50% Nitrox for deco-unsafe?

[Zaixon]

I was about to say I do believe I usually switch to 50% at 70ft...

 

[RJB2014]

Thank you all for your comments. Thats what I thought but figured I'd check. I appreciate the help.

 

[Scared Silly]

There is the right way, the wrong way, and then there is the military way.

 

[nadwidny]

There is the right way, the wrong way, and then there is the military way.

Which needlessly killed 2 of their own back in Feb/2013.

 

[Scubagolf]

Mark Powell, in Deco for Divers at p. 127-129, discusses the use of EAN 50 as a deco gas at 50 FSW with approval. It is worthwhile to note that the MOD for EAN 50 at a PPO2 of 1.4 is only 59', and at 1.6 is 72'.

 

[bada3003]

The part about oxygen toxicity sounds flaky but the point about the benefit of 50% O2 may have some validity depending on one's dive profile.

If you dive below 150', then switching to 50% O2 at 70' reduces your deco obligation by about 2-4 min (specifics depend on bottom time and exact dive profile). The ballpark numbers are similar whether one uses VPM-B or B\"uhlmann with GF. If you blend your bottom gas to match the depth you're diving (e.g., around 0.26 O2 at 150' for fresh water) then your deco obligation is reduced significantly. This is a simple consequence of what engineers call Amdahl's law.

Anyhow, given the hassle of maintaining O2 clean deco bottles and driving to an LDS that handles non-recreational nitrox (such shops don't abound in the midwest), it's must easier to blend an optimal nitrox mix for one's bottom gas using pre-blended nitrox 32/36 at home. By reducing nitrogen on-gassing to begin with, it significantly reduces deco obligation and also the likelihood of DCS.

Not sure your retired Navy guy meant that, but I wouldn't disparage him as easily as some have. Perhaps those who are themselves retired sailors (such as Jim Wyatt) excluded.

 

[PfcAJ]

If you dive below 150', then switching to 50% O2 at 70' reduces your deco obligation by about 2-4 min (specifics depend on bottom time and exact dive profile).

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.

The real benefit of 50% on deep (sub ~130ft) dives is that you have a gas to switch to sooner rather than later. This means more bottom gas that can be used on the bottom rather than gas that needs to be reserved for the ascent portion of the dive.

 

[uwxplorer]

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.

The real benefit of 50% on deep (sub ~130ft) dives is that you have a gas to switch to sooner rather than later. This means more bottom gas that can be used on the bottom rather than gas that needs to be reserved for the ascent portion of the dive.

I have been taught to use a 60% mix for deco (no O2, switch at 50 ft), the rationale being that you get off He earlier than if you were using a O2 switch at 20 ft. The only problem with that is that you need a larger amount of 60% mix than you would need if you used a single switch to pure O2. As mentioned, the deco obligations are usually similar.

 

[PfcAJ]

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.

 

[uwxplorer]

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.

I am in no position to tell whether this is a real issue or not, just mentioning this statement as a possible justification of using a lean mix for deco.
As for why not 50%, it depends on your BT and depth, since, as I reminded the reader, the deeper you switch to your deco mix, the more of it you will need (it might make the difference between being fine with an AL40 of 60% and needing a larger cylinder of 50%).
Then there is all possible kinds of variations if you carry an O2 bottle (or have one waiting for you at 20 ft)...