50% Nitrox for deco-unsafe?

[tomfcrist]

By that logic, using 50% at 70ft would get you off sooner rather than 60% at 50ft.

Fwiw, I'm not convinced that there's a need to "get off" helium at all.

My opinion is similar to yours AJ. The effects of isobaric counter diffusion outweigh getting off helium quickly. Now if you are using light trimix or any mix where o2+He equal less than 50%...maybe EAN 50 would be a sensible switch at 70'.

 

[boulderjohn]

By that logic, using 50% at 70ft would get you off sooner rather than 60% at 50ft.

Fwiw, I'm not convinced that there's a need to "get off" helium at all.

Have you priced it recently? The more you can leave in your tank for the next dive, the better.

 

[bada3003]

Compared to what? Compared to no deco gas at all, even 20mins at 150' (21/35 bottom gas) sees a reduction in total deco time of about 20mins by adding 50%. Compared to just using oxygen at 20ft (same 1.6po2), then yeah there's only a few minutes difference.

Compared to using air as bottom gas. Here's a specific example from a recent dive profile. You can use V-planner with conservatism +3 to verify: depth 145', bottom time 20 min.

plan 1: bottom/travel gas air, deco gas 50% nitrox; total run time 44 min, gas switch at 70'.
plan 2: single gas, 27% nitrox; total run time 47 min

 

[PfcAJ]

a ppo2 of 1.5 at depth is a tad richer than I'd sign up for.

Anyways, I've got Deco Planner on +2 and its 6mins shorter with air (ew, gross) and 50% than 27%. I'm sure you could fiddle with some settings and get them to be even closer, but what's the point? I'd rather take the deco gas and have MORE bottom gas to play with.

 

[tomfcrist]

By that logic, using 50% at 70ft would get you off sooner rather than 60% at 50ft.

Fwiw, I'm not convinced that there's a need to "get off" helium at all.

Have you priced it recently? The more you can leave in your tank for the next dive, the better.

If the price of helium is causing you to create a dive plan that ensures you "get off helium quickly", you shouldn't be diving mixed gasses.

And FWIW, yes helium is expensive these days...but a crafty individual can buy helium direct from a supplier for less than $.85cuft in my neck of the woods, and I have seen it cheap as hell in Texas.

 

[bada3003]

a ppo2 of 1.5 at depth is a tad richer than I'd sign up for.

For this dive profile, the gas was 26.5 nitrox (gives a tad less than 1.4 PPO2, fresh water)
which I round up in practice for conservatism. For our discussion, I should have used 26.5
which increases the run time by 1 min.

Anyways, I've got Deco Planner on +2 and its 6mins shorter with air (ew, gross) and 50% than 27%. I'm sure you could fiddle with some settings and get them to be even closer, but what's the point? I'd rather take the deco gas and have MORE bottom gas to play with.

Indeed. About the only time I use a deco bottle is when I need more bottom time.
But typically I use two LP85 cylinders (sidemount) which holds enough gas for the
40-50 run time dives I plan in Lake Michigan. No gas switching, better balance
(an AL40 cylinder with 1st stage adds 3+ pounds to one side), etc., in my case,
make use of a deco bottle rare.

Anyhow, just responding to the rather one-sided comments on the OP's question
prompted by the retired sailor's input when the answers can be less straightforward.

 

[r4e]

Returning back to original question of:

He then started telling me ... that using 50% below 50 feet was foolish because it raises the risk of Oxtox with little benefit in offgassing... everything I've been taught and read (TDI, GUE, PADI) generally supports O2 mixes up to pp1.6 for nonactive, deco portion of dives... Can anyone comment on the validity of his assertion that the increased risk of using 50% below 50 feet isn't justified by the benefit of increased offgassing?

It is an accepted practise to have O2 in bottom gases with ppO2 of 1.2 or 1.3. This limit has been chosen to keep risk of oxtox during the active portion of the dive reasonably low. Likewise during decompression it is assumed that ppO2 up to 1.6 is reasonably safe *assuming* that this during rest!

So why would a leap of ppO2 from 1.2/1.3 to 1.6 be "safe" at exactly 70ft/21 meters? Depending on your dive you still might be less than restful - so there might be a slightly elevated risk of oxtox due to higher ppO2 during a still relatively active phase of the dive.

Here I am thinking about cave dives of e.g. 40 minutes of bottom time at 150 ft/45 meters, i.e. a lot of swimming instead of just hanging around a wreck. Consequently even with obligatory deep stops, you will ascend to 70ft/21 meters within minutes of ending the active swimming part of the dive. For deeper dives the ean50 itself would cause less oxtox risk elevation, because your body has had more time to relax and adjust. ICD of course is a separate risk.

Personally, I prefer to switch to ean50 at 60ft/18m after the remainder of the team has switched at 70ft/21m. This speeds up things and I can enjoy the slightly lessened oxtox risk. Within 1-2 minutes we'll anyhow ascend to 50ft/15m.

I do switch to ean100 at 20ft/6m, i.e. ppO2=1.6. But conditions and deco ceiling allowing, I prefer to ascend gradually to 10ft/3m as soon as possible and remain there for the rest of the decompression. Some may think that ean100 would be most efficient at 20ft/6m. But, my assumption is that is the *absence* of helium and nitrogen that makes ean100 an effective decompression gas, i.e. NOT the "magic healing powers" of high ppO2. So I am optimizing decompression and minimizing the drawbacks of oxygen (risk of oxtox and oxygen caused damage to tissues).

 

[Agility]

I do switch to ean100 at 20ft/6m, i.e. ppO2=1.6. But conditions and deco ceiling allowing, I prefer to ascend gradually to 10ft/3m as soon as possible and remain there for the rest of the decompression. Some may think that ean100 would be most efficient at 20ft/6m. But, my assumption is that is the *absence* of helium and nitrogen that makes ean100 an effective decompression gas, i.e. NOT the "magic healing powers" of high ppO2. So I am optimizing decompression and minimizing the drawbacks of oxygen (risk of oxtox and oxygen caused damage to tissues).

Maybe the difference between 1.6 and 1.3 bar is not significant, but higher ambient pressure keeps the bubbles smaller :blinking:

 

[Capt Jim Wyatt]

Fwiw, I'm not convinced that there's a need to "get off" helium at all.

Back when we first started Helium diving in the 1980's recreationally we had no way to analyze the contents of a helium mixture. We filled the tank to a certain pressure and based on that we knew the "percentage" of the total filled pressure.

Helium tables were new and by and large untested.

For these two reasons we always wanted to get off the Helium as soon as possible..the two sort of 'unknowns" or perhaps "least knowns".

Nowadays with Helium analyzers & proven algorithms / tables I see no need to get off the Helium so soon either. Some even advocate having a small amount of Helium in deco mixes to slow the Helium offgassing down just a bit.

Especially doing a dive on 14/50 & feeling the need to switch to EAN32 at 130 feet --that switch is not a good idea, but alas it is still being taught by some of the dinosaurs.

 

[boulderjohn]

If the price of helium is causing you to create a dive plan that ensures you "get off helium quickly", you shouldn't be diving mixed gasses.

And FWIW, yes helium is expensive these days...but a crafty individual can buy helium direct from a supplier for less than $.85cuft in my neck of the woods, and I have seen it cheap as hell in Texas.

Guess I should have found a way to make the smiling face more prominent.