Nitrox - 1.40 or 1.60 PO2?

[Kern]

The major reason to add helium is to reduce narcosis.

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If I'm going no deeper than 50/60mts I'd agree. Once I'm going sub 60mts I'd be just as, or more concerned with lowering my ppO2 to an acceptable level.

 

[lamont]

If I'm going no deeper than 50/60mts I'd agree. Once I'm going sub 60mts I'd be just as, or more concerned with lowering my ppO2 to an acceptable level.

You could always use hypoxic oxygen-nitrogen mixtures -- instead of adding pure He to cut the O2 you could add pure N2 (or use a membrane system to increase the N2 instead of decreasing it, etc). You don't require helium to cut the O2.

 

[Kern]

You could always use hypoxic oxygen-nitrogen mixtures -- instead of adding pure He to cut the O2 you could add pure N2 (or use a membrane system to increase the N2 instead of decreasing it, etc). You don't require helium to cut the O2.

Quite right, I could. The next time I want to go hunting those pesky mermaids, that's just what I'll do.

 

[deepstops]

You could always use hypoxic oxygen-nitrogen mixtures -- instead of adding pure He to cut the O2 you could add pure N2 (or use a membrane system to increase the N2 instead of decreasing it, etc). You don't require helium to cut the O2.

Um, well yeah, you could do that. But it wouldn't be particularly intelligent nor have I ever encountered a shop or diver adding N2 to cut the O2 content.

At 50-60 meters, air would be "okay" but obviously not the optimal gas.

 

[netmage]

The major reason to add helium is to reduce narcosis.

And while helium is faster at ongassing, it is also faster at offgassing, so on paper it shouldn't matter if you're using helium or nitrogen from a deco standpoint.

There's a bunch of different opinions on the use of helium, some believe that helium is a bad deco gas, some believe that helium is a good deco gas, some believe that you want to keep them balanced. I've seen some anecdotal evidence that divers on 21% have gotten bent alongside divers on 21/35, which inclines me towards the "balanced is better" or "helium is good" viewpoints. I've also heard that helium may be worse if you don't do your deepstops or have an explosive decompression accident, and don't discount that. Scientific evidence in this area seems to be sorely lacking.

Helium is your friend... It addresses both nitrogen narcosis and work of breathing at depth. Because it is on-gassing faster, you will have shorter NDL times. And because it is also faster on the way out, you will need to pay particular attention to you ability to hold required stops.

 

[deepstops]

Helium is your friend...

The guy that lives down the street from me started that idea. He's usually right.

 

[DoogaDiver]

I'm not going to tell anyone how to dive or what limits to use, but . . .

What once was set forth as a suggestion based upon an increase in oxygen toxicity, for some reason that escapes me, has now become some sort of a scubadiving quasi-law.

When ox-tox studies were done, it was shown that the possibility of incurring oxygen toxicity increased as one exceeded 1.6 PPO2.

So, agencies suggest that to stay a little safer, plan the dives based upon a PPO2 of 1.4.

So now it seems that some think 1.4 is the barrier so they're utilizing an even lesser PPO2.

I suggest you do some in depth reading of the subject matter and come to your own conclusion.

the K

This reminds me of the old Army adage, "Early is on time and on time is late." This mentality generally results in people showing up for formations, meetings, and such 20-30 minutes out of fear of "being late".

 

[Rhone Man]

This reminds me of the old Army adage, "Early is on time and on time is late." This mentality generally results in people showing up for formations, meetings, and such 20-30 minutes out of fear of "being late".

True, except that obviously there is a difference between fear of "being late" and fear of "being dead".

 

[henno777]

Plan the dive to 1.4 or less. Know what depth 1.6 is in case there is a compelling reason to violate the plan. I leave it to you to decide just what "compelling" means.

Mark Vlahos

Agree- the PO2 between 1.4 and 1.6 is only for contingency planning purposes accourding to most nitrox guidelines. The cowboys (technical guys) will probably give you a different answer. Refer to those forums as well.

 

[HowardE]

Look at the CNS Clock Table. The maximum EXPOSURE to 1.6 PO2 for a 24 hour period is 45 minutes. This is the risk of diving at 1.6. At 1.4 the max time is 150 minutes (or 2.5 hours)

A diver having made the first enriched air nitrox dive of the day would find the CNS
exposure percentage on the table. By subtracting that number from 100% they could
determine the maximum exposure for their next dive. Following the second dive, the
CNS exposure percentages from each dive would be added, then subtracted from
100% to determine the maximum allowable exposure for a third dive. While many
authorities suggest that a 90 minute half-time is applicable for surface interval credit
when considering oxygen exposures, such credit is seldom taken into account for
planning recreational dives

For recreational diving with repetitive dives planned, it's best to keep the PPO2 below 1.4 so that you have the time. Decrease your PO2 to 1.3 - and it's 180 minutes (at that PO2). Go below that, and you're approaching 4 hours. Breaching the 1.6 "barrier" is not a ticket to do the chicken dance.

Ox Tox is a function of EXPOSURE (PO2) + TIME.

Certainly - for planning purposes for recreational, it is prudent to keep the main portion of your dive with a max PO2 of 1.4. This provides a good safety margin