Fish_Whisperer:
Slowing ascent rate, the closer to the surface you get, is because of Boyle's Law and its affect on the nitrogen bubbles, correct?
Another way to look at is that in the classic multicompartment dissolved gas model, you have both fast, medium and slow compartments. The purpose of the deeper stops is to offgas the fastest compartments, while not further loading up the slow compartments more than needed. Since you are offgassing the fastest compartments at the deepest stops, they don't have to be long. As you ascend and either do stops or a slower and slower ascent, you progressively are working on offgassing slower and slower compartments/tissues. So you need to get slower as you ascend.
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The faster compartments have significantly higher overpressure limits allowed. The slowest compartments have the lowest allowable dissolved gas supersaturation limits.
At the end of a typical recreational single tank dive, the fastest compartments may be saturated, but still within the limit for surfacing. The medum halftime compartments are nearing their limits. The slow compartments have much lower gas loads, but since their limits are much lower, the are loaded to a significant fraction of their limit.
Popping directly with a fast ascent to the surface will drive the medium compartments right up to their limits. (And keep in mind that it is not a hard black and white limit, but instead a fuzzy "how much are we going to bubble today" sort of limit).
Doing an excessively slow ascent is just the same as doing a long multilevel dive, and the medium to slower tissues will continue to ongass to beyond their limits.
An important concept is that the allowable limit for each compartment is a function of depth. The deeper you are, the higher the limit at that depth.
Heading up towards 40' lets you get to a point where the fastest compartments will start offgassing, but are still well away from their limits for that depth. Meanwhile, you have signficantly slowed the ongassing in the slowest compartments. Depending upon the dive, the medium speed compartments could be either ongassing or offgassing.
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After just a short while at 40', the fastest compartments will have significantly offgassed, and you can now go shallower. Going to some intermediate depth, such as as 20' repeats the situation as above, but you are now offgassing the medium compartments, while the pressure from being at 20' increases the compartment limit higher than what it would be at the surface. Staying well away from the compartment limit minimizes bubbling. Since at this shallower depth, you are now trying to offgas the medium speed compartments, you need to spend longer time than you did for the deepest stops, where you are offgassing the fastest compartments.
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I'll repeat this post with some real numbers when I get a chance.