hypothetical deco regulator failure

[ianr33]

If its deep enough for helium its deep enough for 2 deco gases

 

[DA Aquamaster]

I agree, if you are talking dives in the 170-210' range, it should be a two deco gas dive. Once you get that deep for any useful amount of time, the deco on back gas only starts to get prohibitive and becomes a limiting factor in lost deco gas planning. Another deco gas will allow for deco contingency plans on either remaining gas with reasonable run times and without requiring an enormous reserve in back gas.

 

[BabyDuck]

i wouldn't swap a reg. i'd finish on backgas/travel gas or buddy breathe the o2 with my bestest bud.

 

[kierentec]

with borrowing teammates gas and switching to your back gas to finish off, at what point do you start to worry about counter diffusion? i realize that on a single deco gas dive it might not be as big of an issue, but as i'm preparing to start my trimix training, it pops in my head that this may be an issue. seems like it's more of a theoretical concern, but at what point would you guys start to take it into consideration?

 

[Trace Malinowski]

Lets say a diver is on a normoxic helium mixture and is using a single deco bottle. The diver has ascended to the point where he would switch to his deco bottle, and still has enough backgas to decompress on that if there was a loss of deco gas.

If your deco regulator failed, would you decompress on backgas which would take longer, or would you unscrew your right post and attach it to your deco bottle and deal with the water in the regulator issue later?

Assume that you and your dive team have enough gas to all safely ascend to the surface, you adequately planned for deco off of backgas if necessary, your deco and backgas mixes are correct and you planned your dive and dove your plan up to this point.

Also, please note that I am not asking anyone on this board to teach me a technical diving skill or plan my dive for me.

Prudent use of decompression gas is a means to accelerate the decompression schedule. Adequate reserves of back gas should always remain for each diver to complete decompression. One strategy to maintain adequate reserves, even in the event of the extremely unlikely failure of both or all deco bottles in a team, is to employ a bottom stage for the dive while leaving the back gas untouched for emergencies.

For example, if a diver is using twin AL80's as back gas with a mix of 21/35, and carrying another AL80 stage with 21/35, the 77 cubic feet of gas normally found in a stage becomes "all usable" gas for the dive. This could provide between 20 and 30 minutes of bottom gas until the gas switch. While switching to Nitrox 50 at 70 feet is a common gas switch, DAN studies seem to point to 100% oxygen being the better gas of choice during a single deco gas dive.

If staged bottom gas is depleted earlier than anticipated or if there is a deco bottle failure, the 154 cubic feet of reserve gas on the back is more than ample to handle travel time, lost gas or gas sharing emergencies.

Some divers fear that back gas decompression is "sloppy" as compared to higher oxygen mixtures, but of greater concern than what theory or model one uses during their ascent, is the ability to control the speed of ascent and complete all time at the required stops. To answer Jon's question, for me, personally, Isobaric Counter-Diffusion is simply lopped into the category of decompression-related stress or illness and carries no greater or less concern

I prefer to use a nominal V-Planner ascent profile as my worst case bail-out. While a ratio decompression schedule or +2 or greater conservation factor on V-Planner might be my primary schedule, gas used at depth when doubling the stops on back gas according to ratio decompression may be unnecessarily long. Being able to get out of the water quickly in an emergency and being able to clear deeper stops quickly to conserve gas while at the same time following a practical ascent schedule is beneficial.

For repairing fixable failures of a deco bottle, a wrench and some spare parts secured in a pocket are highly advantageous. Twice this year, I didn't have a wrench in my pocket when I could have fixed a manifold and a first stage underwater. Diving legend Moe Hunt once told me, "Always carry a wrench."

Spare parts to be carried in your pocket may include:

- 1 or 2 adjustable wrenches
- spare second stage diaphragm
- O-rings
- Spare bolt snaps with cave line for a temporary tie of any hose
- Spare stows for deco reg (normally spares are carried on bottle)

Keep it simple. The best solutions to problems underwater are usually the easiest. Easy often means safe.

 

[limeyx]

While switching to Nitrox 50 at 70 feet is a common gas switch, DAN studies seem to point to 100% oxygen being the better gas of choice during a single deco gas dive.

Hey Trace, do you have a link/reference to those studies ?

 

[Trace Malinowski]

Hey Trace, do you have a link/reference to those studies ?

Nick, I first learned about this while having dinner with a cave instructor friend and a DAN research physician. The data was given to JJ at GUE. I was invited to obtain a copy which kind of ended up on the back burner of my priorities list - until now. I sent an E-mail requesting the information so that I can better educate my students in an upcoming advanced nitrox and triox course. What I gleaned from the conversation was that the decompression strategies that we associate with DIR diving have greater success attributed to tissue saturation such as that pioneered by the WKPP in Wakulla Springs. While the same strategies have been passed down to beginning technical divers, the study being conducted seems to point toward the fact that fast tissues respond more favorably to 100% oxygen decompression than to 50% oxygen. Another factor I was told to take into consideration is that on-gassing may actually occur as we make switches using Nitrox 50 at 70 feet during the deeper portion of the intermediate stops when we come off trimixes such as 21/35 or 18/45 when using ratio deco. When I obtain a copy, I'll ask whether I may share the information or not.

