Metric and Imperial tanks.

[jonnythan]

Nope. I mean, yes, your logic is correct. but nope.

Again as I understand it, an 80cf tank refers to the physical size of the tank, not how much gas is in it. The 80 cubic feet from which the nomenclature is derived assumes a pressure, I think, of 3,000 psi. But an 80cf tank that has, say, 3,500 psi in it (or, for that matter, 2,800 psi) will still be referred to as an 80cf tank... even though it now holds more (or less) actual gas. Does that make sense? :06:

Just like a 12-liter tank is still a 12-liter tank, no matter how much pressure it holds.

Maybe we do need to go to metric...

--Marek

Kim had it correct, but you misunderstood his explanation.

An 80cf tank holds 80 cf when it's full to its rated pressure. An 80cf tank with a rated pressure of 3000 psi will have a larger internal volume than an 80 cf tank rated to 3500 psi.

Kim definitely understands as far as I can tell. It is kinda difficult to convert actual tank pressures into gas volume using the imperial measurements. However, we do have some tricks.. for instance, tank factor. Let me explain.

An Al 80 has a tank factor of around 2.5. To use this number, you take the current psi, multiply by 2.5, then divide by 100. So an Al 80 at 3000 psi has 3000*2.5 = 7500 / 100 = 75 cf (it's approximate.. the actual volume at 3000 psi is 77.4 cf). An Al 80 at 2500 psi has 2500 * 2.5 = 6250 / 100 = 62.5 cf (actual volume is 64.5 cf).

To get the tank factor for a tank, just divide the rated full volume by the rated pressure and multiply by 100. So for an Al 80 it's 77.4 / 3000 * 100 ~ 2.5. For a PST E8-130, which holds 130 cf at 3442 psi, it's between 3.5 and 4.

So, while Kim does understand that it is difficult to go from pressure to volume in the imperial system where we give tanks volume measurements based on *full* capacity, it's not impossible. "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. Quick, which holds more gas... a 12 liter 232 bar tank or a 9 liter 300 bar tank?

 

[Boogie711]

Marek - I'm sorry but you're wrong.

Kim is correct. We refer to tanks as their capacity when filled to rated pressure.

You can have an HP100 which is actually smaller in phsyical size than an AL80... because it's filled to a higher pressure, it holds more gas.

 

[jonnythan]

I believe Marek is the most confused out of all of us

 

[Marek K]

I believe Marek is the most confused out of all of us

Yeah... no kidding...

Thanks, you all, by the way, for finally waking up over there Stateside...

Now I understand. I think. The nomenclature of a U.S. tank is derived from how much gas it can hold at its rated pressure, when that gas is un-compressed. With most tanks, that's at 3,000 psi. And a higher-pressure-rated "80cf" tank would be smaller than a 3,000 psi-rated one. Duh.

Thanks for the smack upside the head. And, Kim, sorry for the confusion.

--Marek

 

[Marek K]

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

Is this a trick question? Are you making fun of me now? Huh? Huh?

Oh... it's kind of a test. OK, then, they're pretty close. Assuming they're both exactly their rated volume, and are filled exactly to their rated pressures, then the first one (12 liters) holds more molecules of gas, barely... the 12-liter tank has 1.33 times more empty volume, while the 9-liter tank has only 1.29 times more pressure. Right? :06:

 

[miketsp]

Is this a trick question? Are you making fun of me now? Huh? Huh?

Oh... it's kind of a test. OK, then, they're pretty close. Assuming they're both exactly their rated volume, and are filled exactly to their rated pressures, then the first one (12 liters) holds more molecules of gas, barely... the 12-liter tank has 1.33 times more empty volume, while the 9-liter tank has only 1.29 times more pressure. Right? :06:

No, they're not pretty close due to the non-linearity of the gas laws above 200 bar.
To answer with any precision you will need to know the gas composition.
See this page for a nice calculator:
http://www.hull.ac.uk/php/chsajb/general/vanderwaals.html

 

[Kim]

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

So I did *get it*! I feel like I passed a Maths class!

As for the above statement I can see what you are getting at - and for full tanks it works out well. It gets very different though if tanks aren't quite full.

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!

 

[Marek K]

No, they're not pretty close due to the non-linearity of the gas laws above 200 bar.
To answer with any precision you will need to know the gas composition.

Well *ahem*, yes of course... I was just making the obvious assumption that the gas... um... compressed linearly above... uh... 200 Bar. Yeah. that's it.



Are you falling for that?

Didn't think so.

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?

--Marek

 

[Marek K]

So I did *get it*! I feel like I passed a Maths class!

Kim--

I did misunderstand what you were saying. That was based on my missing the fact that U.S. tank nomenclature is based on the tank's rated pressure, whatever it's rated at; not on the assumption that the tank pressure necessarily is 3,000 psi.

But in my own rather puny and futile defense, I think I was partly correct... if a tank is, say, an 80cf (rated to 3,000 psi), then it's still called an 80cf even if it's under-filled to, say, only 2,800 psi. Or if it's slightly over-filled.

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!

And I used to get confused diving in Europe and the Red Sea, with my U.S. gear -- all feet and psi! You should see the divemasters, when they see my gear...

--Marek

 

[Kim]

But in my own rather puny and futile defense, I think I was partly correct... if a tank is, say, an 80cf (rated to 3,000 psi), then it's still called an 80cf even if it's under-filled to, say, only 2,800 psi. Or if it's slightly over-filled.

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