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tripwire

Wanted: DIY sensor waterproofing ideas

6 posts in this topic

Hi,
 
I'm hoping some of you might have had experience of waterproofing sensor nodes or individual sensors, and can make suggestions.

 

I have a project using the SensorTag to measure altitude, and need to seal it up enough that it doesn't fail after the first rain shower. It doesn't need to be completely watertight as it should never be submerged, but it needs to resist heavy and persistent rain.

 

It can't go in a hard-shelled airtight container as that would interfere with the pressure measurements. It would probably be a bad idea to get conformal coating in the sensor port too. I'm not too sure whether conformal coating would protect the coin cell and holder adequately either.

 

To make it even more difficult I have a wire running to an external reed switch that counts wheel revolutions, so that entry point needs sealing too.

 

My current "plan" is to mummify it in plumber's PTFE tape and hope for the best, unless there are any better ideas out there :)

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Basic guidelines would dictate that if it can't be hermetically sealed, then it needs to shed water and breath. If attitude can be guaranteed, then an open or screened/perforated bottom should do.

 

As a totally different thought, that I have not tried yet, a full hydrophobic coating might do the job, like one of the relatively new hydrophobic coatings for driveways/sidewalks.

 

Or, go old school. Tie the thing up in a condom. The urethane ones hold up well over time, better than the latex ones, and are a bit tougher. A dab of silicone on the sensor wire at the appropriate location before a zip tie around the opening. As long as it is not drum tight (some slack) and has little air in it, it shouldn't effect pressure readings. The Sensortag may be a little too big for this, but similar solutions can be worked out with other schema, like a silk/rayon bag coated with tent waterproofing spray, whatever that is called these days (I don't camp much anymore, so it has been a number of years since I had to use the stuff). Again, if there is a bit of slack, it shouldn't significantly affect pressure readings.

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Forgot to mention another possibility that I have used: nitrile mechanics gloves. Closing the wide (wrist) end can be done with silicone: I have clamped smoothly and tightly using aluminum bar about 5mmfrom the edge roll and run silicone into the exposed end without clamping. Leaves a decent seal. You could trim a finger end to run the wire through and seal that with a dab of silicone and a zip tie.

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Should be able to find a NEMA 3 or 4 enclosure fairly easily, then have a small hole (1/2" or so) at the bottom covered with Gore-Tex. Gore-Tex is breathable so shouldn't affect actual air pressure and humidity readings but also helps prevent sizeable moisture droplets from permeating the fabric; drips and splashed water shouldn't enter but forced spray from a hose might.

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@@tripwire

 

You might try the plastic bottles that food and/or pills come in like these:

post-45284-0-96186600-1467577073_thumb.jpg

 

You can turn them upside down and punch holes in the soft plastic caps for wires and/or breathing and seal with hot glue if you like.  A downside for long term use outdoors is that some of them will deteriorate in UV light.  I  also used a small Tupperware plastic container for a few days once in the rain with no damage to the electronics inside.

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@@tripwire

 

Since you are trying to measure the ambient pressure with your sensor then you will have to allow the air pressure to equalize between the inside and the outside of the water proof container.

 

This is normally accomplished with a Gore membrane. They are water proof yet they allow air molecules to pass through both ways.

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