Converting liters to cubic feet

[Kendall Raine]

(The bilingual in the crowd will notice 2600 L ~= 90 cuft. Not surprising, as it's the same tank and approximately the same pressure, after all.)

Exactly. Well said. See also post 18 @tursiops . Tomato tomato. The real issue is duration planning and for that you need real gas content. Everything else is just chauvinistic jerking off.

 

[JuLe]

Personally, I'm skeptical at tables in safety-related applications. As I see only the results of somebody elses' math who I may or may not trust to do it properly. I don't see the calculation / their formula.

Agreed. Doing the math yourself, you are taking the responsibility of safety in your own hands.
My point was to use the table to better understand stuff being discussed here for example. I see a lot of comments on tank sizes in different threads, but don't know what to compare a HP100 to as it relates to my diving. For stuff like this, a quick reference is handy.

 

[JuLe]

It was just bugging the heck out of me that I couldn't figure out why the conversion calculations seemed so far off and made no sense.

I'm gradually working my way into the metric system but just like trying to learn even a few words of Greek, it's a process.

My brain is like a sieve nowadays.

I hope we managed to point out what was making the conversions so far off and not give you a splitting headache with all the extra math examples spanning over 6 pages.

 

[LI-er]

I hope we managed to point out what was making the conversions so far off and not give you a splitting headache with all the extra math examples spanning over 6 pages.

Yes in between all the extra math and off topic posts, you and a few others clarified this for me quite well.

 

[Sascha314]

Normally, you don't have those decimals. Take a 12l tank filled with 300 bar, so you get 3600l of usable air (at surface pressure). You can do it even in your head, without rounding or decimals.

Be careful: For a 300bar filling you cannot use the ideal gas equation, you overshoot a lot. Realistically its more like around 3200 liters. Relevant for pre planning.

Yep, then it works exactly as the metric. Sometimes the math works out even easier. I'd rather multiply by 3 (Imperial tank factor) rather than 13 (metric tank factor) for an LP85/13L cylinder. Or 6 instead of 26 when doubled up!

I dont know, why you make complicated. In metric you do not need to know the tank factor.
Lets assume you have 200 bar.
With a 12l its 12x200 = 2400 l (approx).
With a 15l its 15x200 = 3000 l (approx)

If you have doubles its 2x.

Both the metric and imperial systems for naming tanks are deficient. But imperial is much worse.

A diver wants to know 4 things about a tank: its physical size, the amount of gas it currently holds, the amount of gas it can hold, and whether it is steel or Al so you know roughly how buoyant it is.

Metric gives you the physical size. If you know the current pressure, it also easily gives you the amount of gas it holds (size x bar = liters of gas). But since it doesn't spec the material or normal fill pressure, you don't know the buoyancy or how much gas it can hold. These problems do go away if everyone is using steel cylinders filled to a standard pressure (232 bar?) which appears to be the norm for much of Europe. The problems also go away if the cylinder material and fill pressure are separately stated.

Why do you need the full filling for your buyoncy calculation?
I want to know the buyoncy at low pressure so I can still safely stay at 1m with say 20bar left.
I can then memorize/write down
12l with empty weight of 14kg I need x amout of lead.
AL80 with 15kg similar.
Now I rent a tank and look at the stamp weight and so approximately I need to add or "ditch" the difference in weight to my reference tank. Its not entitely true, and just round up for the first try. Anyway we are usually have only 1kg modifications to lead possible for rental lead.
You can also calculste it: internal volume, density of material anf weight gives you the displacement of the tank. Add weight for filling at a given pressure, take into account silinity and voila you can exactly calculate the buyoncy.


Question is always: can you use a calculator, or do it in head. You need to do that quick and underwater or have time?

 

[Bento.Box]

These problems do go away if everyone is using steel cylinders filled to a standard pressure (232 bar?) which appears to be the norm for much of Europe. The problems also go away if the cylinder material and fill pressure are separately stated.

At least in Switzerland it's not the case.
We have 300 Bar (probably standard nowadays), 200 Bar (old school) and I've seen some public compressors filling to 230 Bar as well.

Most of our tanks are steel, but Sidemounters who actually do sidemount stuff (as in cave diving etc.) like to use Aluminium as it makes it much easier to handle in the water. I don't because I use it for redundancy only and thus never detach my tanks in the water - they are also part of my weight system.

I've never seen carbon reinforced tanks in the wild, probably because they have no real application in diving.

Not sure what other countries are doing in Europe.

Whenever I'm travelling I ask for material (for ballast computation), tank size (this I have to put in my computer anyway), fill pressure (gas calculation and verification of correct filling) and (nitrox) mixture (computer algorithm and depth limit) anyway.

Be careful: For a 300bar filling you cannot use the ideal gas equation, you overshoot a lot. Realistically its more like around 3200 liters. Relevant for pre planning.

Yes, you're right.
I typically make sure to fill the tanks up by letting them cool down intermittently, and I also use the final pressure for my calculations (typically around 270-290 Bars). And of course, there are reserves that I don't calculate.

E.g. a more realistic calculation would be:
12l x (220 Bar / 1 Bar) = 2640l of usable air during the dive
(270 Bar filling - 50 Bar PADI reserve)

And if you do the same for 200 Bar tanks (easier to fill to 200 bar)
12 x (150 Bar / 1 Bar) = 1800l

So the non-ideally filled 300 Bar tank holds around 47% (2260l/1800l) more usable air (reserve not counted) than the 200 Bar tank of the same size.

Better to take more air and not need it than the other way around.

 

[clownfishsydney]

Remember, there are only three countries in the world that do not use the metric system. The good old United States of America, Liberia (established by ex-US slaves) and Myanmar, a dictatorship country.

Metric is so much simpler to use. Same with scuba tanks. Also, apart from USA, most tanks are now rated to 232 bar or higher. No such thing as a low pressure tank in Australia for many, many years. In addition, the standard tank here is mostly steel now, very few people have aluminium apart perhaps from some dive shops or charter boats.

One last thing, apart from USA (and maybe Canada), aluminium is the correct spelling and pronunciation of the metal that beer cans are made from.