Why are AL80 tanks often refered to as 12L tanks (rather than 10L?)

[JohnN]

A metric, steel 10L tank will have 2320 L of gas at rated pressure (232 BAR)
An imperial, 11L AL80 will have 2277 L at rated pressure (207 BAR)

In terms of gas carried, an AL80 is therefore equivalent to a 10L steel, I believe that is where the verbal shortcut has come up. The tank is in fact 11L but breathes like a 10L steel.

Ahh. . . those volumes of gas would be true, if air behaved like an ideal gas (which it doesn't)

so at the 200 bar mark, at 80 oF (~27 oC) you lose a little over 3% (3.26%) because those damn molecules keep banging into each other. . .

If you are fortunate to own one of those European 300 bar tanks, the derating factor goes to almost 11%

In pure fortnight-per-furlong units, that AL80 only holds a bit over 77 cuFt

 

[tursiops]

Many dive boats have a similar rule. I doubt you'll find it in any training materials, some boats have such a rule and some don't. Probably more common on boats that cater to vacation divers than boats who cater to technical divers.

Yeah, I know they do. And some disallow reverse profiles. And some demand you go the the stern with your fins off, and some demand you slide along with your fins on. And some won't let you touch the O2 analyzer. What a dive boat does is its own business, of course, but I wonder about some of their practices. 50 bar and 700 psi would be a better "rule," since they are more equivalent. But that represents a huge fraction of one's gas....is that really necessary? Now, a "rule" about getting to your safety stop with no less than X pressure would make more sense. Or leaving the bottom....

 

[Storker]

Ahh. . . those volumes of gas would be true, if air behaved like an ideal gas (which it doesn't)

so at the 200 bar mark, at 80 oF (~27 oC) you lose a little over 3% (3.26%) because those damn molecules keep banging into each other. . .

If you are fortunate to own one of those European 300 bar tanks, the derating factor goes to almost 11%

As long as you keep to 200 or 232 bar tanks, the effect of compressibility/non-ideal behavior/molecules banging into each other is negligible compared to other factors like fill station temperature vs water temperature, depth profile or whether you're a mite more or less stressed that day.

And if you - like me - have reasons for preferring 300 bar tanks, there's an easy fix: subtract 10% from nominal gas volume. Easy, peasy. As long as you aren't planning on PP blending. I really don't want to do PP blending math for a 300 bar tank (or twinset)

 

[Storker]

50 bar and 700 psi would be a better "rule," since they are more equivalent. But that represents a huge fraction of one's gas....

It may be region-specific. When I've dived abroad, my gas consumption has been a lot lower than it is back home. I've had just as long dives on a 12x200 down there as I've had on a 10x300 or a 15x200 up here. I don't think I'm the only one who's using quite a bit more gas when I'm wearing a drysuit and carrying about twice as much weight as I do when I'm in a 5 mil wetsuit. So if I'm fine with 500 surface liters at the safety stop when I'm up here, I ought to be be fine with a little less when I'm down there.

 

[kelemvor]

Now, a "rule" about getting to your safety stop with no less than X pressure would make more sense. Or leaving the bottom....

I suspect it's more of a practical concern. Unless they're in the water with you, they can't really check your gas any time other than when you're back on the boat.

Personally, I've never seen a boat crew actually check. Since I use WAI, they'd have to ask to see the computer on my arm. I think they just create the rule to justify themselves if they decide to tell someone who was completely OOA to sit out.

"back on the boat with 13cuft (368l)" would make more sense.. that's what I calc for an al80 at 500psi. But most divers would have no idea what he was talking about. With my lp108 tanks, 500psi would be 20cuft (566l). However, you do need more pressure than your reg's IP if I'm not mistaken.

 

[Diving Dubai]

Is that really a rule or is it a myth? Does it have a written source in training materials?

I believe it's more of a conventions that's been around so long it's almost a rule.

And you're right, your 500psi is nearer 30bar - plus all the SPGs' I've ever seen have the last 50 in red

My understanding is that the reserve comes from 1/4 of a cylinder

The pedants would of course argue that if 50 bar is the reserve on an 11l (550l), then if they were diving a 15l they should be allowed to drop to 36 bar having the same volume remaining.

 

[Storker]

The pedants would of course argue that if 50 bar is the reserve on an 11l (550l), then if they were diving a 15l they should be allowed to drop to 36 bar having the same volume remaining.

If that's pedantry, I'm proud to be a pedant.

Because math(s).

 

[BlueTrin]

Tank factor never made any sense to me.

I would calculate based on SAC rather than RMV, and on volume rather than pressure. e.g. if I know my sac is 1.0cuft/min I don't need to factor in the tank size. All I need to know is the volume of gas in the tank (150cuft / 1cuft per min = 150 minutes).. although my sac is slightly less than that.

As I said, I don't routinely use RMV. I seem to be remember something else confusing about it but I'll be darned if I can recall what it is right now.

I think it’s easier once you put units.

12L is a volume.
Bars are the quantity of air per volume (pressure)

2 bar means that you compressed 2L of air at surface into 1L

12L at 200bar means that you have 2400L of air at surface pressure (1 bar)

To convert your depth in meters into bars, you divide per 10 and add 1

0m = 1bar
10m = 2bar
20m = 3bar
33m = 4.3bar

If you know your SAC in litres per min, you just need to multiply by the number of bars to know how much air you’ll use at depth.

So if your SAC was 20L per min at surface. At 20m, you are at 3bar so you’ll use 60L per min so 2400L is 40mins of air at this constant SAC rate.

 

[JackD342]

I suspect it's more of a practical concern. Unless they're in the water with you, they can't really check your gas any time other than when you're back on the boat.

Personally, I've never seen a boat crew actually check. Since I use WAI, they'd have to ask to see the computer on my arm. I think they just create the rule to justify themselves if they decide to tell someone who was completely OOA to sit out.

"back on the boat with 13cuft (368l)" would make more sense.. that's what I calc for an al80 at 500psi. But most divers would have no idea what he was talking about. With my lp108 tanks, 500psi would be 20cuft (566l). However, you do need more pressure than your reg's IP if I'm not mistaken.

On two different Explorer Venture liveaboard trips I've taken, they were religiously writing down the time in, the time out, and the air remaining when boarding the boat after the dive. I don't remember if they were also asking maximum depth or not, but I think they were.
Not only do they have a quick profile already accessible in the case someone gets bent, but they were monitoring for divers that need get back on the boat sooner, or in the case of one with my group switch to a larger cylinder.
Now, were they actually checking independently, or just taking our word for it? I didn't think to observe if they were eyeballing any gauges or not; I know for me my word was good enough. But I bet they were checking at least a few divers when we weren't looking to stay safe, probably when running the fill whip around from tank to tank after the dive. Crews like that get pretty skilled at spotting who they need to keep a closer eye on, and who they can just hand a towel.

 

[Mike1967]

Interestingly, the tanks I usually dive with are steel and they are real 15Lt (marked and called so).
I've only seen 13.6Lt ones only once in Bunaken/Indonesia when I asked for "15lt" tanks (I use a lot of air) during my first diving trip. And they were aluminium.

Hmm I've booked 15lt tanks for a Bali trip, I was hoping they were going to be Faber steels like I use at home but then they said they were aluminium. Not too happy that they're actually 13.6lt.