Isobaric Countertransport And Mix Switch Strategies

[flw]

Genesis once bubbled...
[
I can certainly see the value of a rebreather in that kind of environment, in that a CC RB effectively holds the relationship between N2 and He constant, as the diluent is a constant, and the FO2 is varied to give the desired PPO2 from depth all the way up to the 20' stop (where it begins to fall off towards 1.0), where essentially the entire loop is full of O2. It would seem that you'd get MUCH more efficient decompression this way. The same dive computed on VPM-B with a maximum 1.3 PO2 and diluent of 15/55 gives me a total runtime of 84 minutes and only 42% CNS O2! That's a MAJOR difference! My concern on a RB dive like that would be OC bailout if the loop was to fail; carrying enough bailout around to cover a potential failure has to become a problem, does it not? [/B]

Assuming you're not going to go the alpinist route, a RB dive still has to be done assuming bail-out from the worst possible time. On a CCR, the commonest diluent ( around here) is a 10/50 heliar. Personally I carry a 7l of 16/50 and then a 7l of 50% and OC bailout on the 3l of O2 - which I feel suits most things - my buddy carries the same as me - I use this down to around 85m or so, sometimes swapping the 7 for a 10 of bottom gas.
It can become a problem, but you only need to carry enough to bail out with, rather than enough OC gas for the whole dive -

I have a 'normal' set of tables - working on keeping the 10/50 back until 6, a bail-out schedule, which using these gasses is very similar to the normal one, and I'd probably do that, at least until up a bit, and then an 'oh Sh*t' set of bail-outs which have no deep stops, but are essentially bend and mend type. In practise I've only ever thought about bailing out once, because the solenoid jammed on at 79m, but managed without coming off the loop in the end
Originally we used to swap diluents on ascent, but someone ( don't know who) noticed they felt better if they didn't, so we gave up swapping completely - which now has a more scientific explanation.

rgds

F

 

[Genesis]

.... once one gets into "deep" diving the total package of a CC rebreather starts to make sense.

Never mind the "what 'ya gotta carry" issues, which are significant. There is also the gas cost issues, which especially if you start using Heliair for the 70' bottle start to climb even more than they already are. The "stretch" of the RB becomes even more compelling as exposures increase, in that gas costs are rather high for He based mixes, and really, between the cost of several sets of doubles, stage bottles and such the cost of a CCR starts to look pretty comparable.

Why would one NOT go to a CCR RB in these situations? It seems to me that the case is quite compelling, if you don't have the "pollution" of a particular dogma to start with.

As someone who is dabbling in the truly "deeper and longer" realm, and considering the formal training path, if any, that I want to embrace to go along with it as my comfort level increases, I am seenig more and more to like in the rebreather category, especially when one starts thinking of this in terms of deep/long Trimix diving as opposed to Nitrox-based shallow stuff...

 

[MikeFerrara]

There is a lot going on with rebreathers these days. Just because not every one is using them doesn't mean nobody is.

With what we spend on helium we could justify the cost fo sure.

I'm still using OC because I understand it.

 

[Genesis]

.... to steal a GI phrase may be in my future.

That plus a hammerhead is starting to look awfully attractive; the only biatch is that I might decide I like it so much that I'd end up abandoning OC for anything other than spearing (its illegal to spearfish on a RB in Florida.)

That'd lead to a major closet-cleaning and eBAYing operation around here...

 

[flw]

There are lots of reasons for choosing to use a RB, however cost shouldn't be among them - they do carry significant risks, the biggest of which is CO2, common among all units, which as it stands will render you incapaple quicker than you will realise you have a problem.
Shallow the risks are greater than open circuit, deep, like somewhere around the 75m and deeper the risks are probably greater on OC - depends on who you ask - on an Inspiration it is very easy to overstep your skill levels early on particularly if you;ve only ever had problems in drills rather than actually happen. Having said that, I got tangled up today in an old line and some wire 24 minutes into a 68m dive, on OC it would have been an issue over gas, as it was, it was simply a case of disentangling then doing the extra stops - and being a bit cold along the way
My buddy has a hammerhead, it had two attempts at failing in the first 10 hours of use, one a frozen screen, I can't remember what the other one was, he's awaiting the new upgrade, and has gone back to the AP head inbetween - I'm going to wait for the new AP head, and trade in my unit 6 months later

 

[Genesis]

is why there isn't more effort on a CO2 monitor. Its a difficult problem from what I understand, but is it insoluble? I don't know....

 

[BRW]

Yo, Genesis, Mike, FLW and Others,


Could not agree with you more about using RBs across the
mixed gas and deco arenas.

As you ponder pros and cos, might I alert you that
Steam Machines (Prism, Topaz, USN Invader) is implementing
full up RGBM in their RB computer units.

Bueno,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr

 

[Genesis]

The Steam Machines RBs look interesting from a number of other aspects..... somewhere down the road, if I decide to get SERIOUSLY into deco diving, I might end up with one....

 

[BigJetDriver]

MikeFerrara once bubbled...
There is a lot going on with rebreathers these days. Just because not every one is using them doesn't mean nobody is.

With what we spend on helium we could justify the cost fo sure.

I'm still using OC because I understand it.

Yeah, if you look at a chart of average cost versus gas useage, it doesn't take long to pay for your CCR in helium cost savings, IF you go deep a lot!

 

[Genesis]

I start looking at how much the "T"s of He cost, and suddenly that nice new RB starts to look pretty attractive.....

I'm not where its important (yet), but in a couple of years I might be....