Diving with gradient factors for a new recreational diver

[crofrog]

The debate around deep stops, to me, has always hinged around the idea of efficiency, i.e., for a fixed time how quickly can I get out of the water, and in the context of recreational diving, I don't think GF Low does much of anything. You start talking about very deep dives I think academically it starts to make a much more significant difference.

In the tech 1 range, it doesn't make a huge difference what GF Low you pick for total time:

170ft for 20min, 18/45, Nx50

Gradient Factor	Total Deco Time	First stop Depth	20ft time
20/85	27	90	17
50/85	25	60	17
85/85	24	40	19

I say that to illustrate further how little difference (no difference) it makes to NDL times on recreational dives:
Nx32, 100ft. It was calculated using subsurface and looking at when a calculated ceiling first appeared.

Gradient Factor	NDL Time (mm:ss)
20/85	21:10
50/85	21:10
85/85	21:20
100/100	29:30
30/75 (shearwater high conservatism)	15:30
40/85 (shearwater medium/default)	21:10
45/95 (shearwater low)	26:50

In any of these recreational dive profiles, you don't start off-gassing your slow tissues till around ~5-10ft...

 

[Capt Jim Wyatt]

Good way to look at it @crofrog - Thanks!

 

[L13]

170ft for 20min, 18/45, Nx50

Gradient Factor	Total Deco Time	First stop Depth	20ft time
20/85	27	90	17
50/85	25	60	17
50/75	27	60	19

I edited your table to show the line you should have added (to highlight the efficiency you mentioned). For the same deco time, 27 min, 50/75 significantly reduces your SurfGF (and your peak GF99's during the deco) vs. 20/85 and therefore your risk of DCS.

It is not just academic, the recent NEDU research shows it has a real effect on DCS.

 

[crofrog]

I edited your table to show the line you should have added (to highlight the efficiency you mentioned). For the same deco time, 27 min, 50/75 significantly reduces your SurfGF (and your peak GF99's during the deco) vs. 20/85 and therefore your risk of DCS.

It is not just academic, the recent NEDU research shows it has a real effect on DCS.

I get 32 minutes of total deco for 50/75.


Is the difference between a gf high of 85 and 75 significant? What is the probability of being bent at 85 vs 75?

 

[LFMarm]

I get 32 minutes of total deco for 50/75.


Is the difference between a gf high of 85 and 75 significant? What is the probability of being bent at 85 vs 75?

It depends on a bunch of factors, many of which are biological differences between individuals. Only thing that is certain is that risk of DCS decreases as GFHigh decrease, quantifying by how much for an individual diver has not been done yet.

 

[dmaziuk]

It depends on a bunch of factors, many of which are biological differences between individuals. Only thing that is certain is that risk of DCS decreases as GFHigh decrease, quantifying by how much for an individual diver has not been done yet.

How do you know? Do you actually know what the expected incidence of clinical DCS is at GF100, GF85, and GF70? Do you have a cite to show for it, or are you just saying it because you read it on The Internet So It Must Be True(tm)?

 

[Capt Jim Wyatt]

Do you actually know what the expected incidence of clinical DCS is at GF100, GF85, and GF70?

No one actually knows. However common sense coupled with understanding what these values actually mean tells us that the expected incidence would be higher at GF100 versus GF70.

 

[dmaziuk]

No one actually knows. However common sense coupled with understanding what these values actually mean tells us that the expected incidence would be higher at GF100 versus GF70.

Yes, but if the expected incidence "is usually assumed to be one hit in a few thousand dives"(*) at GF100, and lowering it to GF70 results in one hit in a few thousand and one dive, then why bother. It maybe would matter if it makes it one hit in a few tens of thousands of dives, -- depending on one's personal take on risks, -- but we have no data to show it's one or the other, or anything in between.

*) The Theoretical Diver – Theorizing about scuba diving

 

[Capt Jim Wyatt]

I just look at it as a long safety stop if at GF70 vs GF85 or GF100.

 

[LFMarm]

How do you know? Do you actually know what the expected incidence of clinical DCS is at GF100, GF85, and GF70? Do you have a cite to show for it, or are you just saying it because you read it on The Internet So It Must Be True(tm)?

Yes, the way the Buhlman model works, the lower the GF High the lower the incidence because you get out of the water with lower residual inert gas. As mentioned above, nobody has quantified the incidence of DCS against gradient factors for individual divers. David Doolette and his team have done a huge amount of work on this for larger diver populations — search for iso-risk in the forum.

Feel free to dive whatever GF you prefer.