Gas Management Questions?

[ianr33]

I'm trying to think how the Rock Bottom can be adopted to recreational multi-level diving, as having a single minimum pressure at the max depth just would not do it. We do multilevel dives and hang around at different depths. So what you need is a table with you on a slate of Rock Bottom psi vs depth at say 10 foot increments, so at each depth you make sure you're within the safe gas supply for the depth. If you find your tank pressure below the Rock Bottom it's a sign you need ascend to the next level and so on. The Rock Bottom is in fact the table version of the Remaining Air Time information you get with an AI computer, albeit in a different format.

Diving a single 80 I always figure Rock Bottom as 10 times depth +300.
e.g. at 100 feet its 10X100 +300 = 1300

That works great for a multi level dive. Leave 100 feet at 1300 psi, leave 60 feet at 900 psi etc. I ran the exact numbers long ago using a SAC rate of 0.6 .

Has the great advantage of simplicity.

 

[Warhammer]

While we're on the subject, I'm curious as to what SAC you guys use for your rock bottom calculations. Aquaregia, quoted some numbers, so looking at his numbers it appears he is using about 1.0 cfpm. That's what I've always used, even though my relaxed rate would be much less than that. I believe the rock bottom concept specifies using a stressed rate but I'm curious what figures you guys use as it relates to your actual relaxed SAC.

 

[aquaregia]

I've managed to hit 1.2cf/min before, stressed. I do use 1cf/min, banking on not being entirely stressed the entire way up and having a buddy who wasn't as bad as me on gas (most of my dive buddies use about 75% of the air I do). My relaxed SAC is usually 0.7-0.8cf/min.

 

[TSandM]

In all my GUE training, we used 1.0 cfm as a stressed rate. I believe this is based on an experienced, otherwise confident diver, whose normal SAC rate is considerably lower. I know I have dived with novice divers whose "unstressed" SAC rate was higher than that . . . which is why I throw rock bottom calculations away when diving with newbies, and simply supply myself with a ridiculous amount of gas for the proposed dive.

I think it is easy to underestimate what stress does to your breathing. One of my first practice dives with a reel was done at night, and my SAC rate was almost THREE TIMES normal.

 

[Doc Harry]

...If all is progressing normally and both of us have sufficient air to complete our dive plan as we hit the "rock bottom" pressure, should we still thumb the dive , or can we continue our dive until we hit our deco limits (or until one of us hits their "personal" rock bottom pressure)?

Remember, your "rock bottom" gas changes with depth. So If you ascend when you hit "rock bottom," then you will have a new "rock bottom" at the shallower depth and you will be able to stay at that new depth for a while. You don't have to ascend all the way to surface just because you hit a "rock bottom" limit.

Example:
At 110 feet you hit your "rock bottom" gas of X p.s.i. You ascend to 75 feet. Your new "rock bottom" gas at 75 feet is Y p.s.i. Now you stay at 75 feet until you hit Y, then ascend to 50 feet...

 

[Thalassamania]

Rock bottom is rock bottom (actually we've called it Bingo Air, after the aircraft fuel concept, since long before there ever was a "Rock Bottom"). There is no individual, buddy, team, etc., concept. Bingo Air is the pressure at which you must turn and head back in order to be carrying enough gas to get you and your buddy to the surface. I would not worry about more that a 200 psi reserve for Bingo Air.

 

[Doc Harry]

...actually we've called it Bingo Air...

That's funny. A few years ago I was looking for a more appropriate name for "rock bottom" or "minimum gas" and decided to use "Bingo" as well, for the same reasons.

Except the concept of Bingo in aviation is not quite the same as "rock bottom" in scuba. But it's close enough.

 

[Thalassamania]

I don't recall who coined it, I may have, since I come from a military flying family and learned to fly at a young age.

 

[Hatul]

Diving a single 80 I always figure Rock Bottom as 10 times depth +300. e.g. at 100 feet its 10X100 +300 = 1300 That works great for a multi level dive. Leave 100 feet at 1300 psi, leave 60 feet at 900 psi etc. I ran the exact numbers long ago using a SAC rate of 0.6 . Has the great advantage of simplicity.

I like Ian's idea the best, because I can't remember a lot of numbers but I wanted to validate it myself. So I made the calculation assuming:
1. 30 secs to swap reg
2. total SAC of 2 cu ft/min for the two of us. My ave California SAC is 0.75 and I peak at 1.3 so 2 is about right
3. 3 min safety stop
4. forget the deep stop
5. tank reserve of 300 psi. If necessary it can be breathed down to almost zero at surface.
6. ascent rate of 30 ft/min. The ideal ascent rate according to my Galileo Sol manual is faster than that below 60 ft and a little slower at shallower depths. For example at 128 ft the ideal ascent rate is 56 ft/min. So on the average 30 is conservative.

at a depth of D:
1. swap reg 0.5* 2 * (D/33+1) cu ft
2. ascent to surface (D/30) *2 * (D/2/33+1) cu ft where D/2 is average depth
3. safety stop 3*2* (13/33+1) = 8.7 cu ft

sum those up to get the RB in cu ft and to get PSI multiply by (nominal tank pressure)/(tank vol) = 3442/80, 3442/100, and 3000/77.4 for HP80, HP100, and al80 respectively.

so I did this on a spreadsheet and plotted the numbers vs depth. as you can see from above the formula is of the form aD^2 + bD + c in other words it's a parabola,

but the section from 0 to 100 ft or so can nicely be fitted by a line as Ian noted and 10*depth in feet is not that bad an approximation for the slope of the line fitting the parabola.

My only correction is that his base of 300 slightly underestimates the RB as calculated above. Here is the RB in PSI for the three tanks according to the assumptions above:

HP80 600+ 10*depth(in feet)
al80 500+ 10*depth
HP100 400+ 10*depth

For each tank you only have to remember one number, the constant of the line, the slope of 10 psi/ft being the same for the three tanks.

at 100 ft with al80 this gives a RB of 1500 psi in agreement with Aquaregia. At 66ft 1160 psi, and at 33ft 830 psi, compared to his 1000 and 600 psi.

Adam

 

[Thalassamania]

You're measuring with a micrometer, marking with chalk and cutting with an axe. I agree with all your numbers, more or less, now ... show me an SPG that makes that all meaningful. 1160 PSI, 830 PSI, really?