Aluminum 80s versus larger steel tanks

[Lorenzoid]

It would be very interesting to see these statistics or ratio of divers with DCS incidents with operators with steel tanks vs. AL80s. My understanding is that there are some operators that use the typical AL80 that have much higher rate of DCS incidents but I don't know if that is true.

It certainly seems plausible. My guess is that divers who seek out the ops that offer steel tanks in Cozumel fall into two categories, both of which seem to have good safety records: (a) very experienced divers who want more bottom time, and (b) very inexperienced divers who heard this particular dive op is "the best." There are probably not that many middle-of-the-road vacation divers itching to dive steel 120s.

I suspect there are a lot of relatively inexperienced vacation divers out there who, if asked, would not be able to tell you how many cubic feet their rental tank holds and aren't familiar with the terms "aluminum 80," "steel 120," etc. They might answer "sure" when asked if they want to rent a "bigger tank" because they profess to use a lot of air. These are the people who are likely to get into trouble.

 

[Skittl1321]

Same 70' on EAN32 but for 45 minutes. You have a longer bottom time but are now 10 minutes from your NDL.

Then you aren't diving an air profile, as postulated by awap.

But okay- you can increase safety while slightly increasing bottom time. But if you use nitrox for bottom time, and go to your 5 minute NDL like you would on air, you don't increase safety. Since it sounds like a Steel HP120 should get you substantially more bottom time than an AL80, if you are only using nitrox for safety, not to extend underwater time, it seems like you'll be wasting at lot of that air.

Your 45 minute dive example is half of the 90 minute dive Aldora tends to promise. (Though I do realize they don't dive square profiles. I'm not sure if anyone on Cozumel does.) Does Aldora automatically give divers nitrox, or are those tanks air? If nitrox isn't standard- how do all the nitrox divers feel when they have to come up as a group due to the one guy on air? I can see why a lot of people prefer ops that go up as their tank/computer require.

 

[gopbroek]

actually you can get both. The more you increase your bottom time, the less you increase your safety, and vice-versa. However, if you increase your bottom time by a little bit, you are still increasing your safety (with respect to NDL).

Take an 70' dive for 35 minutes. You would be within 5 minutes of your NDL.

Same 70' on EAN32 but for 45 minutes. You have a longer bottom time but are now 10 minutes from your NDL.

I agree, a real world (Cozumel) example I know of is looking for Lionfish at Chun-Chacaab. With a floor around 70', air will give a 'hard' NDL at 40 minutes and EAN36 will give a NDL of 60. Using an AL80 (which I always have) due to air consumption with that deep an average depth you could probably get away with either due to total gas volume limits as opposed to NDL limits. This is a dive that is normally gas limited on an AL80(this is one dive that some day I would like to try a HP120 on). The problem I have seen with many divers who use air is that the second dive becomes very limited due to the residual load they are carrying, after a standard 1 HR SI.

 

[Mossman]

Then you aren't diving an air profile, as postulated by awap.

But okay- you can increase safety while slightly increasing bottom time. But if you use nitrox for bottom time, and go to your 5 minute NDL like you would on air, you don't increase safety. Since it sounds like a Steel HP120 should get you substantially more bottom time than an AL80, if you are only using nitrox for safety, not to extend underwater time, it seems like you'll be wasting at lot of that air.

Your 45 minute dive example is half of the 90 minute dive Aldora tends to promise. (Though I do realize they don't dive square profiles. I'm not sure if anyone on Cozumel does.) Does Aldora automatically give divers nitrox, or are those tanks air? If nitrox isn't standard- how do all the nitrox divers feel when they have to come up as a group due to the one guy on air? I can see why a lot of people prefer ops that go up as their tank/computer require.

Here's the thing. Most every time I've been on a boat using 120s, there's a mix of air and nitrox divers and we're all doing the same time profile. Yes, I can stay down deeper and longer, but it's rare I'll reach my NDL on nitrox. So even if I don't extend my dive time, I may extend my depth beyond what the air divers can safely do, yet still end up "safer" because I end the dive with more available time to my NDL.

Then there's the little bit at the end. During my safety stops, I'm breathing a higher percentage of O2 in my mix than the air divers. That means I offgas a little better than they do. That, while so far unproven in actuality based on the limited statistics and experiments, in theory means I'm safer because I end up with less nitrogen saturated into my tissues than the air divers.

