Question Calculate gas cubic feet

[rx7diver]

Actually, that one makes sense: it's 80.1 cf ideal, so within rounding. (It's 77.4 cf including compressibility effects.)

Hmmm. I suppose the only way to know for sure is to measure the water capacity of a random sample of Luxfer Al 80's, and then do the computations taking into account the compressibility factor associated with air, and then compute the average.

rx7diver

 

[Tanks A Lot]

Hmmm. I suppose the only way to know for sure is to measure the water capacity of a random sample of Luxfer Al 80's, and then do the computations taking into account the compressibility factor associated with air, and then compute the average.

rx7diver

The math is pretty straight forward here.

The S80 has an internal volume of 11.1L (678in³) and is rated to 207bar (3000psi).

The ideal gas volume this cylinder holds would be (11.1L x 207bar)/1.01325bar = 2267.65L (80.08f³)

At 207bar and 20°C air has a compressability factor of 1.0325 when using REFPROP. Other algorithms like GERG2004 would give slightly different numbers, e.g. 1,03235852380005. The real gas volume would be (11.1L x 207bar)/(1.01325bar x 1.0325) =2196.27L (77.56f³)

This tracks very well with the specifications that Luxfer gives in their schematics, though they probably used a slightly different algorithm for the compressability factor.

 

[rx7diver]

The math is pretty straight forward here.

The S80 has an internal volume of 11.1L (678in³) and is rated to 207bar (3000psi).

The ideal gas volume this cylinder holds would be (11.1L x 207bar)/1.01325bar = 2267.65L (80.08f³)

At 207bar and 20°C air has a compressability factor of 1.0325 when using REFPROP. Other algorithms like GERG2004 would give slightly different numbers, e.g. 1,03235852380005. The real gas volume would be (11.1L x 207bar)/(1.01325bar x 1.0325) =2196.27L (77.56f³)

This tracks very well with the specifications that Luxfer gives in their schematics, though they probably used a slightly different algorithm for the compressability factor.

I thought Catalina made the "S80" (rather than Luxfer). Do the Catalina S80 and the Luxfer 80 have the same water capacity?

But, yes, I agree, the calculation is straightforward once you know the water capacity and the compressibility factor for air at 3,000 psig. Doesn't Dive Gear Express (for one) show how on their web site?

rx7diver

 

[Tanks A Lot]

[...]
Re-measure the pressure in bar.
Multiply this number by the internal volume (13 L).
That is how many bar-litres you have in the cylinder.
Every bar of pressure added will be 13 more litres.
Why is it so easy? Because it was designed to be simple.
[...]

While this is a great approximation, it is not true and can in extreme conditions lead to mistakes of over 20%. Take Trimix 10/50 at 0°C as an example.

A 10.0L cylinder with Trimix 10/50 charged to 300bar.

• Your back off the hand approximation would give us: (10.0L x 300bar)/1bar = 3000L of free gas. This would assume an ideal gas and additionally make the wrong assumption that 1bar = 1atm.
• Correcting for the 1bar = 1atm mistake we get: (10.0L x 300bar)/1.01325bar = 2960.76L. This ideal gas calculation is not correct, as Trimix 10/50 at 0°C and 300bar is far from an ideal gas.
• The compressability factor of Trimix 10/50 at 0°C and 300bar is 1.19677. Correcting for the ideal gas mistake and calculating the real free gas volume we get: (10.0L x 300bar)/(1.01325bar x 1.19677) = 2474L

The real free gas volume differs by a whooping 526L, close to a 21% mistake. Yes, this scenario is extreme and a Trimix 10/50 charged to 300bar is probably out of the question, but it illustrates the point well.

I hate the imperial system, it is absolute garbage. But just because we use the metric system does not mean that it absolves us from using the proper formulas where appropriate. The back of the hand calculation is fine with air and pressures around 200bar, but it rapidly deteriorates when using higher pressures or different mixtures.

I thought Catalina made the "S80" (rather than Luxfer). Do the Catalina S80 and the Luxfer 80 have the same water capacity?

Yes, both make an S80, which are very similar to each other. They do differ slightly with regards to wall thickness, weight, etc.

 

[justinthedeeps]

While this is a great approximation, it is not true and can in extreme conditions lead to mistakes of over 20%. Take Trimix 10/50 at 0°C as an example.

Z-factor! I am gonna run these calcs for my next high helium blend.

How much does that non-ideal behavior affect the mix? A blending concern?

I guess I haven't yet gotten to conditions where this is has /seriously/ thrown me off, or at least it hasn't caused a high error in a mix yet. Might be swallowed up by other sources of error/variation in pressure, temperature, gas fractions etc for less exotic blends.

For up to say ~16/45, I have experimented and succeeded with non-Z-factor partial pressure blending, topping up 3L dil cylinders with a bit of O2, then helium (slowly), then air over that to desired full pressure (in bursts to mix). Close enough for any diving I'm up to. For higher He% undershoot on the air, test, top it again if/as necessary. "Empirical approach"

The up to ~5% variations seen in recreational diving mixes that are attributable to non-ideal gas characteristics are often about the same as the pressure changes we see due to air vs. sea temperatures, drop the cylinder temp ~5-10ºC upon descent and wow where'd my "3000psi" or "237 bar" go?

[Rec diving] I stopped worrying about 10%+ differences as soon as I started carrying a pony bottle.

 

[justinthedeeps]

Anyone post this link yet

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

Or why does my Aluminum 80 SCUBA tank only hold 77 cubic feet of air? ©2018-2020 by Mark Derrick, Gas Blending Instructor at Dive Gear Express ...

www.divegearexpress.com