Metric and Imperial tanks.

[Marek K]

I agree with that....I even said it myself in post #16!

Yeah... but I was misunderstanding what you'd written in post #3.

See? I told I'd have a futile defense...

 

[Azza]

I have to admit to being confused by imperial measurments as well. Frequently I just shake my head when listening to yanks talk about their tanks.
Well here is my understanding

An AL80 is a 11 litre tank. AL80 refers to the tank holding 80cf at its rated pressure. 11 litres refers to the tanks WC or water capacity. They are generally rated for 220 Bar.
An AL95 is a 11.5L ali that is generally rated to 240 Bar. They suck for buoyancy.
A 12.2 litre is a steel 95cf that is generally rated to 232 Bar.
A 15L is a steel 105cf that is also rated for 232. they suck for trim aparently because of their length. Never used one myself so I am going off hearsay for that.
A 18L is a 123cf, also rated for 232 Bar. These are hard to find around these parts.

Working out the gas volume is easy in Bar.

220 Bar x 11 Litres = 2420 Litres of gas.
240 Bar x 12 Litres = 2760 Litres of gas.
232 Bar x 12.2 Litres = 2830 Litrs of gas.
232 Bar x 15 Litres = 3480 Litres of gas etc etc etc

So if you have a SAC of 15L per minute, and you are diving at 40msw (5 ata) you are using 75 Litres per minute. Hows that for easy?

Hope I was of some help...
Azza

 

[grazie42]

Originally Posted by Kim:
One of the reasons that I started this thread is because of the rule of thirds applied to overhead penetrations. As you normally always have to swim to the entry point - be it a cave or a wreck - then your starting gas when you apply the rule will not usually be a full tank. As the minimum amount of gas that you need is usually measured by volume, while I realize you can work that out before hand in bar/psi, sometimes it is very handy to be able to calculate the actual volume on the fly. This can help a lot when 'gas matching' with a buddy using a different cylinder.
Now - I can do this easily in metric, but I was trying to work out how to easily get there in Imperial as well. If I ever go to Florida to continue cave training, then I will obviously bring my own gear - it's all metric! I already got confused to some extent last year in Hawaii using American tanks!

I´m with you Kim...The way I see it, for diving, I´d rather have the "problem" of initially not knowing the rated bar pressure*(see below) and have easy time calculating volume/air consumption/"time left", "on the fly" during a dive than the other way around...

I took a metric tech-class in Florida ...For me doing those kind of calculations in metric is an added safety factor that I lose if I go Imperial...Maybe because all my experience is metric I only get an intuitive feel for metric numbers...60 feet is just a number until I convert it to metric, the same goes for cft...loosing that safety check during dive-planning/diving is, IMO, a very bad idea...

I´m also looking into cave-training in Florida sometime during the next year so I feel you pain Kim but, for me, I propably won´t do a course with an instructor who isnt "metric"...

*not really a problem as you can pretty much look at a tank and see the difference...also...for "tech" applications 200/232 bar is the norm as most people can´t fill 02/He to more than 200 bar

 

[grazie42]

Azza you posted while I was typing...
For what it´s worth I dive twin 15ls and find it hard to see why someone would have a problem with their length...theyre almost exactly the same length as 12l tanks...

a friend of mine dives twin18s...all I can say is that they require a fair amount of effort to move around...with a full setup (decotanks etc) his equipment weighs about 150kgs/300lbs...call me a sissy but thats a bit much for me...

 

[Azza]

Azza you posted while I was typing...
For what it´s worth I dive twin 15ls and find it hard to see why someone would have a problem with their length...theyre almost exactly the same length as 12l tanks...

A mate told me he dove them for a while and made him really head heavy. I dive twin 12's and find they are slightly head heavy and my mate was saying that while the 15's are the same length, due to the extra width of the 15's, they made him really head heavy. As I said I am only going on hearsay... I would like to try them one day.

friend of mine dives twin18s...all I can say is that they require a fair amount of effort to move around...with a full setup (decotanks etc) his equipment weighs about 150kgs/300lbs...call me a sissy but thats a bit much for me...

Yeah they are huge! Not sure if I would like to lug them around. Twin 12's are heavy enough

 

[miketsp]

..snip..
Seriously... that means that over 200 Bar, gasses start compressing more (exponentially?)? Like, a given volume at 300 Bar holds more than 1-1/2 times as many molecules than at 200 Bar? And what the exponent is depends on the gas?
..snip..

Almost - but the inverse. The gas molecules have a physical volume and different binding forces for each gas type.
So what happens is, as you have more gas in the cylinder, adding just a little more produces a large pressure change because it's as if a lot of the space is already physically occupied by the molecules already in there.
So at 300 bar you will not have 1.5 times the gas you had at 200 bar, you will have less.
In the case of N2 a little less, in the case of He a lot less, which is why Trimix fills at 300 bar are not really an advantage.

I am attaching 2 curves from one of the many sites that treat this subject in detail.
There are many versions of these curves. You can generate them for yourself, for your own cylinder size very quickly in Excel.

 

[Kim]

I am attaching 2 curves from one of the many sites that treat this subject in detail.
There are many versions of these curves. You can generate them for yourself, for your own cylinder size very quickly in Excel.

Mike - do you have a link to a good site that deals with this?

 

[miketsp]

Mike - do you have a link to a good site that deals with this?

Try this one:
http://www.babilim.co.uk/pages/gas_laws.html

 

[TX101]

"Our" system is kinda handy because you can look at one glance at the relative capacities of tanks. A 100 cf tank holds 20cf mroe than an 80 cf tank, regardless of fill pressures.

eh?

Fill pressures and even temperatures are critical in the imperial system. 100cf and 80cf can be compared, assuming they are filled to their rated pressure. But with what I read, particularly regarding tech diving in the US, tanks are rarely filled to their rated pressure.

Quick, which holds more gas... a 12 liter 232 bar tank or a 9 liter 300 bar tank?

Even quicker, which holds more gas... a LP95 overfilled to 4300 psi or a PST120 with a bad fill at 2600psi?

 

[jonnythan]

eh?

Fill pressures and even temperatures are critical in the imperial system. 100cf and 80cf can be compared, assuming they are filled to their rated pressure. But with what I read, particularly regarding tech diving in the US, tanks are rarely filled to their rated pressure.

Clearly I meant "regardless of rated pressure."

Even quicker, which holds more gas... a LP95 overfilled to 4300 psi or a PST120 with a bad fill at 2600psi?

I never said the imperial system was easy! If I knew the tank factors offhand (actually both are about 3.5, which instantly tells me that both have similar volumes at similar pressures, which makes it clear that the overfilled 95 has a ton more gas in it) I could figure out the volume of each tank. 4300 * 3.5 is about 15000 in my head, so ~ 150cf of gas in the overfilled 95 and 2600 * 3.5 is a little over 85 cf.

It's not exact but it gets the job done and from what I'm hearing it sounds about as exact as those metric measurements for about the same level of mathematics.. you just have to know your tank's tank factor, then it's the same simple multiplication you have in the metric system.

I haven't worked with metric tanks on a working basis, so I can't really say which is easier or more useful.. but I think both systems have their plusses and minuses and either works just fine.