Generic question

[kinetic]

Hello all,

I haven't dived a rebreather but would love to when cash prevails....but in the mean time, could anyone help with a few questions? I have been trawling various sites but one or two short answers from those at YD could be useful.

I know so little but can someone sweetly explain the various rebreather options, OC, trimix etc. OC vs CC is OK...I have that sussed in my head.

I am a bit curious, knowing little about rebreathers at this time but, I understand that the comp on board top range models, will adjust the pp of O2 as you descend/ascend. Am I right in thinking that the trimix rebreather does exactly the same as diving trimix OC and just keeps the N down?

Also, what happens with a big helium load....surely that can bubble too....any posts ot thoughts on that?

Thanks all.

kietic

[nigelH]

Originally Posted by kinetic

I am a bit curious, knowing little about rebreathers at this time but, I understand that the comp on board top range models, will adjust the pp of O2 as you descend/ascend. Am I right in thinking that the trimix rebreather does exactly the same as diving trimix OC and just keeps the N down?

You have two gases available to a CCR. Pure Oxygen and a Diluent. The diluent supplies the inert gas fraction of the loop but has some oxygen in it to make it breathable. If your diluent is air the loop is always a nitrox with the oxygen percentage adjusted by either a manual, automatic or mechancal mechanism. If the diluent is a trimix the ratio of nitrogen to helium in the diluent is mirrored in the loop. So if you have 20/40 trimix 40% N2 and 40% He on the surface the loop might be 70% oxygen so the rest will be 15% N2 and 15% He. At 70 meters the oxygen would be adjusted to something like 1.3 bar so 16% and the remaining inerts would be about 42% each. The ratio of the inerts in the diluent is the ratio of the inerts in the loop. Only the oxygen changes.

I think you need to research trimix before this makes much useful sense.

PS: Stay away from rebreathers. They are adictive.

[jptaylor9]

Originally Posted by kinetic

I am a bit curious, knowing little about rebreathers at this time but, I understand that the comp on board top range models, will adjust the pp of O2 as you descend/ascend.

They will aim to keep your ppO2 constant, at "set point" you've selected.

Defaults on the APD units are 0.7 & 1.3.

[Odin]

Then there is SCR..
choose your mix.
Dive to the safe MOD of that mix.
Not as complex - not as ££££!
Lots of modifications to keep you amused...
maintence/set up time roughly similar..
But I can get a w/e diving out of a 10l - and get nitrox fills from the club.
Not a deep dive set-up, but can (modified) run a high deco mix.
Warm air too!

[Freeflow]

Originally Posted by Odin

Dive to the safe MOD of that mix.

And you can play with the bottle mix and flow rates to tune your mixes.

Originally Posted by Odin

maintence/set up time roughly similar.

Having looked at an Inspo I would say that the Dolphin [my unit] takes a lot less time to set up and test. Also you get the duration of twin 12's for the weight of a 15. I went semi closed as a gas extending device [although I also dive OC trimix].

CCR's do offer a lot of advantages over semi closed, but as I am a luddite I won't trust electronics to control the mix for me. SCR's are passive systems in that you set the cylinder mix and that will maintain a fairly standard loop mix that you breathe. You can dive a SCR without a pO2 monitor, but it's a better idea to have one.

If you fancy it you can modify a SCR into a CCR [Narked@90 is the man to talk to on this], and it is something I will look at in a couple of years.

Also SCR divers are a nicer bunch than the CCR oiks...