Then again, I have paid attention to what the computer has told me over my 100 or so dives with it, and compared that to my experience of all my dives (closing in on 1500), and am satisfied that the 500 psi reserve is good for anything down to 100-120 feet with an AL80.
Good under certain condition, right? I mean, is it going to be enough if you have to deploy an SMB? What if you think that a slower ascent rate would be better (after several days of diving)?
If I am in a spot where AL80s are the only thing (lots of places) then I crank the surface reserve to 750 for deep (120 +) dives which will be enough. Remember, the computer's reserve pressure is not "our" gas, it all belongs to my buddy, because the computer sends me up with an additional amount sufficient for me to reach the surface. So, I have dumbed it down somewhat compared to true rock bottom math, but it is not entirely uncalculated, either. I do have a fallback plan (never had to use it). Knowing my 500 psi reserve, if my ATR at depth is approaching zero at more than 1000 psi, then I will take note and ascend, because this means I personally need more than 500 to reach the surface and so my buddy might be short.
This is the part that I struggle with. In some cases, you are doing math anyway. If my ATR is going to go to zero with more than 1000psi, so on and so forth… You are deducing relationships there between the ATR value, the existing tank pressure and what you would need to get you and your buddy to the surface. So like I said, you are doing math anyway, just not in a way that a lot of people might understand. Whereas rock bottom calculations are very easy to understand. No need to tie into something as seemingly obscure as ATR.
And as far as setting your reserves to 750psi for deeper dives, yes, that is more than 500psi. But is it more enough? How do you know without doing some math?
As for allowing for doing things at depth, that's not going to happen. If my buddy is truly OOG and rushes to me with eyes as big as saucers and the reg out of his mouth, reaching for mine (yes, that's how it happens), we beat feet to the surface the second he has my reg, no dallying on the bottom searching for anchor lines or deploying smbs or anything else. I can blow the smb at the surface. Seriously, everything is secondary to reaching the surface.
Its should not happen for you because of the way you plan your dives. You don’t set aside enough resources to do it.
We do recreational dives at home all the time where your ascent could take around boat traffic. Shore dives and boat dives. We do dives where if you don’t ascend at the anchor line, current will take you pretty far away from the boat. So yes, getting to where there is more gas is the most important thing. But it is not the only thing.
Incidentally, I reject the notion that you have to take pencil to paper before every dive. In reality, if you are diving the same tanks over and over again, you wind up doing the math once for several depths.
And after about a dozen dives, you remember what your rock bottom is when you are at 100ft and what it is for 90ft and 80ft and 60ft and so on.
---------- Post added July 31st, 2015 at 02:04 PM ----------
If you are flying the ATR that your computer is showing you, you will never be at depth with 500 psi in your tank. Caveat*: That is at least true with the computer I am using. When my computer says 0 ATR, that means that if I leave the bottom now and do a normal ascent, I will arrive at the surface with 500 psi. Hitting 0 on ATR does not mean that I am NOW at 500 psi - unless I'm already on the surface or practically so.
Yeah, I understood that. 500psi is the reserve. ATR is the amount of time you have left before you have to begin your ascent.
Feel like re-doing the math? I have not done it, but if I'm at 130', a normal (conservative) ascent would mean 4 minutes, 20 seconds (roughly). I think my buddy would have to be really breathing hard to use 500 psi from an AL80 in that amount of time, with an average depth of 65 feet, wouldn't he? I mean breathing HARD?
No, its scuba math.
I had 4 minutes of travel time.
But, I did not make the assumption that as soon as the buddy comes to me for gas, I start the ascent right away. We usually give ourselves some time to get oriented when something like an out of gas event happens. Your buddy comes to you, you donate, you put your backup reg in your mouth. When everyone is breathing again, you get your wits about you. WTF just happened? What is the right next step? Sometimes, it is to put away equipment like your camera. Either that or you just ditch your multi thousand photo/video rig to save your buddy’s life. I give myself 1 minute at depth to get situated and to begin the ascent.
So now, it will take 5 minutes to get to the surface from 130ft.
You can use 3ATA as the average depth if you like.
Your buddy will need 5 minutes x 3ATA worth of gas. And you still have to multiply that by their consumption rate. Average diver who is stressed out.. 1 cf per min sounds good to me.
15cf based on this scenario for your out of gas buddy. Assuming you are going straight to the surface and are not going to do anything else (swim to the anchor line, deploy an SMB or anything else).
500psi in an al80 is about 12.5cf-13cf.
Again, this assumes an al80. The last time I was in Hawaii, the DM asked me to dive an al63. Your 500psi is definitely not enough reserve for an OOG diver in that tank. Although to be fair, no sane person would bring an al63 to a dive that deep. The only point is that a fixed number like 500psi is enough only if you know what assumptions were made.
It's also pretty handy that I don't have to pay attention to the NDL and ATR separately. The computer tracks NDL, ATR, and OTR (O2 Time Remaining, for when diving Nitrox), and displays whichever number is lower as the DTR (the Dive Time Remaining), which is the big number on the main display.
So, if I really just don't want to pay attention, all I have to do is watch the big number on the main display and leave the bottom when or before it hits 0.
I'm not advocating this. But it does seem like it would make a dive safer for someone who doesn't (for whatever reason) do a good job of keeping up with all those separate pieces of data while they are on the bottom. I have caught myself, on occasion. checking my SPG but forgetting to check my NDL, or vice versa.
Man, I don’t know how I ever got by without all that information.
Does that big number tell you when to turn around to head back to the anchor line? Does that big number know when you are swimming up current or down current?
Out of curiosity, what would your plan be if your computer craps out while you are on a live aboard and they don't have any air integrated computers to rent you?
