The amount of time a
diver can spend
underwater is not
static.
Depth plays a major part in determining bottom time, as a result of two factors:
Air consumption and
nitrogen absorption.
The first
factor is fairly
straightforward. The deeper you are underwater, the more
air you consume with every breath. This is a result of water
pressure. Deeper underwater = higher water pressure = decreased
volume. The only thing that remains
constant is the size of your lungs. Thus, the deeper you go, the more air is compacted into every
breath. Greater depth means greater air consumption.
The second factor, nitrogen
absorption, is also direcly related to depth. Common,
everyday air is mostly nitrogen, so when you breath
pressurized air you absorb a great deal of nitrogen into your
bloodstream. Why is this a problem, you ask? Well, if you absorb nitrogen at depth, and then go to the
surface, the nitrogen in your bloodstream expands and makes bubbles. That's a really really Bad Thing. Those bubbles can cause an
embolism and kill you
dead. So, you can only spend so much time on the bottom before you've absorbed too much nitrogen and have to
surface before it reaches certain levels. Sometimes if you've been at depth for too long you'll need to stop for a bit
partway to the surface and let some nitrogen bleed off. Depending on how deep you were for how long, you might need
multiple stops.