NITROX for any and all dives?

[wildbill9]

Dive tables but what do I know? I am not a dive master....

 

[boulderjohn]

Extends bottom time how? A 80 cf tank has x number of breaths depending on your depth. Are you saying you take fewer breaths with nitrox? I doubt that. So how can nitrox extend your bottom time?

25 min on air, 30 min on nitrox. Problem is there is not enough gas either way to stay down that long without running out of air. Your limit is a factor of gas in the tank, not what is in it.

There are a couple of facts that will apparently shock you to no end.

• Not all people breathe air at the same rate. Some people can last much longer on the same amount of gas as others.
• Not all tanks are the same size--some hold more gas than others.

When I do recreational dives in South Florida, I use LP 85 tanks, and the shop that fills them puts a minimum of 3,000 PSI in them, which gives me about 100 cubic feet. I often get much more than that. When I do dive there, I usually go right to the very limits of the nitrox NDLst, and I invariably surface with more than 1,000 PSI in the tanks. My dives would be much, much shorter with air.

 

[mi000ke]

Extends bottom time how? A 80 cf tank has x number of breaths depending on your depth. Are you saying you take fewer breaths with nitrox? I doubt that. So how can nitrox extend your bottom time?

I have an average SAC rate of .42 cf/min. over my last 50 dives.

I dive with AL80s, which have a capacity of about 77 cu ft @3000 psi

I try to surface with about 500 psi, so my usable capacity is about 64 cu ft

The average depth of my last 25 deep (100’) dives was 55’. That represents 2.56 ATM

With those parameters, I can get about [64 cf/(.42*2.56)] = 60 minutes based on my gas consumption, which is about what I typically get.

The NDL based on PADI tables is 63 min for 50’ and 47 min for 60’ – so for 55’ let’s average them and call it 55 min. The NDL for EAN32 at 55’ is 87 minutes. By using nitrox I am not limited by NDL but rather by my gas consumption, and I gain 5 minutes on average, allowing me to use all of my gas, and giving me a better beginning gas profile for my second dive for any given SI.

Let’s assume I did a squarer profile and my average depth was 80’. At that depth I would have about 45 minutes of air (again assuming I surface with 500 psi). My air NDL would be 25 min. My EAN32 NDL would be 36 min. I gain 11 minutes of NDL, which is the limiting factor in this case.

Now assume I did a shallow dive with an average depth of 33’. My air consumption would allow 76 minutes. My NDL for air would be about 140 min and for EAN32 it would be greater than 150 min (the table goes only down to 45’). In either case, EAN would have no effect as gas consumption is the limiting factor.

While the benefit of EAN32 on the deeper dives (5-11 minutes) seems small, it it still a significant percent of the overall dive time, and more importantly may allow me to dive a bit longer and deeper for my next dive. And as I mentioned in an earlier post, at $6/tank it’s worth it to me. On the other hand, if I am doing only shallow dives (< 50’) I use air.

 

[AdivingBel]

(As to increasing BT) When I'm diving typical 2-tank Coz dive outings (on the FUN boat ) on air/AL80 by the end of my second dive I'm riding shallow with a screaming DC. On nitrox/AL80, not so much. I know which gas I'm diving...no brainer YM (or gas consumption) MV. N=1

 

[Magnus Lundstedt]

Yep. But the tire dealer doesn't make anything and you don't get green valve caps so people know you are environmentely sensitive.

I can paint my valve caps green myself. ;-)

 

[Diving Dubai]

I've never been a believer in the anecdotal discussion that EAN makes you less fatigued.

However (anecdotally) if I've spent most of the day in the pool teaching and using EAN which might be kicking about rather than the usual air then I'm certainly less tired. That said there are too many variables (sleep, hydration, students etc) to make a distinctive claim.

The only time I dive air is if on the OW course dives, which are generally shallow and short. Otherwise it's EAN all the way.

The only "hassel" is that on my usual 2 day trips (5-6 dives) I like a mix suitable to the site (because most are subject to currents I like to MoD head room, meaning I'll drag some 27%, 32% and a 40%, but because we can't predict which sites are divable until we get there, I'll haul 3 extra cylinders so that I've always got options (as most of us do on our trips). Given I don't load or unload my cylinders and once the cylinder's filled it's filled it's not an issue. If a quick 2 tanker pops up midweek I've always got an appropriate mix lying around at home.

Do I care about an additional $2 per fill? Nope.

 

[kelemvor]

I can paint my valve caps green myself. ;-)

They sell 'em at wally world for about $2

 

[Charred]

Don't forgot that Nitrox also extends your 2nd/next dive since you are loading less N2.

Thus it may be extending your bottom time, just maybe not on your first dive.

 

[MXGratefulDiver]

Nitrox shortens surface intervals and reduces nitrogen absorbed in blood. It does not extend bottom time in most cases.

I would dispute that last comment in "most cases". For shallow profiles or heavy breathers I'd agree ... because in those cases your gas supply will limit your dive time before NDL does. But for moderate depths (anything below about 60 feet) and "normal" gas consumption rate (where NDL takes over as limiting factor instead of gas supply), using nitrox most certainly does extend bottom time ... assuming you want to avoid a deco obligation. On most of my recreational dives I use EAN32, because diving air would put me in deco long before I wanted to end the dive. This is true even when I'm only doing a single dive in a day.

It costs more so sometimes we don't use it for that reason. 2 dives a day? Paying extra makes no sense, now if you do 3 or 4 different story. So no, in that scenario you don't need nitrox, if its free take it

Cost is the only downside to using nitrox, assuming it's readily available. In many parts of the world, availability will be the limiting factor.

... Bob (Grateful Diver)

 

[MXGratefulDiver]

Only a less industrious scuba agency (or shop) would promote that, a more industrious one will jump straight to promoting recreational trimix as the only safe approach to all diving...

I'll assume that was meant to be a joke, because promoting recreational trimix for anything other than profiles close to the edge of recreational depth limits would be foolish. Cost considerations aside (most folks wouldn't want to pay $60-$100 for a fill), helium will bend you MUCH quicker than nitrogen will ... so whenever you're breathing trimix you really need to pay attention to your ascent rate. Having watched what most recreational divers do once they've completed their safety stop I think promoting trimix at the recreational level really needs to come only with training and experience that the majority of recreational scuba divers are unlikely to have.

... Bob (Grateful Diver)