[This is a re-post of a blog entry I made on February 27, 2005, slight changes, including an annotation.]
One of the first skills we develop as divers is buoyancy control. Yet, there are so many certified divers out there who clearly have not mastered this basic however vital skill. Out of the many possible reasons for this, one stands out most: heavy weighting. Whether it is the excitement to get down quickly or just a misunderstanding of how weight plays a role in SCUBA diving.
Bottom line, too much weight is never a good thing. Being overly weighted causes fatigue, higher air consumption rate and unnecessary safety risks.
Being negatively buoyant at surface will expedite your descent, but it will hamper your diving and your enjoyment. Divers with too much weight will require more air in their BC to achieve neutrally buoyancy. As a heavy weighted diver descend to depth, they will have a difficult time compensating to the change in pressure and will usually bounce up and down searching for that “neutral” sweet spot.
An initial test for proper weighting can be done at the surface. With your regulator in mouth completely deflate you BC, relax and begin to breathe slowly. When your lung capacity is full, your eyes should be at water level. If you sink with lungs full of air, you’ll need to remove some weight. Once you completely exhale, you should sink. If you do not sink, add more weight. This should give you a good start point.
(For you new divers, the most important word in that last paragraph is “relax”. When excited or put into a new environment, people tend to hold their breath and breathe shallow. Full lung capacity can make you anywhere between six(6) to 12 pound positively buoyant.)
The one thing to note is that the proper weighting for SCUBA diving is being able to hover at 15 feet with 500 psi in your tank and no air in your BC. Aluminum tanks begin to be positively buoyant once capacity is less than 1500psi (this is also known as “corking”) and will affect your buoyancy. Consequently, this will require the need for additional weight to the amount found when testing your buoyancy at the surface with a full tank.
When returning from your dive, take special note of your buoyancy control to see if additional weight is needed. The recording of this information should be a standard line item in your individual dive logs.
As you dive more and more you’ll get a better understanding of how much weight you specifically need to dive comfortably. As you change your equipment configuration (i.e. exposure suits, aluminum versus steel tanks, etc.) you should always test your buoyancy to insure proper weighting.
When teaching students we say, "add a knife to your equipment, check your buoyancy." Though we say it jokingly, we're somewhat serious.