PARIS pumps up a Mk 2 release mechanism

Standard threaded rods?

A real boffin would have tapped his own rod.

Note: Any pedantic bugger who points out a tap is for holes and a die for rods can go get a sense of humour.

Forget antifreeze!

Use a low-temperature oil instead - but be aware that it could damage the rubber bung, so you may need to use a non-rubber bung instead.

Magnificent bodging

Lovely stuff Reg. It's great to read of some proper boffinry. Boffinry with bits and bobs from the "bits of string" drawer.

GPS

Isn't this thing going to be GPS tracked? Can't you run a signal off that to check that altitude and then just use a simple solenoid?

Anyway, do like the mechanical way of solving things, all good fun!

I know you've probably planned everything already but...

...coil the tube up into a spring shape for space savings? It'll still expand inside whatever drum or coiled tube you put it in. Also I'll second Parax; that PVC tube can probably have more than 50% of its mass drilled out and still retain enough shape and strength to function. That's if you don't decide to go posh and use a glass or carbon fibre tube (or old fishing rod).

If you want something super-light and you think you can form it around something (dowel and some grease paper possibly), model shops will sell a kit of thin glass fibre sheets and resin that's designed for constructing helicopter bodies and suchlike. It'd take a bit of work and sandpaper, but you would end up with a very lightweight fibreglass tube that can be as thin as its task allows. It probably wouldn't blow the budget either - I think I remember it being a tenner or so for about 4 1m/sq sheets plus bottles of thin adhesive/hardener.

Last but not least, I hope you can get a couple of nice telescopes to film this thing through. That or let any commentards know where and when with enough notice that they can bring theirs. This is daftness on a scale of painting a Robin Reliant up as a radio-controlled space shuttle and sending it heavenward on the most powerful non-commercial rocket built in Europe. It'd be a shame to not have a video of the fun.

How to smooth the threaded rod

You can use termoretractile plastic to smooth the threaded rod, and also to help seal it against the rubber.

2 cents.

What, no system test?

Its all well and good that you've unit tested the oxygen tube, but you must test the full assembly!

Otherwise, solid engineering.

Bah!

*Nobody* knew enough to take some smooth nickel plated brass rod and a die and make a suitable threaded piece?

What the hell is on the school curricula these days? (Latin wasn't on mine, but over the years the basic engineering they taught me in metalwork has proved of more use than a dead language).

Christ on a bike, so much for England, the cradle of engineering. This is what a "point, click, ship" culture gets you.

Antifreeze/antiboil

Remember antifreeze etc are hygroscopic (is that the right word? Water magnets, anyway) which may behave unpredictably at altitude - I'd look at an oil-based substance of some kind which at least should be less likely to trap water. Just a thought.

Here's a thought.

Do you even need the release mechanism to be inside the aircraft? That's a lot of weight saved if you were to, say, use an eye-hook or something on the back to pass a loop of thread through. Use some kind of double tubing to stop the string from tangling with itself as it goes down from the release mechanism, through the eye-hook and back up. On release, one end of the string is released/cut/burned through/whatever. You'd lose the release mechanism but have an aircraft that's a lot lighter.

Or alternately, attach a parachute and transponder to the release mechanism as well and have it find its own way down. Rubber Tubing Released Into Space doesn't have quite the same ring to it though.

Well, too bad...

... for the glass syringe idea then. I thought it was really elegant, simple, robust, light and small, and some vacuum grease would have solved both the leak and stiction problems (the one you used here would most probably have worked, did you test that?).

But at least with with this latter design you can use duct tape! So please, do.

When it boils it's gonna pop.

-- And you're worried about freezing?

Bah!

What's wrong with releasing it by tether? A few tends of thousands of feet of monofilament attached to a simple pin-and-loop and Bob's your mother's bother.

Talk about overthinking a problem.

Bike valve

Scheader valves have a rubber washer inside which might get screwed up by the cold or the antifreeze. the other kind of valve "presta", or sometimes called "high pressure" uses a plastic washer instead, and can hold more pressure.