Pros and cons - steel tank vs aluminum?

[NJMike]

Okay, yes it's true, I started the thread on the pros and cons of weight belts vs WI BC's.

THAT thread led me to thinking about the tanks. I have never dove a steel tank but understand that it requires carrying less weight, since it's heavier. That's a 'pro'.

But the inside of the tank can corrode...that's a 'con'.

But wait, is the chance of it corroding only if you allow water inside? If you take "normal" precautions can this be prevented?

The steel tank is heavier to carry, though...another 'con'.

What are the other pros and cons? I suppose I could wait till next year, then rent two tanks...one steel, one aluminum...and dive them back to back on the same day. That might help.

Here's where I'm going with this. I will probably by the OceaPro BC. It's weight integrated and can carry up to 20 lbs. But I dove with a 5MM farmer john and needed 16 lbs (at Dutch Springs, PA). I'm renting a 7MM farmer john next week, and the LDS said I would need 2-4 lbs more, and I am renting the complete setup of what I am thinking of purchasing, which includes the BC mentioined above.

If I need 4 lbs more, that adds up to 20, the max of the BC. If, for any reason, I need more, well, it won't go in the BC, which means a full BC plus weight belt. Hmm.

However, I am also buying a wetsuit and it will most likely be a Pinnacle Elite, a 7/5 MM. That should cut down on the amount of weight I need, so the BC should be able to carry all the needed weight.

Who needs to go to 130' to get narcosis? I'm dizzy right here just thinking about it! But I tell you one thing, it sure is a heck of a lot of fun shopping for scuba gear!

 

[Rick Murchison]

But the inside of the tank can corrode...that's a 'con'.
But wait, is the chance of it corroding only if you allow water inside? If you take "normal" precautions can this be prevented?

Any tank can corrode inside, especially if salt water gets in there. My oldest steel tank was manufactured in 1965 - current hydro and VIP, dove it last weekend.

The steel tank is heavier to carry, though...another 'con'.

No, steel tanks aren't heavier - they are smaller and displace less water, which makes them more negative.

What are the other pros and cons? I suppose I could wait till next year, then rent two tanks...one steel, one aluminum...and dive them back to back on the same day. That might help.

For back gas, in my opinion the sole advantage to an aluminum tank is that it's cheaper than steel. Aluminum tanks are my tank of choice for stage and deco bottles because they swing around neutral buoyancy and so can be carried and dropped without any major buoyancy adjustment.

Here's where I'm going with this. I will probably by the OceaPro BC. It's weight integrated and can carry up to 20 lbs. But I dove with a 5MM farmer john and needed 16 lbs (at Dutch Springs, PA). I'm renting a 7MM farmer john next week, and the LDS said I would need 2-4 lbs more, and I am renting the complete setup of what I am thinking of purchasing, which includes the BC mentioined above.
If I need 4 lbs more, that adds up to 20, the max of the BC. If, for any reason, I need more, well, it won't go in the BC, which means a full BC plus weight belt. Hmm.

However, I am also buying a wetsuit and it will most likely be a Pinnacle Elite, a 7/5 MM. That should cut down on the amount of weight I need, so the BC should be able to carry all the needed weight.

Bottom line here is that you need more lift in your BC for diving thick wetsuits comfortably. 20 pounds is plenty for tropical diving, but if you're in a 7 mil, you need more - probably a minimum of 30. Remember, you have a five pound swing in buoyancy just from gas use over the course of the dive, so 20 is really only 15... Whether the weight's on a belt or in a tank is irrelevant - the buoyancy shift from a full tank on the bottom to an empty tank on the surface with a 7 mil is at least 20 pounds, likely more, and you need a BC that can cover it.
We can discuss the issue of steel tanks and wetsuits another time
Rick

 

[Walter]

If properly cared for, steel tanks will outlast aluminum tanks. I have two tanks in service from 1955.

 

[NJMike]

Rick, I appreciate you picking apart my post...really. I kept up with it until the end. I got a little lost.

Didn't follow the part about 20 lbs in the tropics, or why going from a 7mm suit to a 5mm suit, all else being equal, would add 10 additional lbs. That just seemed a bit much, and I do say that tongue in cheek because I know you are vastly more experienced than me.

 

[Rick Murchison]

Didn't follow the part about 20 lbs in the tropics, or why going from a 7mm suit to a 5mm suit, all else being equal, would add 10 additional lbs. That just seemed a bit much, and I do say that tongue in cheek because I know you are vastly more experienced than me.

