(NB: experienced technical rebreather divers might find this post “boring” while beginning technical divers might find it interesting and provide you something possibly new to think about and non-divers will probably just frown and wonder what am I talking about). 🙂
I don’t really know any technical divers who aren’t also “gear hounds.” I’ve lost track of how many tanks I own.
One of the big advantages of diving a rebreather is that you generally don’t use your bailout gas. That means that you fill the tank once and it could be literally years before you either want or need to refill it. Of course, you need different tanks with different gasses based on the depth and expected decompression obligation. Hence, the proliferation of tanks…
My “usual” deep bailout tank is a 6.8L Carbon Fiber tank filled with something like 21/35 (yes, I know they aren’t DOT approved).
They are light, they have good “trim” and buoyancy characteristics, and can carry quite a bit of gas. The working pressure is 300 bar (about 4350 psi). At that pressure, a 6.8L tank can hold about 72 cubic feet of gas (6.8 L * 300 bar / 28.3 L/ft3). So, I get 72 cu ft of gas in a tank that weights less than an AL40 and has better buoyancy characteristics. Practically, I usually fill to about 4200 PSI and get about 65 cu ft.
Planning bailout for a 200′ dive
Normally, that tank would be perfect as my “deep bailout” for dives up to 200 feet deep without a lot of penetration (i.e., the types of dives I typically do here in Southern California).
If I do a 200 foot rebreather dive for 30 minutes at a PO2 of 1.3 with a Gradient Factor of 50/80 and a Diluent of 10/50, I get 60 minutes of decompression for a run time of 90 minutes. Perfect!
However, if I have to bailout and all I have is my 21/35, it would require about 245 cubic feet of gas!
Therefore, on a dive like that, we carry a second cylinder of gas with a higher oxygen content. The “standard” gas for that would be Nitrox 50 (50% oxygen). In that case, I would need about 20 cubic feet of 21/35 and about 80 cubic feet of 50%.
So, I would use relatively little of the 21/35 and require (at least) a standard AL80 tank full of 50%. I’ve done plenty of dives with this exact configuration, but then I started thinking about trying to balance my bailout tanks (I do the left/right tanks and not both on the left) and thought “why not just buy a second carbon fiber tank?”
Then, I started to think about this idea of balancing gas usage between the tanks and realizing that I can’t practically carry enough 50% in that 6.8L tank…
The Bailout Gas Tradeoff
There is an implicit tradeoff that happens when you carry bailout gas.
If I carry 50%, I can start breathing that “oxygen rich” gas at 70 feet deep and stop using the “oxygen lean” gas 21/35. This will shorten my decompression time significantly (207 minutes to 96 minutes).
You might ask “why not just bring 100% pure oxygen?” That would reduce the decompression obligation even more, right? Not really.
The problem is that you can’t start using the 100% until you are at 20 feet deep which means that you need more 21/35 gas to fulfill your deco stops until you get to 20′ and the shallower you go, the less effective that gas is. So, you gain efficiency once you get to 20′ but you are less efficient by not being able to switch to a richer gas at 70 feet. Also, note that you would need 92 cubic feet of the deep gas.
Here is a table that summarizes the different scenarios mentioned above:
(only deep b/o)
|21/35||0||245 cu ft||21 cu ft||92 cu ft|
|50%||0||83 cu ft|
|100%||0||39 cu ft|
|Runtime||90 minutes||207 minutes||96 minutes||110 minutes|
Which gas to use in my new tank?
I bought my second carbon fiber tank and started to think about what gas to fill it with given that most of the dives I would use it for are in the 150-200 foot range and that I would be pairing it with an identical tank filled with something like 21/35.
Nitrox 50 would be the normal choice but you can see from the above table that I wouldn’t have enough capacity. I started to analyze the idea of using Nitrox 70 or Nitrox 80. Note that “old school” rebreather divers used 70% quite a bit as a deco gas but I don’t see it very often anymore and most people have “standardized” on 50%.
Here is what the dives would look like for both 70% and 80%:
|21/35||0||51 cu ft||68 cu ft|
|70%||0||62 cu ft|
|80%||0||52 cu ft|
|Runtime||90 minutes||99 minutes||103 minutes|
In the end, I decided to fill the tank with 70%.
One potential downside to this is that Nx70 is not a popular or “standard” gas that other divers carry. So, if I need to donate my gas, the other diver needs to know that it is only breathable above 40 feet AND that they probably need to program that gas into their computer.
Now I need to go dive with the configuration and see how well it trims out. 🙂