Posted by Nick Radov on March 08, 2001 at 23:44:52:
In Reply to: seahunt's DIRt on the DIR Demo. posted by seahunt on March 05, 2001 at 16:02:01:
I can't claim to be a real DIR diver (have only been through the entry level GUE class) but let me see if I can clarify the issues related to using steel tanks and wetsuits. The fundamental DIR rules for open water diving are that you must be able to hold a shallow safety stop at the end of a dive (empty tank) and must be able to get back to the surface if your BCD fails at the beginning of a dive (full tank). In other words balance your rig. Now let's run some sample buoyancy calculations to see how we achieve those two goals.
For the first case assume a 7mm two piece wetsuit (including hood, gloves, and booties) and a PST HP steel 120 tank. On the surface at the beginning of a dive your body is close to neutral buoyancy (+0lb), the neoprene may be as much as +30lbs, the full tank and regulators are about -10lb, and figure another -2lb for fins and miscellaneous. At a 15fsw safety stop the neoprene will be down to about +23lb and the tank could be at -1lb. This means you will wear -20lb ditchable weight to maintain neutral buoyancy and we have met the first goal. Now consider what happens if your BCD completely fails right at the beginning of the dive when you are at 100fsw. Your wetsuit buoyancy is now about +8lbs but your full tank and regulators are still about -10lbs. Adding the weight belt and fins gives us -24lbs net buoyancy. If you ditch the -20lb of lead you still have -4lb buoyancy. Can you swim that up from 100fsw? Maybe so, but what if your leg cramps?
For the second case lets switch to using an AL 80. Now the full tank and regulators are about -3lbs, and about +2lbs when empty. So to hold the safety stop the ditchable weight increases to -23lbs. If your BCD fails at the beginning of the dive and you ditch the -23lbs your net buoyancy is +3lbs. No matter what happens after that you will make it back to the surface (just remember not to hold your breath).
Now I just pulled those numbers out of my ass to make a point; they may be a little off. You need to run the same calculations yourself for your own rig and you may find that in some cases a single steel tank actually is safe with a wetsuit. For double tanks, though, it will never come out safe.
Hopefully one of the more experienced DIR divers will jump in here if I mangled the explanation too badly.