Using Nitrox...

OK, had the class, got the cert - still have a question.

So Nitrox increases your NDL, at the expense of adding some other risks and complications. Got it. Nitrox does NOT let improve your air consumption. Got it.

So, my question is, and I know the answer varies wildly depending on particulars and personal characteristics - so I'm just looking for SOME idea (very ballparkish):

where is the trade off between the benefit of using nitrox to increase the NDL and your rate of air consumption?

In other words....on a 60' dive, if your personal rate of air consumption is such that, you're going to have to be back at the surface in, say, 40 minutes, is using Nitrox worth the cost? What if you're making two such dives, or 3? What if your air consumption lets you get 50 minutes out of such a dive?

Or is the answer really, "well, you'll need to look at the tables, calculate it all out and figure it out, depending on whether you're doing repetitive diving and so forth, and there is really no reasonable "rule of thumb" to use ?

What I'm really asking is there some reasonable way to develop a "crossover" point between one's air consumption for the particular dives one is planning, that lets you weigh the NDL improvement benefit of Nitrox to increase dive time vs your typical dive time based on air consumption without regard to nitrogen issues?

My air consumption rate is not great - I tend to suck air. My suspicion is that my dives will always be more likely to be limited by the fact that I am running out of air, rather than I am running up against the NDL - with or without nitrox. I'm just trying to get a feel for whether or not my suspicion is correct in general, or whether is is correct or incorrect for particular circumstances - such as deep (how deep?) dives, or repetitive dives on a liveaboard, etc.

I know I'm wording this very poorly - hopefully someone can figure out what I am trying to ask

Well, the answer is pretty easy . . . What's driving you to the surface now? NDLs, or gas supply? If the answer is gas supply, Nitrox isn't going to make any difference for you at all.

Whether you are using tables or a computer, when your gas consumption gets good enough that you are having to truncate your dives because you are running out of no-deco time, Nitrox will be useful.

In some cases, it's possible that your first one or two dives of the day would be limited by gas, but beyond that, it would be NDLs, in which case you are at the borderline of where Nitrox will be advantageous.

NudeDiver:

What if you're making two such dives, or 3?

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Using EAN also reduces surface interval on multiple dives; so that would be an advantage in that scenario

Hi ND,
I don't think I can completely answer your question but I will offer some thinking behind my current use of nitrox to see if it helps.
I usually dive AL 80's in open water 60' or less (primarily interested in observing and collecting flora and fauna). My gas almost always runs out before I reach my NDL so for me a dive takes a tank no matter what it is filled with.
A fill of nitrox costs between $2-$5 more per tank than air. That $2-$5 offers me:
a.) an extended NDL (not really needed)
b.) a conservative dive profile if I dive it as air only (which I usually do)
c.) less fatigue and a clearer head after the dive
d.) shorter SI when using EAN/air conversion tables
The Nitrox doesn't increase the time that the gas in the tank lasts (for that I need to practice relaxing, controlling my breathing rate and/or get a bigger tank).
The added expense of nitrox isn't an issue for me (don't know about prices in your neck of the woods). What is a break even issue for me is the risk/reward ratio of using nitrox at or near the maximum O2 exposure depth. Say 100+ft. for EAN32 for example. If I am going to approach those depths I have to decide whether the extended NDL is worth the increased O2 exposure and tighten my dive plan accordingly.

I see TSM and Tortuga posted while I was posting so there is some repetition here but there is something else related that I remember.
If you are thinking about extending your bottom time by getting bigger tanks (say 130cuft. for example) be cautious about blowing your NDL's (as I have learned the hard way). Your first dive may be within bounds but a second dive with a big tank gives you so much gas that you can easily go over the limit if you don't pay attention.
That's all, carry on!

Well all above being said above Ill put my two cents in. I had a problem early on with consumption of air. Part of it was I was a little anxious to dive so I was breathing a little rapidly. I cured that though by doing a serious of shallow dives usually 30 feet or less and my whole focus was my breathing. I noticed I could slow my breathing to a slow inhale then a slow exhale and it made it to where my gas was outlasting my NDL at depth.

