Difference in bottom time between LP77 and AL80?

[volyblmn]

I've seen a lot of debate about the pros/cons of LP77 and AL80 tanks. What I'd like to know is this -- is there a difference in the amount of breathable air you get from each tank? I know the Worthington 77 is rated to about 2400psi and the standard AL80 is 300psi -- does that mean you really get that much less air to breathe? I wasn't sure if more psi necessarily meant more air, or if the cubic feet was the important number. I'm a newbie, so go easy on me if this is a dumb question. (This may need to get moved to the Worthington LP77 thread, but I wasn't sure.)

 

[dsteding]

Same.

Cubic feet is at rated pressure (for the LP 77 that is 2640 psi because it is a "2400 +10%" tank). So the LP 77 holds 77 cubic feet at 2640 psi. An AL 80, btw, also holds 77 cubic feet (the "80" is from rounding up a bit). It does so at 3000 psi.

Now, not that I would EVER recommend such a thing, but if you buy the LP 77 and treat it like an AL 80 (i.e., fill it to 3000 psi) then you will get a bit more gas, probably 7-8 cubic feet (I haven't done the math).

The nice thing about the 77 is its buoyancy characteristics. If I was still allowed to buy tanks, I'd pick up a couple for a set of small doubles . . .

 

[Walter]

An AL 80 actually has about 77 cu ft of air at 3000 PSI. The difference will come if/when the steel tank loses its plus rating. I'm seriously looking at a couple of those 77s.

 

[mike_s]

I wasn't sure if more psi necessarily meant more air, or if the cubic feet was the important number.

Cubic feet is the more important number.

The LP-77 is rated at 77 cubic feet at 2640psi.
The AL80 is actually a 77 cubic foot tank at 3000psi. It just stored at a higher pressure, so it's the exactly amount of breathable air.


Between the two tanks, I'd choose the LP-77's. The cost isn't that much more.

The LP-77 is on sale for $189 which includes shipping. Dive Sports has them on sale.
An AL80 costs minimum $150 plus you pay for shipping, which could run $25 to $45/tank.

 

[tstonedvr]

If the steel 77 is filled to 2640 and the AL80 (really 77) is filled to the rated 3000psi then you have the same amount of air in both....but you will have to add extra weight to the AL80 dive in order to make up for the buoyancy swing as it becomes empty due to the nature of AL....so the steel becomes better as the swing is far less and you are in effect using the tank as weight and you have to take it anyway..so steel becomes a better option...this and the Al will tend to get butt light as it becomes empty and you trim will change...less so with the steel...the steel will require a bit more watching as rust both outside and inside can form, but that's not that big of a deal....right now www.divesports.com is running a intro special I see for $189.00 shipped on the steel 77's.

 

[H2Andy]

the great advantage to the LP 77 is that if you have access to a tech or cave shop, they will normally fill the LP 77 up to 3500 psi, in which case you end up with 102 cubic feet

no one in their right mind will do that to an AL 80

 

[ClayJar]

....but you will have to add extra weight to the AL80 dive in order to make up for the buoyancy swing as it becomes empty due to the nature of AL....so the steel becomes better as the swing is far less and you are in effect using the tank as weight and you have to take it anyway..

You are commingling two concepts here.

When going from one cylinder to another, you must add or remove weight based on the difference between the two cylinders' empty buoyancy. Going from an AL80 at around +4 pounds empty to an LP77 at around -1 pounds empty, you can remove five pounds from your weight belt and maintain the same final buoyancy.

The buoyancy swing, on the other hand, is the difference between the full and empty buoyancy of the tank. As the tank displaces the same amount of water (within reason, we know what a hydro is) whether it's full or empty, the change in buoyancy is only related to the change in weight, which is a function of gas capacity. If an AL80 and a steel LP77 contain the same amount of gas, the buoyancy swing is identical, regardless of any other aspects of their respective construction (i.e. how they trim out).

Anyway, just figured it had to be said.

 

[SparticleBrane]

the great advantage to the LP 77 is that if you have access to a tech or cave shop, they will normally fill the LP 77 up to 3500 psi, in which case you end up with 102 cubic feet

no one in their right mind will do that to an AL 80

Why not? Minimum burst pressure on a Luxfer Al80 is 7500psi.

 

[Hoosier]

Why not? Minimum burst pressure on a Luxfer Al80 is 7500psi.

Because AL is more vulberable to overfill.

 

[divinginn]

ok what is the cf of air @ 3000 lbs on a 77,im seriously thinking of getting a few.