Posted by R Bear on March 07, 2001 at 08:18:59:
On weighting, DIR, and steel tanks while diving wet.
I have read repeatedly on this board that steel while wet is not DIR. This is simply not true. I have read (yesterday) that JJ waffled when asked this question. There is a good reason for that. There are steel tanks that are a terrible choice for a wet diver and there are steel tanks that are an excellent choice for a wet diver. Also since DIR advocates the use of a back plate, a DIR diver tends to be more negative prior to tank selection. I would like to help you make an educated decision on this topic, which is quite frankly not a black and white topic.
A little about me so you know where I am coming from…
I am a Florida cave diver trained by WKPP members in good standing. I dive DIR in these caves. Florida cave diving is what DIR was made for and I believe DIR is unparalleled in this environment. I don’t insist on DIR in open water. I usually end up with some hybrid that incorporates most but not all of the DIR ideas. I also free dive. Some of you will define that as a type of diving, others won’t. It is certainly NOT DIR diving though. Basically I know DIR, and I dive DIR (usually), but I am not a fanatic about it, and I don’t really care whether you dive DIR or not.
DIR weighting commandments
Thou shall weight thyself to be neutral at the final stop with minimal air in thy tanks and no air in thy BC
In open water thou should be able to drop something of thine so that you can swim to the surface in the case that thy BC fails thee.
These two weighting commandments apply to all divers, DIR or not. It is the second commandment that dictates the appropriateness of steel tanks. The second commandment is a redundant ascent mechanism so if you dive dry you already have it covered.
In general, if you satisfy both of these constraints with your configuration then your weighting is correct irrespective of your use of steel tanks. Some of you will find what I have said so far to be obvious. I will now give some examples using my own buoyancy characteristics for those of you who don’t find it obvious.
I am three pounds negative in salt water with my mask and fins only. I have a twelve pint/pound lung capacity so while breathing in and out I average six pounds positive buoyancy from my lungs. So I use three pounds positive as my buoyancy while breathing normally in salt water. At the 15-foot safety stop, my wetsuit is sixteen pounds positive and I weight myself to have only one pound of air remaining.
Three plus sixteen minus one means I need 18 pounds of negative buoyancy to be weighted correctly. Assuming a neutral BC (some are some aren’t) I will achieve that 18 pounds of negative weight through some combination of tanks and weights. As we have already stated that the tank will have 1 pound of air, I will use empty tank buoyancies in all of my examples. Here are some examples:
Al 80 (+3) I need a 21-pound weight belt to be neutral.
PST 104 (0) 18 pound weight belt.
Heiser 104 (-7) eleven pound weight belt.
Heiser 104 (-7) with 12-pound SS back plate… Whoa! That is 19. Too heavy even with NO weight belt! This is part of why JJ couldn’t unconditionally agree.
Double AL 80s with Al back plate (-8) ten-pound belt.
Double PST 104s with Al back plate (-14) four-pound belt.
Doubles Heiser 104s with Al back plate (-28). Too Heavy! Need a ten-pound float belt.
All of these examples were only trying to satisfy the first rule i.e. neutral with empty tanks and BC at the final stop. The second rule is a little trickier. I can swim up from the bottom while 30 pounds negative. But this is hard work and implies I have lots of air. Not likely in an emergency ascent. For an unlabored swim I use the figure of nine pounds negative. I free dive wet and I know that I can swim against that load without having my pulse increase too much. Some will argue that for emergency ascent you should drop something and become neutral or even positive. My wetsuit gains 18 pounds of buoyancy on the trip from 100 feet to the surface. Because of AGE, I don’t want to start that trip neutral and end it 18 pounds positive. I would rather start the trip nine pounds negative and end it nine pounds positive. Although some of you may want to start the trip less negative than nine pounds, I will argue that NOBODY should weight themselves to start a buoyant ascent. Everyone can swim at least a few pounds up with very little effort. Having said all of that let me roll this up into a problem and do some examples. The problem is that I want to have enough droppable weight to get myself to be no more than 9 pounds negative at 100 FSW. I also don’t want to be neutral or positive at 100 FSW after dropping.
I will still be roughly (depends on air in lung) 3 pounds buoyant. My wetsuit at 100 FSW is only 6 pounds buoyant. We have agreed that I need 18 pounds of tank/tank buoyancy. There is a new wild card thrown in. My tanks may be anywhere from empty to full. So for each tank type I will include empty and full numbers. Remember, the goal is to get to (-9)
Al 80 (-6) to (-12) 3 pounds droppable
PST 104 (-9) to (-17) 8 pounds droppable
Heiser 104 (-16) to (-24) 15 pounds droppable but it was only an eleven-pound belt so this won’t work.
Double AL 80s with Al back plate (-3) to (-15) six pounds droppable
Double PST 104s with Al back plate (-9) to (-25) 16 pounds droppable but it was only a four-pound belt so this won’t work either.
I hope I have given you enough data here that you can tailor it to your own personal configuration. DIR divers will note that for the Al 80 an (undroppable) 12 pounds SS back plate is an excellent choice. Feel free to ask questions about any of my numbers although I probably won’t be able to check back on this until tomorrow.
DSAO
Ron