My first (and unpleasant) rebreather experience

[Jax]

I wouldn't be able to say but I know it has been a lot. My try-out dive was scheduled for 20:30 and they've basically been doing it since 11:00. From what I understand, the unit would have been in almost constant use (by noobs like me) for most of the day.

Gil,

If they had been using the same scrubber all day long it more than likely was CO2 breakthrough, which would lead to the hyperventilation etc. After going back through the posts I recall deefste also said that he felt short of breath during his one minute prebreath.

Chris

How many hours would one expect on a scrubber?

 

[TechBlue]

How many hours would one expect on a scrubber?

I don't know what the scrubber duration is on the Poseidon each unit (and) scrubber is different.

 

[battles2a5]

In the video released with Bill Stone, he states the scrubber should be changed when the O2 supply needs to be refilled. Not suppose to do one without the other.

 

[Gill Envy]

I wouldn't be able to say but I know it has been a lot. My try-out dive was scheduled for 20:30 and they've basically been doing it since 11:00. From what I understand, the unit would have been in almost constant use (by noobs like me) for most of the day.

Gil,

If they had been using the same scrubber all day long it more than likely was CO2 breakthrough, which would lead to the hyperventilation etc. After going back through the posts I recall deefste also said that he felt short of breath during his one minute prebreath.

Chris

If that is the case, it's super lame and inexcusable that they would not change out the scrubber with at least a modicum of conservatism.... LAME!

g

 

[Jax]

So is this one of those questions to ask someone familiar with a Poseidon?

 

[Cave Diver]

**** General Info and not intended to reference a specific unit ****

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.

Pure 02 has a metallic taste to many people. Depending on the injection method, you may have been tasting that. Another possibility is that you were tasting anti bacterial cleaner (they WERE cleaning the loop between people, right? :shocked2 or possibly some of the chemical scrubber.

Breathing issues could be from not knowing how to breathe on the loop and having too little volume in it. I had a ton of OC experience and when I did my first few hours on CCR I always felt a little uneasy because I didnt think I was getting enough air.

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.

Unlike OC, exhaling does not help you sink because you're exhaling into another BC (counterlungs). Odds are you were underweighted for CCR. You're hyperventilating and swimming to stay down. My guess is you had CO2 breakthrough because of lack of dwell time (I'll explain later).

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.

I'm guessing dwell time CO2 breakthrough for the headache and nausea.

PO2 of .4 for this dive isn't something I would have done. We set PO2 @ .4 for pre-breathe and leave it low for immediate descent. Anytime spent shallow and PO2 is set to .7 min. At a .4, if the O2 sensors are wet, or out of whack and have a .2 variance, you're run the danger of running your PO2 too low. Not everyone follows this theory tho.

Goodness, the responses are flooding in faster than I can respond.

I've answered some of these questions in my previous post but just to comment on this one, I think it was indeed a "wham bam thank you maam" approach. We were briefly explained the basic workings of the rebreather, what minimum loop volume meant and how to achieve it, what the readings on the console meant and that sort of thing.

I should also think that it was patently obvious that I was not having a good time. At one stage I went back up to the surface and told the LDS owner that I was short of breath and struggling to get down. She said that she didn't know why I'd have been short of breath and that I should just exhale through my nose to get down. I did, but like I said, every next inhalation would then result in diluent being added.

Like HowardE I'm also curious to know why ppO2 setpoint of 0.4 is too low.

All of this explained in my previous points.

I don't know what the scrubber duration is on the Poseidon each unit (and) scrubber is different.

Considering the short duration of most of these types of dives and the time spent between them prepping the user, I doubt the scrubber was expended. More likely that it was partially used and rapid breathing decreased dwell time causing a momentary breakthrough.

In the video released with Bill Stone, he states the scrubber should be changed when the O2 supply needs to be refilled. Not suppose to do one without the other.

I'm hardly in a position to argue with Bill Stone, but have to disagree with this as a hard and fast rule. Maybe a rule of thumb, but both need independent monitoring. Some typical cave profiles can burn through 02 MUCH faster than scrubber life and I can do a long deep dive and have a half full 02 bottle, yet still need to change scrubber before diving again.