 

[luscioman]

I have switched over to Faber HP Steel 100s last year. The reason was 2 fold. First buying using steel I was able to drop 6# off in lead which was nice. Usually my first dive of the day is 90-120 feet using EAN32. I usually max out my NDL on the computer and get on the boat with 1000-1200 PSI of air when I start with 3300 PSI. It is nice to have that extra air for many reasons. One of which if somebody is not so good on air and uses my air I know that I have enough to spare for them including a safety stop. Why not have the extra air even if you don't use it?

 

[Mossman]

The problem I have seen with many divers who use air is that the second dive becomes very limited due to the residual load they are carrying, after a standard 1 HR SI.

This is mitigated somewhat by the 120 shops providing a longer SI, but you're right. I've seen air divers have to limit their divers or at least limit their depths due to NDL concerns when I'm happy as a lark staying deeper through the end of the dive.

---------- Post added May 19th, 2014 at 11:43 AM ----------

Why not have the extra air even if you don't use it?

Now we're going in circles. The only disadvantages are weight/distribution/buoyancy - some people don't like the characteristics of steel tanks and/or larger tanks - and cost, since steel tanks necessarily mean more maintenance and replacement expense and I'd imagine the DM and crew want to get compensated for spending the additional time in/on the water.

 

[Mike]

Here's the thing. Most every time I've been on a boat using 120s, there's a mix of air and nitrox divers and we're all doing the same time profile. Yes, I can stay down deeper and longer, but it's rare I'll reach my NDL on nitrox. So even if I don't extend my dive time, I may extend my depth beyond what the air divers can safely do, yet still end up "safer" because I end the dive with more available time to my NDL.

Ding, ding, ding

The 2nd factor is depth, not just bottom time.

As said before, Aldora doesn't force divers to all dive hand in hand in a big string. You're free to stay shallower or deeper and use your computer to manage your dive and your NDL.

 

[boulderjohn]

I teach both nitrox and advanced nitrox classes. Here is the instructional thinking behind some of the arguments about the comparative safety of nitrox and air.

1. We teach that diving on nitrox using an air table is not safer than diving air on an air table. That is, however, a matter of statistics only. The incidence of DCS while diving air is something like 0.002%. Let's say that you are diving a nitrox mixture that is 50% safer than air. That only takes it from 0.002% to 0.001%--a statistically insignificant difference in overall probability that explains why I personally think it is silly to dive with nitrox on an air table. I will much prefer to have longer bottom times. On the other hand, although the difference in safety is not significantly greater on an overall statistical basis, it is still safer to stay away from the NDLs.

2. Nitrox is no safer than air if you go all the way to the NDLs on nitrox because getting close to the NDLs carries the same risk no matter what you are breathing. However, you aren't required to go all the way to the NDLs when diving on nitrox. According to the PADI tables, the NDL for a 60 foot dive on air is 55 minutes. According to the PADI tables, the NDL for a 60 foot dive on nitrox 32 is 90 minutes. According to the PADI tables, the NDL for a 60 foot dive on nitrox 35 is 115 minutes. That difference is huge. You can therefore do dives on nitrox that are longer than air dives without getting close to the NDLs.

Some of you are acting like there are only two choices--dive nitrox to the edge of the NDLs, thus getting no benefit in safety, or diving nitrox on air tables, thus giving you no benefit in bottom time. Folks, there is a lot of room in between those two choices. You can indeed get longer bottom times without getting close to the NDLs and without having to go to the extreme of diving air limits.

 

[halocline]

There's really not enough data to say that diving with nitrox is demonstrably 'safer' than using air; we assume that anything that keeps N2 loading down is likely to help prevent DCS, and while that's a fairly safe assumption, there are so many other factors that it's very difficult to establish actual causes of DCS in many specific cases.

But, I would still be willing to bet that with enough widespread use of bigger tanks in Cozumel, eventually DCS incidence would rise. Currently it's just a few dive ops offering these tanks to a relatively low number of divers.

 

[Mike]

I understand the basis of your logic, but to be the devil's advocate what would the offset of everybody diving big tanks now is also now diving with a computer? Would that alone be enough to lower the cases of DCS?