Practical difference between AL and Steel?

[davidbaraff]

I'm going on a boat dive where they said they only have steel tanks, not AL. The operator gave me a long explanation about why he like'd them better, but I got a bit lost. As I understand it, if I dive with a steel tank, I'm going to simply reduce the weights I normally carry by aboutu 4Lbs.

But I'll still have the same total amount of weight to lug around when walking, as well as once I'm in the water. So what's the difference and why would someone prefer steel over aluminum? (I feel like i must be missing some basic physics, but the drop in weight from beginning to end of dive due to air is still going to be 6lbs, no matter what your tank is made of, right?)

 

[Ken abucs]

Spectrum said it best...

SCUBA Knowtes, The Evil Aluminum 80

 

[Charlie99]

You are correct in assuming that the change is buoyancy during the dive is due to the weight of air and that whether the tank is steel or aluminum doesn't affect that.

The total dry weight will be less with the steel. Not only are you getting rid of 4 pounds of lead, but the steel tank is most likely lighter than the equivalent capacity aluminum tank. This is the main advantage of the steel tank. The steel tank is also a bit smaller in size than the equivalent aluminum. Since the steel tank has thinner walls, it is smaller and therefore displaces less water, so therefore is less buoyant.

Not all steel tanks are identical, and it would be a good idea to find out which specific manufacturer and tank model number the boat will be supplying. Then look at a site like Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan to figure out the difference in weighting.

 

[davidbaraff]

You are correct in assuming that the change is buoyancy during the dive is due to the weight of air and that whether the tank is steel or aluminum doesn't affect that.

The total dry weight will be less with the steel. Not only are you getting rid of 4 pounds of lead, but the steel tank is most likely lighter than the equivalent capacity aluminum tank. This is the main advantage of the steel tank. The steel tank is also a bit smaller in size than the equivalent aluminum. Since the steel tank has thinner walls, it is smaller and therefore displaces less water, so therefore is less buoyant.

Actually, the key seems to be that they have different buoyancy's, not different weights. if the two thanks had the exact same bouyancy, it wouldn't matter that the steel one weighed less -- i'd still need more weight to sink myself, to make up the difference.

I figured that since they keep the same amount of air at the same pressure, meaning the volume displaced by the air is the same, they'd have the same bouyancy. But evidently the thickness of the tank walls themselves must make a difference, so that in the end, the steel tank simply displaces less water, hence isn't as bouyant. Who'd a thought it...

Ok, thanks.

 

[spectrum]

But I'll still have the same total amount of weight to lug around when walking, as well as once I'm in the water.

Not Likely. My page that Ken Abucs cited is based on the HP steels that I mainly dive. These are very efficient packages and will reduce you lead requirement and be lighter weight to boot. This means you become lighter on your feet in both respects. A LP steel cylinder won't be as light but will still remove lead and maybe even more so than the HP steel with a net result still being a diver that's lighter on his or her feet.

(I feel like i must be missing some basic physics, but the drop in weight from beginning to end of dive due to air is still going to be 6lbs, no matter what your tank is made of, right?)

You are correct that the swing is no different.

We're talking about the Empty Buoyancy value in terms of your lead requirement. For the actual tank weight you can compare from the table as well. Do note that for aluminum you usually need to add the valve weight while it is generally included for steels. Go figure!

Pete

 

[Garrobo]

Probably doesn't make that much difference since you will rarely find an operator who supplies steel tanks. On the other hand this is good to know if you decide that you want steel for your personal tanks.

 

[charlesml3]

I always get a chuckle out of how emotional some people will get over the composition of the tank material. This comes up on these forums regularly. I remember a post several months ago where the guy just went religous over it. Said he REFUSED to dive anything but steel and it was part of his weight program PERIOD.

Please. No way he's going to sit on the dock and cry while his buddies knock 4 pounds off their weight belt and get on the boat.

The best divers I have ever seen were not the most experienced, nor the ones with the best gear, nor the ones that had been to the most exotic locations. The best ones I've seen were able to adapt quickly to different situations and extrapolate what they need to know and do based on their training and previous experiences.

-Charles

 

[fdog]

Given your location I'm making a leap here, and you're going out with the Monterey Express.

Tim runs a great boat and works hard to show you good dives. The steels on the boat are LP 80's.

On a boat there's less advantage to steel cylinders for the diver than shore diving; however boat operators I've talked with report less weighting issues with steel, mostly because low-time recreational divers tend to be underweighted when they first journey to cold salt water. The steel tanks are more negative and cover up for a diver who would be underweighted with aluminum.

Although I use steel cylinders personally, and getting on the Express is no different in weighting for me, I do add 6 lbs of lead for an aluminum 80 when I'm forced into it, so going backwards (from aluminum weighting to a steel) I'd subtract 5 lbs.

Beyond the pure weight difference, I do find my trim is better with steel, as the weight is placed on my body differently than with an Al 80 and a weight belt.

Have fun!

All the best, James

 

[SPKelpDiver]

I think this post sums up this issue a lot better than a lot of the other posts I have read.

 

[Teamcasa]

...The best divers I have ever seen were not the most experienced, nor the ones with the best gear, nor the ones that had been to the most exotic locations. The best ones I've seen were able to adapt quickly to different situations and extrapolate what they need to know and do based on their training and previous experiences.

There is great truth in that! Thanks.