Volume and Pressure in Steel LP Tanks

[Adam Samansky]

I'm considering replacing the old ScubaPro 72's on my doubles with some larger steel tanks. I'm debating about what volume to go with... There is some confusion as to listed pressures and resultant volumes. For example, the PST LP 80s are listed as holding 80 cubic feet of air. They are rated for a working pressure of 2400psi, with a "+" rating of 2640psi. At which pressure (disregarding temperature) do they have the 80 cubic foot capacity? Is it 80 cu ft at 2400 or 80 cu ft at 2640?

Thanks for the help!
Adam

 

[Spectre]

They hit their rated volume at the 10% over. So yea, the 2640 is where it's 80 cuft [according to the tank specs, it's 80.6 BTW]

 

[Genesis]

PSTs.

The HP100, for example, is 100 cubes at 3500 PSI. At 3000 PSI it is about 85 cubes. If you only pump them to 3000 PSI, and are happy with 85 cubes, you can even use a 200 bar DIN mainfold (since 200 bar is ~3000 psi) and be "within spec".

Those HP PSTs are very, very nice tanks. Small, light, and with good buoyancy characteristics. The "objection" people have in technical diving has to do with the extra stress of 3500 psi service. But if you don't need all the cubes, you can avoid the stress and the pressure, and just take the smaller capacity. If you're happy with 170 cubes of backgas, a pair of HP100s at 3000 psi are pretty tough to beat.

 

[Adam Samansky]

Gents:

Thanks for the quick reply. Now I guess I'm on to thinking about the OMS/Faber 85s or the larger 98s or 112s. If I need the 98s, I figure that I may as well get the 112s, seeing as doubled they're only about 6lbs heavier than the 98s....

What I really need to do is figure out how much gas I'll need for 2 EANx34 dives at about 95' maxing out what the computer will allow me. I really don't want to have to change tanks on the boat, but want to dive at least thirds on the first dive (penetration) and have some margin on the second (fun).

More food for thought, thanks! Now off to crunch some #s...
Adam

 

[Genesis]

you REALLY want to consider PST tanks, regardless of whether you decide to do LP or HP.

Why? Hot-dip galvanizing. They're the only company doing it. And unless you like chasing rust, they're the only real option for saltwater.

 

[O-ring]

Adam Samansky once bubbled...
Gents:

Thanks for the quick reply. Now I guess I'm on to thinking about the OMS/Faber 85s or the larger 98s or 112s. If I need the 98s, I figure that I may as well get the 112s, seeing as doubled they're only about 6lbs heavier than the 98s....

What I really need to do is figure out how much gas I'll need for 2 EANx34 dives at about 95' maxing out what the computer will allow me. I really don't want to have to change tanks on the boat, but want to dive at least thirds on the first dive (penetration) and have some margin on the second (fun).

More food for thought, thanks! Now off to crunch some #s...
Adam

Isn't that the NDL for EANx34 at 95'? I don't have any tables in front of me, but I think that's close... I would start with figuring your SAC and working from there...with a SAC in the neighborhood of .6 you could do those dives on your 72s with a little extra juice in them or a set of double 80s. IMHO, I wouldn't want to be lugging those huge tanks around unless I needed that much gas.

 

[Big-t-2538]

At 95', on a 34% O2 blend, his PPO would be 1.32...without the tables, I don't know what the CNS clock says for total time in a 24 hr period at that PPO, but it is probably in the neighborhood of 150min with a single exposure of not more than 60 min. (I'm really guessing at those #s). If he is diving thirds on the first dive, he'll probably come nowhere close to the 60 min...so long as he doesn't blow his plan and go below 95ft.....

FYI--MOD at 1.4 PPO on a 34% O2 blend would be 102ft

 

[pescador775]

I use a more liberal FO2 which gives 122 feet max depth at %34. As to the bottom time, no decompression limit, figure on 32 minutes @ 100 feet depth. Your calcs, computer may differ somewhat. Twin 100's should allow two dives to 100 feet if the dive is not too strenuous, but will fall short if you follow the 'thirds' rule. Why carry 1/3 of your gas back on a no decom dive? However, that's your business, obviously.

This type of dive is what makes those HP 100's so nice. They're light and compact. Not so good for partial pressure mixing however. No sweat if you have access to shop with blender or booster. Don't think much of boosters for home mixing, though. Too much complication and hassle. If you go with low pressure tanks be prepared for lugging and swimming with those monsters. The PST LP's are somewhat negatively buoyant but the 'tech' drysuit divers seem to like them. The Fabers are less negative but have those paint chip hassles. anyway, if you jack your own tanks some folks overfill them. Anyway, if the tank dia is kept to outside limit of 7.25 inch you would be less subject to drag while diving and would find them more convenient for transport in some cases.

 

[pescador775]

Rereading Adam's message I see that he doesn't need to return with 1/3 but merely wants the cushion to exit a wreck. I see no problem with this scenario, especially since he would have plenty of gas in the second tank for contingency. Twin 100's should be fine.

 

[Adam Samansky]

Thank you all for your replies. I think I'll wait a bit to see what those new PST "E" series 100s sell for. I'd prefer those, as I could keep the manifolds and bands (all OMS) that I have on the 72s. If those prices are way out of line, it'll either be the PST 80s, OMS 85s or 98s; or, I could just sell my set as is, get a new manifold and bands and just set up a set of PST HP 100s...

Patience is a virtue, I guess...

Adam