Tank Factor: Neutral Buoyant AL80

[ringmod65]

I recently built a doubles rig with Luxfer Neutrally Buoyant AL80s. They trim out nicely for me and I have no problem getting full 3300 psi fills.

Here's the issue: I'm used to using "5" as a tank factor when dealing with doubled AL80s, but recently realized that this factor might not be accurate in re: the neutrally buoyant cylinders, which have a true capacity (77.4) identical to that of a standard AL80, but a higher service pressure (3300).

__________________________

Capacity / Service Pressure = Tank Factor

AL80
77.4 / 3000 = 0.0258
2.6 x 2 = 5.2

ALN80
77.4 / 3300 = 0.0234
2.3 x 2 = 4.6

__________________________

That strikes me as a pretty wide swing (.6 cu ft per 100 psi) for cylinders with identical capacity ratings, even with a 300 psi difference in service pressure.

Have I calculated this correctly?
Is there enough "fudge factor" built in or should I use 4.5 as a tank factor?

Thank you.

 

[HenrikBP]

Looks correct to me ...

I'd probably use 4.5 as tank factor for the Neutrals and I do use 5 as tank factor for regular AL80 doubles.

Henrik

 

[Blackwood]

In the water, I'd probably call it 9 per 200 (obviously reflecting 4.5).

 

[RTee]

Ringmod...out of curiosity, may I inquire what do you use that number for as I normally use SAC rate as the foundation of my gas management for a specific dive?

 

[lobstah]

for conversion between psi and cuft and back

cuft / tank factor = psi
psi * tank factor = cuft

 

[kanonfodr]

Your math looks right to me. Keep in mind that scuba math is allowed fudge factor as it makes life a lot easier when you may be doing this stuff in your head underwater, so just use 4.5 for your neutral AL80s.

Peace,
Greg

 

[HenrikBP]

Ringmod...out of curiosity, may I inquire what do you use that number for as I normally use SAC rate as the foundation of my gas management for a specific dive?

I use it for figuring out the amount of gas each diver in the team has available in case of dissimilar tanks. Also makes it easy to figure Rock Bottom pressure for each tank configurations.

During the dive I keep track of my air consumption based on my average consumption (what I expect it to be in cu.ft/min at that depth converted to PSI/min) to check nothing unexpected is going on.

Henrik

 

[Blackwood]

Queue a post about "metric cylinders" not needing tank factors (and misunderstanding the fact that L/BAR IS a tank factor).