I made my first dive with a dry suit in open water on December25th, 2005 off the east side of Big Mosquito Island in Jeju Korea. I had done a couple pool exercises the weekend before and figured it shouldn”t be a problem.

Our first dive was to 100 fsw. I was using Nitrox EAN 32 for this dive. The islands off this area of Jeju are old volcanic vents which the magma cooled and solidified in. Over a few millennium, the outer dome eroded off and left the solidified magma like a monolith rising from the sea bed almost 200 fsw below. This is wall diving.

I did a buoyancy check and found I was much too light. My drysuit is 5mm neoprene so I wound up wearing 32 lbs on my weight belt. We made the dive and everything went as planned. When I got to my safety stop, I couldn”t maintain neutral buoyancy. I was too light and had to hang on to the rocks during the safety stop and fight the surge.

Since my weight belt was full, the dive operator offered me a steel 98 to dive with. I thought the steel would add the weight I needed to maintain neutral buoyancy at the safety stop. I was wrong. Though it was a little better, I still had to fight to stay down. So what went wrong?

Every diver is taught how to do a buoyancy check as part of their Open Water Certification. Once you have your buoyancy neutral, we add 4 lbs. and go dive; assuming you have an aluminum 80. This is to counter the weight of the air you breathe during the dive.

At sea level, one cubic foot of air weighs approximately .08 lbs. So, a full aluminum 80 has approximately 6.4 lbs. of air in it of which you consume a large portion during your dive. When we add 4 lbs. of weight after we complete a buoyancy check, we are assuming we will have 2.4 lbs or 30 cubic feet of air in our tank at the end of the dive. For those of you who may be wondering, this is called swing weight.

What happened on my dive is instead of carrying the customary 6.4 lbs of air in the standard aluminum 80, I had 7.84 lbs of air in the steel 98. Of course, I started my ascent with 800 PSI in the tank which means I had already used 5 lbs of air. The extra 4 lbs of weight weren”t helping me at all. Had I been thinking, I would have added 6 lbs after I did a buoyancy check. This would have allowed me to hold my safety stop with little or no air in my BC with out having to fight to stay down.

So there you have it. Not only do you know why we add the extra four pounds when diving aluminum 80′’s, but now you can figure out how much (or little) to add for different size cylinders.