Aluminum vs Steel & why?

[KY_BOB]

Very interesting, thanks! I've only rarely dived steel tanks since going AL in the early 80's so my statement was anecdotal, not personally experienced.

Like what weighs more, a pound of lead or a pound of feathers.

That is true. 80cuft of air weighs about 6#. It doesn't matter what it's in.

AL tanks may go from negative to positive during a dive but steel tanks go from way negative to less negative the same amount for any given volume used.

 

[Oceanus]

stephencilento
Generally divers move to steel for

• increased gas supply (bigger tank)
• buoyancy characteristics (may weigh less & finish more negative)
• perceived increase in durability/value

The best thing to do is educate yourself and then barrow/rental a few options. Faber offers 13 sizes of steel scuba cylinders not counting the ones for rebreathers.
Until you know why you need/want a steel cylinder the AL80 is fine.

This seems to be a big misconception for a lot of divers. There is no difference in buoyancy shift in equally sized tanks reguardless of the tank material. The change in buoyancy of a steel 80 and an AL 80 is exactly the same, you use the same amount of gas from equal sized tanks so the buoyancy shift must be equal as well. Granted the amount of weight you need to achieve neutral buoyancy is different but the shift in buoyancy from full to empty on equal volume tanks is the same.

WTF!?
This is one of the big problems with forums and their 'experts'. You just butchered a basic part of [scuba] science and people are thanking you it!?

Only the change in weight/mass would be exactly the same.
Buoyancy is more about relative density than it is about weight.
To say that the tank material is irrelevant is completely wrong because the tank material can be a significant factor of the object's density. Since density is determined by weight (mass) AND displacement (size) a cylinder’s buoyancy swing may be primarily, but completely, determined by its working volume.

LP120 goes from -4.87 to 4.07 = 8.94 swing (19L displacement)
HP120 goes from -8.82 to 0.65 = 9.47 swing (12.9L displacement)

The difference above is primarily explained by the HP cylinder's smaller displacement.

Buoyancy - Wikipedia, the free encyclopedia

 

[Walter]

You just butchered a basic part of [scuba] science and people are thanking you it!?

Herman got it right. Density is a combination of weight and displacement. The displacement of a SCUBA tank does not change. The weight will change by the weight of gas used during a dive. As you use the air, the total weight of your tank decreases by the weight of the air used. If you use 6 lbs of air you've lost 6 lbs of air and it does not matter if that air was stored in steel or aluminum. The laws of physics don't change depending on what material was used in making your tank. The difference between steel and aluminum is a steel tank will typically be about 6 lbs negative when full and neutral when empty while an aluminum tank will typically go from about 4 lbs negative when full to 2 lbs positive when empty

 

[herman]

stephencilento
Generally divers move to steel for

• increased gas supply (bigger tank)
• buoyancy characteristics (may weigh less & finish more negative)
• perceived increase in durability/value

The best thing to do is educate yourself and then barrow/rental a few options. Faber offers 13 sizes of steel scuba cylinders not counting the ones for rebreathers.
Until you know why you need/want a steel cylinder the AL80 is fine.



WTF!?
This is one of the big problems with forums and their 'experts'. You just butchered a basic part of [scuba] science and people are thanking you it!?

Only the change in weight/mass would be exactly the same.
Buoyancy is more about relative density than it is about weight.
To say that the tank material is irrelevant is completely wrong because the tank material can be a significant factor of the object's density. Since density is determined by weight (mass) AND displacement (size) a cylinder’s buoyancy swing may be primarily, but completely, determined by its working volume.

LP120 goes from -4.87 to 4.07 = 8.94 swing (19L displacement)
HP120 goes from -8.82 to 0.65 = 9.47 swing (12.9L displacement)

The difference above is primarily explained by the HP cylinder's smaller displacement.

Buoyancy - Wikipedia, the free encyclopedia

On that point we totally agree.

