Nitrox and the Myth of the Longer Dive

[GDI]

At a dive shop yesterday and some divers were talking about using nitrox. While this is not a unusal discussion What often comes out of it is a statement of a diver who believes they can and are diving longer becasue they are using nitrox.

As I listened to the discussion I felt the need to clarify something (maybe the need to do this is a ego thing - I don't know-Anyway)
Several of the divers in the discussion believed that they could and were diving longer because they are breathing nitrox. (implied that they are using the same equipment ie size of cylinder by volume)

Now think about this for a moment. While a diver MAY be more relaxed using nitrox and thus their breathing rates improve (no proof to this exists yet) they are not lengthening their dives.

Yes nitrox permits longer dive times, less nitrogen absorbtion equals longer bottom times but it does not lengthen the dive. Your lungs are the same size and will breath at the same rate whether you are on nitrox or air or trimix or whatever you suck back.
The cylinder does not get bigger just because it is now cleaned for O2 use. A 80 cu Ft Al is still 80 cu ft. So with the two mechanical devices (lungs and cylinder) being the same, your breathing rate for the most part being the same it is not likely that you will find your bottom times at depth having a major increase. One should not confuse the allowed times of using nitrox to the breathing times of when using air or nitrox or again whatever you suck back.

With this in mind new divers need to look at why they want to go and take a nitrox class right away. While I am not against them doing so they really need to see if they will benefit from using nitrox. If you have a 1.5 cu ft/min SAC rate then you may want to just wait out on the costlier nitrox gas mix until you get your skills down and the bottom times start to improve. Going on deeper dives where it is possible that you may find your times ending sooner then your air supply and causing you to make an ascent earlier then wanted well then yes nitrox may be the gas of choice (with correct training). BUT if you are diving deeper and your gas management skills are not up there and you do not have the practical application of the gas management knowledge down then you might want to rethink your diving development. Breathing Nitrox does not reduce risks of DCI (too many varibles here thus another myth)

Nitrox while it may allow for longer bottom times DOES NOT translate into an actual increase in bottom times. Not until you can change your lung's volume and SAC rate or increase the amount of gas you carry. With the later having it's own set of issues. Divers should also learn through correct nitrox training that it is at times better to use a mix other then the standard 32 and 36 %, I feel many classes fail to illustrate this point in the practical application of nitrox gas use and management

 

[D_B]

I think that that is a common misconception for many people that have not taken a nitrox class, that breathing gas with a higher O2 % would translate into needing to breath less .. I did not know that some still beleave that even after taking the class ... I know it was one of the things talked about in my class and quickly dispelled by my instructor

 

[DIR-Atlanta]

It's true that Nitrox does not make your tank bigger, and the comments that you heard are a common misconception regarding the use of Nitrox. However, there are two benefits to Nitrox that are available to all divers, even the ones that have "a 1.5 cu ft/min SAC rate".

First is the physiological benefit of breathing a gas with less Nitrogen in it. This reduces Nitrogen loading and any decompression stress associated with it. That's certainly a benefit, whether you get more bottom time or not.

The second benefit actually can translate to longer bottom time (even for the same SAC rate), but not in an obvious way. As you pointed out, most newer divers tend to end their dive due to running low on "air time", rather than "nitrogen time". On repetitive dives however, it is often the case that they hit the limit on nitrogen time rather than air time, particularly when diving with conservative dive tables.

For those folks, Nitrox probably won't provide much benefit on the first (non-repetitive) dive of the day, but they could certainly pick up a few extra minutes of bottom time on the second and subsequent dives. As to whether that is worth the extra expense for the training and gas fills - only the divers themselves can decide that.

But it certainly is possible to get more bottom time from Nitrox, even if your SAC rate is not that great.

 

[The Kraken]

Well put, GDI.

I have to explain this all the time.

Nitrox changes only the nitrogen/oxygen ratio of the gas one dives and yields a resulting change in the depth the body perceives it is diving based upon the nitrogen content.

It does not change the volume, all things being equal, that a diver uses on a given dive.

the K

 

[Web Monkey]

At a dive shop yesterday and some divers were talking about using nitrox. While this is not a unusal discussion What often comes out of it is a statement of a diver who believes they can and are diving longer becasue they are using nitrox.

They get a longer dive if they were previously limited by the NDL (which is now longer) and not by tank volume, which has not changed.

Terry

 

[D_B]

The Physiological benefit of breathing Nitrox over air ends when you go to the same No Deco time limit on nitrox as you did with air .. your body then has same nitrogen loading (just takes longer time to get there)

 

[fisheyeview]

Lets not forget the benefit of a shorter surface interval. For those diving in areas of rough seas this can be a huge benefit.

 

[fisheyeview]

The Physiological benefit of breathing Nitrox over air ends when you go to the same No Deco time limit on nitrox as you did with air .. your body then has same nitrogen loading (just takes longer time to get there)

With the hours you keep you probably were sleeping.

 

[D_B]

Question .. is there a shorter surface interval if you've dived all the way to the NDL .. wouldn't your nitrogen loading now be the same as it would be with air ?

 

[deco_martini]

I got certified to use Nitrox before my SAC rate was under control. The horror.