Re-Evaluating My GF

[Celt]

I think whats clear from this tread is that until such time that personal decompression computers take into account the individual divers metabolism, fitness and gas consumption rate people are going to get bent. So ill wait another while before buying one.

 

[Michael Guerrero]

I dive 70/70, but would dive 90/70 or more without concern. Get off the bottom and hang on O2 in the shallows with air breaks. The end.

 

[leadduck]

This only makes since with regard to the He. Once you through in the N2 loading in parallel your increasing your TTS. Some compartments may even begin on gassing N2 at the switch.

The switch from 18/45 to EAN50 is most obvious as inspired He drops to zero and getting rid of He in medium-fast compartments is your main concern at that moment. But also on an air+EAN50 dive, the inspired N2 drops instantaneously from 79% to 50% , hence your trade-off between "get shallow to protect slow compartments" and "stay deep to protect fast compartments" will shift at the same moment. Although we don't know the optimal schedule, we do know that the trade-off depends on the inspired fraction of inert gas. So we can expect that the (unknown) optimal schedule will include some extra stop time right after a gas switch to a higher O2 fraction.

Practically I'd say that on an air+EAN50 dive the idea of Andy Davis' blog will probably work well: choose GFlow so that your first stop is at 21m. I think the NEDU study results suggest cutting back deep stops on bottom gas but do not justify going shallow quickly while on EAN50.

 

[J-Vo]

Practically I'd say that on an air+EAN50 dive the idea of Andy Davis' blog will probably work well: choose GFlow so that your first stop is at 21m. I think the NEDU study results suggest cutting back deep stops on bottom gas but do not justify going shallow quickly while on EAN50.

Not sure I agree. On air, I wouldn't have been very deep so 70' is still more than half depth. Unless it's an extremely long dive my medium and slow tissues will still be on gassing with 50%.

 

[leadduck]

Here I tried again, with an air dive for 20min to 50m (164ft) with EAN50 deco from 21m (70ft):

Three ascents of same runtime,
with GF20/80, GF50/70, and GF77/77X (staying at 21m on EAN50 until the 9m ceiling clears):


GF20/80:

depth stop run gas
----------------------------------
↓ 50 1 1 Air
┄ 50 19 20
↑ 27 3 23
↑ 24 1 24
↑ 21 1 25
┄ 21 1 26 EAN50
┄ 18 2 28
┄ 15 2 30
┄ 12 2 32
┄ 9 4 36
┄ 6 5 41
┄ 3 12 53
----------------------------------

GF50/70:
depth stop run gas
----------------------------------
↓ 50 1 1 Air
┄ 50 19 20
↑ 21 3 23
┄ 21 1 24 EAN50
┄ 15 2 26
┄ 12 3 29
┄ 9 4 33
┄ 6 6 39
┄ 3 14 53
----------------------------------

GF77/77X:

depth stop run gas
----------------------------------
↓ 50 1 1 Air
┄ 50 19 20
↑ 21 3 23
┄ 21 8 31 EAN50
↑ 6 2 33
┄ 6 7 40
┄ 3 13 53
----------------------------------

The ISS chart (normalized to the values of GF20/80):

Going from 20/80 to 50/70 as expected, shifting between slow and fast, while 77/77x is better than 20/80 in every compartment. The N2 loading by EAN50 is not very strong at 21m.

 

[J-Vo]

I'm curious how the 77/77 ISS chart would compare with itself for prescribed stops rather than the 3 long stop approach.

 

[leadduck]

I'm curious how the 77/77 ISS chart would compare with itself for prescribed stops rather than the 3 long stop approach.

It'd take only 49min and not be comparable to the others. Anyway, this example was air to 50m and even there the effect is small. It just proves that "follow the ceiling" is not optimal even for air+EAN50. But I agree that if you don't dive deep on air and 70ft is more than half your depth, then staying longer at 70ft doesn't make sense. This effect is significant only for trimix dives, where you achieve maximum helium off-gassing gradient with EAN50 at any depth. With air+EAN50 I'd follow Andy Davis' idea.

 

[leadduck]

I found something interesting playing with these delayed ascents: EAN80 is actually a useful decompression gas and EAN40+EAN80 can be better than EAN50+oxygen, but only if you use EAN80 to shift decompression time from 3m to 9m, like this:

   depth    stop     run       gas
----------------------------------
v     60      2       2        18/45
-     60     18      20  
^     30      3      23  
-     30      1      24        EAN40
^     27      1      25  
^     24      1      26  
-     21      2      28  
-     18      3      31  
-     15      3      34  
^      9      1      35  
-      9     12      47        EAN80
-      6      9      56  
-      3      6      62  
----------------------------------

This one gives me significantly less ISS in every compartment compared to anything I can get out of EAN50+oxygen with the same TTS. Actually you're using the same idea already when decompressing with oxygen at 6m (20ft) only instead of going to 3m (10ft).

Three elements are important:
1) EAN40 switches to deco gas, off helium, earlier than EAN50
2) stay longer at 15m until you can go straight to 9m for EAN80; no need for the 12m stop
3) use EAN80 at 9m to the full extent. Do not go shallower as long as EAN80 has a sufficient off-gassing gradient at 9m even if the ceiling would allow you to do so.

Whereas if I use the traditional ascent rule "always stick to the ceiling", then EAN40+EAN80 is worse than EAN50+oxygen.

 

[J-Vo]

How do you know you have a "sufficient off-gassing gradient"? Is this simply that all tissues are off gassing? Seems to me you would start some significant N2 on gassing at the switch to 40%.

Are you calculating combined ISS of He+N2 or just He?

 

[leadduck]

How do you know you have a "sufficient off-gassing gradient"? Is this simply that all tissues are off gassing? Seems to me you would start some significant N2 on gassing at the switch to 40%.

The ascent rule was: as long as (p_tissue_N2 - p_inspired_N2) + (p_tissue_He - p_inspired_He) > 1.25 bar for any tissue, the current off-gassing gradient is considered sufficient. Ascend to the ceiling if every tissue's off-gassing gradient is <1.25bar or if ascending to the ceiling allows switching to the next gas. Else, stay 3m (10ft) below the ceiling.
This is of course a rough approach and better rules may exist. Of course some slow tissues are still on-gassing even if one tissue off-gasses with >1.25bar. However this simple rule produces ascents with smaller overall ISS.

Are you calculating combined ISS of He+N2 or just He?

Always combined He+N2, integrating max(0; (p_tissue_N2 + p_tissue_He - p_ambient)/p_ambient) for every compartment separately for the whole dive until 48h after reaching the surface.