RonR
Contributor
Water is not compressible, but the atmosphere is- so most of that 63 miles is near vacuum. So you can’t look at this in terms of a % of the 63 miles, unlike water depth, the atmospheric pressure change with altitude is not linear. Close to the earth’s surface the change in pressure is much greater for each added foot of altitude.33 ft of seawater pressure = approx the same as 63 miles of air pressure + the water pressure. When you went to altitude, what percentage of that 63 miles did you achieve? The depth reading will reflect that but it is minuscule in the water environment. The altitude setting only really adjusts for the off-gassing at the surface and the fact that the pressure change will be greater than it might be a sea-level. Therefore, for safety, it reduces the no-stop time so that you are less loaded with nitrogen back at the surface.
As others have pointed out as well, going to 5,000’ / 1500m will drop the atmospheric pressure about 170 mBar, equivalent to 5.5’ / 1.7m of water depth- you would be that deep underwater before reaching sea level pressure. That’s not exactly trivial if you are trying to map something, but it is irrelevant for decompression calculations. What matters for deco is only the difference between the pressures the diver experiences on the dive and the ambient pressure they will see on exiting.
-Ron