Is Rebreather diving inherently more dangerous than OC?

Janos

Yes. But not by that much. My own theory is that this is due to the loop being saturated with water. Analox say that their analysers need to be adjusted by 5% if used in 100% humidity. I can't see why the same doesn't apply to breathers.

Janos

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Genesis

All oxygen cells are sensitive to humidity in the gas being sampled. There is no humidity compensation circuit in these devices (nor could there easily be on a self-powered fuel cell due to power requirements).

Is this a big deal? Only if you push your PO2s (reading low by a consistent amount presents you with a hedge against DCS, but at the cost of increased CNS O2 risk.)

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Gilles

Maybe not such a big deal, but it makes me feel a lot more comfortable when cells are reading what they are supposed to. In the last 6 months or so, I don't do any pre-dive calibration. I like to check on the way down @4.5m, and again on the way out. If any discrepancy, I'll calibrate right after the dive while cells are wet.

Result is .23-.25 and 1.05-1.1 in ambient air/O2 respectively, and about 1.6 @4.5m when cells are dry. Re-Cal isn't needed until a good 6-10 hours operation have passed. I take a marginal DCS risk to protect against under-estimating cells. Is that wise?

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Genesis

Depends.

CNS hits are of course really bad but DCS hits can be too.

Why not dive with the "understated" PO2 and fly at a "claimed" 1.0 - 1.2? You're under 1.4 this way and yet your deco is computed for a 1.0 to 1.2, which gives you both the CNS safety AND the DCS margin......

If going "to the wall" is important, then you need exact numbers. But is that really a good idea to start with?

Between sensor and measurement uncertainty you likely have a 3-5% error anyway. O2 sensors are only guaranteed to 1% within a 1.0 PO2 and 2% at 2.0 PO2 in most cases anyway, and the additive error of any amplification and ADC stages in the middle likely pushes this into the 3-5% range in total.

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket

Dawktah

As a newbie to the concept of CCR and only to take MOD1 in 2007. I think the inherent danger of CCR is its expense. Why? If one had an unlimited supply of money one could buy, boxes of sensors and cases of sofonolime for example. Are incidents on CCR attributable in design to these elements? I took inherent to mean its design not its use or user. OC being relatively inexpensive has less inherent dangers, except those mentioned freezing and free flow.

Just from reading, CCR is safer because you must take a bailout. Thus the inherent design includes bailout. OC has no safety other than a buddy, who isn't physically attached to you. All other errors are user and not inherent. Just a newbie ramble, I welcome responses. I want to learn as much as I can before I step into this realm.

Drmike

I dont think so.


Is this what Leon teaches???

IMHO you should calibrate and/or flush O2 and validate before every dive with dry stabble cells. You shouldnt be getting such a big error due to wet cells - they shouldnt be getting wet.

In my experience it is not easy or fast to get 100% in the loop during a dive - especially at the end of the dive during 6m stop. it can take quite a while to fully flush the loop of all inert gas - plus you now have 100% humidity/off gassing to deal with

I would trust surface cal result over those at 6m anyday.

IMHO and in my experience calibrating with wet cells = disaster

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Janos

I think there should be a 5% difference [1] between wet and dry cells. Ie cells calibrated dry underread by 5% on the bottom. And I don't mean wet as in flooded I mean breathing a loop at 100% humidity.

That said I like to calibrate in an "easy" environment rather than on a boat immediately post dive. Plus it tends to be a week or so between dives I would calibrate anyway.

But then, what's a ppO2 difference of 5% or so between friends? It makes bugger all difference to deco, for those with integrated computers, and for CNS it's not a huge issue either.

Janos

[1] - Source: The little table that comes with Analox analysers.

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Drmike

Yes but hes talking about calibrating post dive with wet cells. They are not stable if wet (water can drip off one or more of them, they can dry out - they WILL change)

Flush enough O2 at 6m and eventually you can get prity close to 1.6 - but ot takes time and uses a lot of O2. If I get 1.55 I am more than happy. If I get 1.6 then something is wrong! I presume flushing with O2 will get rid of some of the humidity as the fresh gas replaces the moist - at least for a moment

I agree that it makes bugger all difference anyway. Certainly not risking dodgy calibration (wet cells). At the end of the day if theres humidity in the loop then you havent got 100% - so why should you want/expect to see 100% on the displays?

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Get a girlfriend you sad twat - a Rebreather is an unfaithful mistress - dont blind yourself to her faults just because she goes down on you