Deco Gas Question

[TheDecoStop]

What are the advantages/disadvantages of diving a deco gas of 50% vs 36% when doing a dive around 175 – 190 on 21/35? I generally run a 21/35, then switch to 36 then to 80 on a 190’ dive. By running 36 instead of 50 I get a shorter run time since I am getting off the helium sooner, and moving to a higher % O2. I know that certain agencies advocate nothing but 50 and 100, is this the main reason that people would run a 50 over a 36 or is there another reason? The reason I ask is that some good old dir guys gave me crap about my 36 mix, actually all my mixes… but I will leave that discussion off this board.

Thanks for any input.

Safe diving,

Rick

 

[WreckWriter]

TheDecoStop once bubbled...
The reason I ask is that some good old dir guys gave me crap about my 36 mix, actually all my mixes… but I will leave that discussion off this board.

You'll get the same thing here, particularly about using 80%. Good luck.

WW

 

[Doppler]

Hi Rick:

I am going to resist the temptation to give you the gears about your dive and deco mixes <LOL>.

OK this is the very short version... you'll hear more, so take this as a overview or precursor.

Your decompression will be more successful if the partial pressure of oxygen during your ascent is somewhere between 1 and 1.6 ata. By the time you switch to that 80 mix at 30 feet, you're at about 0.75 ata. In effect, you have just executed a couple of stops with marginal efficiency... There's way more to this topic -- mostly vacant partial pressure (the oxygen window) -- but this is the answer to your original question.

Something to think about too is that the dive schedules that we cut on our computers are only mathematical models. Few programs take into account the partial pressure effect and therefore, mostly they will tell you that if you deco on 80/20 and your EAN36 you will get you out of the water sooner. Well, from the perspective of the algorithm... the calculus used to model what's happening inside you... that may be the case.

What the math doesn't tell us is how we actually feel after the dive... basically how close to the edge we are...

There's a whole essay topic here about silent bubbles activating the complement complex (immune system) and the drop in partial pressures from the mouth to the avioli but for the time being, I hope I have answered your question.


Take care and dive safe

DD (Hogarthian diver... Doing It Longer)

 

[TheDecoStop]

I appreciate the answer. Makes plenty of sense. I will rethink my methods after a little more research.

Thanks again.

 

[AquaTec]

Doppler once bubbled...
Hi Rick:


Your decompression will be more successful if the partial pressure of oxygen during your ascent is somewhere between 1 and 1.6 ata. By the time you switch to that 80 mix at 30 feet, you're at about 0.75 ata. In effect, you have just executed a couple of stops with marginal efficiency... There's way more to this topic -- mostly vacant partial pressure (the oxygen window) -- but this is the answer to your original question.

What the math doesn't tell us is how we actually feel after the dive... basically how close to the edge we are...

DD (Hogarthian diver... Doing It Longer)

If high partial pressure is you answer then what is the difference between 36% at 113 feet and 50% at 72 feet. which ever gas you choose to use double your stop time at the gas switch were the PO2 is the highest

the longer deco time is because he is on helium for the additional 40 feet, if he gets off the helium then his deco is a bit shorter.

yes there are arguments to the contrary, but that may be outside his question.

as for feeling better, I will pressume you are talking about nitrogen loading, perfect reason to get onto the 80% shallower, and flush that out. do the balance of your deco at the 30 foot stop, and just your added safety deco time at the 20 foot stop.
with the 80% there is no back switching required for those longer stop times.

36% and 80% are better choices for deeper diving, until it is time to add in that intermediat trimix, but consideration then is given to added tanks to carry and beyond his question

 

[TheDecoStop]

Here is some interesting info that I have come accross while doing a little extra research on this subject.

"Furthermore, trying to maintain high partial pressures of oxygen with multiple mixtures results in
vasoconstriction (a reduction in off-gassing potential), increased pulmonary irritation (likely with reduced
diffusion efficiency), and increased CNS toxicity risk."

Just a portion of the story, but interesting. The rest of the story points towards using minimal gases with the best calculated results.

I always like to hear peoples practices and the reasons behind them, so keep them coming.

Rick

 

[omar]

It is an outdated notion that you need/should get off of helium as soon as possible. If you are basing this statement on running different scenarios on the available models then you need to realize All of the models out there except RGBM do not adequately account for the ongassing/offgassing and bubble formation and growth with respect to helium. Even RGBM does not do a good job for the shallow stops as well.

Selecting the proper deco gases ALSO includes selecting the appropriate bottom gas. It is a symbiotic process, you can not properly do one without the other. For the first dive to 190 the proper gas would be 18/45. I would only take 50% for the first deco gas starting at 70 ft. Subsequent dives would be done with 50% and 100% at 20 ft.

The use of 36% vs 50% and 80% vs 100% are based on the oxygen window and utilizing it to maximum benefit to offgass. To go over the mechanism would take considerable space which would still be inadequate.

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[Divesherpa]

For dives in the 190 range for times between 30-70minutes, I prefer to jump on 32% or 36%, then 100%.
I feel much better after the dives.

 

[voidware]

Few programs take into account the partial pressure effect and therefore, mostly they will tell you that if you deco on 80/20 and your EAN36 you will get you out of the water sooner.

Doppler,
What is the partial pressure effect? I have never heard that term used before. While I could understand the benefits of using pure O2 to eliminate N2 in the breathing media, I do believe that whatever get's you out of the water fast would be beneficial (which for many dives means using 80).

brandon

 

[Doppler]

voidware once bubbled...


Doppler,
What is the partial pressure effect? I have never heard that term used before. While I could understand the benefits of using pure O2 to eliminate N2 in the breathing media, I do believe that whatever get's you out of the water fast would be beneficial (which for many dives means using 80).

brandon

Hi Brandon: should have been "VACANT partial pressure effect... also known as Oxygen Window... sorry for the mis-type... gotta start sense-checking this stuff!!! Very, very simply put in the current context... why breathe something that contains 20% of the stuff you are trying to eliminate?