Posted by Gerry Smith on March 11, 2001 at 19:24:01:
I’ve been thinking about diving wet with steel bottles. It’s my favorite way to dive but some say it’s dangerous so I’m interested in where the danger comes into play. I did a bit of analysis and thought I’d run it up the BBS flagpole to see what I might have forgot. I want to use this, or something similar, in my classes in the future.
The chart below shows reserve buoyancy for depths in 10’ increments to 130’ and for rigs weighing 2 to 16 pounds in the water. I made the following assumptions:
1. The danger from diving wet with steel comes from the possibility of the rig (non ditchable weight) weighing more than the buoyancy of the compressed wetsuit can overcome. So in the case of a catastrophic BC failure, I couldn’t get buoyant.
2. My wetsuit (Kettenhoffen ¼” farmer john) has 25 pounds of buoyancy at the surface. This is reduced as it is compressed by depth according to the gas laws. It contains 1 pounds (in water weight) of neoprene. The mechanical stiffness of the bubble walls reduces compression by 10 percent.
3. The non-ditchable weight figure includes tank, valve, backpack, attached doodads and gillhickies and will decrease as breathing gas is used up. Hence the start of the dive is the most dangerous because of the weight of compressed air.
4. Ditchable weight is used to offset reserve buoyancy.
5. My body is neutrally buoyant.
Reserve Buoyancy
Depth Wetsuit Non Ditchable Weight (Pounds in seawater)
2 3 4 5 6 8 10 12 14 16
Surface 25.0 23.0 22.0 21.0 20.0 19.0 17.0 15.0 13.0 11.0 9.0
10 20.1 18.1 17.1 16.1 15.1 14.1 12.1 10.1 8.1 6.1 4.1
20 16.1 14.1 13.1 12.1 11.1 10.1 8.1 6.1 4.1 2.1 0.1
30 13.4 11.4 10.4 9.4 8.4 7.4 5.4 3.4 1.4 (0.6) (2.6)
40 11.4 9.4 8.4 7.4 6.4 5.4 3.4 1.4 (0.6) (2.6) (4.6)
50 9.9 7.9 6.9 5.9 4.9 3.9 1.9 (0.1) (2.1) (4.1) (6.1)
60 8.8 6.8 5.8 4.8 3.8 2.8 0.8 (1.2) (3.2) (5.2) (7.2)
70 7.8 5.8 4.8 3.8 2.8 1.8 (0.2) (2.2) (4.2) (6.2) (8.2)
80 7.0 5.0 4.0 3.0 2.0 1.0 (1.0) (3.0) (5.0) (7.0) (9.0)
90 6.4 4.4 3.4 2.4 1.4 0.4 (1.6) (3.6) (5.6) (7.6) (9.6)
100 5.8 3.8 2.8 1.8 0.8 (0.2) (2.2) (4.2) (6.2) (8.2) (10.2)
110 5.4 3.4 2.4 1.4 0.4 (0.6) (2.6) (4.6) (6.6) (8.6) (10.6)
120 4.9 2.9 1.9 0.9 (0.1) (1.1) (3.1) (5.1) (7.1) (9.1) (11.1)
130 4.6 2.6 1.6 0.6 (0.4) (1.4) (3.4) (5.4) (7.4) (9.4) (11.4)
Negative numbers (in parentheses) indicate zero or less reserve buoyancy. Use a full width screen to make sense of the chart.
The "danger area" in the chart is simply where the reserve buoyancy drops to zero or below. Obviously you can swim up against a moderate amount of negative buoyancy, make your own adjustment as you see fit.
As you can see, within sport diving limits the danger area for my reserve buoyancy situation comes into play only with rig weights (in water) of 5 pounds or more. Many steel bottles weigh more than this underwater when full, and if you factor in the weight of a big brass regulator and maybe a dive light or two there is very little room for safety. In fact, certain aluminum cylinders fall into the too heavy range (Catalina’s compact 100 for example.) The old 2250 PSI steel “72” however, comes in at a nice light two lbs. And I have quite a few of these.
An interesting point: There is no tank, aluminum OR steel, with which I could safely use a stainless steel back plate with single tank adapter.
Your results will, of course, vary according to your personal characteristics and the wetsuit you wear.
OK, Who wants the first shot?
GS