OP
pnw_diver
Contributor
Gotcha. Math fail on my part.
Gas Planning
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DaleC
Contributor
I echo Lynnes thoughts.
I dive independant twins and many dives I surface with 1000-1500psi in one and 500-1000psi in the other, and that's with a starting pressure of 2450psi.
For the Sask you should plan to return to either the bow or midship mooring lines (preferably the one your boat is on). I would not want to plan a bluewater ascent there.
The good news is that it is a square profile for the most part making your calculations somewhat straightforward.
A very conservative gas plan for the Sask:
Stressed SAC of 1cuft/min.
5 minutes to solve problem at depth (for example a lost upline): (5 x 4ata = 20cuft)
total ascent @30'/minute:
99'-66' (1 x 4ata = 4cuft)
66'-33' (1 x 3ata = 3cuft)
33'-0' (1 x 2ata = 2cuft)
Stops:
1 min @ 50' (1 x 2.5ata = 2.5cuft)
1 min @ 40' (1 x 2.5ata = 2.5cuft)
1 min @ 30' (1 x 2ata = 2cuft)
3 min @ 20' (3 x 2 ata = 6cuft)
This gives a total of 44cuft. Close enough for an 40cuft pony but with a 19cuft pony you only have enough for a direct ascent (no problem solving) with a 3 minute SS and 4cuft to spare. With a 100cuft tank that gives you 60cuft of gas at depth. With a sac of .6 that gives you (.6 x 4ata = 2.4cuft/min 60cuft / 2.4cuft) 25 minutes bottom time. Some people may calculate slightly different values depending on how exact they calculate their ata's etc... It's ok as the final volume is just a working estimate of what one will need and not an exact number.
You can use a less conservative gas plan of say 2 minutes to solve a problem at depth and no deep stops (only 3min SS) which would give you a revised rock bottom of 23cuft. 100cuft - 23cuft = 77cuft or (77cuft / 2.4cuft) 32minutes bottom time.
Most people probably find themselves somewhere in between the two (hopefully).
I dive independant twins and many dives I surface with 1000-1500psi in one and 500-1000psi in the other, and that's with a starting pressure of 2450psi.
For the Sask you should plan to return to either the bow or midship mooring lines (preferably the one your boat is on). I would not want to plan a bluewater ascent there.
The good news is that it is a square profile for the most part making your calculations somewhat straightforward.
A very conservative gas plan for the Sask:
Stressed SAC of 1cuft/min.
5 minutes to solve problem at depth (for example a lost upline): (5 x 4ata = 20cuft)
total ascent @30'/minute:
99'-66' (1 x 4ata = 4cuft)
66'-33' (1 x 3ata = 3cuft)
33'-0' (1 x 2ata = 2cuft)
Stops:
1 min @ 50' (1 x 2.5ata = 2.5cuft)
1 min @ 40' (1 x 2.5ata = 2.5cuft)
1 min @ 30' (1 x 2ata = 2cuft)
3 min @ 20' (3 x 2 ata = 6cuft)
This gives a total of 44cuft. Close enough for an 40cuft pony but with a 19cuft pony you only have enough for a direct ascent (no problem solving) with a 3 minute SS and 4cuft to spare. With a 100cuft tank that gives you 60cuft of gas at depth. With a sac of .6 that gives you (.6 x 4ata = 2.4cuft/min 60cuft / 2.4cuft) 25 minutes bottom time. Some people may calculate slightly different values depending on how exact they calculate their ata's etc... It's ok as the final volume is just a working estimate of what one will need and not an exact number.
You can use a less conservative gas plan of say 2 minutes to solve a problem at depth and no deep stops (only 3min SS) which would give you a revised rock bottom of 23cuft. 100cuft - 23cuft = 77cuft or (77cuft / 2.4cuft) 32minutes bottom time.
Most people probably find themselves somewhere in between the two (hopefully).
