Do you think Nitrox is a deep diving gas?

[DandyDon]

[c]:lol:[/c]

 

[fppf]

The question is, "Is NITROX a deep gas?" I say no ... but then, for me, deep starts at 190.

Then go over to the deco stop, or at least in the technical forum. When the majority of people here say "deep" they mean within recreational limits.

 

[Crowley]

Nitrox certified? Hah! I had to make the two Nitrox dives when I got my card and my Instructor told me not to descend below 70 ft in Santa Rosa's Blue Hole since that was the altitude adjusted MOD for the mix. :silly: I tried to explain that altitude would not affect the MOD, but he wouldn't listen to this student - so I made my two 65 ft dives and got my card. Found a better Inst.

If the Maximum Operating (or Oxygen) Depth of a gas is based on its partial pressure (absolute) then how can diving at altitude NOT affect the MOD of a particular blend?

Taking air for example - ppO2 of .21 ata at the surface - at 56.67 metres the absolute pressure is 6.67 ata, 6.67 x 0.21 = ppO2 of 1.4 ata

If we're diving at altitude where the atmospheric pressure is - let's just say 0.8 ata then at 56.67 metres the absolute pressure is therefore 6.47 ata, therefore the ppO2 at that depth would be 1.36 ata. So in terms of O2 content, diving at altitude does indeed affect your MOD.

(Note: the definition of MOD varies - I'm using the 1.4 ata limit as a "standard")

Definition of "Nitrox" = any gas mixture of oxygen and Nitrogen
Definition of "Enriched Air Nitrox" = a mixture of oxygen and nitrogen with an oxygen content greater that regular air.

Just for a reference, let's define "deep diving" as the recreational limit of 40m
Q: Is Nitrox a deep diving gas?
A1: Enriched air Nitrox is most certainly not - it significantly reduces your maxiumim operating depth as compared to air - for example the MOD for air (assuming a recreational limit of 1.4 ata partial pressure of O2) is the aforementioned 56.67 metres. A 32% mix gives you an MOD of about 33 metres, off the top of my head.
A2: Well, technically, "hypoxic" - i.e. lower percentages of O2 than 21% - nitrox is required for deep diving. If 21% is toxic at 56 metres, you need less than 21% to go deeper. 6% nitrox would suffocate you at the surface, but would be more than plenty down at 90 metres. This is, however, impractical for the reason below:
A3: Nitrogen and Oxygen are both narcotic. This is partly why Helium is added to deep diving gases to counter the effects of the narcosis - as well as extend NDLs. Helium has it's own problems at depth, although even the deepest of deep divers are unlikely to encounter HPNS (High Pressure Nervous Syndrome) during their careers. There are also 'off-gassing' issues just as with Nitrogen.

Q: Does Enriched Air Nitrox reduce incidences of DCS?
A: Well, theoretically it should reduce your predisposition to DCS since you are inhaling less Nitrogen. Statistics shmatistics - people get bent. If you dive within air limits on Enriched air then in theory you are reducing the risk of DCS - but decompression theory is exactly that - a theory. I can give you circumstantial evidence in the resort area where I work that regular use of Enriched air does indeed reduce the incidence of DCS amongst the staff here - "pub chat", if you will, but no hard and fast evidence.

Q: Does EANx make you feel better after a dive?
A: Well, there seems to be some circumstantial evidence that this is true. Older people and those divers who are in less than tip top condition often report this. I used to say the same thing, however now I dive every day I notice very little (if any) difference. Lots of people say it, and I have to go with the opinion that they can't *all* be wrong.

Q: Are recreational EANx courses a marketing gimmick?
A: Are free toys with Big Macs a marketing gimmick? Are cup holders a gimmick for that SUV you want to buy (who cares about the engine or the wheels!? Must have cup holders!!). But - it's a marketing gimmick that has some definite advantages (however minor they are perceived to be) with minimal drawbacks (as long as one does not push oxygen limits)

Now - let's all get some popcorn, sit back and watch this thing go round and round and round again!

Happy gassing,

C.

 

[wedivebc]

A3: Nitrogen and Oxygen are both narcotic. This is partly why Helium is added to deep diving gases to counter the effects of the narcosis - as well as extend NDLs. .

I am not aware of helium extending NDLs. What tables are you using?

 

[CompuDude]

Just for a reference, let's define "deep diving" as the recreational limit of 40m
Q: Is Nitrox a deep diving gas?
A1: Enriched air Nitrox is most certainly not - it significantly reduces your maxiumim operating depth - for example the MOD for air (assuming a recreational limit of 1.4 ata partial pressure of O2) is the aforementioned 56.67 metres. A 32% mix gives you an MOD of about 33 metres, off the top of my head.

