Nebie Rebreather Question

[caveseeker7]

The purpose for the rebreather was to get colse enough, however I am not interested in deep diving (100-150ft max) and not able to get pure 02 in most places as I go to budget resorts for my diving.

I apologize if I sounded hard referring to you as a troll, but the question seemed just too odd.

Pure oxygen might be easier to source than nitrox in many places as almost all dive outfits have (and many are required to have) medical O2 for emergencies. Sometimes it's available through commercial suppliers, sometimes through hospitals. Some places have restrictions on O2, but with some careful planning most problems can be avoided. Have a look at links section of the site above, you'll find one to an article by Jeff Bozanic about his RB trip to the Bikini Atoll.

The Sport Kiss is a unit worth looking into. Small and fairly lightweight, designed for the diving you inteent to do. Without the tanks it'll fit into a JanSport backpack as a carry on. It is also the least expensive CCR available, and the operating principle is not unlike most SCRs. Quite simple and easy to maintain. You'll find several articles in RBW's article section, with more coming soon.

Also, have a look at the New To RB section, it'll aid in a better understanding of how they work, risks, pros a and cons.

 

[DA Aquamaster]

Yes but we all know that practice is dangerous and foolhardy. The risk of spiking your po2 should you require a fush or even clear your mask you could tox out very easily.

Dangerous? Foolhardy? I'd argue it's a lot safer from an oxtox perspective to dive to 150' on 25-26% Nitrox than it is to dive to the "approved" limit of 132 ft on 32%. If you have to flush the loop, at 150 ft on 25% your spike will be to no more than a PPO2 of 1.38. With 26% it would be 1.44 and both are much lower than the 1.6 spike you would get at 130 ft on 32%. With either 25% or 26% the loop FO2 for the normal portions of the dive will be around 21-22% with a PPO2 of 1.16-1.22 at 150' and it is essentially an "air" dive.

The critical limiting factor at 140-150 ft is instead the flow rate and you will need to moniter the loop FO2 so that you do not pull the FO2 below 21%. This is a potentially serious consideration if you are using an orifice intended for 32%, and perhaps not even an option if your O2 consumption is high in the first place. If your activity level is higher than planned, it may cause you to have to flush the loop and possibly abort the dive.

If you think about it, the 132 MOD for a 32% nitrox SCR rebreather is based on the idea that the loop O2 is going to drop about 4% as the PPO2 is then reduced to 1.4 at 132 ft rather than remaining at 1.6 for the entire deep portion of the dive. Afterall, no one is going to advocate a continuous 1.6 PPO2 for the working portion of the dive.

Going slightly deeper on 32% (loop FO2 of 28%) does elevate the O2 exposure as it would give a PPO2 of 1.47 and a potential spike of 1.68. But 1.5 for brief periods is not problematic and a spike to 1.68, while higher than the recommended 1.6, is not going to produce an immediate O2 hit and in many cases could be avoided by ascending 8-10 ft. before clearning your mask, etc. Is there more care and planning required? Yes. Does more consideration need to be given to O2 exposure? Definitely. Is it dangerous or foolhardy? Not anymore than choosing a rebreather over open circuit in the first place given the larger number of things that can go wrong, the greater knowledge required to operate it, or greater number of factors and operating limitations that need to be considered.

What needs to be stated to any diver considering a rebreather is not some type of "safety terrorism" comment but rather the fact that diving a rebreather demands more than just taking a course and learning the procedures specific to a particular rebreather. It also demands that you thourougly understand what is happening with the unit, that you fully master the planning involved and that you fully and clearly understand the impact a specific situation will have on the system as a whole.

If a diver is not comfortable with math, physics or technical concepts, a rebreather, whether it is SCR or CCR, is not for them.

 

[wedivebc]

Dangerous? Foolhardy? I'd argue it's a lot safer from an oxtox perspective to dive to 150' on 25-26% Nitrox than it is to dive to the "approved" limit of 132 ft on 32%. If you have to flush the loop, at 150 ft on 25% your spike will be to no more than a PPO2 of 1.38. With 26% it would be 1.44 and both are much lower than the 1.6 spike you would get at 130 ft on 32%. With either 25% or 26% the loop FO2 for the normal portions of the dive will be around 21-22% with a PPO2 of 1.16-1.22 at 150' and it is essentially an "air" dive.

Since your orignial post suggested using 32% at 150ft the po2 spike one could encounter is 1.78 which is well above the 1.6 limit recommended by NOAA and allowed by any dive training agency I know of. I suggest that recommending this type of behaviour on scubaboard is against the TOS and I am quite surprised you are endorsing it in the first place. There are ways of extending the range of a scr rebreather but exceeding MOD is not one I would use.
You should recheck your math. If at 150ft a diver has a need to flush he has to ascent 20ft to be at 1.6. I don't deny in an emergency a brief po2 hit is very low risk but I one should not put them selves in that position. There are a number of reasons one might need to add fresh carry gas including a suspected scrubber breakthrough. In that case the diver might be physically incapable of ascending 20ft untill he had done a couple of flushes.
I know of no rebreather diver who advocates this practice.

 

[caveseeker7]

When looking at the depth limit/pO2 limit on the Dolphin one has to consider that the basic design of the unit is about 10 years old, as are the ratings. In the past, a pO2 of 1.6 ata was indeed acceptable. Before that, pO2 values of 1.8 ata and higher were thought of as acceptable.

While DAA's practise might not be what is taught in PADI's Dolphin class, a thorough understanding of math, physics and risks involved mean it isn't necessarily a more dangerous way to dive.

Personally I think gas mixing rebreathers are more suited for these profiles as they offer constant pO2 in the mix, however if the diver has the knowledge and skills to use a SCR as described I have no problem with it. Just as long as full responsibility is taken for such actions. Nor do I believe that other people should restrict either the diving or the discussion of it.

 

[wedivebc]

What needs to be stated to any diver considering a rebreather is not some type of "safety terrorism" comment but rather the fact that diving a rebreather demands more than just taking a course and learning the procedures specific to a particular rebreather. It also demands that you thourougly understand what is happening with the unit, that you fully master the planning involved and that you fully and clearly understand the impact a specific situation will have on the system as a whole.
.

I find that "safety terroism" remark offensive. We are all here to promote safe diving practice. If this board's moderators are going to advocate anything unsafe what happened to the TOS?
I don't see any rebreather training in your profile and wonder why you are even taking this position.

 

[wedivebc]

While DAA's practise might not be what is taught in PADI's Dolphin class, a thorough understanding of math, physics and risks involved mean it isn't necessarily a more dangerous way to dive.

.

So who's rebreather class is it taught in? I teach ANDI and they don't advocate it.

 

[BigJetDriver]

Ladies and Gentlemen,

As we all know, Scuba Board has one mission when it comes to diving, and that is to promote safe diving practices. Our TOS plainly states that scuba diving, and, by extension, re-breather diving should NOT be learned from messages on any Board.

That having been said, I would have to point out that any exhortation to a student to seek out a good instructor who works for an established training agency is not "safety terrorism", but merely common-sense wisdom.

We do appreciate the fact that you folks are trying to guide a person with no knowledge of re-breathers in the right direction in a positive and pleasant manner!

R. Davie
Scuba Board Staff

 

[caveseeker7]

So who's rebreather class is it taught in? I teach ANDI and they don't advocate it.

Some people collect hours instead of tickets.

If memory from my CSU training serves me right, ANDI recommends a max. pO2 of 1.45 ata for the dive, but allows up to 2.0 ata for emergency accent (SA50 for bailout from 30msw). That's still well below the pO2 spike of 1.68 ata quoted by DAA, or 1.78 ata you mentioned.

So if a loop flush is needed, and done while ascending to a safe depth, I don't see any more of a problem here as on OC. He pointed out that proper training, skills and understanding are required.

Mind you, I don't endorse it, I might not do it myself, and as stated, I think there are better rebreathers options for Peter.

 

[wedivebc]

Some people collect hours instead of tickets.

.

That's what I like about ANDI, you can't get the latter without the former.

If memory from my CSU training serves me right, ANDI recommends a max. pO2 of 1.45 ata for the dive, but allows up to 2.0 ata for emergency accent (SA50 for bailout from 30msw). That's still well below the pO2 spike of 1.68 ata quoted by DAA, or 1.78 ata you mentioned.

So if a loop flush is needed, and done while ascending to a safe depth, I don't see any more of a problem here as on OC. He pointed out that proper training, skills and understanding are required.

Mind you, I don't endorse it, I might not do it myself, and as stated, I think there are better rebreathers options for Peter.

That is correct but one does not intentionally put themselves in a position where they could be periodically exposing themselves to a higher PO2 possibly without even being aware of it. The high po2 allowance in the CSU is for bailout out. At that point tracking CNS clock is not a big issue. Diving in such a way that might have repeated high po2 (even if they are brief) exposures makes tracking CNS clock impossible.

 

[gjw]

Hi Peter

As a SCR or CCR may not be what you are looking for due to $$$ or availability of Nitrox or pure O2 at your destinations, a double hose reg (as suggested by caveseeker7) might be an option. Have a look at the one made by Aqualunh:
http://www.aqualung.com/products/mistral.html
;-> gjw