My approach to gas planning is fairly conservative, but then I have done mostly cave diving the last couple years and it rubs off.
Basically on a deep OW dive I still plan what amounts to thirds and it works fairly well over all. For example, on a dive to 150' for 35 minutes using 25% for the bottom mix and 50% for the sole deco gas, and assuming a SAC of .65 during the working portion of the dive and .40 on deco, you will have 35 minutes on the bottom and 39 minutes of deco (using DPlan) and use 126 cu ft of 25% and 26 cu ft of 50%.
That leaves about 74 cu ft in reserve - sllightly over the 67 cu ft that would consititute 1/3rd of a set of double 100s.
If I lose the deco gas or am unable to access it for any reason, I can still do a longer deco (62 minutes) on backgas and in that case I will use 167 cu ft of the back gas, leaving 33 cu ft still remaining in the reserve at the end of the dive. Comfortable considering you had a single failure.
But lets add a 5 minute delay on the bottom (tangled in a net, looking for the deco gas, looking for the ascent line, establishing a new upline, whatever). In this case I will use now 144 cu ft of 25% and 32 cu ft of deco gas, due to the longer bottom time and longer deco. But in a lost deco gas situation with the 40 minute bottom time, I am now using 196 cu ft of 25% to finish the 77 minutes of deco required - cutting it awfully close.
In other words, 35 minutes is as long as I want to plan at 150' with double 100s if I consider both the possibility of a 5 minute delay on the bottom and potentially losing access to the only deco gas. Roughly speaking if I have plan for less than a 1/3rd reserve, I know I am at elevated risk of having problems covering the deco in a lost deco gas situation even before I do the actual gas planning. I will still want to run the numbers and do the gas planning, but I know that I am in the ball park if I plan to be on the first deco gas before I have used 2/3rds of the 200 cu ft available.
More likely I will plan to use both 100% and 50% for a deco gas on a dive to 150' for 35 minutes. Since I am adding a shallower gas it does not help me with the bottom gas any as I still use the same backgas to get to the first deco gas. In this case I will use the same 126 cu ft of back gas but if all goes well I get out of the water with only 30 minutes of deco using 12 cu ft of 50% and 8 cu ft of 100%.
Again, planning for a contingency of staying an extra 5 minutes on the bottom, I will use 144 cu ft of back gas, 15 cu ft of 50% and 10 cu ft of 100% during 36 minutes of deco. If I lose the 50% and deco only on 100%, I will use 167 cu ft of back gas and 11 cu ft of O2 during 47 minutes of deco. If I lose the 100% I will use the original 144 cu ft of backgas and 32 cu ft of 50% on 47 minutes of deco.
So in short, adding a second back gas when the deco starts to get long, both shortens the deco but also makes the loss of access to a deco gas a lot less dicey.
In a N FL cave, 50% is not real useful due to the profiles of many of the caves, so it is common to dive with only 100% as a deco mix. Since you are doing the stops below 20' on back gas, you are going to use more under normal circumstances but the back gas only contingency is still a controlling factor. The reserve becomes much more critical when you consider the cave profile which will dictate the ascent and silt or other less than optimum environmental conditions can delay you and add bottom time. Consequently, in some circumstances, a 1/3rd reserve may not be adequate in the event of even a single failure on a deco dive.
A 2 person versus 3 person team increases the risk as you lose a potential "third" in the team resources and in that case, using a stage on the dive may be preferrable to ensure an adequate reserve.
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Tank size, oddly enough, is not the major consideration. For example if my team mates all have HP100s and I have HP 130s, the "third" in the smallest set of tanks is controlling as we all need to ensure that a remaining third in any tank is enough to get any diver out. So a third for each team member will be the 67 cu ft "third" in a set of HP 100s, not any larger third in any larger set of tanks.
When I apply this, I will turn after 850 psi is used, not the normal 1100 psi to reflect a 67 cu ft third, not an 86 cu ft third. This is because I have to turn after 850 psi and 67 cu ft to ensure that if I have a gas failure at that point, the 67 cu ft reserve in a team mate's doubles will be enough to get me out. So in effect, the larger tank capacity does not really get utilized if I dive with team mates with smaller tanks.
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The math to figure this out is fairly easy if you use a tank factor.
If I take the capacity of my HP 130s (260 cu ft at 3442 psi) and divide the 260 by 3442, I get 0.0755. If I take this times 100, I get 7.55, or 7.6. This is the "tank factor". What it measn is that each 100 psi is 7.6 cu ft. and the tnak factor never changes for a set of tanks, regardless of overfills, etc. 100 psi still equals 7.6 cu ft for a set of double HP 130s no matter how many hundred psi you stuff in them.
So if I am diving with a guy with double 100s (200 cu ft) I know he will have a 67 cu ft "third". If I am really good with math in my head, I will figure out that 67/76. = 8.81, or taken times 100, 881 psi. This means 67 cu ft equals 881 psi in my HP 130s. If I am not so sharp with mental math it is no big loss as I can round 7.6 up to 8 and then divide 67 by 8. Unless I flunked out of the third grade, I know 8*8=64 and 8*9=72. Since 67 is just about in the middle, I'll plan to turn after 850 psi. Most SPGs are lucky to be accurate to 50 psi, so the 30 psi difference between 850 and 881 is decimal dust.
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The short story is:
1) Consider what tanks your buddies have as you will be operating as a team.
2) Get a set of tanks that will trim out well for you.
3) Be conservative on your gas planning, especially when just starting out.
4) Don't plan your tank size based on the longest/deepest dive you envision as that will be 5% of your diving and leave you carrying excessive weight on the other 95%. Plus you can easily make up the gas difference with smaller doubles on those 5% with a single AL 80 stage.
