselection (say, one with environmental sealing) or some of the other measures we've presented.
Dalit
#9
May24-09, 03:51 AM
P: 36
MATLABdude, thank you for the file.
The cabinet is not tough to crack as much as I want to keep it low maintanance. I don't want to open it every other day.
The cabinet is sealed in some level, but I can not remove (vacuum) the air trapped inside the "box".
Weather changes will condense the air trapped inside the "box" into water drops, these drops will accumulate on the surfaces of the internal "box" and encourage corrosion.
That's what I am trying to avoid.
MATLABdude
#10
May24-09, 04:06 AM
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P: 1,724
Quote Quote by Dalit View Post
MATLABdude, thank you for the file.
The cabinet is not tough to crack as much as I want to keep it low maintanance. I don't want to open it every other day.
The cabinet is sealed in some level, but I can not remove (vacuum) the air trapped inside the "box".
Weather changes will condense the air trapped inside the "box" into water drops, these drops will accumulate on the surfaces of the internal "box" and encourage corrosion.
That's what I am trying to avoid.
But that's the point of environmental sealing. You don't need to remove the air, because the desiccant will absorb the moisture out of the air inside the box, and prevent moisture from condensing on your electronics. And since you open the cabinet so infrequently, you won't replenish the humidity.

As part of my work, I heat seal moisture-sensitive chemicals in polyethylene bags (ZipLocks with the tops cut off) with (fresh) desiccant packs. Then I freeze the ones that need to be frozen (at -20C), and no moisture ever condenses on the inside of the bags. I keep other chemicals (ones that don't need to be frozen) in a vacuum desiccator kept under (a low) vacuum, so that there is so little air that you have very little moisture. Nevertheless, I still keep desiccant inside the desiccator to remove what little moisture remains / leaks in.
Dalit
#11
May25-09, 08:28 AM
P: 36
Quote Quote by Bob S View Post
you could use a heater inside the cabinet to always keep the temperature inside the cabinet above the ambient temperature.
Shouldn't the temperature inside the cabinet be below the ambient temperature?
russ_watters
#12
May25-09, 09:34 AM
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P: 22,248
Quote Quote by Dalit View Post
Shouldn't the temperature inside the cabinet be below the ambient temperature?
No, that's largely what causes the condensation!
Dalit
#13
May26-09, 01:44 AM
P: 36
That way I might solve the condensation problem, but start a new problem of heat inside the cabinet, which might damage the components inside.
MATLABdude
#14
May26-09, 01:59 AM
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P: 1,724
Think about this for a second: would you leave the heater on all the time? How might you control this?
Dalit
#15
May26-09, 02:19 AM
P: 36
This is not afeasible idea. I am very much aware of that. The cost for constant heating the internal volume of a cabinet will be enormous.
I can not avoid the condensation problem, therefore I am trying to design the cabinet to comply with humidity and water drops.
I am sure there is a mechanical design that I can drain the drops accumulating over time.
I just wonder if there is a thumb rule or know how I can follow.
FredGarvin
#16
May26-09, 06:47 AM
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You can easily overcome and direct the drops in the cabinet. However, you still have to deal with the condensation forming on the electrical components to begin with.

What about some form of heat tape? Can you put a small electrical controller in there and use a heat tape do this?

http://en.wikipedia.org/wiki/Trace_heating
Dalit
#17
May26-09, 08:07 AM
P: 36
The solution implementation can't be electrically controlled.
I am sure all the electrical cabinet on the streets have the same problem, I wonder what is the solution there.
Is it special coating on components, drain or what ever…?
FredGarvin
#18
May26-09, 12:13 PM
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Nuts. Well, it was worth a shot.

I am sure that electrical boxes like the ones you mention have components in them that are rated for outdoors use. Like I said, I have seen some that have protective coatings applied to the PCBs and other components to protect them.


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