While an impressive effort, you are incorrect on this... unless you assume there is no EAN other than 32% (and richer mixes).

A leaner mix ... but still > air, and thus properly called "Nitrox" by any definition ... of EAN28 delivers extended bottom times at the max recreational limit of 40m (131'), making it an excellent candidate gas to use, and arguably a better choice than air.

I'll use 130' since that's the convention here in the US, and aligns nicely with the tables.

MOD of EAN28 is ~131' (at PPO2 of 1.4, as in your assumption)... just like the recreational depth limit. So no change there.

EAD of EAN28 @ 130' is 120', so per PADI's tables, your time limit at 130' is 13 minutes. On air, the limit at 130' is 10 minutes. While 3 minutes doesn't seem like a huge difference, it's actually a 30% increase in bottom time. On a "DEEP" dive.

This discusses a square profile dive, on tables. On a computer, if the dive is anything other than perfectly square, it's likely that the EAN28 would grant an even further extended BT (more than 3 minutes) compared to air.

 

[fppf]

Again, this just proves my point, and that's from a "Master Instructor"

 

[Crowley]

to answer the points above:

webdivecc: by Helium "extending NDLs" - of course I mean that by lowering our nitrogen intake by adding helium to the mix, we are lessening our exposure to N2 and thefore extending our "no decompression limits" - a term which is, I appreciate, not relevant to technical diving, however one that more people would understand.

CompuDude: I didn't say anything about extending bottom time, I was implying that the greater the percentage of 02 in the mix, the shallower your depth limits as opposed to air, where O2 becomes the limiting factor over N2. Of course you are correct, but my point is not to suggest that nitrox has no benefit at all, just to clear the common misconception that the words "Nitrox" and "Deep" should somehow be specifically related. I use 32% as an example because that is by far the most common mix used in the recreational dive industry.

fppf: Not sure if your post is meant as an insult or not!? I'd appreciate you taking the time to repsond (in PM if you prefer) as to your meaning? The subject is complicated but I think I've given a generally accurate overview of the subject for the broader spectrum of forum users.

I've run out of popcorn... pass it over,

Cheers,

C.

 

[CompuDude]

CompuDude: I didn't say anything about extending bottom time, I was implying that the greater the percentage of 02 in the mix, the shallower your depth limits as opposed to air, where O2 becomes the limiting factor over N2. Of course you are correct, but my point is not to suggest that nitrox has no benefit at all, just to clear the common misconception that the words "Nitrox" and "Deep" should somehow be specifically related. I use 32% as an example because that is by far the most common mix used in the recreational dive industry.

It's the "most common" mix, but other mixes are also common, especially depending on where you live. (I've read that 30% is the standard mix in parts of Jersey, for instance, where the deeper wrecks make 32% impractical).

What I took exception to is first you defined "deep" as 130' and as the recreational max, (which I'd agree with) but then you said Nitrox "it significantly reduces your maxiumim operating depth" ... whereas with 28% (a not uncommon mix for divers who need to go deeper than 110'), your MOD and your recreational limit are one and the same, so it's not true that Nitrox lowers the limit, in that situation. It just shouldn't be a blanket statement, and you made it into one, that's all.

Rainer's earlier comments about hypoxic mixes get outside the realm of this conversation and have little relevance here, certainly, but I do consider 28% to be within the recreational realm, albeit at it's limits, and therefore relevant.

 

[Crowley]

fair comment - it was a blanket statement - but I still stand by the statement that "it [as in Enriched Air Nitrox] significantly reduces your MOD" - although I perhaps should have qualified that (but didn't think I needed to) by adding "as compared to air". I see your point, of course, however in my job I hear a lot of divers talk about Nitrox (EANx of course) "allowing you to stay deeper for longer" - a conversation I hear almost daily (literally) and a myth I think needs to be dispelled.

But - we are picking at nits here. I prefer to pick at popcorn!

C.

 

[CompuDude]

fair comment - it was a blanket statement - but I still stand by the statement that "it [as in Enriched Air Nitrox] significantly reduces your MOD" - although I perhaps should have qualified that (but didn't think I needed to) by adding "as compared to air". I see your point, of course, however in my job I hear a lot of divers talk about Nitrox (EANx of course) "allowing you to stay deeper for longer" - a conversation I hear almost daily (literally) and a myth I think needs to be dispelled.

But - we are picking at nits here. I prefer to pick at popcorn!

C.

No disagreement there.