Thanks for all the responses. Herewith a few comments on those and answers.

Oh, it's quite possible that my understanding of minimum loop volume is incorrect and that I was indeed not running minimum loop but, seeing as I was struggling to get negative or neutral, I exhaled through my nose on almost every other breath and on the next inhalation the rebreather would add diluent. That's how I assumed that I was on minimum loop volume. Am I understanding the concept incorrectly?

Min Loop means that on a full inhalation you're practically bottoming out the counter lung. Exhale should re-inflate it, but not fully unless you have a small CL or a huge tidal volume. To me this was a very strange feeling and took a couple of hours to fully get the hang of it where I was comfortable. CCR divers rarely exhale through the nose. This is wasting gas and defeats the purpose of a rebreather.

I wouldn't be able to say but I know it has been a lot. My try-out dive was scheduled for 20:30 and they've basically been doing it since 11:00. From what I understand, the unit would have been in almost constant use (by noobs like me) for most of the day.

Time elapsed does not equate to time on the scrubber. Actual breathing time would have to be evaluated and compared to the recommendations for the particular unit.

The instructor, actually no, the LDS owner who happens to have qualified for rebreather diving recently was in the water with the two of us the entire time. The instructor was sitting by the side of the pool observing.

Better than nothing, but I personally think there should be one on one, not two on one for this type of exercise. Not sure what the standards are on this.

You're probably right. If this was a mild CO2 hit, I'd hate to ever have to experience a severe CO2 hit. I was actually thinking of it just prior to ending the dive. Imagine you feel this way 30m down. I'm not sure I'd have survived the trip to the surface. Pretty compelling argument for how vital it is that you understand your rebreather and get PROPER training.

IMO, it was likely a mild hit, and AMEN to the rest of it.

Yes, we were told to do the 5min prebreathe but in reality we probably didn't do it for much more than 1min, by the end of which I was already short of breath and should have aborted the exercise.

Unless it was really cold, this might not have been that big an issue but I'm still a stickler for following the 5 minute rule. You prebreathe it to get the chemical reaction going on the scrubber and to make sure there are no issues before getting in the water. Odds are the chemical reaction never had time to shut down between users, but still...

Just one more question, am I correct in thinking that the console showing ppO2 of 0.4 means only that there is the correct pp of O2 in the mix (it was set for 0.4) and says nothing about the other gasses. In other words, for all I knew I could have been breathing a mix of 40% O2 and 60% CO2, the console wouldn't have shown me?

It shows the O2 content only.

Dwell time: The amount of time your exhaled gas spends in contact with the scrubber material. The longer its in contact and the more material, the less chance of C02 breakthrough. Rapid breathing on a partially used scrubber means there is less absorbent and the CO2 has less contact time with that limited material. CCR requires slow, steady breathing to be the most effective. Rapid shallow breathing can cause breakthrough even on a new scrubber.

 

[Jax]

Thanks! Especially for the minimal acronyms!

 

[D_B]

Seeing as this thread is likely to be viewed by people with less knowledge of rebreathers , with that in mind, what are some of the books on them would you recommend ?

 

[battles2a5]

I'm hardly in a position to argue with Bill Stone, but have to disagree with this as a hard and fast rule. Maybe a rule of thumb, but both need independent monitoring. Some typical cave profiles can burn through 02 MUCH faster than scrubber life and I can do a long deep dive and have a half full 02 bottle, yet still need to change scrubber before diving again.

The Poseidon MkVI is a recreational rebreather. It was never intended to go in a cave. Per Stone, they chose the rule for their target market (e.g. the snorkels and pretty fishes set) since it is less to track and monitor, not for serious technical dives.

This being said, I have no idea why Stone chose this rebreather as the weapon of choice for his "Deep Cave Exporation" team last year. If I were a few thousand feet underground diving sumps, it wouldn't be with a recreational breather.

 

[Jax]

Okay, I'll bite. What makes a breather "recreational"? smaller?