Tank recommendations

[lowwall]

So, I have seen lots of talk about steel tanks, but very little on aluminum, is there a reason that steel tanks are so favored over aluminum for this type of diving? I also have to wonder what the difference in longevity is between steel and aluminum.

Steel tanks provide more negative buoyancy than Aluminum. For example, you will need to add 5 lbs of lead to your weight belt to achieve the same buoyancy with an Al80 as you would get with a steel HP100.

Steel also lasts at least twice as long as AL given even minimal care (i.e., rinse after saltwater dives, store somewhere dry, and keep at least a couple hundred psi in the tank so water can't get in).

The only real advantage of AL for single tank backmount is initial cost.

 

[Boarderguy]

Steel also lasts at least twice as long as AL given even minimal care (i.e., rinse after saltwater dives, store somewhere dry, and keep at least a couple hundred psi in the tank so water can't get in).

I currently dive two AL80s that are 50+ years old. VIP plus an eddy current and hydro makes sure they are safe and happy. I have 2 local shops that still fill them even though they are old and made from the taboo AL. Gear will only last as long as you are willing to take care of it. Rinse and dry for storage is key for everything.

 

[-JD-]

Devil is really in the details. Which AL80, AL100 and which (?) steel ...

ex. For the data that I have I'll include the relevant section entries.

View attachment 714977

Re: bouyancy ...
A Luxfer 80 is notably different than a Catalina C80 for example.
A Catalina C100 isn't really that much different than a PST HP variant 100 (about 1lb) and the steel is about 12lbs lighter on land.

@myself Quoting me from another thread for the table ---

If you look at the quoted table: Buoyancy and net rig dry weight for a similar gas capacity is generally (significantly) better for steel vs aluminum..

The "common" Aluminum tanks are the Luxfer 80 and Catalina s80. The Faber FX-100s are really the only currently available (as new) high-pressure 100cuft class steels (I dive the old PST 100s.) As pointed out above, you are adding 4-5lbs of additional lead to account for tank buoyancy when empty (in addition to the lead required for exposure-protection and personal buoyancy).

 

[lowwall]

I currently dive two AL80s that are 50+ years old. VIP plus an eddy current and hydro makes sure they are safe and happy. I have 2 local shops that still fill them even though they are old and made from the taboo AL. Gear will only last as long as you are willing to take care of it. Rinse and dry for storage is key for everything.

I agree that durability isn't a good reason to choose one material over another (unless you are stocking a tropical dive shop). AL tanks can obviously last longer than the diver who purchased them. Still, with the level of care you obviously give your tanks, steel would have essentially infinite life. From a purely mechanical perspective, longevity comes down to the fatigue properties of Al versus steel. AL continues to weaken forever when subjected to stress, while steel weakens to a point and then no further.

From a practical perspective, it comes down to getting them filled. Many shops simply won't fill AL over a certain age and it seems likely to become harder and harder to find a shop that will fill 20+ (or 30+) year old AL tanks.

 

[Boarderguy]

From a practical perspective, it comes down to getting them filled. Many shops simply won't fill AL over a certain age and it seems likely to become harder and harder to find a shop that will fill 20+ (or 30+) year old AL tanks.

Shops that don't fill old tanks are just showing how much ignorance drives some parts of our industry. Keeping tanks clean, dry and with 500-1000 psi has kept all of mine in service with the exception of my LP72s. They are again older epoxy(?) coated tanks and were rejected on hydro last year because of some un-inspectable pitting. They just weren't worth the work to strip and inspect.

When I buy new (to me) tanks in a few months they'll be hp 100s or 117s depending on what is available from a local guy in the PNW. Those are the only tanks i would recommend buying. Longer dives and still negative when empty. The AL 80s are great for jumping in the lake for a quick repair or a short/shallow shore dive imo.

 

[Belzelbub]

So, I have seen lots of talk about steel tanks, but very little on aluminum, is there a reason that steel tanks are so favored over aluminum for this type of diving? I also have to wonder what the difference in longevity is between steel and aluminum.

I use both steel and aluminum tanks regularly. Both will do the job well, but I prefer steel.

Steel means I can carry a bit less weight in my weight pockets as the steel remains negative, but a nearly empty AL tank will most likely be positive. If the amount of gas they hold is the same, the buoyancy swing throughout the dive will be the same.

Both steel and AL can oxidize (rust). Aluminum oxide tends to be a bit tougher, so doesn't flake off and helps to protect the rest of the aluminum. Iron oxide will flake off after time if left untreated, so the cycle can continue. In reality, this is not really a big concern. Galvanized steel holds up really well. With my steel tanks, the only areas that tend to show any rust are the Hydro stamp area. This is easily solved, though. When I pick up my tanks from hydro, I just spray it with a bit of Rustoleum primer and spray paint or galvanize spray. AL tanks have square bottoms, so don't need boots to stand up, but many have them anyway. Regardless of the material, knocking the boots off every once in a while is a good idea.

Both AL and steel tanks can last many years, so longevity isn't really a concern. There are, however, some shops that won't fill AL tanks over 20 years old, or made before 1990. I haven't heard of any shops that put the same restrictions on steels, so that may be something to consider. The 20 year limit is not backed by anything. I can only think this policy was put in place around 2010 and never updated. The 1990 limit is more common and comes from issues from the AL6351 alloy. Some manufacturers used this alloy for a time, some never used it, and all manufacturers were no longer using by 1990. So that's a nice round number.

Like I said, I've got a couple AL tanks as well. My Al tanks were post 1990, so made from AL6061. I have no problems getting them filled, and plan to keep them around for a while.

 

[Skip Y]

I have 2 HP 117's. I prefer those to the 120's because they are much shorter