Metric and Imperial tanks.

[Kim]

I have to admit to being confused when trying to relate metric to imperial tank sizes.
We dive metric tanks here. These are sized typically in Liters - 12ltr, 14ltr, 15ltr etc. If you then want to know your actual air volume in liters you just multiply by the bars i.e a 12 ltr tank @ 200bar = 2400 ltrs. Imperial sizes though confuse me completely - for instance the often talked about AL80 - what is it's volume in liters? or in cf? Does anyone have any easy tips to compare the two systems?

 

[Marek K]

I have to admit to being confused when trying to relate metric to imperial tank sizes.
We dive metric tanks here. These are sized typically in Liters - 12ltr, 14ltr, 15ltr etc. If you then want to know your actual air volume in liters you just multiply by the bars i.e a 12 ltr tank @ 200bar = 2400 ltrs. Imperial sizes though confuse me completely - for instance the often talked about AL80 - what is it's volume in liters? or in cf? Does anyone have any easy tips to compare the two systems?

Hi Kim...

The true experts will weigh in, as soon as they wake up on the other side of the world...

...But if you can't wait, then the short answer is, it's a little like apples and oranges.

As far as I know, metric tanks are measured according to their un-pressurized volume.

While U.S. tanks are measured according to their equivalent volume in, um, cubic feet -- assuming they're pressurized to about 3,000 psi (~200 Bar)... right? That is, an AL80 pressurized to 3,000 psi, if all its gas were released, would fill a space of 80 cubic feet to a pressure of one atmosphere (which is, of course, 14.7 psi... which you certainly know, being a proper Englishman. Hey, now that the Euro-Constitution is going down in flames, you can once again be proud of your Imperial heritage!! ).

So it depend on what pressure a U.S. tank is pressurized to.

I believe an 80 cu. ft. tank is roughly equivalent in un-pressurized volume to a 12-liter tank. Though I wouldn't be surprised if I were corrected.

--Marek

 

[Kim]

Yes, that's kind of what I figured. So a AL80 rated to 3000psi is 80 cf when it's full - and if it's rated to 3500psi then its again 80 cf when full - is that right? (although the second higher pressure tank will be physically smaller).
I have to say - multiplying your tank ltr size by the bar on your gauge seems a lot easier to do!
Hey - I might be a Brit but I was all for metrication!!!!!! Plus I left the UK in 1978!

My own tanks here are 14 ltrs - so that would be about 100s?

 

[grazie42]

You are far from alone Kim and I support you in your struggle to make sense of those imperial measurements of tanks...

For what it´s worth an al80 is about a 12ltr 200bar...its also a bit confusing that "their" high pressure is actually equivalent to our low pressure (ie 200/232 bar) while "our" highpressure only seems to be achieved during "cave fills"...I´ll leave it open as to whether that means that those tanks are filled in a cave or used for diving in one ;-)

 

[Kim]

You are far from alone Kim and I support you in your struggle to make sense of those imperial measurements of tanks...

For what it´s worth an al80 is about a 12ltr 200bar...its also a bit confusing that "their" high pressure is actually equivalent to our low pressure (ie 200/232 bar) while "our" highpressure only seems to be achieved during "cave fills"...I´ll leave it open as to whether that means that those tanks are filled in a cave or used for diving in one ;-)

Yes I agree! What made me really have to think about this was a post from H2Andy:

i am NSS-CDS, which mandates 70 cf of gas minimum to enter overhead. with my LP
85's overfilled at 2650, that means my minimum is 1200 psi

If 3000psi is about our 200bar - which is the lowest tank fill I've ever seen in bar. Then what is 2650psi?????? and it's an 'over-fill' !!!! By the way - this fill is what he's using to dive caves! (Sorry Andy - I'm not getting at you - I just don't understand something here!)

edit: also to be clear Andy is talking about double 85s - not a single.

