My first (and unpleasant) rebreather experience

[TechBlue]

0.4 is way too low for a setpoint by the sounds of it you had a few things going on at the same time, as has been pointed out it is doubtful that you had minimum loop volume which would explain the shortness of breath because you are breathing against the static pressure of the counter lungs. You were also hyperventilating which on it's own can cause a CO2 hit which would also explain your symptoms. You may have overbreathed the scrubber too but I don't believe that is the case.

What kind of run through did you get before went for your dive? It sounds like you were at odds with the unit and yourself from the get go which of course is not going ot help your breathing. Was there an instructor in the water with you because from what you are telling us it must have been patently obvious that you were not having a good time and you should have been brought up to the surface to discuss what was going on. It sounds like the whole try dive was a "wham bam thank you maam, next!" kind of deal. Did you prebreathe before you got into the water and how many try dives had been conducted on the scrubber. I am not familiar with the Poseidon but for a shallow try dive the 0.4 setpoint makes me wonder how much attention was being paid to making sure you had any idea about what you were doing, nothing wrong on your part and I am sorry to hear that your first attempt on a breather was such a disaster.

 

[HowardE]

0.4 is way too low for a setpoint

(For those who don't understand who may be reading this... that is the equivalent 40% Nitrox at the surface.)

Why is that too low of a setpoint for a pool dive?

 

[Deefstes]

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

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).

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?

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.

Sounds perfectly reasonable, thanks.

How many people had tried the unit up to this point and how much time had they spent?

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.

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.

Maybe, maybe not. I'm not here to find a reason to blame the instructor though. Even if they did something wrong, the ultimate fault would be mine for doing something I'm not trained for and don't understand fully.

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!

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.

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.

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.

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.

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.

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?

 

[Deefstes]

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

0.4 is way too low for a setpoint by the sounds of it you had a few things going on at the same time, as has been pointed out it is doubtful that you had minimum loop volume which would explain the shortness of breath because you are breathing against the static pressure of the counter lungs. You were also hyperventilating which on it's own can cause a CO2 hit which would also explain your symptoms. You may have overbreathed the scrubber too but I don't believe that is the case.

What kind of run through did you get before went for your dive? It sounds like you were at odds with the unit and yourself from the get go which of course is not going ot help your breathing. Was there an instructor in the water with you because from what you are telling us it must have been patently obvious that you were not having a good time and you should have been brought up to the surface to discuss what was going on. It sounds like the whole try dive was a "wham bam thank you maam, next!" kind of deal. Did you prebreathe before you got into the water and how many try dives had been conducted on the scrubber. I am not familiar with the Poseidon but for a shallow try dive the 0.4 setpoint makes me wonder how much attention was being paid to making sure you had any idea about what you were doing, nothing wrong on your part and I am sorry to hear that your first attempt on a breather was such a disaster.

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.

 

[battles2a5]

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.

So, 1 min pre-breathe resulted in problems. You surfaced and stated that you were not feeling good. And the instructor was not in the pool. This never should have happened. It sound more and more like the scrubber was shot. And it should be obvious by now that qualifying to dive a rebreather does not make you qualified to teach or supervise on a rebreather. This dive should not have happened, and the instructor and LDS owner should have caught any one of the myriad of signs that something was amiss.

As for minimum loop, you would be bottoming or nearly bottoming the lungs with each inhale if you were running ML. I'm not sure what size the lungs are on the Poseidon, but if you are exhaling out of the loop then inhaling from the loop if would likely hit the ADV as you described.

In terms of a .4 being too low, it is just a risk. Rebreathers are riskiest at or near the surface. PO2 changes dynamically with depth and the pressure gradients have the largest effect in the 1st ATM from the surface. So a .4 at 3M is less at the surface. Let's say you had an O2 failure and the PO2 dropped to .2 before you realized it. If you went to the surface you would be hypoxic when you got there and run the risk of passing out. Passing out is not a huge problem, it's the resulting drowning/dying that we try to avoid, especially during pool try-dives.

 

[TechBlue]

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


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

0.4 is normally used at the surface and switched to 0.7 when you enter the water, shallow dives with a rebreather are dangerous because most changes happen within the first ATA breathing a 0.4 setpoint could result in a hypoxic mix quite quickly if the solenoid did not fire.

From the additional information you provided it sounds like the scrubber you were using was shot, if you had prebreathed on the surface you would also have experienced a shortness of breath, every molecule of O2 absorbed creates one molecule of CO2 which needs to be removed by the scrubber, when the scrubber is depleted CO2 will break through the scrubber and if you don't know what to look for you will be none the wiser, a setpoint of 0.4 leaves you absolutely no margins.

A freshly trained rebreather diver has no business conducting try dives how many units were in the water at the same time and how alert was the instructor while all this was going on?

 

[HowardE]

He dived to ~10 feet (and I'm sure that in a pool that probably had a MAX depth of 3m {9.84 feet}) . Regardless of all of the other factors and situations involved with his problems. The .4 setpoint really isn't pertinent to his problems. While it may be a DIFFERENT problem in and of itself. Focusing on this is IMO a disservice to those who follow along who don't understand rebreathers.

 

[TechBlue]

He dived to ~10 feet (and I'm sure that in a pool that probably had a MAX depth of 3m {9.84 feet}) . Regardless of all of the other factors and situations involved with his problems. The .4 setpoint really isn't pertinent to his problems. While it may be a DIFFERENT problem in and of itself. Focusing on this is IMO a disservice to those who follow along who don't understand rebreathers.

?? I don't follow you ??

 

[battles2a5]

He dived to ~10 feet (and I'm sure that in a pool that probably had a MAX depth of 3m {9.84 feet}) . Regardless of all of the other factors and situations involved with his problems. The .4 setpoint really isn't pertinent to his problems. While it may be a DIFFERENT problem in and of itself. Focusing on this is IMO a disservice to those who follow along who don't understand rebreathers.

I don't think anyone is saying this was a factor in his experience. I do think it is emblematic of how poorly these try dives were conducted. You shouldn't be putting an untrained, unsupervised diver in shallow water on a CCR at .4. So it is a separate problem, but all the problems are pointing to the same place.

 

[Deefstes]

I think HowardE is just pointing out that the low setpoint was not part of the reason why I was experiencing these problems. That said, I find the discussion around the setpoint interesting as well. Serves as another reminder that you just don't know what you don't know - until you get a CO2 hit and ask around on ScubaBoard, when it becomes clear that you really didn't know squat

At any rate, thanks for all the responses. I hear what you're saying about this dive being a bad idea and all but I'd have to add that it was ultimately my choice to do the dive and to continue with the dive when things didn't feel right. Ultimately, safety is the responsibility of the diver, not the instructor or DM or boat captain. I won't be diving CCR any time soon (if ever) but hopefully this experience will have reminded me to abort any dive where I'm not comfortable.