Fish_Whisperer:
Please refresh my memory: What does Henry's Law state?
Simplified explanation ... sufficient for the new diver ...
The deeper you go, the more gas your body tissues will absorb due to the increased pressure of the water, and subsequently the density of the gas (partial pressure of N2) you are breathing. As you ascend, your body's ability to retain the absorbed gas decreases proportionally with pressure. Since the rate of pressure change increases (relative to depth changes) as you approach the surface, maintaining a constant ascent rate results in a faster rate of offgassing the closer to the surface you get. As an example
from 99 fsw to 66 fsw, you change the pressure from 4 ATA to 3 ATA
and the density of the gas you breathe changes by 25%. From 66 fsw to 33 fsw, that change is 33%, and from 33 fsw to the surface, that change is 50%.
In effect, holding a constant ascent rate
from, say, 60 feet to your safety stop
will result in more bubble releases, as well as faster bubble expansion, because of the increasing rate of pressure changes with each foot that you ascend.
This is why some computers will allow a certain ascent rate at deeper depths, but will alarm if you hold that same ascent rate at a shallower depth (my Oceanic Versa Pro does this, for example).
Most technical divers will ascend in a manner that they call shaping the curve
which, to simplify, means that the closer to the surface they get, the slower they will ascend.
... Bob (Grateful Diver)