 

[grazie42]

I´m not sure that it matters whether its doubles or sigles when you´re talking pressure...volume yes but pressure no...my double 15 liters (232) still read 232 when full (because they are both filled to 232 bar)...the fact that they weigh 40kgs empty is a whole ´nother discussion...

of course my single 8 liter reads 300+ bar but thats an entirely different matter...even if it´s still technically a 80cft bottle....

I THINK 3000psi = 200/232 bar and 4500psi = 300bar...I´m a bit embaressed about my lack of understanding as I took tech-classes in Florida but my instructor was nice enough to let me dive metric and, as it was a wreckdiving class, neither I nor the compressor where stationed in the vicinity of any caves...

 

[miketsp]

I have to admit to being confused when trying to relate metric to imperial tank sizes.
We dive metric tanks here. These are sized typically in Liters - 12ltr, 14ltr, 15ltr etc. If you then want to know your actual air volume in liters you just multiply by the bars i.e a 12 ltr tank @ 200bar = 2400 ltrs. Imperial sizes though confuse me completely - for instance the often talked about AL80 - what is it's volume in liters? or in cf? Does anyone have any easy tips to compare the two systems?

What you think are metric tanks, are rarely really metric.
I don't have my cylinder tables with me at the moment but if I remember rightly a so-called 12ltr tank is really about 11.2ltr and due to non-linearity of gas-laws starting around 200bar your 12ltr tank will not hold 2400 ltrs but around 2150 ltrs.
The same way that 200 bar is considered 3000psi which would mean that 1ATA = 15psi whereas it's really a little less.
In other words many approximations are used to simplify mental arithmetic.

As for 2650 being an overfill - probably just means he was talking about tanks rated for 2400.

Edit #1: some examples of steels here:
http://www.omsdive.com/cyl_spec.html

Edit #2:
link to luxfor US imperial & metric table
http://www.luxfercylinders.com/products/scuba/specifications/us.pdf

link to luxfor Europe table
http://www.luxfercylinders.com/products/scuba/specifications/europe.pdf

 

[Kim]

No Grazie - you are quite correct - the pressure reading will be the same - however double 85s at 2650psi will have twice as much gas as a single - that's what I meant!

 

[Kim]

As for 2650 being an overfill - probably just means he was talking about tanks rated for 2400.

Yeah....but 2400psi comes out with an equivelent fill in Bar that I have never seen - isn't that around 160 Bar? I've never heard of metric tanks like that.

 

[Marek K]

So a AL80 rated to 3000psi is 80 cf when it's full - and if it's rated to 3500psi then its again 80 cf when full - is that right?

As I understand it, yes. Or more-precisely, an AL80 is an AL80, no matter what pressure it's holding.... despite the fact that the "80" designation does imply a certain pressure. The "80" becomes just an abstract description of physical size.

(although the second higher pressure tank will be physically smaller).

No... Why? The tank would be the same size (still an "80"), just holding more pressure.

I have to say - multiplying your tank ltr size by the bar on your gauge seems a lot easier to do!

I agree... if determining the actual amount of gas is important, like in technical diving. For weenie rec diving (like I do), it's pretty esoteric. (*cringe!*)

Hey - I might be a Brit but I was all for metrication!!!!!!

For what it's worth, so was I. You know we (U.S.) had decided to introduce the metric system back in the 1970s... that's why all our cars still have speedometers in both MPH (big numbers) and km/h (small numbers). Didn't take long, though, for all those plans to get dumped. Folks just didn't want to change.

Unlike Canada.

Me? Having been in the (U.S.) military, I got used to thinking in metric for certain things. Distance, for instance... military maps have meter grids. Or weapons range... battlesight range for an M60A1 tank using APDS ammunition was, I think, 1,600 meters. (Dates me!!

)

But water depth while diving? Feet, please. I can visualize 60 feet; I can't visualize 18 meters. Same with tank pressure-- psi, please.

My own tanks here are 14 ltrs - so that would be about 100s?

I'm not sure... if you assume a 12-liter tank = an "80" tank, then a 14-liter tank would be more like a "93." A 15-liter tank would be a "100," unless there's some inaccuracy...

--Marek