Too Long, Didn't Read (and I wouldn't blame you - it's a long post): My AOW Deep Dive included no discussion of gas management. But I do not in the slightest feel my recreational OW/AOW instruction has failed me.
I've been following this thread with extreme interest, from when I was lurking SB before recently registering, because the topic of gas management fascinates me. And now, as one of the posters recently publicly admitting to a SAC rate of 1cu.ft/min as a beginner without emergencies, I would like to add my 2psi to the conversation (it's all I can spare from the tank due to my SAC rate
)
@MacDuyver didn't share the whole dive profile with us, and I'm not suggesting it. However, with my limited experience I can successfully plan a variety of multi-level dive profiles even at my own SAC rate that would allow me to go to a depth of 36m/120ft, stay for the length of time stated at that depth (which from what I see in Subsurface at various GF rates and on my own dive computer is within the NDL for the first dive of the day), be at shallower depths for a bit either before or after the max depth and surface with the reported amount of gas in the tank. There is no indication one way or the other that they came straight to the surface from that depth, with or without a safety stop. I have no reason to doubt that the divers on that dive executed a safe dive.
For
@jadairiii the OP, I have not taken the PADI Deep Diver course, just the AOW Deep specialty which I have just learned from following this thread is apparently Dive 1 of Deep Diver. I can confirm that as of December last year there was no discussion of gas management for that, at least by my PADI instructor, and since it seems from previous posts in this thread that it has been removed from the curriculum I am happy to know that he was not technically being negligent in not discussing it. It appears to be a common complaint from experienced divers not just here on SB but elsewhere on the Internet that gas management is not being taught by the agencies, and I can certainly understand the concern.
However, I don't expect any agency to teach me everything there is to know about any of the specialties in the amount of time that I have available for training - there is no local dive shop where I live; I have no choice but to do my practical training on-site while on vacation, which of necessity limits how much can be taught in person. While the recreational dive training that I have received has certainly tended to be light on some of the more "technical" aspects of scuba diving (as I recall the OW treatment of gas management was limited to the very basic discussion of turn pressure required by the curriculum), I feel confident that where my OW and AOW instructors have absolutely
not failed me is in stressing over and over, in almost every single instruction session/knowledge review/pre-dive-briefing/post-dive-chatting, is that as a certified recreational scuba diver
I am responsible for my own safety.
So, when doing the AOW deep dive I looked over at my buddy at our depth of 30m and thought to myself "If her first stage regulator fails right now or some other catastrophic failure happens to either of us, do we have enough air to get to the surface?" I looked at my SPG and saw a comfortably high number (it was early in the dive) and thought "I think so?" And then, in the same dive at the 5m safety stop, I looked at my SPG and thought "So in OW the instructor said to give him the low on air signal at 500psi. And now the DMs at this operation say to give it at 700psi. Why the difference?". I meant to ask the AOW instructor about both things, and simply forgot about it. I was on vacation, after all...
But once back home from the dive trip, and especially now that we are planning the next one, it came back to me. So, since
I am responsible for my own safety I started researching gas planning, and many (not all) of the hits from Google searches led me to various SB posts. And so the lurking began...fascinating stuff. And after lots of reading and mind stretching to get my head around the principles of minimum/rock bottom gas planning (whatever we want to call it), just like
@Para Goon I built my own spreadsheet for calculating what I decided to call my "minimum ascent pressure", based on what I know of my SAC rate and the principles as I understand them, and came up with numbers that seemed eminently reasonable to me. I learned a lot in the process. Among other things, the difference between the two shops was
not because of a difference in tank size (AL80 in both cases), although it possibly could have been. I added the ability to factor in light deco if for some reason we happened to blow our NDL (or had agreed to it in advance with the DM during planning / briefing). For 95% of all the dives that I have done so far, 700psi is my own calculated NDL "minimum ascent pressure" with a safety stop, which boosts my confidence in the "safety first" policy of the shop I currently dive with. No reflection on the shop we did our OW with - most likely our (excellent) OW instructor was confident we would never hit 500psi during our OW course due to shallow depths and short dive duration. He was wrong - I was (and is) an air hog...
So for me personally, the GUE standard of 20L/min is too liberal, and the 3m/min ascent rate is too slow. My buddy and I, at least at first, would probably be sucking gas like hyperventilating elephants running a marathon up a mountain in 90F weather in a true OOG emergency, and as we gradually ascended and realized that the world was
not coming to an end and we
will most likely survive this situation, we would calm down and start breathing more normally again: me as if my mother's maiden name was Hoover and her with her built-in gills breathing at just slightly above 0.0cuft/min... Would that average out to 20L/min over the ascent? Maybe, maybe not - I tend to think not. Regardless, almost for sure neither of us would feel comfortable ascending at 3m/min. I was glad to see the more detailed explanation about the GUE policy above - it makes sense to me. We would likely try to ascend at ~9 or 10m/min up to safety stop depth, and then 3m/min from there. That, actually, is already my standard practice, OOG emergency or not, and doing what I always do would reduce stress in that situation.
And then I came across the wonderful spreadsheet created by
@wetb4igetinthewater which, given the same inputs, gives just about the exact same numbers as mine -
even though his work
appears to be have been done in Imperial (and seems to enforce a minimum ascent pressure of 700psi
) and mine is purely in metric with a conversion to PSI at the end because that's what the rented SPGs at the dive shop display).
Ultimately it's all academic - from what I can see, at 36m my NDL on air is going to have me "shallowing up" long before even the highest psi number posted previously: somewhere around 2000 PSI. But, depending on the level of conservatism that I am willing to accept, my own calculations for my "minimum ascent pressure" from 36m /120ft with an AL80 tank are 1500psi at the most conservative (definitely the number I would use if this was my one of first few times at that depth) and 1200psi at the next level (what I would possibly use with more experience and a better SAC rate). At the lowest level, the one that I actually use for the depths I am currently comfortable at, it is just slightly less than 1000psi (including time for the 5m, 3min safety stop) - and if I was a marine scientist / master instructor with 100x more dives in my log than I currently have and almost assuredly by then a far lower rate of gas consumption even in an emergency I likely would be perfectly fine using that level of conservatism for a dive to 36m.
All of these numbers appear in previous posts in this thread (rounded up for conservatism), and I personally can accept any of them as correct, for the people posting them. With other levels of conservatism my "700psi for 95% of my dives" becomes 800psi, so I can also accept that number just as readily if that's what someone else wants to use - I will ascend when he/she does if we're diving together. Dive and let dive, as The Chairman of SB likes to say.
Are my "home-grown" numbers right? Maybe, maybe not. The only way to know would probably be to do as suggested in a previous post and actually test it. That test dive is probably not going to happen, but that would likely be the best way to proof my calculations.
Of course, I have no business being at 36m/120ft right now anyway. I'm not trained for it, and our OW/AOW instructors have also been excellent at reminding us to always dive within our limits. And even if at the depths I
am trained for and hence comfortable at my buddy and I were to hit the "once in a quarter century of diving" OOG emergency jackpot (as mentioned by another long-time member of SB), because I accept that I am responsible for my own safety, I know two things - (1) because I now have a way to calculate my personal "minimum ascent pressure", we will (theoretically) have enough gas to get us safely to the surface in a controlled way, from any depth, with a safety stop if we both feel comfortable actually stopping for three minutes at 5m in that situation instead of going right to the surface where the amount of gas available is infinite, and (2) any of the DMs at the shop we dive with that happened to be leading that dive would be right there with us all the way up, because they are totally with it and dedicated to a "safety first" policy.
That, after all, is why I dive with them.
