An 80 is an 80 right?

[Linedog]

As I progress in my scuba learning curve, my wife bought me 3 AL80's. OK they hold 80 cf of air more or less, bear with me here for a moment. A LP80 hold 80 cf of air, a HP80 holds 80 cf of air. The differences being size, buoyancy life span etc. Yes I know I need more weight for AL80's but am I on the right path that they all hold the same amount of air? So if I had an AL80 and a HP80 and my sac was exactly the same with just different weighting, I could stay down for the same amount of time, right?

 

[Scared Silly]

Yes you are on the right track. An AL80 at 3000psi actually holds 77.4 cuft whereas a modern exempt HP80 at 3442 psi truly holds 80 cuft. Now to add to the mix some of older HP80 made by PST at 3500 psi holds 82 cuft. But for all practical purposes they are all 80s.

Now for the LP80s. At their working pressure of 2400psi they hold 72.7 cu ft. But if they have a plus rating i.e. a + mark after the hydro date the cylinder can be overfilled by 10% which then makes the working pressure 2640 psi and a full 80 cuft.

 

[Stu S.]

You are correct. (To be more exact, an aluminum 80 is really a 77 cu. footer.) The volume of gas, not the shape determines bottom time.

 

[Tortuga68]

nevermind

 

[JBRES1]

Its kinda like the old story about what is heavier, 1 lb of feathers or 1 lb of lead.

So, yes all of the above 80's hold about 80 cu ft of air. The cylinders differ in how you will trim out with added weight, and the physical size of the cylinder.
The hp 80 is the smallest of the 3, and the lp80 and the al80 will be close in size.
And if this doesn't sound bad enough, some cylinders are bottom heavy while others are not.

Isn't buying gear fun.

To me scuba tanks and golf clubs have alot in common. You search for the perfect mix of gear to make the sport fun, just to find that there are too many options and too little money to try them all.

Jim Breslin, golf hack and owner of too many scuba cylinders.

 

[JahJahwarrior]

Disregarding the fact that 80's are actually 77's, then at their rated fill pressure, an 80 is an 80 is an 80.

However, other issues come in to play. Will your HP80 be filled to the rated pressure when it cools? Will your LP85 (closest common LP steel to an 80) be filled to the rated pressure, or will the fill person only go to 2400? Or, will he fill it to 3442 on accident?

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)

Just do all your planning in terms of actual cubic feet, and then convert to pressures if you must.

 

[JBRES1]

One last thing that I missed when I went over this post.

You said your wife purchased the tanks for you.

HOLD ON TO THAT WIFE. she is a keeper.

A wife that is buying you scuba gear either really loves you or is trying to kill you.
My vote is that she loves you, but I always see the glass as being half full.

Jim breslin

 

[toddthecat]

A snake is still a snake no matter the size or shape?

 

[SkimFisher]

Can't take credit for this, I found it online a while back, but it's very useful when determining tank specs.

Tank Specs

Click download in the upper left hand corner once the link opens.

 

[Linedog]

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.