The Significance of Relative Changes
At first glance, the reader might well observe that the changes associated with Boyle’s Law occur in a simple straight-line progression. Ambient pressure increases by 1 bar for every 10 meters of depth, or by 1 atm for every 33 feet of depth, in seawater. This information seems to suggest that changes in pressure, volume, and density always are uniform over various depths, regardless of the depths involved. Such is true, if one considers these changes only in terms of actual pressure.
More importantly, the reader should focus upon the relative changes in pressure, volume and density that occur over a range of depths. For example, ambient pressure doubles when the diver descends from the surface to 10 meters or 33 feet (which is an actual change of only one 1 bar / 1 atm), but ambient pressure does not double again until the diver descends from that prior depth of 10 meters or 33 feet to a new depth of 30 meters or 99 feet (which is now an actual change of 2 bar / 2 atm). The prior table is repeated here, for ready reference:
|Depth in Seawater||Ambient Pressure||Volume||Density|
As another example, volume increases by only 25% during an ascent from a starting depth 40 meters or 132 feet to a new depth of 30 meters or 99 feet (which is an actual change of 1 bar / 1 atm), yet volume increases by 100% during an ascent from 10 meters or 33 feet to the surface (which also is an actual change of only 1 bar / 1 atm).
As demonstrated, the greatest relative changes occur at the shallower depths. In many cases it is these relative changes that will be most apparent to the diver, because the results can be dramatic. This situation emphasizes the importance of a diver equalizing early during a descent. It helps explain why a new diver may have a bit more difficulty in effectively controlling buoyancy at shallower depths, and why he or she might unexpectedly pop to the surface. It also demonstrates the need for vigilance in maintaining a slow rate of ascent throughout the entire ascent, especially as the diver nears the surface.