Interesting...
But first, this is a hypercapnia drill, not a hypoxia drill. IE: It is the crude equal to trying a rebreather on the couch without a scrubber, not with a scrubber loaded and not replenishing the 02.
And it's a good point, as I was just going to moot the question "how many have done a couch dive to experience hypercapnia?" (which in my estimation is far more dangerous... this is the thing that can trigger a cardiovascular event...)
As for symptoms in hypoxia, what we teach in aerospace is that you will never "lose" a symptom from experience to exerience, but you may add new ones as you continue to gain experiences. Example: I do my chamber ride every two years, and every year I see the same set of symptoms but now and then I add a new one to my matrix.. and two years later I can go and find that same symptom again.
Everyone is different, but here are mine:
(1): a flash of "heat" across the back of my neck, followed by a "pins and needles" feeling in my scalp.
(2): the sensation of "sun glare" as my vision deterioriates (a very early symptom, MINUTES before any other for me at 0.10 PP02)
(3) Basic sense of unease.
(4) This sense is lost and I feel "better"...
(5) recognition that coordination requires more effort
(6) then the spiral into where I cannot assist myself.
At a PP02 of 0.10 this takes me about 15 minutes (at rest).
At a PP02 of 0.05 this takes me about 2 minutes (at rest)
But EVERYONE IS DIFFERENT which is why personal knowlage is something that the aerospace community INSISTS on.
Dave
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Yes, I am aware he's talking about hypercapnia when he is describing breathing out of the paper bag in the chamber. He was using that as an example of something else the clearance divers used to have to do but no longer do. However, I understand that ANDI has a drill where they remove the student's scrubber in the pool?
You apparently know your symptoms of hypoxia very clearly. Now, you've obviously done hypoxia exercises numerous times throughout your aviation career. I highly doubt you knew with such certainty what your symptoms were after your very first hypoxia drill. My concern would be that someone does this exercise once, thinks they know, then experiences something else when the real thing happens but they ignore it because it's different or something they didn't experience the first time. Someone previously mentioned what about when coupled with narcosis and at this point you're experiencing mixed symptoms? How do you interpret those? There are many variables. Doing this exercise once could cause someone to falsely feel armed with information when they, in fact, have incomplete data.
I understand completely why the aviation industry would conduct hypoxia training with its participants. My brother is a commercial pilot and he underwent this training as well. He describes two purposes for this training, the first you've already mentioned, to recognize symptoms and therefore the onset of hypoxia and the second, to be able to function with it (i.e. deployment and donning of oxygen masks). The chances of someone in the aviation industry encountering hypoxia is a very plausible possibility, whereas, this is something that a recreational diver should never encounter. It circles back to proper training and diving practices. Once, when I first got my Meg, I did not push the QC connection for the solenoid in far enough. The distance from the truck to the dive site entrance was a bit of a walk and I was prebreathing as I walked to the entry. One of my predive checks is to let the loop breath down below a 0.4 setpoint and confirm that the solenoid is functioning. As I walked I kept hearing the solenoid fire but no PO2 increase on my handset. After it fired 4 or 5 times and my PO2 only diminished, I knew something was up with my solenoid and asked my buddy to verify that the solenoid was connected, it was connected but not pushed in all the way. Key points:
1) Prebreath your rebreather BEFORE you get in the water.
2) If an eCCR, prebreath at a setpoint of 0.4 so even if the solenoid does fail to fire, you SHOULD notice before the loop reaches a hypoxic level.
3) As anarchista states previously, work towards designing equipment that prevents hypoxia; the newer Meg CE version no longer uses a QC connection, they use a LP hose fitting that screws into the head, if not screwed in all the way, a leak would be noticed during the predive checks.
4) I had TWO indications of a problem long before I was even close to a state of hypoxia.
You stated previously:
"Hypoxia in rebreather use is caused by technical failure (IE: a freeflowing ADV with hypoxic diluent near the surface), or human error (IE: not turning on 02), but not by any deliberate act."
A free flowing ADV with hypoxic DIL is something that should be caught during the predive checks and if not, the prebreath. At depth, this would not be such a big issue, but would cause a change of buoyancy. One of the first thing you learn on an entry level CCR course is that any unaccounted change in buoyancy on a rebreather is a cause for concern and a further investigation is required, the corrective action would be to execute a BOOM drill. At the level where you are diving an hypoxic DIL, you should have the experience and knowledge to know how to notice and deal with a free flowing ADV: shut down the problem valve and thumb the dive, feather if necessary to exit an overhead environment. Again, if proper diving practices are followed, with this scenario there are two indications of an issue long before one should become close to hypoxic. If someone cannot recognize the the sign that they are on the road to hypoxia, then what makes you think this person has the situational awareness required to recognize the symptoms of hypoxia as they succumb to them? To quote one of the men I admire most in my life, "while we try to make things as idiot proof as possible, there is always a smarter idiot".
We work SO far away from hypoxia that's its almost a non issue in my opinion, except at the surface. Free flowing adv with 10%, surface, waves, foam making it hard to see displays, struggling with attaching a stage... That's a hypoxia mishap looking for a place. Would you recognize it? Not if you've not trained to detect it.
There is no excuse for not always being aware of your PO2. If you're carrying hypoxic DIL, you should be even MORE aware. I don't buy "foam making it hard to see displays", make it possible, it's your PO2, it's important, or look at your HUD if you have one. The group I dive with drops a line off the back of the boat and we clip our bailout tanks below the surface. This is particularly advantageous in big weather as you can drop down below the surface chop and attach your bottles off in much more manageable conditions. Those who struggle with bottles at the surface in big weather are more likely to end up with a bloody nose courtesy of the dive ladder or swim grid or a clonk on the noggin from bouncing against the hull. This is again, where good diving practices are beneficial. We are trained to thumb a dive when the first issue develops, rather than waiting for it to snowball. With this scenario, a number of things would have had to have been neglected prior to entering the water.
With respect to failure to turn on your O2 bottle, again, this should be caught during the prebreath on the surface prior to entering the water. This should also be something that is double checked just prior to entering the water. Once again (do I sound like a broken record yet?) proper diving practices. I have an APECS and always confirm my setpoint just prior to splashing. It's like a seatbelt. You have to put it on for it to be effective, most of us put it on without thinking about it. You might drive 100,000km without one and nothing will happen but when it does it could save your life. If you make it a habit to confirm your O2 is on/set point set prior to entering the water, it will save you life and prevent you from becoming hypoxic.
So, no, I do not see the value in a diver doing a "hypoxic couch dive" to acquire an incomplete set of data as to what a divers unique personal set of symptoms would be in the event of hypoxia, not when the diver should NEVER be allowing his/her PO2 to come anywhere close to the clutches of hypoxia, at the surface or underwater. I see more value in using the time teach the little tricks that keep them as far away from hypoxia as possible (i.e. checking O2/setpoint, donning bailouts just below the surface in the event of big weather, prebreathing on the surface, etc.).
So agree to disagree I suppose? I have my teaching methods, you have yours.
