An 80 is an 80 right?

[Stu S.]

When you are at 1500psi on your hp80, how does your air supply compare to your buddy at 1500psi on his aluminum 80? (You have less air than him).

There's an important concept. Off the Florida Coast, boat operators often say "head back to the anchor line and go up when you reach 1,000 pounds" (or some similar number). That's a habit from when 95% of the divers they saw had an aluminum 80 that filled to 3,000psi.

With a 80 cu. footer that fills to 3,500, 1,000psi represents a different volume of air than the "80" that fills at 2640psi. So that 1,000psi "rule of thumb" is handy, but not exact.

We consider our tanks and plan our own dive accordingly.

 

[Doc Intrepid]

OK my thinking is sorta on track. Thanks, yes my wife loves me, I don't have enough insurance for her to try to kill me. Thanks for the tank spreadsheet, I was looking at one like it.

Luxfer also makes "neutral" aluminum 80s, which have the advantage of not being (as) bouyant when empty (or nearly so). Luxfer achieves this by adding thicker walls and base to their tanks (which increases empty weight). While the external dimensions are identical, the interior dimensions are somewhat smaller. Ergo, you need to compress gas to a higher rate to fit the same amount of gas into a smaller volume, and (shazam!) accordingly the rated fill pressure for these tanks is 3300.

Ordinarily this isn't a problem for most shops, however, you may need to bring it to their attention that you want the fill to 3300 instead of 3000 if they aren't paying attention to the data on the tanks.

Just FYI