Nitrox instead of air for lower DCS risk?

[ArcticDiver]

At this point in our understanding of human physiology no one can say for certain what specific circumstances will cause DCS. We have some very good ideas; but no one Knows.

Is this important? Yes, because when that fact is combined with the very low rate of reported DCS the inescapeable inference is that the chances of decompression sickness in recreational scuba diving are very low.

Since nitrogen absorbtion by the body is one factor contributing to DCS, we think, it makes sense to reduce the nitrogen in our breathing gas. But, nothing is for free. When we reduce the nitrogen by increasing the oxygen we also increase the risk of CNS toxicity. So, that has to be monitored. That is done by using a specialized algorithm. Generally that is the Nitrox setting on your computer or Nitrox paper tables.

To me the lesson is: The risk of DCS(not DCI) is so low in the population at large it can nearly be ignored by the population at large. But since the population at large doesn't dive, individuals do, and the consequences of DCS on that individual can be catastrophic; each individual diver should do the literature research themselves. Then, with a thorough understanding of the theory, at least at the "black box" level make their own decisions about using nitrox, or any other breathing gas.

In other words: Don't blindly trust the "experts". It is your body and your family that is at risk, however small that risk may be.

 

[carlthecat]

Ok. Here is my 2¢.

If you are planning a dive with air dive tables and using nitrox, you are adding a degree of safety (according to Hal Watts Adv Nitrox Manual) as long as you make sure your depth is NOT beyond the MOD for the EAN mix. This part is fact. This idea is guided toward technical diving since DCS in recreational diving is very, very rare (see next paragraph about PADI). Think about it... you make 2 decompression dives. One on strictly air and the other on air with EAN 50 as the deco mix. The air + EAN 50 combo will greatly decrease your decompression time (see dive profiles below - 70 minutes deco air vs 44 minutes deco air w/ EAN 50). Why is that? Less nitrogen to off gas because the EAN you are breathing during decompression has less nitrogen... wow, what a concept. Although EAN reduces the problems with nitrogen (narcosis and DCS) it adds the problem of Oxygen Toxicity which in my opinion and the opinion of others is more dangerous than the dangers of nitrogen. DCS you are most likely going to make it to the decompression chamber and recover from being bent. OxTox will shake your regulator out and drown you at depth. On the plus side, EAN is know as the “geeser” gas because it is energizing to older and out of shape people.

Now PADI says there isn’t a meaningful increase is safety regarding DCI between air and nitrox because the occurrences of DCI using air recreationally is so low. Here is a quote from the Enriched Air Diving manual:

"Safety Advantage? Because you absorb less nitrogen using enriched air, you might expect that using enriched air within normal air no decompression limits would substantially improve your safety. Interestingly, the decompression illness (DCI) incidence rate is already so low in recreational diving that simply reducing nitrogen is unlikely to produce a meaningful safety improvement. Although there's been no experimental study of this, statistical estimates suggest that using enriched air within normal air limits only reduces the incidence rate a fraction of a percent" (Enriched Air Diving, PADI Manual, 2004).

My disclaimer is I do not know everything. In fact, I realize I know nothing. However, I like to think I at least have a clue on dive physics.

Hope this helps and clarifies at least something,

Carl the Cat


Diving Profiles

DIVE PAN 1
V-Planner 3.72 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 180ft (3) Air 50ft/min descent.
Level 180ft 20:24 (24) Air 1.35 ppO2, 180ft ead
Asc to 110ft (28) Air -15ft/min ascent.
Stop at 110ft 0:20 (29) Air 0.91 ppO2, 110ft ead
Stop at 100ft 1:00 (30) Air 0.85 ppO2, 100ft ead
Stop at 90ft 2:00 (32) Air 0.78 ppO2, 90ft ead
Stop at 80ft 3:00 (35) Air 0.72 ppO2, 80ft ead
Stop at 70ft 3:00 (38) Air 0.65 ppO2, 70ft ead
Stop at 60ft 4:00 (42) Air 0.59 ppO2, 60ft ead
Stop at 50ft 5:00 (47) Air 0.53 ppO2, 50ft ead
Stop at 40ft 8:00 (55) Air 0.46 ppO2, 40ft ead
Stop at 30ft 9:00 (64) Air 0.40 ppO2, 30ft ead
Stop at 20ft 13:00 (77) Air 0.34 ppO2, 20ft ead
Stop at 10ft 20:00 (97) Air 0.27 ppO2, 10ft ead
Surface (97) Air -15ft/min ascent.

