Rate of ascent vs. rate of change of pressure?

[Reg Braithwaite]

In the sport/recreational world, there is much talk of a safe rate of ascent. I am ignorant of the way decompression models work, so I apologise in advance for asking a ridiculously naive question. Here goes:

In a simple case (diving air, no gas switch, ascent from depth to the surface without pause), is it safer to choose a constant rate of ascent or to choose a constant rate of change to the pressure felt by the diver?

My constant rate of ascent, I mean something like 6m/minute from depth to the surface without stops of any kind. By constant rate of change to the pressure, I mean slowing down as you near the surface such that the change to pressure remains relatively constant.

I am not explaining the latter well, so here is my hypothetical example:

One diver is at 23.75m ascends to the surface at approximately 4m/second, taking six minutes to reach the surface. His pressure changes from 3.375 ATA at depth to 1 at the surface over six minutes.

Another diver is at 23.75m and wants to reduce the pressure by 50% every two minutes. Therefore, he would take two minutes to ascend to 12.5M, then slow down and take two more minutes to ascend to 5M, then slow down and take two more minutes to the surface. Thus, his pressure also changes from 3.375 ATA at depth to 1 at the surface over six minutes, but his change to pressure remains relatively constant.

Is the second diver "safer" in any way?

 

[ianr33]

The second diver will be safer,although for an NDL dive you would never be able to measure the difference.

The 4m/second ascent rate you mention is very slow. Ascending that slowly from the bottom portion of a dive means you will be ongassing nitrogen.

This is an excellent article http://www.hhssoftware.com/v-planner/decomyths.html

It is mostly about deco dives but the principles are the same.Read the "Slow Ascent in NDL dive" section

 

[Dr Deco]

Hello Reg Braithwaite:

In the example that you gave, it would not make much difference, although the second dive would probably be safer. It would be hard to detect in a controlled study.

Dive tables are calculated for a set ascent rate, however. If the dive is long and requires stops, these are similar to "slowing the ascent rate. "

Dr Deco :doctor:

 

[TSandM]

You are thinking along valid lines. If you look at the output from decompression programs for technical dives, you will always see the ascent rate slow as the diver gets closer to the surface. This is due both to the exponential nature of on and off gassing, and the bigger pressure changes with the same absolute depth change as you approach the surface.

For NDL dives, it probably doesn't matter, although I was taught a 30 fpm ascent to half depth, and 10 fpm after that.