Another comment on isobaric counter diffusion is that ICD hits often target the inner ear, which is not just a risk to mixed gas divers, but also has been recorded in recreational or air divers who have ventured below 70 feet. I believe DAN studies also show that inner ear hits would make one wonder if a PFO was involved since inner ear DCS, neuro hits, skin bends, etc., are higher in divers who have right to left shunts. But, while arterial bubbles may be common in studies DCI is rare so some other causal factor may be involved.

 

[TSandM]

Hmm . . . in my classes which have discussed staged decompression, it was never suggested that 50% was a "better" gas for decompression from 150 feet. I was taught that the loss of decompression efficiency was offset by the improvement in rock bottom requirements. Rock bottom from 150 feet or more to a 20 foot gas switch becomes quite impressive, especially when you factor in lost deco gas requirements.

 

[Thalassamania]

Yes, it's not surprising that 100% works better than 50%, naive physics would tell you as much ... I hope that they didn't spend a whole lot of money or time figuring that one out.

Lynne's other point about calculating your turn point to get you and your buddy on back gas to either 20 feet if your carrying pure oxygen, or 80 feet for 50% (OK, I cheat a little and am OK with a ppO2 approaching 1.8) is a rather telling one.

That's why I favor a travel gas, say EAN-32 for deep dives (I can make my switch here at about 150) or EAN-36 for a shallower dive (switch at 130). But I am going to end up on 100% at 20 no matter what. Green gas is good gas.

 

[Kevrumbo]

Nick, I first learned about this while having dinner with a cave instructor friend and a DAN research physician. The data was given to JJ at GUE. I was invited to obtain a copy which kind of ended up on the back burner of my priorities list - until now. I sent an E-mail requesting the information so that I can better educate my students in an upcoming advanced nitrox and triox course. What I gleaned from the conversation was that the decompression strategies that we associate with DIR diving have greater success attributed to tissue saturation such as that pioneered by the WKPP in Wakulla Springs. While the same strategies have been passed down to beginning technical divers, the study being conducted seems to point toward the fact that fast tissues respond more favorably to 100% oxygen decompression than to 50% oxygen. Another factor I was told to take into consideration is that on-gassing may actually occur as we make switches using Nitrox 50 at 70 feet during the deeper portion of the intermediate stops when we come off trimixes such as 21/35 or 18/45 when using ratio deco. When I obtain a copy, I'll ask whether I may share the information or not.

Decompression from an N2-based dive is longer with N2 containing deco mixes because some N2 is continuously diffusing into tissue during deco. Decompression from a He-based dive can be longer with N2 containing deco mixes because N2 is diffusing into tissue as He is diffusing out of tissue. The decompression obligation of a tissue compartment is based on the sum of gas partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being removed, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . .

Furthermore, inert gas elimination is independent of depth during oxygen breathing.The gas partial pressure gradient for movement from tissue into blood is not controlled by ambient pressure; it is controlled by the gas partial pressure in the tissue and in arterial blood. As long as the arterial [inert, non-metabolic] gas partial pressure is zero, the gradient for [inert, non-metabolic] gas removal from tissue is maximal . . .It should be intrinsically obvious that removal of a gas from tissue can be speeded by elimination of the gas from the inspired mixture. If the arterial partial pressure of a gas is zero, then no gas will diffuse into tissue while the gas is diffusing out of the tissue. . .
p.11-12, Gas Exchange, Partial Pressure Gradients, and the Oxygen Window Johnny E. Brian, Jr., M.D.

See also this thread from Bruce Wienke regarding ICD and Mix Strategies . . .from seven years ago:
Isobaric Countertransport And Mix Switch Strategies

--------------------------------------------------------------------------------

All,

Got some emails about optimal switch strategies in modern
deco frameworks, and impacts of isobaric countertransport.

So for those who asked me, here's my read using dual
phase dynamics (RGBM)), what tec sectors dive and report,
some earlier experiments and observation by Chris Lamberstsen,
field tests, plus building on earlier posts.

1) -- light-to-heavy switches are the ONLY safe switches,
and once started, should never be reversed back
the other way;

2) -- good way to deco overall is riding He to 20 fsw zone
and switching pure O2 or 80/20 if you can (you will
most likely feel better on He deco, be in better position
to repet, and have roughly equal deco time as with some
switch to nitrox earlier);

3) -- the lighter the He mix, the less benefits of switching
to nitrox;

4) -- heavy nitrox switches (depths beyond 70 fsw) are not
an optimal (nor good) strategy for short or long
exposures;

5) -- EAN50 at 70 fsw is usually N2 light enough for longer
exposures

What this all adds up to is the following isobaric switch
prescription, which is, of course, outside of most dive
logistics. But useful on first principles -- and some places
DO have the support in place for just such logistics.

So, it goes like this.

Best deco strategy is to increase O2 on way up in same proportion
as He is reduced, while keeping N2 relatively constant. Switch to
a nitrox mix with less N2, and thus outgas both He and N2. The
He to O2 changes look a little like an RB, and the lower N2
nitrox switches avoid gradient slams and further ingassing of
N2.

Practically, this means all the above plus EAN50 at the 70 fsw
level for long exposures, and/or He to the surface for shallow
exposures, plus O2 in the 20 fsw zone either way.

Cheers, and safe diving always,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr
__________________
BRW
http://www.scubaboard.com/forums/as...c-countertransport-mix-switch-strategies.html