How does Nitrox work?

[Tassie_Rohan]

I've got no problem with people who it will benefit taking the class... but most new divers... and even many experienced ones don't fall into that category.

I think Offthewall hit the nail on the head.


I found Nitrox to be a very informative course, and would recommend doing it simply for the theory it provides.

However - I managed to dive without a nitrox certification for 10 years, and in the 10 years since the course I still only rarely use it - for occasional extended dives, rare multi-dive liveaboards and, nowadays, the odd deco stop. It's a tool to wheel out when conditions require it.


Cheers,
Rohan.

 

[offthewall1]

Cheapest, easiest, and in many ways rewarding card: $100 - 3 hours class in many places. Sorry to see Inst-offthewall1 talk against it. Get your AOW & Nitrox cards ASAP. Too often divers "wait until they need one" but don't have it when they actually do.

Don,

If DAN says it has no benefit for most recreational divers... who am I to disagree.

My personal experience as it relates to Nitrox. I'm certified and an Instructor in it's use. I've been Nitrox Certified since 2000. I dove it in 2000 and 2001 because back then I didn't know any better. I had been convinced by my LDS I had to have it.

Then I did some research and read articles in DAN's magazine over the years and learned that it was for the most part (and lack of a better term) a bunch of fooey.

I stopped using Nitrox because it cost an extra $100 or more a week on a trip and for tanks locally it was $3 - $5 more per fill if I was diving locally. Based on what I had researched the minimal benefit or lack thereof entirely was not worth the extra investment.

Now as I've said... it does have it's place in recreational diving... but I even bring that into question and here is why. The only real benefit is an extra added measure of safety... which if you're diving within limits... is minimal.

My wife and I do liveaboard trips with many friends. They all dive Nitrox except for us. We've been doing it for years now (8 - since 2001.) We all get in the water at the same time, we all do the same dives and we all get out of the water at the same time. Virtually identical profiles. Same surface interval times etc... We'll sometimes do 5 or even 6 dives a day... In the last 8 years there has not been one dive where I couldn't do the exact same profile as any one of them.

Now I'll grant you that my nitrogen loading is higher than theirs... however my loading is still within my computers limits.. and the computers algorythym limits are so conservative they make the Navy Tables look like jumping off a cliff.

I know that putting my post on as I did will draw some attitude from those who push Nitrox on every customer. That is to be expected. They're all out to make a buck and posts like this one don't help that ideology. My posts are there to educate the consumer on the truth. This truth is something they can just as easily obtain through diligent research.

DAN has been very forthright about Nitrox use in recreational diving and I believe that as one of the top authorities in dive research... theire opinion carries far more weight than mine.

The whole thing about Nitrox making you feel better after a dive has also been refuted by DAN. At best it has a placebo effect... and I'll agree that may be enough for some people. We all know perception can be far more powerful than reality.

So if you truly believe you feel better because you dive Nitrox - you probably do... but it has nothing to do with the Nitrox...

Cheers to "feel good" diving!

 

[herman]

Here in NC it is very common for off shore dives. Our dives tend to be on the deeper side and it is common for a lot of divers to be diving larger tanks- 100s and 120s are common. The combination of multiple deep dives and larger tanks easily puts us into deco without nitrox.
OP- Here is some online info on nitrox you may find interesting. Nitrox Course Outline

 

[DA Aquamaster]

Lets look at an average "just got his AOW" diver with a SAC of .75 CFM on an average NC two tank boat trip where the first dive is to 110 ft and the second is to 85 ft.

With an AL 80, that SAC rate will allow a maximum of about 19 minutes of gas at 110' with a common recreational reserve of 500 psi. At 85 ft under the same conditions, an AL 80 would give you a gas limit of about 24 minutes of bottom time.

With an HP 100 however under the same conditions, you'd get 26 minutes at 110 ft and 32 minutes at 85 ft.

For a fairly new AOW diver with his SAC more or less under control, nitroxcan make a great deal of sense. In the NC depth examples above, 32% will reduce the equivalent air depth by around 15-20 ft and increase your NDL's fairly significantly.

On air, the first dive to 110 ft will give you an NDL of only 16 minutes, 3 minutes short of your gas limit, so even an AL 80 can get you into deco trouble with a SAC of .75. The second dive, after a common NC surface interval of 90 minutes, to 85 feet will give you an NDL of 21 minutes - again three minutes shorter than your gas limit placing you into a potential deco situation if you use all your available gas.

In comparison, 32% nitrox on the first dive to 110 ft will give you an NDL of 25 minutes, 6 minutes more than your gas limit of 19 minutes. The second dive, after a 90 minute surface interval, to 85 feet will give you an NDL of 28 minutes, 4 minutes more than your gas limit.

So with an AL 80 and 32%, you are gas limited rather than deco limited and will be far less likely to get yourself into an unintentional deco situation on your available gas. On the other hand, you are only picking up 6 minutes more bottom time over the course of two dives if you are limited to an AL 80 so many divers may correctly argue nitrox is not really worth it, as long as they are careful to mind there shorter NDl's.

