oversea:
I will soon be diving with single 119's. Eventually I had thought about doubles. Maybe doubling up the 119's. Having never dove anything larger than an 80, would doubling these typically be good for (2) dives at a max depth of 110' or would that typically be running short on gas? I would assume based upon gas consumption with an 80, the doubled 119's should work. I realize my sac rate and physical condition and physical stress affects this, so it is just a ball park. Oh, and the main point for me is "do I need to have two sets of doubles" or will one set work for diving within NDL?
I am going to assume that you would have the appropriate gear and skills needed to dive doubles, it is a little more complicated than in might seem at first. If you don't... bring your wallet, it's really expensive.
An aluminum 80 tank actually holds only about 77.4 cubic feet of gas when it is filled to its rated pressure of 3000 PSI. Your 119 should hold about 119 cubic feet of gas at its rated fill pressure of (I am assuming here) 3442 PSI.
When you dive an aluminum 80 many boats like you to get back onto the boat with between 500 PSI and 700 PSI. At 500 PSI you should have about 13 cubic feet of gas left in the tank, at 700 PSI you should have around 18 cubic feet of gas left in the tank. Remember, this is the gas left in the tank when you are finished with the dive, you need to safely calculate when you need to begin your ascent so as not to use this small reserve of gas. If you have doubled your 119's and want to have 18 cubic feet of gas left when you finish your second dive you will want to save as a reserve a little less than 300 PSI. This pressure of 300 PSI is too low for me to be comfortable with when you consider the little details like your regulator, so lets go back to the 500 to 700 PSI reserve most boats like and apply that you your doubles. At 500 PSI with your 119's doubled you will have in reserve approximatley 35 cubic feet of gas, if the boat captain is a stickler and insists on 700 PSI then we are talking about 48 cubic feet or so. Since you are using doubles I would probably be fairly comfortable with the 500 PSI mark to be back on the boat, so lets take that reserve of 35 cubic feet away from your total volume of 238 cubic feet. This leaves you with half of this remaining volume for each dive, remember, if you use more on the first dive you are taking this gas away from you on your second, leave the reserve alone. 238 cubic feet minus your reserve leaves you with 203 cubic feet for the two dives and half of that is, well lets just round it to 100 cubic feet per dive. If you want to look at it as tank pressure then Start your first dive with 3442 PSI and end the first dive with 1971 PSI (or better even round this to 2000 PSI). For your second dive just come out of the water with at least 500 PSI in your tank and you should be fine. The actual pressures are 1471 PSI for the first dive, 1471 PSI for the second dive, 500 PSI in reserve. These values are almost certainly imposible to hit exactly on most pressure guages so the 2000 PSI mark is easy for the first dive and the 500 PSI mark for the second.
The 100 or so cubic feet per dive you have available with double 119's is clearly more gas than the 59 cubic feet you could comfortably work with on a single AL80. The doubles introduce more drag and sheer weight so you will be working harder with these tanks than you would with a single tank, so that alone will burn up some of your available gas, but the net result is yes you will have more gas using the doubles than the single. But you need to remember that the larger amount of gas will not magically change the amount of bottom time available based on either tables or your computer.
If it was me, I would keep the 119 as a single until I had a really good reason to make a set of doubles. Good reasons are Cave diving, or Planned decompression diving, or Wreck penetration, each of these reasons requires additional training. A Deco class is a good place to start.
I hope this helps, and I hope my math is right.
Mark Vlahos
