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Hi i am not a CCR diver just OC. Still that feeling i know it it happens sometimes and actually it did during a dive 2 weeks ago.Hi everyone,
I don't know if this is the right section, but since no incident occurred, I don't believe this categorizes as a "near miss" (well, at least I hope so)
I'm posting this more as a question to understand if something like this occurred to anyone else, and if it was indeed just a psychological/narcosis effect, or something more serious (pre-hypercapnia or bad gas).
A little context about me and the previous dives
A bit of context about me: I'm a lake diver, I dive quite often (2/3 times a week), and I recently started CCR diving. I've just shy of ~50 hours with my rebreather, so I probably qualify as a super-beginner and still have a lot to perfect compared to when I dive OC (and proper weighting is one of the things where I still struggle with, and this usually leads to being over-weighted).
The dive itself was a really simple and pretty "standard" one for me, in a spot that I know like the back of my hand and where I've dived several times, both on OC and CC; it was in the afternoon (so no morning stress or whatever), and with two buddies I trust and I often dive with.
It was a recreational dive; the other buddies were on OC, and we all used air.
I used the rebreather so that I could practice trim and buoyancy, as I had been using a new configuration on my last two dives (a steel 12-liter bailout tank and a lighter undersuit).
It was the third dive on the same scrubber: the first one lasted ~54 minutes with air, the second one ~43 minutes with 20/30; both with an avg depth of 20 meters (66 fsw).
All dives (including this one) were over the same week, and the scrubber was always carefully sealed between each dive; by the way, the three-dives / three-hour rule on a single scrubber is what I normally follow and never had any issues so far.
Setpoint was 1.2 on the bottom, and CNS before the dive was (obviously) 0% with a surface interval of about 42 hours.
The dive
It all started without problems and was pretty straightforward: simple shore entry, then we went down to our target depth (40m - 133 fsw), stayed there just for a couple of minutes, then ascended to ~30m (100 fsw) and continued our dive.
They were slowly riding their NDL being on OC, I was staying more or less at the same depth; I was the last one in the group, the visibility was ok below 25m (82 fsw), and the thermocline was around 16m (52 fsw), above water temp was around 20-22 °C (68-72 °F), below was 9 °C (48 °F).
After 25 minutes or so, the first buddy who was leading the dive signaled to turn back and slowly ascend, and that's where we got separated, due to poor visibility and me being a bit deeper than them, and I didn't think they would turn back so fast.
But, anyway, no big deal, I searched them for a minute, then continued alone to reach the entry point and the anchor we left at 5m (16 fsw) at the beginning of the dive.
I knew that they wouldn't panic or anything, and in our pre-dive briefing, we decided that we would be autonomous in case we got separated and we would reunite either at 5m (16 fsw) or the surface.
Since I was on CC, I decided to stay at the same depth (it was around 25m - 82fsw) to have better visibility.
In hindsight, I probably swam a bit too fast and felt very heavy. In fact, after just 15 minutes, I had already reached the entry point (there is a fixed rope that goes from ~3m (10 fsw) down to 50m (165 fsw), to ease navigation).
"Something bad is going to happen"
And this was when something strange happened: after a quick dip to 35 meters (to see another little part of the rocky wall before ascending and ending the dive), I suddenly felt a sense of dread and fear, as if something bad was going to happen and that I needed to get out of the water quickly.
I don't know if this accounts for the term "a sense of impending doom", but it was unpleasant for sure.
In addition, I felt my heart rate increasing rapidly, and I could sense my heart pounding through the drysuit.
But again, no panic and no "grasping for air" or whatever. I then decided to ascend, and once I reached 16m (52 fsw) all symptoms subsided; so I decided to continue diving following another wall around 18m (60 fsw) for 4-5 minutes, so maybe I would find my buddies and end the dive together (since that wall is the "return" route we usually take).
I didn't find them, but I felt, again, the same sense of dread, this time when I was just 20m (65 fsw) deep; so I decided to go straight up following the floor instead of going back to the rope, then continued to the anchor at 5m (16 fsw), and holding my safety stop.
This time, the sense of dread didn't subside (even though it was "lighter" than the first time at 35m/114 fsw), and I also bailed out to see if something changes (but after a minute or so nothing changed so I decided to go back on the loop).
(and, thinking back about the fact that I was able to hold the 5m/16 fsw stop with a normal volume in the counterlungs, while breathing from the bailout regulator, makes me think that maybe I was surely over-weighted...)
Finally, I exited the water by slowly ascending from 5m (16 fsw) to the surface, and found my buddies waiting for me there (they finished the dive just 5 minutes before me), and almost immediately, all symptoms disappeared.
And no after-dive effects as well: no headaches (as I think I would have had if I had increased CO2 retention), no restlessness or flu-like symptoms like a subclinical DCS, and no "long-lasting" fear.
I have no idea what happened and why; underwater, I thought of everything, including:
- Bad scrubber or channelling
- Increased ppO2 and possible CNS toxicity (but the ppO2 stayed at 1.2 with few spikes to 1.3 when descending)
- Bad loop due to something in the tank (maybe carbon monoxide or any other contaminant)
- Bad loop due to me burping twice during the dive, and so I was rebreathing some hydrogen or methane in the mix
- And honestly, I didn't think about narcosis, when on OC I dove down to 55m (180 fsw) quite often and with no issues; yet I know that increased WOB on the CCR at depth is dangerous, that's why I limit the use of air to a maximum of 35-40m (132 fsw) and use light trimix for everything deeper (or with longer bottom times).
Conclusions
First of all, sorry for my wall-of-text (you could have guessed it would be a long write-up by looking at my other posts on SB).
Secondly, have you ever experienced something similar? And do you think this was a real near miss, and I risked by staying underwater so long after the first symptoms?
Maybe it was "just" a bad episode of dark narc, probably triggered by swimming faster than usual, being over-weighted, a bit tired, and a little cold after ~40 minutes in 9 °C (48 °F) water for about ~45 minutes with a lighter-than-usual undersuit.
Or maybe it was something else, but I still have no idea what
.
Almost all stated by @davehicks I sensed during my last CCR dive, at the end of dive, going shallow from 22 to 17 mfw at 8 deg.C.
