Tank buying time: aluminum vs. steel

[rr75]

I'm in the process of looking to buy a new tank. My buddy bought a steel 130 at a local shop a few weeks back and loves it. I can assure you that tank is heavy, but since I'm diving an aluminum 80 (rented from another shop), I'll let you guess which of us runs low on air first.

So, a couple of questions:

--About how much more air is in a steel 130 vs. an aluminum 80, if both are filled to the same psi? Would it be 130 / 80 = 62% more?(My buddy and I go through 80s at comparable speed, but with a steel tank he still has almost half his air at the end of our dives...figure the tank has a lot to do with that.)

--Aside from the theory, I'm looking to buy a tank in the next month or two. Why would I want aluminum or steel, one over the other? I know about the buoyancy...and since I need a bunch of weight to stay down (about 30 lbs in cold water with full 7 mm), steel is compelling to offset some of the weights. Other than buoyancy, though, are there other reasons? Does one last longer, or require more frequent maintenance/testing?

Thanks for the help.

 

[sheck33]

rr75 once bubbled...
I'm in the process of looking to buy a new tank. My buddy bought a steel 130 at a local shop a few weeks back and loves it. I can assure you that tank is heavy, but since I'm diving an aluminum 80 (rented from another shop), I'll let you guess which of us runs low on air first.

So, a couple of questions:

--About how much more air is in a steel 130 vs. an aluminum 80, if both are filled to the same psi? Would it be 130 / 80 = 62% more?(My buddy and I go through 80s at comparable speed, but with a steel tank he still has almost half his air at the end of our dives...figure the tank has a lot to do with that.)

--Aside from the theory, I'm looking to buy a tank in the next month or two. Why would I want aluminum or steel, one over the other? I know about the buoyancy...and since I need a bunch of weight to stay down (about 30 lbs in cold water with full 7 mm), steel is compelling to offset some of the weights. Other than buoyancy, though, are there other reasons? Does one last longer, or require more frequent maintenance/testing?

Thanks for the help.

Filled to their working pressures a 130 contains 130 CF and an 80 contains 80 CF (ideal gasses assumed for this example).
You will have to do the math if fill pressures are other than working pressures.

130 @ 3500 psi filled to 3000 contains about 111 CF
80 @ 3000 filled to 3000 contains about 80 CF

130 @ 3500 psi filled to 1500 contains about 56 CF
80 @3000 psi filled to 1500 contains about 40 CF

get the idea here.

choice of tank depends on kind of diving done. How long a particular tank lasts also depends on the dive and you.
either way, make sure you can swim your rig up if your BC fails particulary if you are in a wetsuit.

 

[scubasean]

rr75 once bubbled...


So, a couple of questions:

--About how much more air is in a steel 130 vs. an aluminum 80, if both are filled to the same psi? Would it be 130 / 80 = 62% more?(My buddy and I go through 80s at comparable speed, but with a steel tank he still has almost half his air at the end of our dives...figure the tank has a lot to do with that.)

Well, actually, it all depends on how much pressure the tank is rated to....If the AL80 holds 3000 to get 80cu ft. and the 130 holds 3000psi to get 130, then your calculation is correct....But if they are rated to different pressures, the computation is slightly more complex.....
But why do you care about making the pressures equal?

--Aside from the theory, I'm looking to buy a tank in the next month or two. Why would I want aluminum or steel, one over the other? I know about the buoyancy...and since I need a bunch of weight to stay down (about 30 lbs in cold water with full 7 mm), steel is compelling to offset some of the weights. Other than buoyancy, though, are there other reasons? Does one last longer, or require more frequent maintenance/testing?

Thanks for the help.

Actually, the only reason I bought a steel tank is the different buoyancy characteristics. I still dive my neutral buoyant AL 80, as well as also diving my high pressure 100.


One thing you might consider is that shops sometimes miss that you have a high pressure tank, if you have one. It is easier to get a low pressure (2450psi) tank filled to rated capacity than a high pressure (3450psi) tank. Its only the awareness of the LDS that makes that difficult. Some LDS's do it right every time, and others are hit and miss.

Good luck!

Sean

 

[jrmy_1]

Just get the steel 130. They have plenty of gas and will help take a couple pounds off the belt.

 

[Bob3]

If your shop requires a vis+ on your aluminum tank, you get to add that in to your annual maintenance cost.

Steel tanks require tumbling only if you get wet air. Some of my steel 72s have gone over 30 - 40 years w/o needing a roll.

I agree on the LP side, you're more apt to get a decent fill more places you go.

 

[rescuediver009]

Be careful when you are doing the math to figure how much air you get. OMS tanks are stamped as 2400 plus the 10% overfill gives you 2640. That is what their volumes in cubic feet is rated at. Although it is very common to see someone change the discs and run them at 3000. giving you alot more air.

 

[MechDiver]

rescuediver009 once bubbled...

Although it is very common to see someone change the discs and run them at 3000. giving you alot more air.

You don't have to change anything. The burst disk in a LP tank will take quite a bit more than 3000psi.

MD

 

[rescuediver009]

on OMS 2400PSI rated tanks? I don't think that they are anything over 330 PSI, whi0ch doesn't kleave much slack if you are running them at 3000 especially in the summer

 

[FredT]

hydros at 4000 psi.

Burst disk must be rated to fail above working pressure and below hydro, I'd generally use one rated for about 3800 for this bottle.

Remember, the burst disk as defined in US DOT rules is there to protect firefighters. It's definitely NOT there to protect divers as their presence in the rules predate both sport and military SCUBA diving by several decades. Much of the rest of the world does not use burst disks.

FT

 

[ScubaRon]

The basic differences between steel and AL are:
- steel tanks are lighter, thus easier to carry
- steel tanks are less buoyant, allowing you to take some weights off your belt and giving you a better UW trim.
- steel tanks often can be filled to a higher pressure. This varies per type though and be aware that you will also need a DIN regulator and of course your dive center needs to be able to actually fill to the higher pressure.

The maintenance and reliability of steel and AL tanks are the same (theoretically steel is a little harder, but reality is that both scratch and neither will "break" unless you start throwing them around).

So overall a steel tank is preferable. The only downside is the higher price.
ScubaRon