What I want to do
I want to make a way to reliably put sensors through the cap of a PET bottle. ...
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I want to make a way to reliably put sensors through the cap of a PET bottle. the Open Water Project is using bottles for waterproofing, which I really like, and punches sensors through the lid.
Lids are made from HDPE, which is notoriusly hard to glue to for reasons of polymer physics. This makes me dubius about the long-term survivability of sealed sensors in bottle caps. It also makes it hard to switch bottles if they wear out, since caps don't always match.
So I'm interested in making a system that lets:
sensors be exchanged easily between bottles and;
caps seal between sensors and bottles with cheap, off the shelf things.
I got a standard #3 bung (cost $0.40) with a hole through it from a brewery supply store, and some 3/8" outside diameter brass piping from a hardware store (I was looking for PVC but they didn't have it that small).
I pulled the seal out of the cap and drilled a 1/2" hole through it.
Then I sealed the end of the brass pipe with aquarium-grade silicone sealant and waited 24 hours. The idea being that sensors or wires could be run through the tube, and the tube filled with silicone or another sealant. PVC or another non-metalic pipe (that can still be glued to easily) is a better choice, and standard 3/8" schedule 40 PVC is available for cheap, as well as tons of fittings, Ts, etc for any imaginable attachment.
I then shoved the pipe through the bung, pushed the bung into the top of the bottle and worked it down, then I screwed the cap on tight. At that point the pipe wouldn't move and seemed tightly held by the seal.
Then I filled a trash can with 50 cm of water and used a scrap of galvanized conduit to hold the bottle under water for 24 hours.
The bottle stayed dry.
I've got to try deeper tests with PVC chunks, and run some sensors or wires through the pipe so that the seal represents actual working conditions.
I think this bung + 3/8" pipe system will work well and is cheap and easy system to work with. time to test it deeper and longer.
Pop bottles are cheap and everywhere. they'd make great waterproofing if we can get sensors out of them.
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This is a good definition of the problem:
"..sensors be exchanged easily between bottles and; caps seal between sensors and bottles with cheap, off the shelf things."
This suggests that two ports will have to be made waterproof: one for a large opening to pass the electronics into and out of the bottle, and one for the cables to the sensors.
Your design has not two but three areas of contact where leakage can occur: inside the brass pipe, between the pipe and the bung, and between the bung and the bottle. There is probably a good design that has only two areas of potential leakage.
The sensor cable port is easier to make waterproof because it can be permanently sealed. The port to get the electronics in and out is harder because it has to open and close easily.
Two of the contact areas in your design are easy to seal. The long pipe is a great idea because if you figure out how to fill the entire thing with goop it will be pretty reliable. The contact between the pipe and the bung can probably be made to seal well because it can be permanently gooped and/or because the bung will exert inward pressure if it is jammed into a hole.
The contact between the bung and the bottle opening is the weak link. Bottles vary and some will seal better than others. Some bottle necks might not be strong enough to tolerate jamming the bung in securely. The seal could be improved with grease, and made more reliable if the screw-on cap can be screwed on securely holding the bung in place. Are the caps strong enough and reliable enough for this?
Is a flimsy water bottle the right container for this job? It's bigger than it needs to be and the body plastic is a lot thinner than it should be to survive some outdoor deployments. The fact that it is cutely appropriate is nice. It's also good that it is free, but spending $10 for a smarter housing might be quite appropriate since there will probably be a $150+ gadget inside.
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you're definitely right about the long thin piping, it is designed to be completely "potted," or filled with some sealant or epoxy. Its the best way I can think of to encapsulate wires and get them out of an enclosure. I'm less certain that a seal is needed between the tube and the bung. As you said:
"the bung can probably be made to seal well because it can be permanently gooped and/or because the bung will exert inward pressure if it is jammed into a hole."
exactly. it has 2.5cm of vertical contact area in the bung, the top 1 cm of which is under significant compression. Look at this comparison:
I don't think the bottle neck is the week link. PET bottle necks are amazingly strong. The body of the bottle is the weak link, especially when a water bottle is chosen, since many water bottles aren't designed to take pressure and are much thinner than pop bottles. I think PET bottles are up to the job, having worked with them as floats when I was a life guard. With the caps sealed they would last several seasons, often getting tossed on shore with cinder blocks.
What usually breaks on a PET bottle is the cap. The bigger the cap, the bigger the problem. A juice bottle, dropped cap down, will often shatter open. Pop bottles have a smaller cap and are a little stronger, but still an issue. There are reusable caps made for home carbonation that are thicker and stronger. Also, if the electronics are small, PET blanks, called "preforms" or "soda bottle preforms" can be purchased and are incredibly thick and strong (but only 5" long).
I'd like to keep the housing reparable and replacible though, I like having the housing be a bottle-- that said, it might be worth distributing some more durable components, especially caps.
If I were doing this I would use easily obtainable silicone rubber to act as a seal and as a mechanical protection.
I'm using silicone to seal the sensor in the tube, but I really want to be able to get in and out of the bottle without re-sealing it.
one thing I've been thinking about as well is pressurizing the interior of the bottle. A pop bottle will take 9-10 atmospheres without trouble.
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