Last edited:
[OK, I admit I haven't checked the math of the people who have responded -- BUT....]
A while ago I did a "Gas Management" lecture for the instructors at my dive shop and actually used the Sask. as an example for one segment. I broke the lecture into three parts and I think "Gas Management" is really a three part issue.
Part number one is: Rock Bottom -- how much do I need to get me, and my buddy, to the surface safely while making all of my deco/safety stops. That has been discussed at length here.
BUT, that is just ONE part of the whole topic of Gas Management.
The second part is: How much gas do I need for the dive I actually plan to do?
The third, and I think often overlooked part is: How am I going to actually use the gas I have on this particular dive? This is, in some ways, the actual "gas management" part of the dive because it is the one that will actually shape your dive.
Can you go directly to the surface at any time with no problem? Then you can use "all your available gas" -- your common drift dive.
Would you LIKE to get back from whence you came? Then you can use "halves" as a strategy -- your typical shore dive.
Do you really NEED to get back from whence you came? Then you can use "thirds" as a strategy -- your typical "overhead" environment -- except it may not be conservative enough if you are on the Sask. or in some other such environment. (Even though you wrote there isn't much current, I may not really want to go directly to the surface from the stern when the upline is on the bow because of conditions on the surface.)
So your "gas plan" may cover a lot more than just -- "How much do I need to get to the surface?"
PwrPnt of "Air Supply Management" lecture
A while ago I did a "Gas Management" lecture for the instructors at my dive shop and actually used the Sask. as an example for one segment. I broke the lecture into three parts and I think "Gas Management" is really a three part issue.
Part number one is: Rock Bottom -- how much do I need to get me, and my buddy, to the surface safely while making all of my deco/safety stops. That has been discussed at length here.
BUT, that is just ONE part of the whole topic of Gas Management.
The second part is: How much gas do I need for the dive I actually plan to do?
The third, and I think often overlooked part is: How am I going to actually use the gas I have on this particular dive? This is, in some ways, the actual "gas management" part of the dive because it is the one that will actually shape your dive.
Can you go directly to the surface at any time with no problem? Then you can use "all your available gas" -- your common drift dive.
Would you LIKE to get back from whence you came? Then you can use "halves" as a strategy -- your typical shore dive.
Do you really NEED to get back from whence you came? Then you can use "thirds" as a strategy -- your typical "overhead" environment -- except it may not be conservative enough if you are on the Sask. or in some other such environment. (Even though you wrote there isn't much current, I may not really want to go directly to the surface from the stern when the upline is on the bow because of conditions on the surface.)
So your "gas plan" may cover a lot more than just -- "How much do I need to get to the surface?"
PwrPnt of "Air Supply Management" lecture
AfterDark
Contributor
OP
pnw_diver
Contributor
It wasn't covered in PADI OW, AOW, or Rescue, nor any of the 6 or so specialty classes I've taken - at least not past a rudimentary level. Other agencies may differ, but that's been my experience with PADI.
It's not covered, as far as I can tell, in anything not 'advanced'. Not gas planning as we're doing in Adv Nitrox/Deco procedures.
The first good gas planning I've seen was in the TDI Nitrox manual. SDI Solo Diver touched on it with basic planning, but it was the TDI Nitrox manual in which I first saw full-up gas planning.
Have you taken Nitrox yet? If so, try to get a TDI Nitrox (not the advanced, the basic) manual.
I was certified in Nitrox by PADI, but the TDI manual opened my eyes to a lot bigger world out there.
Gas planning of this kind is not covered in the PADI recreational curriculum. This was the answer that Peter got to the question, when he asked it of the instructor for his IDC.
AfterDark
Contributor
We had a 2 hour session on it during the course I took. Sad people are certified to dive without what was once considered basic knowledge. I'm starting to understand why the accidents and incidents site is so busy.nickmarsh7
New
since depth can change the bottle PSI, how much weight do you give to that when thinking about turning around?
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