Hi @Tides 

Looks like @htarold provided the correct answer. These aquatic release mechanisms typically use some form of corrosion/oxidation. They come in both active and passive varieties. These forms aren't easily transferable to terrestrial applications unfortunately.

Wildlife computers also has a couple different releases that exploit different mechanisms. Will post those if I can find the references.

On the note of the release mechanism work of @Rob_Appleby et al. Since much of my work in this area was inspired by their open source project @jared / Margo Supplies has been generous enough to agree to open source our work in this area. It's a difficult problem to solve in both wildlife tech and cubeSATs. Tackling this as a community seems to be the best path forward.

I've been collaborating with CalgaryToSpace at the University of Calgary on a burnwire/thermal knife design that builds upon some of Robs works. This is somewhat premature and it's far from a ready design but since this topic has come up again I thought no time like the present to start sharing.

I will do a full post about this project once V1 has been field tested/validated.

Have seen people successfully use burnwires underwater. Just requires a bit more work to keep things thermally insulated and ingress protected.

"Just requires a bit more work to keep things thermally insulated and ingress protected."

seems to me a nice understatement!

I understand you burn Dyneema fibers with some hot resistors, but this has to be within the pressure housing (we are talking about underwater!) as electronics do not like salt water. Not sure how you can release some weights outside the pressure housing if you burn inside the housing. Burning outside, simply means you cannot use simple electronic parts like resistors. 

Any ideas on how to burn Dyneema fibers in water without the need to replace the burning element (resistor) for each deployment? 

Haha ya my bad I kinda thought that part was implied. Most my projects start out with me thinking they will be easier than they end up.

Was just saying it's not impossible and people have done it before. Not sure I can easily find the one I'm thinking of but have seen more than one. Usually on deep sea cameras.

Bet there's a way you could have the burnwire enclosed within a sealed vessel attached to a preloaded spring/magnet. In such a way where your magnet provided the coupling to your anchor and then when the burn mechanism is triggered the spring provides enough force to overcome the magnet.

I've never tried melting nylon at any real depths under water, but I have successfully gotten it to work in a bucket of water (we briefly looked at a possible system for crocodile cameras). I realise that once you start talking deep water, things might get dramatically different, but the principle was to cover the nichrome coil and fill any voids in dielectric grease and then pot it, with the nylon line in place (I also covered the nylon in silicone spray so that it could slip easily out of the epoxy once it was set). I was actually surprised that it worked to be honest, but it did...It's certainly possible that over time and with pressure changes etc., saltwater could get into any crevices and cause havoc, but you could potentially pot the entire mechanism in a vacuum chamber to help reduce this prospect. 

Anyway, I think you could possibly get it to work, but it'd no doubt take a bit of exploration...

Cheers,

Rob

Very nice work on the cubesat burnwire too @BrettMargoSupplies ! That's really low current to melt the dyneema!

It is not only some water depth (say 30-100 m, as applications that go deeper can afford a full-blown acoustic releaser) but also repetitive deployment. I guess, cubesats are only deployed once. I look for something that can be rearmed solely by replacing the drop-weight (including the attachment rope).  So the part where the Dyneema is  attached must be in water (mabe u-shaped), strong enough to hold a couple of kilos (in water) and maintenance free. 

Thanks @Rob_Appleby. Hasn't been fully tested just yet but the math checks out and we have purchased components that use that amount.

You are correct @WMXZ deep sea release mechanisms have a completely different set of design parameters. Likewise with terrestrial and space mechanisms. CubeSat releases are typically only used once but we design them to be refurbishable/reusable for testing purposes. 

One difference I discovered while testing is the cubesat mechanisms don't need to withstand as much shock loading. For larger mammals I've been experimenting with using Vectran (Used in parachutes) instead of dyneema. Works better but much harder to cut.

When this project is finally done I'm moving onto Shape Memory Alloy mechanisms. Which was what I actually wanted to do before this very lengthy burnwire detour.

Made this a long time ago but never shared it.

Leveraging CubeSats to Design Better Wildlife Technology