A wetsuit is buoyant at the surface, barely buoyant at all by the time you reach a hundred feet or more, due to the compression of the nitrogen bubbles in the neoprene that give the wetsuit its buoyance in the first place. If it takes 20 pounds of lead to sink a 7mm wetsuit at the surface (mine takes 22) then at 100 feet you'll need nearly 20 pounds of lift from the BC to counter the lift you've lost from compressing the wetsuit.
In addition to compensating for the wetsuit, you must also be able to compensate for the change in weight of your tank - about five pounds of gas used on a dive with an AL80, for example. In order to be able to control your buoyancy at the end of a dive near the surface, you must carry enough weight at the beginning of the dive to compensate for the wetsuit and the change in weight of the tank...
Let's run some numbers, ok...
Item................Buoyancy
Me.......................+3
Wetsuit...............+20
Steel 72 (500 psi)...-3 (with regulator)
Adding those up algebraically, the weight I need to carry so I can be neutral at the end of the dive, near the surface with only 500 psi left in my tank and with no air at all in my BC = +3+20-3 = 20 pounds.
But, at the beginning of the dive, my Steel 72, full, is -8 pounds, so to be neutral at the surface I'll have to carry 5 pounds of positive buoyancy (air) in my BC. As I descend, my wetsuit will compress and get less buoyant, for all practical purposes becoming neutral itself by 100' or so, where the negatively buoyant rubber is just balanced by the compressed nitrogen bubbles in it, and I'll need to be able to add another 20 pounds of positive buoyancy to the BC to stay neutral, a total of 25 pounds. (I said I want 30 minimum because I always want a little gravy )
In the tropics without the wetsuit I'd need no weight at all, and only 5 pounds of lift from the BC at the beginning of the dive... so 20 pounds lift in a tropical BC is just fine.
Hope that clears up the concept a bit...
Rick

 

[lamont]

But wait, is the chance of it corroding only if you allow water inside? If you take "normal" precautions can this be prevented?

it really depends on the h2o content of the air that you fill with. i just looked inside my year-old X8-119s and they were nearly spotless, no need to tumble, they've only ever been filled at one fill station... my older E8-130s that i tumbled recently had seen air from a few more fill stations...

 

[Doc Intrepid]

..."I will probably by the OceaPro BC. It's weight integrated and can carry up to 20 lbs. But I dove with a 5MM farmer john and needed 16 lbs (at Dutch Springs, PA). I'm renting a 7MM farmer john next week, and the LDS said I would need 2-4 lbs more, and I am renting the complete setup of what I am thinking of purchasing, which includes the BC mentioned above.

If I need 4 lbs more, that adds up to 20, the max of the BC. If, for any reason, I need more, well, it won't go in the BC, which means a full BC plus weight belt. Hmm..."

My good man...have you ever considered purchasing a high quality wing and 12 lb SS backplate?
Your problems would be completely solved!




My work here is done!

(Doc turns and runs out of the room....)

 

[shakeybrainsurgeon]

I found that I have less tendency to "turtle" with aluminum tanks since the tank is negative buoyant and grows more so as the dive progresses. With a steel tank, a jacket bc, a wetsuit and a lung full of air, that's a lot of weight on the back and a lot of buoyancy in the front. Maybe it's just my imagination, but I think aluminum helps my stability. If you have never used aluminum tanks, beware...the increase in buoyancy as the tank empties is much more palpable than any similar change in steel tank buoyancy.

 

[Rick Murchison]

I found that I have less tendency to "turtle" with aluminum tanks since the tank is negative buoyant and grows more so as the dive progresses. With a steel tank, a jacket bc, a wetsuit and a lung full of air, that's a lot of weight on the back and a lot of buoyancy in the front. Maybe it's just my imagination, but I think aluminum helps my stability. If you have never used aluminum tanks, beware...the increase in buoyancy as the tank empties is much more palpable than any similar change in steel tank buoyancy.

You mean *positively* buoyant and grows more so...
Also the change in buoyancy during a dive (at any given depth) is strictly a function of the gas used over the course of the dive and has nothing to do with the tank material.
However... the concentration of metal towards the neck of an AL tank is more pronounced than in a steel tank, so the AL tank tends to affect fore-and-aft trim as it empties more than a steel, and this is often interpreted as a "much more palpable" change in buoyancy even though it isn't a greater change in total buoyancy.
You are correct in the observation that AL tanks provide better lateral stability than steel.
Rick

 

[Rick Murchison]

My good man...have you ever considered purchasing a high quality wing and 12 lb SS backplate?
Your problems would be completely solved!




My work here is done!

(Doc turns and runs out of the room....)

Ahem... well... yes... there is that option
Rick