Let’s look at each of these four points.
1. Lungs are primary for buoyancy control.
A properly weighted and balanced diver will
be neutral at every depth. This means small
changes and adjustments in buoyancy can be
easily and quickly accomplished by changing
lung volume. On descent, gas is added to the BC
and the dry suit only to keep the diver neutral.
Initiating a descent or ascent is by lung volume
– want to go up? Inhale (then start exhaling as
soon as the ascent begins). Want to go down?
Exhale. Depending on body size, most lungs can
adjust buoyancy from between four and eight
pounds (two and four kgs). This is plenty to start ascents or descents, or make small changes to adjust for
swell or other water movement.
2. The BC carries the weight of the gas. It is not used for buoyancy control (it’s a buoyancy ‘compensator,’
not ‘controller’).
A properly weighted diver should be neutral at 10’/3m with the cylinders almost empty. This allows for
precise buoyancy control at the shallow stops. When weighting a diver, we do it with the BC empty and
NO GAS IN THE TANK. If using aluminum tanks, we do it without the tanks. If using steel tanks, which
are part of your weighting system, we do it with 500 psi/35b. We get the diver neutral at 10’/3m. Then,
when we add a full tank, the weight of the gas is ‘compensated’ for in the BC. So with a single AL80, the
divers adds about 5lbs/2.5kgs of lift to the BC.
3. The dry suit has nothing to do with buoyancy – the dry suit inflation system is only to maintain the suit
at the same buoyancy throughout the dive.
If the diver is wearing a dry suit and a single 80 cft/11l tank, the BC starts at 5lbs/2.5kgs and is vented
throughout the dive, so at the end of the dive the BC is empty. With a wet suit, the BC also has to carry the
loss of positive buoyancy of the wet suit at depth, but that’s temporary, as the wet suit will expand on the
way up and the BC will have to be vented to compensate, again leaving the BC empty at the shallow stops
with the suit is not compressed.
4. The breathing cycle continues regardless of whether there is a regulator in your mouth.
This is where most people unravel in the water. Some divers have been taught to blow a small stream
of bubbles any time the regulator is removed from the mouth, in order to prevent a lung over-expansion
injury in the event of a loss of buoyancy control. But if you start neutrally buoyant and take the regulator
out of your mouth and blow a stream of bubbles, it will force a change in buoyancy and you’ll descend.
It’s been ingrained into most divers: never hold your breath. But the real rule is never hold your breath ON
ASCENT.
TM