My first (and unpleasant) rebreather experience

[Deefstes]

Hi all,

Yesterday evening I had the opportunity to do a try-out dive on a Poseidon Mk VI at our LDS in their pool. I have to be clear first of all that I know very little about rebreathers and that I am far from qualified to even consider getting training. I am AOW with only 65 dives to my name.

Either way, I thought it might be an interesting experience and decided to take the opportunity. And an interesting experience it was! I would love to know from rebreather divers if what I experienced was really to be put down as diver inexperience or if something wasn't wrong with the unit perhaps (or of course a combination of the two).

From the moment I switched to closed circuit I started feeling a shortness of breath. I wrote this down as unfamiliarity with breathing from a closed loop. I never had any issues breathing from SCUBA equipment, even when I just started my OW training but I know that some people (like my wife) do and I assumed I was experiencing that same anxiety now. The air also had an odd taste to it but I have no idea how to describe the taste. I wrote that down as the result of expelled air being scrubbed and breathed at a higher temperature than with OC.

OK, so those were the warning signs that I ignored (or didn't really know to heed). As I started diving I had trouble getting down (and I did have minimum loop volume). I may have been underweighted but I imagine that I was hyperventilating also which obviously wouldn't have helped. I did manage to get down by swimming but was positively buoyant all of the time. This wasn't a huge problem but it did mean that I had to work a little at staying down which compounded the breathing issue.

At any rate, the dive never exceeded 3m and lasted no longer than 3 minutes, by which time I was desperate to end this ordeal and get out of the pool. By the time I got out of the pool I had a splitting headache and I wasn't feeling very steady. I had to stop the car on the way home to get out and vomit and as soon as I got home I got into bed. By this morning I felt much better although there's still this lingering headache.

Do these sound like the typical symptoms of a noob who dives on equipment that he's only received a very basic explanation of or does it perhaps sound like there may have been something awry with the rebreather? I'm wondering if the scrubber didn't get flooded (or at least wet) sometime during the course of the day. Many non-rebreather divers have tried out this unit throughout the day and it wouldn't surprise me at all. The console however did show a ppO2 of 0.4 throughout my dive which, if I understand correctly, means that the mix was considered to be good by the two analyzers.

My wife also did the try-out dive at the same time (on a different unit of course) and she had no issues whatsoever. One of the divers before me was very vocal that he didn't enjoy the rebreather at all but he didn't elaborate on his experience of it and he didn't look the way I felt. I also don't know which of the two units he dived so I can't make any conclusion based on that.

I'm not trying to find a recourse or anything, I merely want to clarify this for my own sanity. I am fully willing to accept that I did things wrong and that this is the result of someone doing things wrong on a rebreather (even if only for 3min at 3m). But I would like to at least have some understanding of what happened.

 

[D_B]

Just based on what I've read here on SB about rebreathers, I would have been very worried about those symptoms, and after reading Jeff Bozanic's book "Mastering Rebreathers" , even more so.

Edit: I also am interested in doing a rebreather tryout dive .. just to do so, I'm pretty sure that I do not/will not ever have the discipline necessary to dive one (not to mention my dive number, or the cost)

 

[battles2a5]

Sorry you had a bad experience. Hard to say exactly what happened without being there, but here is what I imagined happening:

- Shortness of breath- this is a typical first reaction to getting on the loop. You are used to the positive pressure of an OC second stage. You need to breathe a little differently on CCR; long deep breaths and KEEP BREATHING. RMV means nothing on CCR so no need in trying to reserve gas. You get over this fairly quickly
- Odd taste- You are scrubbing gas with a chemical, so you will get a little bit of an odd taste. It may also be residual from the cleaning agents. Again, you get over this pretty quickly

At this point, nothing sounds out of the ordinary. Keep in mind that the MkVI won't go into dive mode without performing the checks (including pos/neg) so I doubt there was an issue with the unit. Your next comments were interesting to me:

- Minimum loop/ hyperventilating/ positively bouyant- First off, you probably were not running minimum loop (not a knock on you, just very odd that someone with no RB experience would have this nailed on the first go). Since you were that positively bouyant, you probably had a full loop (or just way underweighted). At this point, you are going to have to fight the unit to dive it. This will cause two problems; 1- you have to overwork to swim it and you can overbreathe the scrubber and 2- you were likely swimming in a head-down trim which will usually cause poor work of breathing, exacerbating point 1. If you were hyperventilating, you likely had a poor tidal volume (short breaths, not a full inhale and exhale) which will also cause CO2 retention.

