Equalizing the pressure in a sealed box

They make these nifty Goretex vent things that let air through but not water. Depending on what "low cost" means, it is worth a look. They work fairly nicely.

An air vent from a coffee bag?

Thanks MooseFET, I did not know the correct name. I will look into these.

:) I also thought of those things you get in the top of squeezy sauce bottles - a rubber disk with a cross cut in it, so it forms 4 flaps that seal when pressure is neutral. But I am really looking for an accepted "industrial" solution. But still cheap :)

I assume you want to maintain the water seal. How about a long flexible plastic tube or bladder(inside) with one end sealed, the other end of which is vented outside? The pressure would be exerted on the tube, crushing it and expelling air to outside?

Really good idea! I like that!

...Jim Thompson

We actually did think of that one! But it seemed a bit cumbersome, unless we can buy something ready made.

Drill a hole in it?

Have Fun! Rich

Just get some poly hose, glue a stopper in one end, fit a nipple through a hole in the side of the box (facing inward) and glue the nipple to the box and the hose.

The other poster said to use a sheet of plastic membrane. You could make a false wall by inserting a plastic ring or frame against the wall, drilling a small relief hole and glue the plastic sheet (diaphragm) over the frame.

Either should work depending on the geometry of your free space inside the enclosure and the volume needed to be displaced.

Someone else mentioned Goretex, I don't know if that violates the IP67 spec, but it might work as well.

The free space is quite large but awkwardly shaped - I don't know if we could get the membrane/tubing to flex enough to compensate for the change in air volume. But perhaps we can reduce this volume too, will think about it.

Yes, don't know either but will find out.

Thanks,

Yes, it's a common problem.

Eg.

Best regards, Spehro Pefhany

Thanks Spehro, I knew there must be something. They look like just what we need. I just didn't know the correct terminology to search for...

Aircraft compasses (the 'whisky" type) have a rubber seal covering the entire back, itself covered by a plate that clamps it to the cast housing, making the seal; said plate has a small hole in it.

This is a perfect application for Gore-Tex(r). Specify the maximum immersion depth needed to W. L.Gore & Associates and they can tell you if their standard product is suitable. IP67 should be no problem. The Gore material is microporous polytetrafluoroethylene. The surface tension of water prevents it from penetrating the pores. Gases, however, pass easily through the material. This includes water vapor (which is why Gore-Tex clothing lets sweat evaporate), so if the box is subject to elevated temperatures in high humidity and then cooled, moisture can condense inside the box. At one time, virtually _every_ sealed hard drive had a small piece of Gore-Tex which allowed pressure equalization while blocking dust and water ingress.

You will likely get the best answers to your questions from Gore's filtration products division.

Dr. Barry L. Ornitz

I was going to suggest filling in the airspace--no air, no expansion. That's cheap and easy.

Even a partial fill should help.

Cheers, James Arthur

... also you could try researching 'dessicator' and even 'copper bellows'. I don't know the details but I have vague recollection of copper or beryllium-copper sealed bellows being placed inside sealed enclosures to compensate for pressure variations on account of temperature change. I could be wrong but I seem to recall the bellows were filled with a Freon - at what pressure I don't know - and who manufactures them I haven't a clue.

Chris

It would help, but not much for us I think. There is a space between the LCD and the window. Unless the remaining volume can be made much less than that of the space, the air in the space will still expand proportionally and make the window bow out. Essentially, to help we would have to make the total remaining volume less than that created by an acceptable amount of "bowing out".

Thanks, that is very useful to know.

We will contact them.

Depends on the box size. There are those 1.5 inch diameter military chassis relief valves in use on gear everywhere...

They are not small footprint items, there has to be clearance inside the box. They have a small, manual operation button on them about 2.5mm diameter in the center of the vent on the outside as well.

So, 1.5" x 2" with about .25" to .375" of that on the outside.

Also known as an auto-equalization system. Same thing is used for transformer oil except the other way around. Called an expansion bladder.

I think Pascal has you guys beat on these inventions though.