The Truth About Nitrox

[Bob]

I find myself in agreement with Rick again. The reason I use Nitrox is the extended BT plus the surface intervals between dives are reduced. All one has to do is plan two dives of equal depths, one on EAN, the other on compressed air. The difference will be right there in the numbers. The extended BT is one of the reasons I've gone to LP steel 104's.
:tree:Bob

 

[Warhammer]

Rick….I agree with you that nitrox does have benefits for shallow depths, especially for instructors that have multiple classes on the same day and don’t have time for much of a surface interval. But the recommended surface interval for diving nitrox is 1 hour. So if you took that recommendation to heart and followed it, you’d still have more than enough bottom time for your 49’ dive for 50 minutes. Going by the RDP, you’d have 59 minutes at 50’ for the second dive. But as you know, each diver is different, dives different profiles, and should choose the best “tool” (air, nitrox or whatever) for their dive plan.

Greg…..I’d say nobody in their right mind dives 36% nitrox to 130’. That would give you a partial pressure of 1.81, well above the recommended 1.4 and the contingency 1.6. What you can do though is dive say a 26% nitrox to that depth and still have a little margin for error.

 

[TexasMike]

Thanks for the compliment. And I will do my best to keep you (and the others) informed of the classes I take. I am hoping to start my SDI advanced coursework soon, which at the end will provide me with certs in 5 specialties and be further along toward actually calling myself an advanced diver.

Interesting comment there about your instructors. My next question would be "Why not?" Now that more and more blue water centers and live aboards are offering it, I beleive that I will almost always avail myself of the extended time that Nitrox will afford.

And as before to others, I would definitely recommend the TDI nitrox course. I checked http://www.tdisdi.com and found at least 5 different shops in the greater Houston area that offer SDI/TDI courses.

 

[TexasMike]

Greg:

What Warhammer says is accurate according to my tables.

But I wanted to take the opportunity to state for your (and the other folks) benefit that the more common blends of Nitrox is EAN32 and EAN36 (32% and 36% oxygen respectively), also known as NOAA Nitrox I, and NOAA Nitrox II.

These blends have been established both theoretically and practically by NOAA and many others to provide the most benefit with appropriately reduced risk. Also, becuase of these being so common, the folks that blend/fill the tanks have the O2 vs. air proportions down cold.

If you were to request a different mix, say 29%, they would have to consult their calcualtor or mix table.

Oh and a bit of trivia....you are, right now, breathing Nitrox. Regular old air can be described as EAN21 (21% O2). Interesting, eh?

 

[Rick Murchison]

While air can accurately be described as Nitrox (21%), there is no "EAN21" because a 21% mix ain't "Enriched."
A tiny point....
Rick

 

[Iguana Don]

"Also, becuase of these being so common, the folks that blend/fill the tanks have the O2 vs. air proportions down cold."

Mike,
This may be true if all you fill is standard 80 aluminum 3000 psi cylinders. But you add in to the equation LP (2650)
HP (3500) LP (2250) Luxfer's new (3300) and you have nothing down cold.

I am a TDI Certified Nitrox Blender and I never take anything for granted, I check the tables and revert back to the calculator on EVERY tank. I take this very serious, to many variables to have anything down cold.

ID

 

[greg somers]

Never a dull moment here....Meeiioowww.....
Cheers ears

 

[turnerjd]

I know this is slightly off subject, but where does the ppO2 of 1.6 max, and 1.4 for longer dives origionate from???

In terms of a CNS hit, a ppO2 of upto 2.1 is thought to be tolerable without significant* risk of a CNS hit. Someone somewhere obviously sat down and decided on the 1.4 / 1.6 figure. Who, and when? - was it the NOAA when they did their first manual? - what evidence did they use to choose this as a safe value rather than any other? How did they choose the margin of safety? - I don't know of any scientific study where they measured chances of CNS hits vs ppO2.

(this is just idle curiosity, none of the courses give the reason why 1.4/1.6 rather than 1.5/1.7 or any other range of figures that would still be relatively safe)

Jon T

* please note, by significant risk I mean > about 4-5% chance of a hit (per dive, not in a diving career) - about the same risk as the old US navy tables, and the UK RN tables.

 

[100days-a-year]

Lessee,a 1 in 20 chance of becoming unconscious while underwater and at depth.That'll do for a reason.Most of the"rules"when discovered by Darwin candidates.

 

[turnerjd]

Originally posted by 100days-a-year
Lessee,a 1 in 20 chance of becoming unconscious while underwater and at depth.That'll do for a reason.Most of the"rules"when discovered by Darwin candidates.

Great informed response!!!!!

Some-one somewhere came upwith the number 1.4 / 1.6 and I am certain it wasn't from a large number of people 'evolving' under darwinian mechanisms. These numbers represent some risk of CNS toxicity, as do heigher ppO2's (they just represent a heigher risk). Who chose them? and how did they choose the margin of safety?

A ppO2 of upto 2.1 has NO SIGNIFICANT risk of CNS oxygen toxicity - this is less than the risk of getting bent on the old US navy tables - plenty of people used these tables for quite some time, and some people still do (they were designed to give a bends incidence of 5% or less).

Someone somewhere chose the 1.4/1.6 value as being more conservative than 2.1 Who? and how did they choose it?
The french (FFESSM) tables are based on 1.9 as a ppO2, and people aren't keeling over dead using this value!

We all get taught 1.4 and 1.6 (unless you are french) and that this is for safety, but no-one tells you how this number was derived. I suspect there is NO SCIENTIFIC evidence that sais that reduction of ppO2 to 1.4/1.6 (from the ppO2 of 2.1 where you start to have problems) reduces the chances of a CNS hit from X% to 0.X%

Jon T