- How many people had tried the unit up to this point and how much time had they spent? I will give the instructor the benefit of the doubt and ASSUME that he used a fresh EAC catridge (scrubber). It is possible that the scrubber was exhausted and you were just rebreathing your own CO2. This would definitely accelerate the scenario I pointed out above. The fact that another diver complained on the same unit makes me suspect this a bit more, but I really want to believe that an instructor or group of instructors could not be that careless.

Another factor here is instruction. Were you in the pool alone? Whoever was running the try dive should have been in the water with you helping you through this. Also, I think it is irresponsible to have you in the pool alone w/ a fully functioning rebreather! Running a .4 is too low. Most try dives that I know of are done with the unit to be configred as a O2 rebreather so you elimiate the risk of going hypoxic. Had you been given proper instruction on trim and breathing, I don't think this would have happened.

So in summary, my hypothesis is that you overworked and were breathing in a manner that is not conducive to CO2 removal. Swimming in a position with poor WOB only made it worse. If the catridge was spent, it would happen very quickly. The symptoms you describe sound a lot like a mild CO2 hit.

NOTE: One other question I would have- was there any exhaust near the pool (e.g. compressor, boats, car running nearby)? Another thing you need to be aware of on CCR are fumes. If you take a deep breath and jump on the loop, you can recirculate and CO in the air and take a nasty hit that way. At 3M this is unlikely to be the culprit but it is possible. It's important to take a full exhale just before getting on the loop.

 

[HowardE]

Hi.

Without having tried the Poseidon Rebreather... Let me say this.

Diving any rebreather at such a shallow depth will have an increased WOB as compared to deeper depths, since the pressure against the counterlungs is what aids in easier WOB. At shallow depths like 3m a normal scuba regulator would breathe MUCH EASIER than a rebreather. The max depth of 3m is extremely shallow, and wouldn't be a pleasant first experience if you ask me.

As for the vomiting, and headache. It sounds like a CO2 hit. Now the big question is. How many "try me" dives did they do on that unit, and on that scrubber? You also state you were practically hyperventilating which would push more co2 through the loop, and could most certainly be more co2 than the scrubber would be able to handle; especially if it was on the "more used" side of its use.

 

[battles2a5]

You know what, after re-reading your post I am suspecting a spent or flooded canister even more. But some of this doesn't make sense. EAC's are probably the MOST flood tolerant scrubber material out there. So it would really need to be flooded in order to knock it off line completely. You would prob be heavy in the water w/ a flooded can as well. If there were a problem w/ the scrubber, it would prob be spent and not flooded.

Another question- did the instructor have you do a 5 minute prebreathe on the unit? This is standard protocol and meant to catch any CO2 issues before jumping in the water. If it were a scrubber issue it should have been caught during the pre-breathe.

 

[TSandM]

It's interesting to read this. My one rebreather experience was on an MC90 (modified Meg) and was done in water between 10 and 15 feet. I was definitely underweighted. Nevertheless, I did not feel short of breath or as though I was working hard, and I didn't get a headache. Sounds like something just wasn't right with the unit.

 

[Deefstes]

Just based on what I've read here on SB about rebreathers, I would have been very worried about those symptoms, and after reading Jeff Bozanic's book "Mastering Rebreathers" , even more so.

Edit: I also am interested in doing a rebreather tryout dive .. just to do so, I'm pretty sure that I do not/will not ever have the discipline necessary to dive one (not to mention my dive number, or the cost)

Would you care to elaborate? I have not read much here on rebreathers yet and wading through all of it now to find what applies to my experience would just not be practical.

 

[HowardE]

The poseidon doesn't use EAC's. It uses a pre-packed scrubber from Molecular products.

 

[Jax]

[ignorant] What is WOB, please? [/ignorant]

 

[D_B]

As you can see from the other and much more knowledgeable posts, I have no experience with them .. but I do know that taste is one of the things to watch out for, and also any "funny" feelings ... but again, I have no experience with them

WOB = Work Of Breathing ... resistance in the breathing loop