Open source underwater distributed sensor network hackaday o gastronomo buffet

One way to design an underwater monitoring device is to take inspiration from nature and emulate an underwater creature. [Michael Barton-Sweeney] is making devices in the shape of, and functioning somewhat like, clams for his open source underwater distributed sensor network.

The clams contain the electronics, sensors, and means of descending and ascending within their shells. A bunch of them are dropped overboard on the surface. Their shells open, allowing the gas within to escape and they sink. As they descend they sample the water. When they reach the bottom, gas fills a bladder and they ascend back to the surface with their data where they’re collected in a net.

Thus far he’s made a few clams using acrylic for the shells which he’s blown himself. He soldered the electronics together free-form and gave them a conformal coating of epoxy. He’s also used a thermistor as a stand-in for other sensors and is already working on a saturometer, used for measuring the total dissolved gas (TDG) in the water. Knowing the TDG is useful for understanding and mitigating supersaturation of water which can lead to fish kills.

He’s also given a lot of thought into the materials used since some clams may not make it back up and would have to degrade or be benign where they rest. For example, he’s been using a lithium battery for now but would like to use copper on one shell and zinc on another to make a salt water battery, if he can make it produce enough power. He’s also considering using 3D printing since PLA is biodegradable. However, straight PLA could be subject to fouling by underwater organisms and would require cleaning, which would be time-consuming. PLA becomes soft when heated in a dishwasher and so he’s been looking into a PLA and calcium carbonate filament instead.

I sympathize with both of you. I ruled out spending time on underwater communication until the project reached the next level. I believe that it would be too difficult for me to make anything reliable, it would drain a lot of time and was unnecessary to gather water quality data. There are a lot of complications with communicating underwater (reflections, different densities, et cetera), and I do not want to do this haphazardly and produce a lot of sound pollution (it’s unlikely it would deafen any sea creatures, but it just seems tasteless to blunder into it as a novice). Having said that, it would be a fantastic capability. It could extend GPS underwater by triangulating from clams on the surface. Many of the problems with ultrasonic underwater communication are similar to what hams do all the time, so if you have any suggestions, I would be grateful to hear them. The only thing I can offer right now is the WUWNet mailing list, which has a lot of information on underwater communication and a community of academics working on the problem.

1. Chemical reactions can be affected (slowed down) by the high pressure. This is especially the case for reactions creating gas since emitting gas against the ambient pressure will require a certain amount of energy. This energy must come from something (the chemical reaction) and may drastically slow down the reaction / shift the chemical equilibrium towards the non-gaseous state.

2. The amount of gas required to create buoyancy will strongly depend on the diving depth. Outputting 1 mole of gas directly below surface (still at normal atmospheric pressure) will fill up a volume of 22.4 liters and therefore create ~224N of buoyancy. Creating the same amount (1 mole) of gas at a depth of 1000m will only fill up 0.22 liters and therefore create a buoyancy of only 2.2N (100 times less than that the same amount of gas would generate near surface). The expanding gas volume during the ascent also means that it will probably require a way to release excess gas during the ascent.

It may still work well enough for moderate diving depths – but higher diving depths will probably require another solution. One option would be just dropping a (non-toxic) ballast after reaching the intended diving depth. One option for that is to hold a piece of iron by an electromagnet (and turn off the magnet to drop it). This solution has a built-in failsafe mechanism since the ballast is automatically dropped in case of a power failure.