Aluminum 80s versus larger steel tanks

[Ron Lee]

When my Nephew warned me that the Zombie Apocalypse was coming, I never expected that it would first appear on a Scuba Board Post. Will this thread never die?

No. Here is one reason.

"which is one reason I dive NITROX on all Cozumel dives,"

I am not saying that this approach is bad but I will say that I do not subscribe to it. My thinking is that a typical first dive is at a place where a down current or diver issues might (rarely) cause them to get below suggested limits of nitrox. Using air provides me more flexibility in responding to an occurrence of this nature...even if it never happens. I know my reasonable acceptable depth limit on air. I have not determined it for nitrox 32 or 36.

 

[scubafanatic]

No. Here is one reason.

"which is one reason I dive NITROX on all Cozumel dives,"

I am not saying that this approach is bad but I will say that I do not subscribe to it. My thinking is that a typical first dive is at a place where a down current or diver issues might (rarely) cause them to get below suggested limits of nitrox. Using air provides me more flexibility in responding to an occurrence of this nature...even if it never happens. I know my reasonable acceptable depth limit on air. I have not determined it for nitrox 32 or 36.

Ron, I respect your viewpoint, I ask for EAN 32 (not EAN 36) which gives me a max depth limit that's more than adequate for any dive profile one will encounter in recreational diving in Cozumel. I'm much more concerned with benefiting from the preventative effects of NITROX than I am worrying about a freak downdraft that VERY rarely happens, and would likely be a very short-lived event (minimum exposure time) IF it did happen.

 

[boulderjohn]

Er, isn't three all of them? I mean for the bigger than 100 AL tanks.

Beats me.

Some people can be fooled if they do not have a firm understanding of the relationship between tank volume and pressure. For example, if you fill a LP 85 steel tank to to 3,000 PSI, you have nearly 100 cubic feet of gas.

 

[Ron Lee]

scubafantic, your approach is well-reasoned and safe. However I base my nitrox use based upon other factors.

Imagine diving Devil's Throat. Some dive ops will continue into the swim through that used to have the yellow tube sponge...even though you seldom see anything in a swim through.

At some point the divemaster is unlikely to directly monitor people near the end of the group so if the next to last person starts to descend and I am at the end as I usually am, I have plenty of depth on air to assist before I get to an elevated ppO2 (oxygen partial pressure). The same ppO2 on air that I would tolerate would only get me to 119 feet on EAN32. Actually, the exit at Devil's Throat is deeper than that. This was not a good example but the idea applies to other some dive sites as well.

Even though the chance of needing to render assistance to someone is low, I prefer air on the first dive for that reason. There are other things that I do "just in case." Whether that is carrying a PLB on a hike or changing a vehicle water pump, hoses and fan belts pre-emptively, it is one way of approaching things.

---------- Post added May 24th, 2014 at 03:58 PM ----------

I only know of three dive shops using 120 cu ft tanks. Aldora, Living Underwater and Liquid Blue. There could be more.

 

[boulderjohn]

I only know of three dive shops using 120 cu ft tanks. Aldora, Living Underwater and Liquid Blue. There could be more.

I stand corrected on my previous statement. I have dived with Aldora and Living Underwater, which were two of the three I was thinking of. The third I was thinking of is Deep Exposure. I was remembering using LP 85s filled to 3,000 PSI, but I just checked and realized that was only when I was diving doubled LP 85s. I believe they use AL 80s for single tank divers.

---------- Post added May 24th, 2014 at 04:10 PM ----------

The standard MOD for nitrox 32 is 111 feet using the 1.4 standard. The contingency MOD for that (1.6 standard) is over 130 feet. You should also be aware that there is a time factor involved in that--you don't suddenly go into convulsions if you dip below the MOD, even considerably so. The only cases of oxygen toxicity I know of occurred with technical divers who made serious errors and were well below their MOD for extended periods.

 

[Mossman]

The standard MOD for nitrox 32 is 111 feet using the 1.4 standard. The contingency MOD for that (1.6 standard) is over 130 feet. You should also be aware that there is a time factor involved in that--you don't suddenly go into convulsions if you dip below the MOD, even considerably so. The only cases of oxygen toxicity I know of occurred with technical divers who made serious errors and were well below their MOD for extended periods.

Thank you John. A lot of people have this conviction that they will surely die if they exceed their MOD just as they will surely die if they exceed their NDL. While that may be a healthy approach for the unaware "vacation" diver, it can also cause irrational fears and behaviors even among experienced divers.

 

[mikeycanuk]

Scubafanatic said it a lot better about the work involved in getting ready/cleaning up. We got the equivalent time of 3 dives in just 2. A third dive in a regular AL80 dive day would mean another hour of boat time and shlepping gear around. Plus in my case another round of sea sickness meds and probably ephedrine as well, not to mention another round of possible sinus equalization issues once in the water- boy that hurts.

There are 3 of us so guess who gets to haul the 3x wetsuits, masks, computers, spare clothes, cash, dry bag.....!

As for the MOD of 1.4 I believe PADI took the 1.6 standard from ???? and simply lowered it to be extra cautious. From my understanding 1.6 is fine for easy tropical dives, 1.4 for more demanding cold water, low vis ones.


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[boulderjohn]

As for the MOD of 1.4 I believe PADI took the 1.6 standard from ???? and simply lowered it to be extra cautious. From my understanding 1.6 is fine for easy tropical dives, 1.4 for more demanding cold water, low vis ones.

PADI did not create the 1.4 standard. It is in use almost everywhere. I can think of only one agency that does it differently, and that is a very small one.

Most technical divers use the 1.4 standard for the working part of the dive and the 1.6 standard for the decompression stops. That is based on the frankly unproven belief that physical activity increases the risk of toxicity. During decompression stops you are usually just hanging around, looking at your buddy. Some people bring games like Connect 4 to pass the time, but that does not require a lot of effort. It is very possible that at the end of a decompression dive, the diver will spend a half hour or so at 1.6.

 

[Mossman]

Most technical divers use the 1.4 standard for the working part of the dive and the 1.6 standard for the decompression stops. That is based on the frankly unproven belief that physical activity increases the risk of toxicity.

If unproven, while still so widely held?

From the Winter 2013 Alert Diver article by Petar Denoble, MD: ""Symptoms of CNS oxygen toxicity, which include seizures, may occur after short exposures to partial pressures of oxygen greater than 1.3 ATA in exercising divers, which equates to breathing pure oxygen at 10 feet of seawater. Resting divers in comfortable conditions tolerate 1.6 ATA of oxygen well. . . . The best protection is to keep inspired oxygen pressure at or below 1.2 ATA while physically active and at or below 1.6 ATA during decompression." -- Alert Diver | Understanding Oxygen Toxicity

 

[Ron Lee]

So now the standard is a max ppO2 of 1.2 during the active part of the dive?