 

[F106A]

I used Catalina HP AL100's for years then got a couple of PST HP ST100's. The PST diameter is 3/4" less, they're about 8# lighter and 3"+/- shorter and $100 more per. I like the Cats and love the PSTs. But ... diving in Key Largo with Ocean Divers using their steel 80's I notice a top/back heaviness (see post #5) wearing mostly just a BC, flippers(!), mask and a grin. Diving the PSTs in NJ with dry/wetsuits, heavy-assed fins, and 100#+ of other goodies the steel weight on my back isn't noticeable. If you're just thinking about warm water, minimal gear, shallow depth diving the AL 80s might be worth considering. But if you figure on getting into the deeper, colder, more complicated stuff, think about going for the steel 100s or 130s.

 

[Oceanus]

The change in buoyancy of a steel 80 and an AL 80 is exactly the same, you use the same amount of gas from equal sized tanks so the buoyancy shift must be equal as well.

I agree a pound of air is a pound of air and the displacement of a rigid cylinder does not change significantly (it changes a little).
IF Herman meant displacement instead of size, fine. But that is not what he wrote is it? No

Not extacly equal displacement or volume, but close enough to prove bouyancy is not EXACTLY the same as some would state.
Catalina AL80 goes from -1.8 to +4.6 = 6.4 swing
Faber HP80 goes from -8.05 to -1.74 = 6.31 swing


All else being equal:
The density of AL does not equal Steel and therefore the buoyancy must be different.

 

[stephencilento]

Thanks for the assistance everyone. While researching this topic, I have come to the conclusion that I really like the steel tanks. The down side is, at this time I cannot afford to invest in them. The upside is, as of today I am the proud new owner of a pair of aluminum 80's compliments of a wonderful family member who decided I should just dive and stop worrying about cost and equipment, and purchased them for me. (I wasn't going to argue). Since I am still diving warm and moderate depths, they should do just fine. When I start getting deeper, I'll look into the steel tanks. Thanks again everyone.

 

[Orlando Eric]

I keep a set of aluminum air shore dive tanks. I have a shore dive reg set also because often over a weekend I'll jump in.. get sand all over the back of my truck and leave the rig in there for the next day. My shore diving here is max depth of 18 - 20'. THe aluminum tanks require zero maintence. But for my actual Diving tanks the steel tanks are way better. I'd actually like to get a set of 120's and retire my st100's because they are too short.

 

[Jax]

I used Catalina HP AL100's for years then got a couple of PST HP ST100's. The PST diameter is 3/4" less, they're about 8# lighter and 3"+/- shorter and $100 more per. I like the Cats and love the PSTs. But ... diving in Key Largo with Ocean Divers using their steel 80's I notice a top/back heaviness (see post #5) wearing mostly just a BC, flippers(!), mask and a grin. Diving the PSTs in NJ with dry/wetsuits, heavy-assed fins, and 100#+ of other goodies the steel weight on my back isn't noticeable. If you're just thinking about warm water, minimal gear, shallow depth diving the AL 80s might be worth considering. But if you figure on getting into the deeper, colder, more complicated stuff, think about going for the steel 100s or 130s.

:worthless:


One aspect about steels that I like is they don't go buoyant like the AL80s . . . in a 7mm with aluminium, the last few feet are light!

 

[Tom Winters]

Enjoy the aluminum tanks and forget steel envy. I dived with aluminum tanks for a lot of years - I even mailed 14 aluminum tanks from Hawaii to Seattle when I emigrated to the United States. Then when I left the US for the capital of Central America in Mijami-Dade county, I Amtraked all those babies there too.
They're all long gone now - I only have aluminum deco tanks these days. Since I stopped getting humongous fills in my steels, those tanks have lost some of their charm. Plus steel tanks are magnetic, and I have some problems with magnetism occasionally.
After I have humped the 12th or so steel tank of the day, I'm ready to throw all the damn things over the side. Even the LP85's sometimes feel like they're filled with lead.
As a sideline, aluminum tanks are so light that with practice, you can actually juggle them like Indian clubs.