Wednesday, April 9, 2008

Cave Skills Part 3 : Gas Planning (part 1)

Easter 2004 and I found myself in the one situation no cave diver wants to be in, stuck at 152 meters, 10 minutes away from my nearest support diver and 15 minutes away from my staged gas. This was not supposed to happen! I was attempting to reach 160 meters in a narrow incline (well actually decline) shaft at Badgat. It had taken a 3 minute swim at 100 meters to get to the entrance and whilst narrow, the last descent was relatively simple – a quick in and out. My planning was (as always) a balance of practicality (there are only so many cylinders you can beg borrow or steal) and conservatism (I want as much air as possible to breathe). The problem was that all my planning had revolved around losing gas and then being able to head back to the safety of a support diver or staged gas. I had not planned to be trapped at depth having to rely only on what I had brought along to get me through. No wonder Sheck Exley had gas planning as his number two factor for surviving in a cave.
Sheck’s rule for gas management is most commonly referred to as the rule of thirds. Simply put, you use a third of your gas to get in, reserving a third to get back out and leaving a third for emergencies/ your buddy. The assumption is that you should use the same amount of gas (if not less) to get out and that if there is a problem, you will use gas at the same rate as your entry. These assumptions do not hold true all the time (which is always the problem with assumption) but as a guideline it is solid and gets divers thinking along the right lines – plan for the unknown.Do I use the thirds rule ? Yes, on every day, conventional dives (and there lies the risk… what dives conform to that definition ). But on a deep or extreme dive I use critical pressures to manage my gas requirements. For those of you who are unsure, the critical pressure is the number on your pressure gauge at which you must turn around. Put another way, the critical pressure tells you when you have reached that point where the gas that is left will get back to the surface and includes your ascent rate, travel time and any decompression obligation. It also includes that third reserve for emergencies. If you are doing a dive that uses staged gas to get you out, your critical pressure is that pressure that will get you to your next set of staged gases a opposed to the surface.Actually, as I write I realize that gas management is not as simple as using a flat rule of thirds. It is a complex three dimensional problem that includes parameters like the number of gases, how deep you can stage, what your decompression is, how fast you are going to ascend…. Let me explain.
My dive was to 221 meters. Now I know I can not carry my entire gas supply with me (well I could, but what is the point if I am in a cave and can stage), so I will carry enough gas to get me from a specific point (35 meters) to 221 meters and then back to a different point (my first decompression point and deepest staging point 130 meters). Going in is easy, I use my nitrox 36 until 35 m (yes, I run partial pressures of oxygen at 1.6 bar on open circuit). Then I swop over onto my bottom mix (I am going to avoid the discussion of which gas to swop onto, travel gas or bottom mix as that is an entirely different conversation). I know my gas consumption rate (I plan for 20litres per minute, I use around 12) and I know the time that I have allocated to get myself to 221 meters (13 minutes). This gives me the litres I need to get from 35 m to 221 m (at a conservative descent rate of 20 m/s) - namely 4900.The next step is to determine how I am going to carry that gas ? I physically can not manage twin 18’s so use twin 15’s with an assortment of side slungs. I like this approach because I protect my gas in nice neat and isolated containers where if something goes wrong with one, I still have others. At 250 bar, that means I can carry on my back 5600 litres BUT that gas has to also provide buoyancy for the whole dive so I can not plan to breathe my back mounts dry. I can however plan to swop onto a side slung at 35 meters and breathe that to 150 meters (a personal cut off point for managing stress and task loading).
That would mean that I have used approximately 1800 litres of gas from another source, which means I only need to get 3800 litres from my back mounts… to get there. Either way I do not have enough gas in my back mounts to get me there and back with a reserve. I know I need another side slung.Realistically I can not stage deeper than 130 meters (I do not have divers who can get that deep), so I will need enough gas to get back to 130 meters, which was 2500 litres. Each side sling manages 1800 litres of gas, so without a reserve, I need 2 and a half cylinders (I plan to be only able to get 1500 litres out of a cylinder). Translated this means that just to get from 35 meters to my first stage cylinder I need to carry a total of 7500 litres and that is without gas for buoyancy and a reserve. So, just to get there and back – if I were to breathe everything dry- would need a full twin set and two 10 litre side slungs. In order to feel safe I need my full exit gas in an isolated and protected reserve, i.e. 2500 litres which translates to at least one extra cylinder (total # of side slungs now required 3 or 1 to breathe in and two to get me out to my staged cylinders).
BUT I still have not accounted for the fact that I am not breathing my gas dry, so add another 1800 litres (an extra stage, total count is now 4 stage cylinders).
My gas consumption calculation requires that I have 7500 litres to breathe. I am planning on carrying 11250 litres which is a healthy extra 6300 litres over twin 15’s and four side slungs. Should I carry an extra stage ? Probably but that puts the descent at risk. Side slungs create drag and will slow me down. Am I happy with the risk ? Yes, because if I am coming out I have at least one if not two dedicated cylinders that I can use that I will not touch on the way down. I can also go straight to any number of staged cylinders as there is no swim required (I carry extra power inflator hoses and have independent bc’s inflated from independent sources to manage buoyancy risk).
Even if the calculations above indicate that in order to really be safe I should have an extra cylinder, my fail safe is my critical pressure. This will ensure that I leave with enough gas in my back mounts to get me out even though I have planned to use at least 3 extra cylinders (I build in a lot of fail safes). I know that if I am able to breathe my twin 15’s dry I will need 2500 litres to get to my staged gas (and that is at a conservative ascent rate, not an “oh my god things just when wrong, abort, abort, abort” ascent rate). That means I absolutely must turn my dive when my back mounts have 90 bar in them. Absolutely, because if I do not I absolutely do not have enough gas to live and will be doing my ascent on a wing and a prayer. Ad then there had better be a good reason why I pushed the limit so far because at that pressure in my back mounts I have no reserve and it is going to be real tight and real hard going to get to my staged gas.
My planned critical pressure is a more comfortable 140 bar. If I use gas at the planned rate (which is higher than my actual consumption) then I will get to my staged gas with 50 bar left in each 15 and three unused stages, oh and three stages that are full. Assume that something goes wrong with one stage, I still have two left. How much gas do I need to get to my staged gas at 130 meters, one cylinder. So to be in real trouble I would have to lose an entire back mount cylinder (I dive isolated manifold) and three side slungs. That scenario sounds highly unlikely but even if it does happen (gotta love Murphy), then I have one back mount and one side slung to get me back out. Those are odds I can live with.
Shew! Did you get all that ? It takes time to work out a proper gas management plan. It also takes an understanding of what the real, probably risks are. And there is a difference between a probable risk and a possible risk. I plan for probable risks because there is a good chance they will happen as opposed to possible risks which hardly ever happen. And then I work out what will happen if the unthinkable occurs… if I am happy with the risk, then I dive, if not I go back to the drawing board.Next week I will go into less extreme examples of gas management. Hopefully you have an understanding now of the importance I place on having enough gas, and how much thought goes into getting it right.