Why the small tanks?

[DA Aquamaster]

Most adult males on an AL80 run low on air just before they run out of bottom time. "Bigger" divers and the "less experienced" burn it up sooner. Running out of air is easier for the divemasters to watch than running out of bottom time, so I don't blame the dive operations for erring on that side. Kids and small women can get away with 72s and smaller, but it is sloppiness on the dive boat's part to start a full grown guy's dive with anything less than 80 cu ft at 3000 psi. Such a shop deserves to get the maximum possible negative publicity for it. I own a pair of steel 120s personally; the extra weight is worth running out of bottom time instead of air. It would be nice if a dive boat would let folks specify a bigger tank and/or Nitrox mix for a set extra fee. As a capitalist, I accept the concept of extra bottom time = extra money. Anyone else know why they don't use that tactic for the extra revenue?

You may have answered your own question.

When I started diving the general rule, with US Navy tables, was that if you stayed above 100' you could not get yourself bent on a single steel 72 in a 12 hour dive day. I had very good SAC, so after playing with the tables I realized I had to adjust the depth upward to 90 feet for that to be entirely true.

With AL 80s (77 cu ft) and Nitrox it gets a little more complicated, but at shallower depths, even with current tables, the same is mostly true - but only for the first dive.

Assuming a 500 psi reserve, you have 64 useable cu ft in an AL 80.

With a SAC of .6, on air you would have:
38 minutes at 60 ft (50 minute NDL)
34 minutes at 70 ft (40 minute NDL)
31 minutes at 80 ft (1 minute over the 30 minute NDL)

And with 32% Nitrox, considering the equivalent air depth and rounding down on the same table, you get:

60ft - 70 minute NDL
70ft - 50 minute NDL
80 ft - 40 minute NDL

Placing all three dives well within the NDL on the first dive. However even with 32% nitrox after a 90 minute surface interval, you would have to stay above 60 ft to stay within the NDLs for the second dive with the gas you have available in an AL80.

Air is less forgiving on the second dive requiring much shallower depths in the 40 ft range on the second dive to stay within the NDL with an AL80.

So from a deco perspective, I can see why an operator may want to only supply smaller tanks if the depths are greater than 80 ft for the first dive or 60 ft for the second dive (and 60ft/40ft on air) - if they are operating with a boat load of pretty fish tourist divers who don't know enough to limit their bottom time.

If it were me having to deal with a boat load of unknown or inexperienced divers with dependents who have attorneys on retainer and the depths were in the 80'/60' range, I'd be tempted to provide AL80s with 32% or 36% to the nitrox certified divers and AL50s to the non nitrox certified divers.

 

[klavdivs]

Actually, on an 80 I've been directed out of the water early several times since I reached that ability. One company in Australia even stuck me with a much smaller tank (63?) on the second dive. Being a "big guy" with 30+ pounds of "natural insulation," it took a fair amount of diving to stop being the first guy back on the boat. I've gone back to the dive ops that DIDN'T short me air. Maui Dive Shop has treated me right regularly on Maui. Nice to know B&B has larger tanks. I will have to try them...

 

[KrisB]

A SAC of .6 is extremely good for a new or tourist diver. I've regularly seen divers that seem to be closer to .8 or .9 here.

And I don't know about you guys... but it seems to me, having such a diver (out of practice / inexperienced) run out of air at depth is far more likely to have a poor outcome than the same diver getting bent. Furthermore, from a risk-management perspective, I'd suggest that getting bent looks a lot more like an accident (providing tables or a computer are followed) than having someone run out of air diving a 50, because you insisted they take a smaller tank so they could stay within NDL's based on their tank pressure.

 

[DA Aquamaster]

I agree entirely. Providing a larger tank puts the responsibility on the diver - where it should be. But sadly when a diver has a stupid attack and gets hurt, kills themselves, etc the victim or the survivors often hire an attorney who points out to the non diving and basically ignorant and sympathetic jury that if the dive operator had not provided them the opportunity to get bent with the larger tank in the first place, the accident would not have happened and/or they suggest the DM should have noticed and told the diver when to come up - which is just not practical - and consequently the accident was not the result of stupidity on the part of the plantiff but rather negligence on the part of the operator or DM. And that approach is so effective that 80% of the time the operators insurance company will force them to settle before it ever gets to court - which just feeds the cycle where attorneys continue to make money off of bogus lawsuits.

Personally, I like the "you are pretty much on your own" North Carolina dive boat approach where they take you out, put you an a wreck and give you a ride back home with no operator supplied tanks, no gearing up assistance, no fruity drinks and no possible miss communication about anything other than that they will verify you are back on board before they pull the hook. I go down with 200 plus cubic feet and no one looks twice at me.

 

[Peter Guy]

TSandM (Lynne) did NOT go out with L. Divers -- we went to Lanai with Ed Robinson.

 

[cuddlefish]

For easy, current-free dives in less than 60 feet of water, I don't understand why the average diver would need anything bigger than an 80. 72s and 80s should be fine for most. Of course deep dives, etc, could warrant a larger tank. But come on...100s and 120s on most easy Hawaii dives is just a bit much for most. I don't think a dive op should be shunned for having 72s. Of course on the other hand, 50s and 63s should not be the main tanks offered.

 

[Blazinator]

. . . But come on...100s and 120s on most easy Hawaii dives is just a bit much for most. I don't think a dive op should be shunned for having 72s. Of course on the other hand, 50s and 63s should not be the main tanks offered.

I agree with you in general, and I certainly cannot expect dive ops to have large tanks at the ready for people like me.

I'm an isolated case, and am not bothered by AL 80s, just lesser tanks. I'm at the 80 dive mark, and still suck up an 80 in 40 minutes, but I really like the extra weight and extra air that a 100 provides for 40 - 60 foot dives.

About five dives into my upcoming Hawaii trip, when I really begin to relax, I expect to get plenty of bottom time on an 80, especially for shallower shore dives.

I have a condition called "pidgeon chest" diagnosed when I was younger, where my sternum sticks out about 3" or so more than the average person, so I suspect that my lungs are slightly larger than most and I may breathe higher volume per lungful than most (need to measure that). Couple that with the fact that I'm eight weeks past my ruptured achilles tendon surgery and am really out of shape since I can't really exercise, so I'll likely be sucking an 80 in a half hour initially (gotta do some biking before May).

Once I'm past 500 dives like most of you, I'll probably have the same attitude as you all.

 

[DiveWestOahu]

biking will make your air consumption worse,,,, you need to start smoking cigarettes,,,, j/k hope your trip turns out well!

 

[DiveWestOahu]

interesting fact,,,, in december when we have the honolulu marathon, those are always the quickest trips for us,,,, boy those runners sure can suck down a tank in no time, i've seen them go thru an 80 in less than 20 min (@ 40 feet)

 

[Blazinator]

interesting fact,,,, in december when we have the honolulu marathon, those are always the quickest trips for us,,,, boy those runners sure can suck down a tank in no time, i've seen them go thru an 80 in less than 20 min (@ 40 feet)

Are you serious!!! That's amazing.

You mean to tell me that increasing your cardiovascular workout increases your lung capacity, which means you will be less likely to get winded underwater by being in shape, but what you do breathe will be higher volumes per lungful, thus draining the 80's faster!?!?!

Holy cardiogaspulation Batman!