why we use two GF？

[ginger bear]

hi guys , since I knew the GF/L is indicated the first stop we need , but I still confused why we need two GF ? what I mean is how can we make sure our body could tolerant much more pressure in the shallower water than the deep ? why don't we have like 30/30?

 

[tbone1004]

you can dive 30/30 if you want, but it is to shape the ascent curve of your decompression profile. All depends on your risk factors. Many divers are starting to have their GF-lo creep towards their GF-hi with one of the leading decompression scientists running 50/70 for his personal gf's. For everything except deep trimix however the gf-lo doesn't change a whole lot for the ascent profile

 

[rossh]

hi guys , since I knew the GF/L is indicated the first stop we need , but I still confused why we need two GF ? what I mean is how can we make sure our body could tolerant much more pressure in the shallower water than the deep ? why don't we have like 30/30?

GF is applied like this:

 

[ginger bear]

thank you very much rossh it make sense now

 

[atdotde]

The idea with gradient factors was to model the outcomes of bubble models in diffusion based models (like Bühlmann): The idea behind the bubble models is that even small bubbles that you create at depth grow thanks to Boyle-Mariotte when you get closer to the surface and create problems there. So you must be extra careful there as problems from bubbles are amplified whereas bubbles created a shallower depth cause less trouble. So you want to apply extra conservatism at the deeper part of your deco schedule. Of course, you could apply the same amount of conservatism everywhere but that would lead to very very long deco schedules. The GF-solution is to vary the conservatism (measured in percentage of allowed supersaturation compared to the classic Bühlmann model) between a value at depth (GFlow) and a value at the surface (GFhigh).

In a dive planner, for example the one built into Subsurface (http://www.subsurface-divelog.org), you can see how changing the gradient factors influence the time you spend at various depths.

In recent years, people raised doubts that considering these bubble effects outweighs the problems caused by spending more time at depth (i.e. the deeper stops) where maybe the leading tissue desaturates but in particular slower tissues would still on-gas. As a consequence, there is a movement of people who propose to reduce the conservatism at depth (to get to shallower depths faster) and then add more conservatism closer to the surface. In terms of gradient factors, this means that people who buy into the bubble story would use gradient factors like 20/80 whereas the "don't waste too much time at depth to stop the slow tissues on-gassing" party is using the 50/70 mentioned above.

 

[UWSojourner]

...In terms of gradient factors, this means that people who buy into the bubble story would use gradient factors like 20/80 whereas the "don't waste too much time at depth to stop the slow tissues on-gassing" party is using the 50/70 mentioned above.

See this post for a discussion about "using gradient factors to mimic bubble models".

The basic point of the post is that simply switching to GF provides some of the benefit of deemphasizing deep stops even if the user attempts to "use it like a bubble model".

 

[atdotde]

I agree. Of course there are many ways of associating gradient factors to a bubble schedule, GF vs depth is only one (and matching first step - which in itself is pretty meaningless as a 1min stop is almost indistinguishable from a slower ascent and total runtime is another) but you could also plot GF as a function of time (rather than depth) or do (as subsurface does when it computes "effective GFs" for VPM-B profiles do a linear regression here of GF vs depth).

BTW, in VPM-B the GF varying with depth is mainly due to the fact that VPM-B allows slower tissues much larger gradients so in the last part of the deco that is dominated by slow tissues you see those crazy gradients as I explained here: VPM-B Gradients as Gradient factors – The Theoretical Diver