However if you instead use an HP 100 with 26 minutes of gas at 110' and 32 minutes of gas at 85' the situation is very different. With NDL's on air of 16 minutes at 110' and 21 minutes at 85 feet you have enough gas to over stay your NDL's by at least 10 minutes and acquire some fairly significant deco. Potentially more than just a "little' bent if you stay to your gas limit and blow off your deco, and enough deco where a 500 psi/15 cu ft reserve may be pushing it.

With an HP 100 and 32%, the gas limit of 26 minutes at 110' is 1 minute longer than the 25 minute NDL and the 32 minutes of gas available at 85' is 4 minutes greater than the 28 minute NDL. So in effect, if you use an HP 100 with 32% you will get a total of 53 minutes of no deco bottom time compared to only 37 minutes if you dive on air, regardless of whether you dive with either an HP 100 or an AL 80. In this example, 32% combined with a larger tank effectively gives you 43% more bottom time with no deco required. And the mix and tank are not badly matched in terms of NDL versus gas limit at a SAC of .75.

The moral is that with enough tank, you can get about 40% more no deco bottom time with nitrox, but also nitrox will not do you much good on a standard AL80. People on both sides of the argument are correct depending on the tank they use.

The opposite is also true however. A larger tank will not do you much good if you want to stay within the NDL's unless you use nitrox. So if you plan to pony up the money for a larger steel tank, it is worth your time and money to take a nitrox course.

Personally when I go to NC for 3 days of diving, I spend about $100 in fuel, about $60 in nitrox fills, $300 in hotel rooms and $500 in boat related fees. Basically $960 for 6 dives - $160 per dive. You will note the nitrox fills are the lowest cost on the trip and using air would only save about $15-20. But when I am paying $150-$160 per dive, I'll take the extra 43% of bottom time every chance I get by using 32% and a 100 cu ft tank.

In the local quarry where the cost per dive is much lower, I'll happily use air.

* the legal disclaimer - all profiles were developed with a PADI Wheel using equvalent air depths. Your computer or tables may vary and your individual SAC will impact you personal gas limits. You need nitrox training to safely dive nitrox and you need to ensure you do your own dive planning prior to each dive.

 

[DevonDiver]

Do you have a reference for this DAN quote? I would like to read it for myself and see what context it was said in,...


I teach and dive in an area with many wrecks. Our divers are most keen to visit the two 'signature' wrecks.... a Landing Ship Tank (upright at 34m) and the USS New York (armoured cruiser at 30m). They are pretty much square profile dives.

On air, it is impossible to do two meaningful dives on these wrecks in a single am or pm trip. Even people with moderately poor air consumption will find that their dive computers / NDL limit their second dive severely.

So...if you want to take that dive option, your choice is to use nitrox or be content with a second dive on a shallower less inspiring wrecks or a trip to Omer-Seksuall Reef...or learn how to conduct decompression dives.

If you only dive on Omer-Seksuall Reef...and are able to easily multi-level your profile...and happy to spend the majority of the dive in the shallows counting parrotfish...then, fine, don't bother with Nitrox. :mooner:

 

[offthewall1]

Do you have a reference for this DAN quote? I would like to read it for myself and see what context it was said in,...


I teach and dive in an area with many wrecks. Our divers are most keen to visit the two 'signature' wrecks.... a Landing Ship Tank (upright at 34m) and the USS New York (armoured cruiser at 30m). They are pretty much square profile dives.

On air, it is impossible to do two meaningful dives on these wrecks in a single am or pm trip. Even people with moderately poor air consumption will find that their dive computers / NDL limit their second dive severely.

So...if you want to take that dive option, your choice is to use nitrox or be content with a second dive on a shallower less inspiring wrecks or a trip to Omer-Seksuall Reef...or learn how to conduct decompression dives.

If you only dive on Omer-Seksuall Reef...and are able to easily multi-level your profile...and happy to spend the majority of the dive in the shallows counting parrotfish...then, fine, don't bother with Nitrox. :mooner:

First, I went to the DAN website to see if there was an easy way to pull up older articles and I couldn't find a way... but they exist. If anyone knows how to find artcicles from old DAN magazines online - can you please help...

Second... I'm in full agreement that successive deep dives without an adequate surface interval will require Nitrox - to avoid going into decompression mode on your computer or being told no to the dive - if you're using tables. I'm also in agreement that bottom time is limited even on your first deep dive without Nitrox.

The point being made however is that most recreational divers aren't trained for those types of dives to begin with (OW to 60ft) and most never go that deep (100ft +) at anytime in their diving careers.

You, me and many others (experienced) on this board go to all sorts of depths and I'll agree that in that sense, Nitrox is something potentially beneficial. Of course it is arguable as to the benefit being anything other than time motivated.

I do those same types of dives you do... only I do them on air with a short deco stop of usually no more than 10 - 15 minutes. I enjoy this deco stop in NC and Truk Lagoon etc... hanging out with the fishes. so again, Nitroxes benefit is not worth the added expense to me. I have no problem hanging out for 10 or 15 minutes after going into deco.

