Northwest Dive Club

I need some info to settle a debate….

If a diver has a 85cf tank that is stamped 2400psi but has a + rating, is it full with 85cf of gas at 2400psi or 2640psi?

TIA!

2640psi is the full working pressure of that tank.

The direct way to know this is to fill it with clean water to measure its internal volume (e.g. weigh empty vs. full, 1kg = 1 liter volume), then multiply that volume by the number of atmospheres of pressure when filled with compressed gas.

DGX claims a steel LP85 has an internal volume of 13 liters
https://www.divegearexpress.com/library ... capacities

At 165 bar (~2400psi) that would contain
13 L * 165 bar = ~2150 L*bar = ~76 cf*atm

At 183 bar (~2650psi)
13 L * 183 bar = ~2375 L*bar = ~84 cf*atm

This really underscores another area where finally switching to metric would be so helpful! The cylinders are even fabricated in a metric country.

In Florida, it is reportedly commonplace for cave divers to fill their modern steel Faber 'LP' cylinders to ~200+ bar (even 250 bar), which is in the 3000 to 3500 psi range. That would be up to ~110 cf for an '85?'. As far as they are concerned, the US D.O.T. LP 2400(+)psi rating was just a government stamp to get the cylinders imported. No public dive shop here would be crazy enough to risk this so you would need to 'know a guy'

eh.haole wrote: ↑Wed Apr 26, 2023 7:43 am At 183 bar (~2650psi)
13 L * 183 bar = ~2375 L*bar = ~84 cf*atm

Hm.

In other words, you need to fill it to the full+ rating, 2650 psi, to achieve the 85 cu ft.

Thanks everyone!! Y’all helped me win the debate!