What I want to do
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.
My attempt and results
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.
Questions and next steps
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.
Why I'm interested
Pop bottles are cheap and everywhere. they'd make great waterproofing if we can get sensors out of them.