Off gassing starts at 125.1ft

OTU's this dive: 47
CNS Total: 18.7%

DIVE PAN 2
V-Planner 3.72 by R. Hemingway, VPM code by Erik C. Baker.
Decompression model: VPM
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 180ft (3) Air 50ft/min descent.
Level 180ft 20:24 (24) Air 1.35 ppO2, 180ft ead
Asc to 110ft (28) Air -15ft/min ascent.
Stop at 110ft 0:20 (29) Air 0.91 ppO2, 110ft ead
Stop at 100ft 1:00 (30) Air 0.85 ppO2, 100ft ead
Stop at 90ft 1:00 (31) Air 0.78 ppO2, 90ft ead
Stop at 80ft 3:00 (34) Air 0.72 ppO2, 80ft ead
Stop at 70ft 3:00 (37) Air 0.65 ppO2, 70ft ead
Stop at 60ft 3:00 (40) Nitrox 50 1.41 ppO2, 26ft ead
Stop at 50ft 3:00 (43) Nitrox 50 1.26 ppO2, 20ft ead
Stop at 40ft 4:00 (47) Nitrox 50 1.10 ppO2, 13ft ead
Stop at 30ft 5:00 (52) Nitrox 50 0.95 ppO2, 7ft ead
Stop at 20ft 8:00 (60) Nitrox 50 0.80 ppO2, 1ft ead
Stop at 10ft 12:00 (72) Nitrox 50 0.65 ppO2, 0ft ead
Surface (72) Nitrox 50 -15ft/min ascent.

Off gassing starts at 125.1ft

OTU's this dive: 73
CNS Total: 27.4%

 

[JeffG]

The air + EAN 50 combo will greatly decrease your decompression time (see dive profiles below - 70 minutes deco air vs 44 minutes deco air w/ EAN 50).

Yes

Why is that? Less nitrogen to off gas because the EAN you are breathing during decompression has less nitrogen... wow, what a concept.

No

 

[Lehmann108]

Use the dive algorithm that matches the breathing gas you have selected. If it is Air, use the Air Table/Algorithm. If it is Nitrox, use a Nitrox table or set your computer to the correct nitrox percentage. Failure to do this results in not tracking all the pertinent factors that can affect your safety and health.

.

I'm just a newbie here, but what you say makes much more sense than creating a false margin of safety by diving nitox using the air tables. Its like somebody setting their clocks 5 minutes ahead so they won't be late. Just use the right time and leave 5 minutes earlier!

Of course the metaphor breaks down because using the air tables does not take O2 loading into account when diving nitrox.

 

[Lehmann108]

oppps didn't see how old this thread was!!:shakehead

 

[ArcticDiver]

oppps didn't see how old this thread was!!:shakehead

Actually I hereby award you the Silver Star for bravely looking at the mass of information on SB and thereby finding your answer! :11star:

 

[cowdog77]

"As with understanding the EAN tables, divers should be aware that those tables were cut using data from young, healthy in-shape divers..."
_________________________
Good point~

 

[Glycereine]

I know this is an absurdly old thread, but I think what arctic is trying to say that a lot of people are missing is that the MOD in itself does not prevent oxygen toxicity. There is also an oxygen exposure limit associated with how much oxygen your body has been taking in (partial pressures for a given time) over the course of several dives.

The majority of nitrox users do not calculate this on their own, but use a computer to tell them if their "oxygen loading" (to use a term that shows the similarities between excess nitrogen) has reached a critical level. If a person dives nitrox habitually, but uses air tables or an air computer, there will be no monitoring of oxygen exposure.

Sure a person can still calculate their oxygen exposure before and after each dive based on the dives they have done in the last 24 hours (I believe it's still 24 hours being taught in nitrox classes), but honestly how many people who are using air tables are going to do that?

Yes diving nitrox on an air table for a given dive reduces your exposure to nitrogen, but there are tradeoffs. You cannot simply dive nitrox and pretend it's air. To responsibly account for oxygen toxicity possibilities, it is generally a good idea to put your computer in nitrox mode (or use a nitrox RDP). If you want the added benefit of a safety margin, don't dive to the limits of the table or computer, but by putting the computer in the correct mode for the gas you are diving you will be certain you aren't inadvertantly missing something equally as important and that the data you are seeing on your computer is in fact correct.

I personally dive with a computer, but also look up table limits for a depth I am about to dive to and make sure (in the event my computer fails) I stay within table limits as well. Is that overly cautious? Perhaps, but I feel safer doing it that way and the couple times my computer has been stuck in gauge mode I was really glad I did it. Along the same lines, you can dive with your computer in nitrox mode for the blend you are using, but look at an air RDP prior to your dive and base your dive around those values for extra conservatism, or dive with an additional computer on air. In the case you dive with a computer on air and one on nitrox just be sure that if your nitrox computer is yelling at you, you don't ignore it and switch to your air one because there's probably a reason.


To all the people who wished this thread had disappeared I apologize but I felt like (by reading the responses) there was still quite a bit left misunderstood and that newer divers to the forums might gain some benefit from reading this as well.

Please let me know if anything I said was incorrect or you have questions about it.

 

[dschonbrun]

For DCS discussions, focus on the Partial Pressure of Nitrogen.

If you dive for 20 minutes at Depth of X meters on a Nitrox gas with a PN2 of 2.4, that has an effect. If you dive for 20 minutes at Depth of X meters on a different mixture of Nitrox gas with a PN2 Less than 2.4... say 2.0... then all things being equal, the nitrogen saturation of a given compartment will be less in the second case... resulting in a lower probability of complications during off-gassing.

 

[Rolodive]

Let's not forget the safety stop. Nitrox will alleviate micro-bubbles faster than air, and release nitrogen from tissues safely at a higher rate than air. Regardless of your dive profile, your safety stop will have a huge impact on "silent" bubble removal, and the safety stop on Nitrox is clearly much more effective.