Loop plug for rebreather testing

[Captain Starfish]

It's all a bit of a black art but the short version is there's a gas path for exhaled/inhaled air on a rebreather and it needs to not leak.

But often they do, and hunting down leaks can be hard work.

I've just done some modifications to my mouthpiece and something leaks. Rather than leap to conclusions I thought I'd isolate the bit in question with a hose either end, which needed a threaded fitting with two different threads. Printed in ABS at 0.2mm layers, made the internal bore (which seals against o-rings in the hose fittings) a little undersize, skimmed it in the lathe, acetone finished the inside (just washed with a brush) to close pores and get a smooth finish and she works like a charm!

[briankb]

This is very cool!

Are you worried about de-lamination or stress fractures at depths over 50ft?

Also how does that hose assembly attach to the rest of the re-breather?

I'm a diver but never got into re-breather but hope to one day.

[barry99705]

This is very cool!

Are you worried about de-lamination or stress fractures at depths over 50ft?

Also how does that hose assembly attach to the rest of the re-breather?

I'm a diver but never got into re-breather but hope to one day.

This isn't a dive part. It's just to test the mouthpiece for leaks. The hoses are attached to each other, so it's a closed loop. Stick your head underwater and blow into the mouthpiece, look for the bubbles, replace gaskets as needed.

[Captain Starfish]

This is very cool!

Are you worried about de-lamination or stress fractures at depths over 50ft?

Also how does that hose assembly attach to the rest of the re-breather?

I'm a diver but never got into re-breather but hope to one day.

This isn't a dive part. It's just to test the mouthpiece for leaks. The hoses are attached to each other, so it's a closed loop. Stick your head underwater and blow into the mouthpiece, look for the bubbles, replace gaskets as needed.

Pretty much this. Although the mouthpiece has an isolation switch built into it. So as standard pre-dive procedure we do positive and negative tests where we fill or empty the loop, close the moutpiece, and come back a few minutes later to ensure it's holding pressure. I can also blow it up (the corrugations will stretch a little), switch it off, then dunk it to see where the bubbles are coming from.

Considering that these units regularly dive past 500 feet (not mine, I don't dive past about 200 for the moment), their performance under pressure is certainly an issue. Fortunately the loop is maintained at ambient pressure by adding gas during descent and dumping on ascent, much like a BCD, so there's no pressure differential. Whilst I have printed structural parts (some of the frame brackets, the brackets for routing cables and hoses you see in the photo above etc) I haven't yet printed anything that sits inside the breathing loop. Porosity and off-gassing of the plastic after extrusion are the two biggest concerns.

[stonewater]

never done rebreather diving only open water to 120 FT (PADI 1982 Cert) been a few years.... could you post pics of your complete rig? what do you have, a Hollis? rebreather diving is intense, you have to unlearn most of what you learned using standard tanks.

Tom C

[teoman]

Nice

[Captain Starfish]

never done rebreather diving only open water to 120 FT (PADI 1982 Cert) been a few years.... could you post pics of your complete rig? what do you have, a Hollis? rebreather diving is intense, you have to unlearn most of what you learned using standard tanks.

Tom C

Rig's an Inspiration Classic (aka Yellow Box of Death), but pretty severely modified with new case, new electronics, an added OCB (the big mouthpiece in the photo in the OP) and a few other bits and pieces.

[teoman]

So you basically only have the lungs and scrubber

[Captain Starfish]

Well, plus the gas supply (hoses, manifolds, 1st stages, gauges, MAVs etc) and the original Buddy wing.

Really all I've swapped is the harness, case, electronics and front loop (from DSV to OCB).

There's a homebuild somewhere in my list of future projects though