The
Split from A&I Yukon thread: Gas Rules in OW Solo Dives thread has got me thinking about my own gas planning for OW dives with a buddy.
Gas planning hasn't been part of the course material for any of the courses that I've taken, and so I've been left to figure things out on my own. General rules such as "return to the boat with 500psi" haven't been particularly useful.
So, I'd like to get opinions on a particular profile.
Assuming the following - non-penetration wreck dive, wreck upright on a 130ft hard bottom, deck at 95 feet. Plan is to dive to the deck so we'll assume a maximum depth of 100 feet to allow a bit of wiggle room. The site has little or no tidal current. For the curious, the wreck I am using in this example is the
HMCS Saskatchewan.
Diver is using a 3500psi 100cf tank with a 19cf pony. The diver's buddy is equipped similarly.
What is your gas plan for this dive and why?
pnw_diver,
The Rock Bottom gas management is a worst case scenario plan. The back on the boat with 500 PSI is the "you have a DM with you and he's hopefully doing proper gas management" plan. PADI doesn't really cover this topic at all.
Everyone has given good advice but I'd just like to add there are other factors. For example, what is the visibility like on the wreck? Is there a strong current? Not penetrating the wreck is one factor (you'd need less air because you are avoiding an overhead situation) but if the visibility is poor and there is a strong current, you might need to plan pulling yourself along a line to get back to the descent line.
Last deep dive I did was at 118 feet with a STRONG current which would either push you down to 160+ feet or out into the commercial shipping lanes. Experienced divers would drift off the wreck and knew where to go to avoid the freighters and not get pushed down to 160+ feet. Inexperienced divers would pull themselves to the stern of the wreck and back up the descent line. You could easily burn through 30 cu.ft. of air pulling against the current.
Additionally, the water temperature can make a difference. Colder water usually means plan the dive to be 10 feet deeper than it really is. Cold and poor visibility can also affect you psychologically: increased breathing, nervousness, etc.
Bottom line, think about all the things which could use up air. Most likely, there will be some you haven't thought of it you are a new diver. I tend to go to new sites with people who have been diving there for a while.
Maybe you want to ask people what uses up air and how quickly. Proper risk management would be, how quickly XXX uses air multiplied by the odds of it happening. Add up all these figures and that is how much air you need. For example, SAC rate of 0.6 cu.ft./minute at a depth of 100 feet is 2.4 cu.ft./minute. If we dive for 19 minutes we are looking at 45.6 cu.ft. The odds of this happening is 100% or 1.00. Therefore we can assume 2.4 * 1.00 * 19 = 45.6 cu.ft. Having a Great White Shark rip the valve off your tank would cause total loss of air. The odds of this happening in Lake Ontario is 0%. There we can assume 100 * 0.00 = 0. A free flow might use 25 cu.ft./minute. If you were diving the Arabia in Tobermory in May the odds of a free flow are fairly high for some regulators. Let's say historically you find 60% chance of free flow. Then you are looking at 25 * 0.6 = 15 cu.ft./minute.
This is all AVERAGE case. If you get WORST case + WORST case + WORST case happening on a dive, you are in trouble. To handle all worst case you need to add up all the reasonable, possible air consumptions * 1.00 change of happening.
Finally, what is your risk tolerance? Some people are willing to say, "that would never happen to me." The are okay with the risk that the air supply they have would not be adequate for certain emergency situations because they don't believe it would ever happen to them.
Personally, for deep, new, cold dives I find myself coming back on the boat with 40+ cu.ft. of air. So far I've never had an emergency situation. If I did, that 40+ could easily become a 10 cu.ft. (around 500 PSI on an AL80).
