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

[BlueTrin]

@oppo I think it is safer to tell them it is 11L. They can workout themselves the real capacity once they measure the pressure.

If you tell them it is 12L they may multiply 12L by the pressure in bar and double count the overpressure ?

 

[afieldofblue]

@oppo I think it is safer to tell them it is 11L. They can workout themselves the real capacity once they measure the pressure.

If you tell them it is 12L they may multiply 12L by the pressure in bar and double count the overpressure ?

Yep I agree - definitely better, but from a marketing point of view, not sure

 

[kelemvor]

personally I think that cubic feets are pretty useless as tank volume measurement

One could say the same about the liters rating. What seems odd to me is that you measure it at surface pressure, not actual pressure. The "cuft" rating is based on the volume of gas inside at a given pressure.

I.e. an al80/11l tank that's supposed to hold 77.4cuft at 3000psi might actually contain 85.4cuft at 3300 psi. Or another more valuable use is when overfilling low pressure tanks. My lp108's hold 108cuft at 2640psi. I routinely fill them to 3600psi and so they have 147cuft in them. I have no idea how you'd calculate what's actually in there based on the metric system. Maybe 11l/surface pressure (14.7) * actual pressure? So an 11l tank filled to 3300psi is 2469l of gas - is that right?

Seems counter intuitive to measure a tank's internal volume of gas based on how much uncompressed water it holds. Many metric users are happy with the system so it must be simply a matter of what you're used to.

 

[Storker]

I.e. an al80/11l tank that's supposed to hold 77.4cuft at 3000psi might actually contain 85.4cuft at 3300 psi.

Well, that's where water volume comes in handy. My tanks are 10L, period. No matter the fill pressure. If I fill them to 200 bar, I'll have 2000 surface liters. 250 bar? 2500 surface liters. 300 bar? 3000 (minus 10% because compressibility) surface liters.

Seems counter intuitive to measure a tank's internal volume of gas based on how much uncompressed water it holds.

It isn't, really.

10L tank, 200 bar left: 2000 surface liters left
12L tank, 100 bar left: 1200 surface liters left
15L tank, 50 bar left (you'd better be at the safety stop, mate!): 750 surface liters left.

RMV of 15 SLM, 15L tank: 1 bar/minute at the surface, 2 bar/minute at 10m, 4 bar/minute at 30m. 10L tank? 1.5 bar/minute at the surface, 3 bar/minute at 10m, 6 bar/minute at 30m. RMV of 20 SLM, 10L tank? That's 2 bar/minute at the surface, 4 bar/minute at 10m and 8 bar/minute at 30m.

The math is so easy that I can do it in my head at 30m. What I can't do, even stone cold sober topside, is wrapping my head around your feet, cubic feet, psis and tank factors

The only weird thing about the metric tank capacity system is the OWD rule of "be at the safety stop with at least 50 bar left". If I'm diving my 10L single and my buddy carries a D7x300, she'll have 40% more gas than I have when we're both at 50 bar. If she carries a single 15, she'll have 50% more gas than I have.

 

[tursiops]

OWD rule of "be at the safety stop with at least 50 bar left"

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

 

[Schwob]

Ok - I didn't write them down today but they're 100. We don't have a "15L" on tomorrow but I'll take a snap at the filling station if I have time.
I think they're all from dive supply so probably the ones listed on page 93 here -
The 13L has a higher service pressure, but we also fill to 200 bar.

Retail Catalogue 2018-2019View attachment 500730

Thanks.
Just as an FYI, I am asking because my air consumption seemed to have gotten a little higher if those actually were 15l tanks.... but if they were indeed 100cf tanks at a rated 3300PSI and I got essentially 200 bar (2901 PSI) Then it all makes more sense again...
And I am generally curious if there are any other kind of Al100 tanks ... out there in the metric world... or if, by whatever name (a then whoppingly incorrect 15l name i.e.) they are all just the same old Al100s....

 

[Storker]

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

My OWD manual isn't easily accessible, but I'm pretty certain we were taught that during the OW class. I guess it's as much of a rule as your 500psi "rule", but a bit more conservative. If you're carrying a 15L it's a lot more conservative.

Al80 (11L), 500 psi (35 bar): 385 surface liters
10L, 50 bar: 500 surface liters
15L, 50 bar: 750 surface liters

When one of my tanks was a 15x200, I was quite comfortable breathing it down to some 35 bar before I was at my safety stop. Because we used 10 liter tanks during my OW class, and we were told "50 bar at the safety stop". That was long before I learned that you 'murricans are comfortable with 500psi. I'm not sure if that's at the safety stop or if it's back on the boat, though.

 

[kelemvor]

Well, that's where water volume comes in handy. My tanks are 10L, period. No matter the fill pressure. If I fill them to 200 bar, I'll have 2000 surface liters. 250 bar? 2500 surface liters. 300 bar? 3000 (minus 10% because compressibility) surface liters.


It isn't, really.

10L tank, 200 bar left: 2000 surface liters left
12L tank, 100 bar left: 1200 surface liters left
15L tank, 50 bar left (you'd better be at the safety stop, mate!): 750 surface liters left.

RMV of 15 SLM, 15L tank: 1 bar/minute at the surface, 2 bar/minute at 10m, 4 bar/minute at 30m. 10L tank? 1.5 bar/minute at the surface, 3 bar/minute at 10m, 6 bar/minute at 30m. RMV of 20 SLM, 10L tank? That's 2 bar/minute at the surface, 4 bar/minute at 10m and 8 bar/minute at 30m.

The math is so easy that I can do it in my head at 30m. What I can't do, even stone cold sober topside, is wrapping my head around your feet, cubic feet, psis and tank factors

The only weird thing about the metric tank capacity system is the OWD rule of "be at the safety stop with at least 50 bar left". If I'm diving my 10L single and my buddy carries a D7x300, she'll have 40% more gas than I have when we're both at 50 bar. If she carries a single 15, she'll have 50% more gas than I have.

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.

 

[Storker]

I would calculate based on SAC rather than RMV

AFAIU the difference between SAC and RMV is whether it's at depth or topside, but I may well have misunderstood. I've logged my gas consumption since I bought my first gear which included an AI PDC. It varies between 12 lpm and 20+ lpm, depending on conditions and exposure protection (wetsuit or drysuit).

So I'm assuming I'm burning from 15 (best case) to 20 (worst case, unless I'm freaked out) liters per minute. That's actual volume, not surface volume.

Let's say 20 for easy math. That's 20 surface liters at the surface, 80 surface liters at 30m. My tank is 10L. So I'll be burning 2 bar/minute at the surface and 8 bar/minute at 30m. My tank is rated to 300 bar, let's say I have 240 bar left when I'm at 30m. That's 30 minutes to empty, 20 minutes to 80 bar, 10 minutes to 160 bar. If I'm burning 15 lpm, that's 60 SLM @30. That's 40 minutes to empty, 30 minutes to 60 bar, and 20 minutes to 120 bar.

 

[kelemvor]

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

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.