Equalizing the pressure in a sealed box

[John Devereux]

Hi,

We have an IP67 box with a membrane front panel with window. At
temperature extremes the air inside will change pressure, up to ~0.3
bar - enough to cause significant bowing of the window and probably
enough to break the adhesive seal.

I would have thought this is a common problem - is there some kind of
low cost pressure equalization/relief valve that can be fitted in the
wall of the box?

Thanks,

 

[MooseFET]

Hi,

We have an IP67 box with a membrane front panel with window. At
temperature extremes the air inside will change pressure, up to ~0.3
bar - enough to cause significant bowing of the window and probably
enough to break the adhesive seal.

I would have thought this is a common problem - is there some kind of
low cost pressure equalization/relief valve that can be fitted in the
wall of the 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.

 

[Adrian C]

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?
<http://www.flickr.com/photos/elithebearded/sets/72157594574694645/>

 

[John Devereux]

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

An air vent from a coffee bag?
<http://www.flickr.com/photos/elithebearded/sets/72157594574694645/>

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

 

[Eeyore]

Hi,

We have an IP67 box with a membrane front panel with window. At
temperature extremes the air inside will change pressure, up to ~0.3
bar - enough to cause significant bowing of the window and probably
enough to break the adhesive seal.

Fit a thicker window with stronger adhesive. The simplest answer is
usually the best.

I would have thought this is a common problem - is there some kind of
low cost pressure equalization/relief valve that can be fitted in the
wall of the box?

It wouldn't IP67 then I expect ! Is that a problem ?

Graham

 

[John Devereux]

Fit a thicker window with stronger adhesive. The simplest answer is
usually the best.

Yes, we may have to do that. It's not quite that simple since the
plastic membrane was going to be the keyboard overlay too, so we can't
just make it arbitrarily thick.

It wouldn't IP67 then I expect ! Is that a problem ?

Not sure at this point - just looking into what options are available
at what cost.

Thanks,

 

[John Devereux]

Very long time ago, I think I did see that in Elektor magazine,
they used a plastic bag with some fluid and saturated vapour to
make a constant pressure in sealed speaker boxes.
Do not remember what fluid it was.

Hi Jan,

You can do it with an internal "bladder", that has a connection to the
world outside.

 

[John Devereux]

Really good idea! I like that!

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

 

[John Devereux]

How about an elastomer sheet and a drill hole? You don't need to
exchange air, just equalize the pressure.

Thanks - we had thought of using an internal bladder.

Anyone know of something we can buy? The box is smallish, say 2x3x4
inches.

 

[Rich Grise]

We have an IP67 box with a membrane front panel with window. At
temperature extremes the air inside will change pressure, up to ~0.3 bar -
enough to cause significant bowing of the window and probably enough to
break the adhesive seal.

I would have thought this is a common problem - is there some kind of low
cost pressure equalization/relief valve that can be fitted in the wall of
the box?

Drill a hole in it?

Have Fun!
Rich

 

[John Devereux]

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.

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.

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

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

Thanks,

 

[John Devereux]

Box material? Method of assembly?

It's diecast aluminium.

Maybe just a hole with the diaphragm over it... clamped around edge?

Yeah, maybe , see reply to RFI-EMI-GUY.

Thanks,

 

[Spehro Pefhany]

Hi,

We have an IP67 box with a membrane front panel with window. At
temperature extremes the air inside will change pressure, up to ~0.3
bar - enough to cause significant bowing of the window and probably
enough to break the adhesive seal.

I would have thought this is a common problem - is there some kind of
low cost pressure equalization/relief valve that can be fitted in the
wall of the box?

Thanks,

Yes, it's a common problem.

Eg.
http://www.agmcontainer.com/immersion_breathers/pdfs/Immersion Proof Breathers.pdf
http://www.agmcontainer.com/immersion_breathers/immersion_breathers_ta250-2.htm



Best regards,
Spehro Pefhany

 

[John G.]

Box material? Method of assembly? Maybe just a hole with the
diaphragm over it... clamped around edge?

Many yeas ago there was an RF Wattmeter used in the services that was oil
filled for cooling and it had a rubber diaphram for expansion.
The diaphram was slightly heavier but very similar in shape to ones you will
find in dispensers in gas station toilets or mens wallets..

John G.

 

[John Devereux]

Yes, it's a common problem.

Eg.
http://www.agmcontainer.com/immersion_breathers/pdfs/Immersion Proof Breathers.pdf
http://www.agmcontainer.com/immersion_breathers/immersion_breathers_ta250-2.htm

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...

 

[_]

Hi,

We have an IP67 box with a membrane front panel with window. At
temperature extremes the air inside will change pressure, up to ~0.3
bar - enough to cause significant bowing of the window and probably
enough to break the adhesive seal.

I would have thought this is a common problem - is there some kind of
low cost pressure equalization/relief valve that can be fitted in the
wall of the box?

Thanks,

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.

 

[NoSPAM]

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

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

 

[James Arthur]

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.

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

 

[Jasen Betts]

It's diecast aluminium.

if you can mount all the electronics in the tub part of the box you
could drill a hole in the lid and replace the gasket with a synthetic
rubber diaphragm,.

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[John Devereux]

Jim Thompson <[email protected]> writes:

[...]

It's diecast aluminium.

if you can mount all the electronics in the tub part of the box you
could drill a hole in the lid and replace the gasket with a synthetic
rubber diaphragm,.

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:: ## .-#####-. ### #### ## # .-####-. ## ::
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Nice, very neat!

For us it's not quite that simple since the screws that hold on the
"lid" are inside the seal perimeter. (The screws have a seal too). But
that looks a good solution for more standard boxes.

Thanks,