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How big should a hydrophobic vent opening be for outdoor application

How big should a hydrophobic vent opening be for outdoor application

How big should a hydrophobic vent opening be for outdoor application

I've got a water tight (IP-68) plastic communications device with an internal volume of about 50 in^3.  It is vented through a hydrophobic membrane for two reasons.  The first is to prevent pressure build up due to changes in temperature.  The second is to allow humidity fluctuations to happen more quickly to help prevent condensation on the electronics.  The problem is that the vent is being partially restricted due to manufacturing problems.  So my question is how big does the vent orifice really need to be.  It seems that to vent the pressure changes prety much any size would do since they happen so slowly.  As for the humididty issue I have no idea.

RE: How big should a hydrophobic vent opening be for outdoor application


   IP68 means protection against continuous immersion.  I seem to recall that the test for IP68 involves 2m of water.  Your vent is not going to work unless the pressure inside your enclosure is greater than the pressure outside.  If you are going to remove the thing from the fluid periodically, you are probably okay.  

   Is your hydrophobic membrane hydrophobic under pressure?

   Is there a specification showing the pressure drop or head loss of air passing through your filter?  That should tell you something.


RE: How big should a hydrophobic vent opening be for outdoor application

No chance just a membrane alone couldn't work?

Keith Cress
kcress - http://www.flaminsystems.com

RE: How big should a hydrophobic vent opening be for outdoor application

drawoh is right.

Is this really just a rain/spray application, or an honest-to-God submersion scenario? I ask because usually these are heavy rain applications (i.e. top of a phone pole) and the customer simply wants it tested to be fully water tight.

Is there a source for dry air or nitrogen nearby, or is this device located at random standalone locations?

Never forget "cheap" unsophisticated solutions, especially in high volume aps! A 50 cu in box is very small, without much air to compress internally. If it's a rain application I might test a simple 1/32" weep hole. Or, if a location with occasional flooding, I might attach a 12" length of small ID vinyl tubing to the weep hole.  

Need more info.

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