Converting liters to cubic feet

[Sascha314]

Why so? We fill to 230 and even more.

Well I am not filling myself. I am in a club with a monthly fee, tank fillings are then for free unlimited - so I do not really care. Usually the tank heats up quiet a bit, so its like 210-215 and cooled down like 195 plus minus. I also had the same with my previous club, where I lives and also at a lake I visited for a couple of days.
If they have time they let it cool and top up. Then its more like 220-230 after it cooled down.
At vacation basis with steel tanks i also got usually around 200 bars.

The reason? Idk maybe safety margin and time for cooling. A little effect maybe isnduento my spg. It seems to be at the "lower end", but well within uncertainty. Tanks from friends from the same filler show more pressure from their spg, but only by around 10 bar, ans also my Galileo luna transmitter shows a bit more, however is pretty much then equal below 80 bar. So its ok for me.

 

[BoltSnap]

Well I am not filling myself. I am in a club with a monthly fee, tank fillings are then for free unlimited - so I do not really care. Usually the tank heats up quiet a bit, so its like 210-215 and cooled down like 195 plus minus. I also had the same with my previous club, where I lives and also at a lake I visited for a couple of days.
If they have time they let it cool and top up. Then its more like 220-230 after it cooled down.
At vacation basis with steel tanks i also got usually around 200 bars.

The reason? Idk maybe safety margin and time for cooling. A little effect maybe isnduento my spg. It seems to be at the "lower end", but well within uncertainty. Tanks from friends from the same filler show more pressure from their spg, but only by around 10 bar, ans also my Galileo luna transmitter shows a bit more, however is pretty much then equal below 80 bar. So its ok for me.

Got it, thank you. I just thought there was another reason due to some issues where you live.

 

[Bento.Box]

Only if you always round up or down. Otherwise, they tend to cancel each other out. Look, you are only needing maybe one decimal place, if that....so why fuss with whether the fourth or fift palce is wrong?

Not sure what you mean. Every time you round it's an additional operation. Easier to save yourself the effort of rounding and just do it at the end.

And you only get those odd numbers when you calculate in imperial. But then you're already used to that.
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.

 

[lowwall]

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.

Really the only issue is if you are travelling somewhere, tanks are not specified by anything other than volume, and it turns out that you've hired tanks with different characteristics than you thought you were getting.

Imperial sort of gives you everything, but only approximations and only with prior knowledge and more difficult math. The prior knowledge required is that 1 atmosphere = 14.7 psi (at standard temperature and pressure), AL indicates Aluminum which is typically rated for 3000 psi (207 bar), HP indicates steel rated for 3442psi (237 bar), and LP indicates steel rated for 2640psi (182 bar). Since tanks are (sot of) named by their capacity at the rated fill pressure, you can get the current capacity in cubic feet by dividing the current pressure by the rated pressure and multiplying by the nominal capacity. For example if you are diving an HP100 and your SPG (conveniently) reads 1721psi, then you have 1721/3442*100 = 50 cubic feet of gas left. Another example is for an LP72 "cave filled" to 3500psi, you get 3500/2640*85 = 95 cubic feet of gas (assuming ideal gas laws hold, which they don't). You can get the size of the tank by dividing the nominal capacity by the number of atmospheres it holds at the rated pressure. For our HP100, that's 100/ (3442/14.7)=.427 cubic feet or 12.1 liters.

And then you look up the spec sheet and find out that your Faber HP100 has an actual internal capacity of 12.9 liters, because the actual capacity at its rated pressure is 101.3 cu feet. And, did we mention that an AL80 is 77.4?

Short version. You can have incomplete (European 12l) or complete, but overcomplicated and inaccurate (US AL80).

 

[Edward3c]

Why so? We fill to 230 and even more.

In the U.K. it’s illegal to fill past the working pressure, likely to be the same in other European countries. A fast fill to 232 will cool down to sub 200, 300 fills are even worse cooling to around 260.

 

[justinthedeeps]

@LI-er it sounds like a great opportunity to just dive in metric units, particularly in a place where metric is widely in use. All the computers support it and the math is actually much simpler!

Just call that Al80 what it really is (an ~11L) and go diving.

Had this experience while training in Australia, and haven't really looked back at Imperial units much since. Even when back in the US.

Bar is super simple, 1 bar is about 1 atmosphere and anything over 200 bar is a full tank.

Why even bother trying to work out how many cubic feet of 1-atmospheric gas crams into the tank? It's not even a round unit compared to typical lung volumes or SAC rates (litres is much closer.....)

10-20 litres per minute SAC makes the capacity math super simple.

~2000 litres @ 10-20 L/min is 100 to 200 mins at the surface, or 50 to 100 mins at 10 metres. 25 to 50 mins at 30 metres (~100ft). It's brutally simple. 10 metres add one atmosphere, add 1 to the denominator.

Metric units are actually widely in use throughout many US industries etc.

It's just less publicly advertised since the Reaganites (and now MAGAs) grabbing fringe votes by pretending its a great idea to do a national 1800's reenactment (do you know what conditions and wages were for normal people back then? Pretty sh!t, and guess what, it was all based on immigrants........)

Feet miles pounds etc isn't even American, it is an English system and they've already largely moved on from it

What was that about tea partying to cast off the Imperial English etc?? "Imperial system"

 

[JoeTPhilly]

Using Tank Factors makes imperial calculations pretty easy.

 

[inquis]

And, did we mention that an AL80 is 77.4?

Hey now, don't pick on the poor AL80! It's only because of compressibility that it's not 80 cuft (80.1, technically) at its rated pressure. Same reason a 12L at 200 bar isn't actually 2400 L, but rather 2326L.

 

[inquis]

Using Tank Factors makes imperial calculations pretty easy.

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!

For those who don't know, the tank factor is the ratio of capacity to pressure. LP85: ideal capacity of 82.4 cuft at rated pressure of 2640 psi (or 26.4 hundred psi to avoid decimals in the answer) 82.4/26.4 ~= 3. (Yes, they rounded up when naming it.) A 3000 psi fill has about 3*30=90 cuft of air (ignoring compressibility).

The only problem is we have to memorize the tank factor. It is stamped on the side of the Euro tanks (aka the water volume). That's because a 13 L cylinder holds 13 L of gas at 1 atm (1.013 bar). Tank factor = 13 / 1.013 ~= 13. A 200 bar tank therefore has about 2600 L (again, ignoring compressibility).

(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.)

 

[LI-er]

Why even bother trying to work out how many cubic feet of 1-atmospheric gas crams into the tank? It's not even a round unit compared to typical lung volumes or SAC rates (litres is much closer.....)

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.