New divers - as this thread began with - won't (or I should say - shouldn't) be doing this type of diving right out of the box and maybe never will at all. I specifically stated to decide what type of diving you'll be doing before dropping the money for Nitrox. That is my only point.

It is not for everyone... but it is for the few who do certain types of diving... and even then, still arguable as to whether or not it is needed. I suppose I would need it if I didn't want a 15 minute deco stop... but that is truly not a need... but rather a choice. I'm completely safe no matter which choice I make.

Cheers

 

[DevonDiver]

The point being made however is that most recreational divers aren't trained for those types of dives to begin with (OW to 60ft) and most never go that deep (100ft +) at anytime in their diving careers.

You, me and many others (experienced) on this board go to all sorts of depths and I'll agree that in that sense, Nitrox is something potentially beneficial. Of course it is arguable as to the benefit being anything other than time motivated.

You must deal with a very different clientele that I have encountered before. I find that most divers are keen to take their AOW quite soon after OW...and that when they do so, they have no hesitation in exploring underwater within the full range of their depth limitations.

This is very true here in Subic, but was equally true when I worked at Sipidan and in Thailand.

For me, I would rather use nitrox than have to pay a deco penalty. The end result is the same, but I prefer to be on the boat with a coffee than hanging beneath it. Of course, as 90% of my dives are with recreational customers, I cannot conduct deco dives. In fact, the only time I tend to do decompression when working with recreational customers, is when I teach nitrox..... because I prefer to be on air to teach that course for safety reasons.

 

[scubadude79]

simply put

less nitrogen in a mix = more bottom time at a given depth

Given that you don't run out, because that works pretty much the same as air.

 

[Rhone Man]

Personally when I go to NC for 3 days of diving, I spend about $100 in fuel, about $60 in nitrox fills, $300 in hotel rooms and $500 in boat related fees. Basically $960 for 6 dives - $160 per dive. You will note the nitrox fills are the lowest cost on the trip and using air would only save about $15-20. But when I am paying $150-$160 per dive, I'll take the extra 43% of bottom time every chance I get by using 32% and a 100 cu ft tank.

My numbers would be different, but that sums up my approach. For as much as you end up paying to get under the water, why would you not pay just a little bit more to dramatically extend your bottom time?

If you are putting in 3 dives a day, you get dramatically longer bottom times at the back end (or shorter surface intervals, which can be more important when it is rocking and rolling). Given the total investment you make to get under the water in the first place, I say it is money well spent.

 

[DA Aquamaster]

I do those same types of dives you do... only I do them on air with a short deco stop of usually no more than 10 - 15 minutes. I enjoy this deco stop in NC and Truk Lagoon etc... hanging out with the fishes. so again, Nitroxes benefit is not worth the added expense to me. I have no problem hanging out for 10 or 15 minutes after going into deco...

...It is not for everyone... but it is for the few who do certain types of diving... and even then, still arguable as to whether or not it is needed. I suppose I would need it if I didn't want a 15 minute deco stop... but that is truly not a need... but rather a choice. I'm completely safe no matter which choice I make.

That is to some extent the point of nitrox - it allows greater bottom times without the need for the additional equipment, redundancy and training required to do intentional decompression diving safely, which makes it well suited for experienced OW or perhaps less experienced but AOW certified divers who are diving to the deeper half of the recreational limits and need to stay within the NDL's.

For recreational divers who never intend to advance to any type of technical diving, nitrox is ideal in the 80-100 ft range simply because it gives them bottom times with no deco similar to those they would achieve with air and required deco.

I also agree with you that nitrox is even better for a more advanced diver properly trained to do deco. I normally dive double 100's in NC and I like to be one of the first people off the boat and one of the last people back on board. With a good SAC rate that strategy allows me to exceed the NDL's even on 32%, get a lot more bottom time, do a 5 or 10 minute deco stop and still get back on board without delaying the boat's departure. Plus I still use less than 1/2 the 200 cu ft of gas available on the first dive and consequently I can do the second dive on the same set of doubles. It works slick and requires no separate deco bottle.

To do the same bottom times on air, I'd have a lot longer deco on backgas and would need an an accellerated deco gas to shorten the deco to keep the boat crew happy. Alternatively if the crew was not in a hurry and I did the deco on back gas, I'd have to either shorten my bottom time to allow enough reserve with the back gas deco, or I'd have to bring along a stage or another set of doubles to ensure adequate gas for the second dive.

Nitrox potentially solves all kinds of problems regardless of experience level. But I agree it's utility exists within a fairly narrow depth range for most recreational divers (80-120 ft) and has to be matched by enough tank capacity and/or a good enough SAC rate for the diver to benefit from the increased bottom time it offerrs.

I also agree that nitrox is often promoted to many divers who are not yet ready to benefit from it, or who's profiles won't be dramitically different because of it. On the other hand, it is a fairly inexpensive course the way it is currently done, it makes sense as part of an AOW course and it does give the diver more knowledge and more options as they develop as divers and expand their horizons and enjoyment and as such keep them diving in situations where they may have otherwise dropped out.