Volume of air in tank at a given pressure/temperature

[Jim Lapenta]

My lp 85's have 3200 in em right now. Which gives me 103 cu ft each or 206 cuft on my back. Even if a compressor will only fill to 3000 there's no reason for you to get short fills other than laziness on the part of filler.

 

[Papa_Bear]

LP95s are full at 2640, not 2450.

I see you got my point, but not to point out that pimple on your nose, it depends on if there is a + or not. But thanks for your impute! Percentage is the easiest in my book and no formula to remember! Remember if you have air continue the dive if your gage says low, in the red, or it gets hard to breath the dive is over!

 

[SparticleBrane]

Or better yet, understand the equation and just memorize it.

Memorization -- the lowest form of learning.

Problem solving -- the highest form of learning.


Learn how to solve the problem, rather than memorize a formula.

 

[peterbj7]

Tanks are generally rated at some overfill pressure...
A LP 85 holds 85 CF at 2640 psi. At 2400 psi it holds about 77 CF.
An AL 80 holds 80 CF at 3100 psi; 77.4 CF at 3000 psi

Rick - are you sure that's right, that under the US system tank volumes are assessed at the rated overfill pressure, not the nominal maximum fill pressure? I have tanks from several different manufacturers and I believe (I'm not sure) that the convention used varies across manufacturers.

Reading through the responses in this thread people seem to hold strong views - the only trouble is that they vary between the two conventions. And what did one person mean by saying that we've only used the kelvin temperature scale since 1968? First I've ever heard of it - of that usage, that is, I'm quite familiar with the kelvin scale.

I've often wondered how the American system of tank measurement ever came about, of measuring the amount of air that can be compressed into the tank at some predetermined fill pressure, as to me it seems wholly illogical and difficult to work with. Witness this thread. The system used almost everywhere else (not just Europe) is to measure the actual volume of the tank, and that is both simpler to understand, intrinsically more meaningful, and much easier to work with.

 

[vixtor]

That's the problem with the imperial units for scuba tanks.

In the metric system, we discuss about the tank volume (10 liters, 12 liters, 15 liters and so on) and the rated pressure (200 atm, 232 atm, etc.). The available volume of air is tank volume * indicated pressure (the value read on the SPG). The maximum available air is the tank volume * rated pressure.

In imperial units you don't give enough information about the tank. You say 85 cubic feet. Thats the volume of air. But without also saying either the tank volume or the rated pressure, the information is incomplete and cannot be calculated.

 

[Rick Murchison]

Tanks are generally rated at some overfill pressure...

Rick - are you sure that's right, that under the US system tank volumes are assessed at the rated overfill pressure, not the nominal maximum fill pressure?

The only tanks I know of that are rated at their fill pressure are HP (and 3442 psi) steels that don't have a "+" rating when they come from the manufacturer - and that's a fairly recent phenomenon.
Advertisers tend to round up, and to round up from the highest possible number - standard AL 80 is 77.4 at 3000; the steel 72 is 71.2 at 2475, and all LP steels that I know of have a CF rating based on a "+" fill.
So... at rated pressure a steel 72 is really a 65, and a LP 85 is really a 77.
Rick

 

[bfw]

On a side note to the original question of the thread.. shoudn't this type of knowledge be explained (if not in school) at least in the most basic "open water" scuba training?

It is.

 

[vladimir]

is the volume always directly related to pressure assuming the same temperature

Ah yes...but...if you re-read my original question, it had nothing to do with pressure or volume of gas with respect to temperature...I know Boyles Law

Oops, my mistake. It sounded like maybe you weren't sure.