Nitrox vs. Regular Air?

[Blackwood]

Its OK if you find your dives are being limited by no stop times rather than air consumption (be that a single dive or a series) - THEN you'd benefit from Nitrox.

Otherwise there's no point unless you get it for the same price as air.

Given the above, what's the point if it is the same price as air?

 

[Blackwood]

As that is a fundamental principle (although not necessarily the only fundamental principle) behind the tables and computer algorithms, the position that nitrox does not reduce the likelihood of DCS is untenable.

I've made much the same argument before.

Given a constant profile:
IF nitrox extends NDL, and
IF diving within NDL provides a safety margin over diving to the limit, and
IF the further one stays from the limit the greater the safety margin is,
THEN nitrox provides a safety margin over air.

No empirical proof is necessary, it's true by definition (assuming the IF statements are true).

 

[MikeFerrara]

Actually, you do not off gas any quicker. Your ending Nitrogen load is lower with a dive at equal depth. If you remember from your class you can calculate what your EAD is (Enhanced Air Depth) and use the standard dive tables.

In other words, if you where diving EAN36 to a depth of 70 feet, you nitrogen load is equivalent to diving air (21% O2) to 50 feet.

Not true.

When you use EAD to dive air tables when using nitrox you aren't given credit for the faster offgassing during ascent and/or stops but you do, in fact, offgas faster...which is why we can use EAN mixes and even pure O2 to accelerate decompression on dives that require staged decompression. For a detailed illustration take a look at any accelerated decompression table or play with some decompression software.

At the risk of being overly blunt, I sure wish the people teaching this stuff would teach the truth about it.

 

[MikeFerrara]

I've made much the same argument before.

Given a constant profile:
IF nitrox extends NDL, and
IF diving within NDL provides a safety margin over diving to the limit, and
IF the further one stays from the limit the greater the safety margin is,
THEN nitrox provides a safety margin over air.

No empirical proof is necessary, it's true by definition (assuming the IF statements are true).

All else being equal, less N2 does mean a lower risk of DCS. But, when it's looked at statisically a dive within NDL's is such low risk that the a little lower doesn't matter and probably disappears in the noise because all else is never exactly equal.

Another "but" here...when we dive we don't really care about statistics do we? We're dealing with a population of one. What's required here is a little common sense. When I dive the local 25 ft deep quarry or the local 15 ft deep river, I don't go out of my way to get nitrox bucause DCS is almost an impossibility.

That all said, I think some of the divers who would benefit the most aren't using nitrox. Those being the middle aged and older, maybe too heavy and out of shape divers, who have moderate to poor buoyancy control skills out doing four or five dives/day at a resort. They have EVERYTHING working against them...lots of dives and lousy profiles and a less than up to the task body.

I really think the agencies spewing their "EAN isn't safer" nonsense is doing these folks a disservice and I think there are some resort locations with pretty busy chambers to prove it.

 

[Zoomer]

Not true.

When you use EAD to dive air tables when using nitrox you aren't given credit for the faster offgassing during ascent and/or stops but you do, in fact, offgas faster...which is why we can use EAN mixes and even pure O2 to accelerate decompression on dives that require staged decompression. For a detailed illustration take a look at any accelerated decompression table or play with some decompression software.

At the risk of being overly blunt, I sure wish the people teaching this stuff would teach the truth about it.

I don't understand Mike.

 

[Blackwood]

Not true.

When you use EAD to dive air tables when using nitrox you aren't given credit for the faster offgassing during ascent and/or stops but you do, in fact, offgas faster...which is why we can use EAN mixes and even pure O2 to accelerate decompression on dives that require staged decompression.

Gradient schmadient

When you dive EAN on EAN tables (at least those I've seen) you aren't given credit either.

 

[ClayJar]

I don't understand Mike.

If you just look at a 15' "safety stop", assuming you make the stop at the same 15' depth on air or nitrox, and assuming you dove "equivalent" profiles such that your nitrogen loading was identical, you would offgas faster on nitrox. (If you did your safety stop at an EAD of 15', you would offgas at the same rate as an air diver at 15', but I've never seen someone do an EAD safety stop.)

Assuming no gases but oxygen and nitrogen, 32% O₂ nitrox gives you a ppN₂ of 0.99 atm at 15 fsw. 21% O₂ air gives you a ppN₂ of 1.15 atm at 15 fsw. The "pressure" gradient between your nitrogen loading and the nitrogen in the gas you're breathing is greater on nitrox, so you will indeed offgas faster.

 

[The Kraken]

Interesting observation, ClayJar . . .

. . . so, should a diver make his 15' safety stop at the EAD equivalent of fifteen feet based upon the gas being used?

IE: Should a diver breathing EAN36 make his safety stop at 27' ??

(PS: I already know the answer, but I just thought it'd be fun to ask)

the K

 

[MikeFerrara]

I think ClayJar's explaination nailed it but if someone still isn't seeing it say so and I'll try.

 

[ClayJar]

Interesting observation, ClayJar . . .

. . . so, should a diver make his 15' safety stop at the EAD equivalent of fifteen feet based upon the gas being used?

IE: Should a diver breathing EAN36 make his safety stop at 27' ??

No. That EAN36 diver would offgas N₂ at the same rate as an equivalent air diver at the air diver's 15' stop, but stops are not set at their particular depths due to the rate of offgassing. Rather, stop depths are set based on the diver's nitrogen loading. A stop must be deep enough that the ambient pressure is sufficient to keep nitrogen in solution (dissolved gas models, such as Haldane-derived tables), to hold any bubbles to sufficiently small size ("bubble methods"), or some combination of the two.

The general assumption with recreational diving is that the ongassed nitrogen is limited to a level such that a normal, slow ascent (generally 30 ft/min) directly to a 15' (or so) stop stays within acceptable levels (for dissolved gas pressures, bubble sizes, or what have you -- NAUI tables and NAUI RGBM tables use different methods, for example, but that primary assumption holds). Diving with a richer mix does not change those assumptions, and so, the ascent rate and safety stop remain unchanged, and the reduced ppN₂ in the nitrox diver's breathing gas allows for more rapid offgassing at the safety stop.

(PS: I already know the answer, but I just thought it'd be fun to ask)

Is this on the test? :biggrin: