Can you do too much deco?

[RedSeaDiver2]

Eliminating The Helium Penalty - Shearwater Research

Our understanding of decompression science has undergone dramatic revision in the last two years thanks to decompression physiologist David Doolette, Ph.D. and the team at the Navy Experimental Diving Unit (NEDU).

www.shearwater.com

Very interesting. Unfortunately the link to the research is a broken link, but some interesting reading on the Shearwater page.
We still seem to be at the point of "we don't know what we aren't 100% sure about"

 

[RedSeaDiver2]

Last night, I figured out an excellent method for determining what gradient factors one should use during a dive. I was watching Netflix. You know that "surprise me" option? That would be great in a Shearwater. Hit the "surprise me" option and the computer will select from among several popular deep stop or post deep stop GF's.

Kind of like Google's "I'm Feeling Lucky" on the search page?

 

[RedSeaDiver2]

Standard question about lower GF-LOW settings: why would you want to on-gas more? The point is to get up shallower where you're not on-gassing.

Also, why such a high helium content, 65% for a 60m/200ft dive? ~45% would be more normal to give an EAD of 24m/80ft -- and cost less. Gas density is 4.0g/litre compared with 5.3g/litre

According to MultiDeco, for a 60m/200ft dive with 30 mins bottom time (CCR with setpoint 1.3):

• 15/45 gas with GF 50:80 is a dive length of 94 mins
  • OTU's: 136, CNS: 51.3%, Gas density: 5.3g/l
• 15/65 gas with GF 30:80 is a dive length of 111 mins
  • OTU's: 161, CNS: 60.8%, Gas density: 4.0g/l

AIUI this is an additional 17 mins in the water at the cost of higher CNS/OTUs which becomes an issue on repetitive dives.

I don't want to on-gas more, but accept that is the penalty for having a nice low density gas/low WOB. If we were diving somewhere with colder water (Norway or the Great Lakes for example) then the additional on-gassing and resulting longer deco would be an issue, but here in the Red Sea cold water isn't such an issue and I'm happy to take the longer deco in exchange for a nice low WOB.

 

[RedSeaDiver2]

One thing about strenuous activity tests is that they usually are focused on lower body exertion in a lab setting, such as peddling. Makes sense because exertion in scuba diving is often related to swimming. But from what I've read, upper body exertion is more likely to create a shunt and that definitely comes into play when commercial diving or possibly doing recovery work. I wonder if deep stops might provide a better cushion to get rid of problematic bubbles in such a case.

Edit: I should add once out of the water, such as climbing a ladder, lower body exertion is more likely to create a shunt.

Interesting point - and very relevant to archaeological work where a lot of time is spent kneeling on the sand working

 

[RedSeaDiver2]

Lurker here among legends
While I too have keenly followed the 50/80 rationales, if you want a pee break at 39 metres and some more shallow hang time, then seems ok, no?

Will that extra 27 minutes mean the difference between a bend or not while climbing onto the boat before you sit down for another hour of relaxed surface deco? Idk. Probably depends a lot on the individual that day.

The END for 65% at 60m with 1.3 ppO2 (~18%) is like 5 metres? Nobody I know intentionally aims for a 5m END

Is there a conflict between:
a) trying to make the deco more conservative/safer, and
b) upping the fast-diffusing gas to levels higher than normal?

All good question.

I wasn't intentionally aiming for an END - just aiming for a lower gas density than others like to dive with.

 

[RedSeaDiver2]

The NEDU data definitively shows that GF 50/80 is SAFER than GF 30/80, and strongly suggests that GF 50/60 would similarly be safer than GF 30/60.

I've no doubt that 50/80 would be safer than 30/80 as the 80 is too fast to compensate for the additional on-gassing created by the 30. 50/60 being safer than 30/60 I'm not so sure about.

 

[tursiops]

Unfortunately the link to the research is a broken link, but some interesting reading on the Shearwater page.

Attached.

We still seem to be at the point of "we don't know what we aren't 100% sure about"

Nonsense statement. We are never 100% certain...but we still know plenty in order to proceed.

 

[RedSeaDiver2]

Hi
Why don't you try to approach other teams like Frank Goddio's which, I believe, did a lot of similar works at similar depths?

Franck Goddio's work was much much shallower - most was done on single cylinder, some with smaller twins.

 

[boulderjohn]

Finally am getting a quiet day to catch up.
That sounds like some very interesting and thought provoking research and conclusions. If they are saying that helium and nitrogen off-gas at the same rate but that we need to be doing longer deco stops then would it be fair to say that what they have concluded is that nitrogen off-gases at the same rate as helium and therefore we need to be doing deco more in line with algorithms for helium than what we currently do? I don't know - I haven't seen the research paper to know how it should be interpreted.

Ater reading that research, I have decided to keep inputting helium into the computer, accepting the extra time that is built into the algorithm. I do not, however, add additional time to that.

 

[boulderjohn]

I've no doubt that 50/80 would be safer than 30/80 as the 80 is too fast to compensate for the additional on-gassing created by the 30. 50/60 being safer than 30/60 I'm not so sure about.

The length of time needed to reach a GF high of 80 is determined by the algorithm, and it includes everything you did on the dive. It will take you longer in your shallower stops to reach 80 if you started with a GF low of 30 than if you started with a GF low of 50. You don't need to make a manual adjustment.