The nitrox is no miracle cure for getting more Minutes at depth versus the air but like the others have mentioned its more of a cure on repetitive dives. With shorter surface intervals and longer NDL its more of a beneifit in that it makes more dives possible with less risk of DCI.

Now another plus that is my real selling point of Nitrox is when I am finished for the day, I usually have an hour at the least drive home. On air It makes the rest stops look mighty dang good because I feel like I want to fall asleep. then with Nitrox I can drive home and do my chores and run errands if need be and still be good to go. All in all I would say I would never go back to air as long as I had nitrox to dive with.

Hope it helps!

I think you are missing one important reason that many dive Nitrox. For any given dive within the limits of both air (NDL), and Nitrox (MOD), your Nitrogen loading will be less using Nitrox, therefore your off-gassing will also be less aggressive. This is easier on bodies, especially ones that are getting older and aren't quite so "efficient" as they once were!

This is why Nitrox is sometimes described as "Geezer Gas"!

If you use a big tank to run the mix to it's full NDL though, you lose this advantage - as your Nitrogen loading is the same as air at it's NDL limit.

For me, I use both gains as reasons to dive Nitrox sometimes. I always dive Nitrox on liveaboard boats, doing 4 dives a day. I generally dive an air profile with the group - but sometimes I can dip a little deeper (within my own MOD) to look at something/take a few pictures, for longer than the guys on air can. This has got me more than a few nice pictures of Leopard sharks, Ghost-pipe fish. Sea-horses etc.

NudeDiver:

... My air consumption rate is not great - I tend to suck air...

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As has been said, a lot of this comes with practice. You don't need to breath a lot more air than you do on the surface. But also, a better regulator will help - like the Scubapro Mk25 first stage, coupled to an S600 second stage (which is what I was advised to purchase). This requires less vacuum (suck!) to get the air to deliver (the S600 is adjustable while diving). This makes breathing from a tank a lot more like breathing on the surface, and hence enables lower air consumption at depth.

Still need to practice relaxed breathing though!

As to how big a difference nitrox makes, I find another factor that is almost as important as depth and dive time. That's whether or not the dive is multilevel.

When doing a dive like Molokini backwall in Maui, air works fine because it's a very multilevel dive that starts deep, but then we work our way back up and spend lots of time shallow. The reef goes all the way up to the surface. My profile on air is very similar to that of a nitrox profile, except I spend a bit more time towards the shallow end of the reef.

In contract, when diving SE Florida's 3rd reef, I'll hit the bottom at 60-85' and pretty much stay right there for the duration of the dive until doing my ascent. Since the reef is all pretty much below 60' there isn't any way to make it mch of a multilevel dive. Nitrox makes a huge difference in how much time I'm hanging out midwater twiddling my thumbs.

All:

Thanks for the replies. What I got out of it so far is, more or less, as long as air consumption is limiting, Nitrox is going to be of little help (which is what I figured), except for the fact that it can give me shorter surface intervals (which I overlooked). So, for the most part, it sounds as if, for me, given my situation, Nitrox isn't too helpful until the next time I am on a liveaboard and want to cram in as many dives as possible. However, I should start paying better attention to what is limiting my dives - air consumption or NDL.

So, for the time being - my dives will be local, drysuit, fairly cold water, not real deep (60' would probably be stretching it), and 2, at most 3 per day (shorter surface interval isn't a big benefit). For this scenario, it sounds like I should stick with air, esp. since my buddy will be diving on air anyway. Does this sound about right, given the advice given here?

But also, a better regulator will help - like the Scubapro Mk25 first stage, coupled to an S600 second stage....

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As it turns out, this is exactly what I have

Cheers!
nd

Nitrox is useful when you're doing several dives a day. 3 can easily be done on air in most cases. Once you start looking at doing 4 or more dives, you're better off going with nitrox because you will be approaching NDLs if diving air. Another useful application for nitrox (and the one I use it for) is when doing shallow (100' or less) decompression diving. Nitrox can significantly reduce your deco obligation.