Is steel safer than aluminum??

[SM Diver]

Upon getting my c-card I purchased three AL80s. I took a speciality class from a different dive shop, and they said "all we use are steel." They went on to explain the difference in buoyancy of an empty AL vis-a-vis steel.

So, what I am wondering is, if an AL80 with only 500 psi turns 5-6 pounds positively buoyant, and a Steel 80 has a buoyancy swing of only one or two pounds, isn't a steel less likely to cause an unexpected ascent due to a more radical, unexpected buoyancy change? Of course, you can plan ahead for that buoyancy change by adding more weight, but wouldn't it be nicer to carry less weight.

Any feedback appreciated.

 

[roakey]

I think you’re confusing a couple things. There are two important stats for cylinders (ok, there’s a third but we’ll skip it for now).

First is the empty buoyancy of the cylinder, the second is how much the cylinder’s buoyancy changes during the course of the dive.

To take the second stat first. Cylinders swing 1 pound for every 13 CF of air you stuff into them. This means an AL80 swings 80/13 = about 6 pounds from full to empty. A steel 80 swings 80/13 = about 6 pounds from full to empty. A huge, multiple thousands of tons nuclear submarine, if you add 80 CF of air to it will change 80/13 = about 6 pounds.

So it doesn’t matter what the cylinder material is, if you compress 80 CF of air into it it’ll weigh about 6 pounds more on land and be about 6 pounds less buoyant in the water.

What’s the interesting difference between an AL80 and a steel 80 is not the swing (it’s the same) it’s the buoyancy endpoints. An AL80 swings from about -3 to +3 from full to empty. Taking a SWAG at a steel cylinder, it may swing from -7 to -1 from full to empty. Since it ends up having 4 pounds less buoyancy (-1 vs. +3) that means you can take 4 pounds off your belt.

Also note that this swing is a slow, linear process throughout the dive. It’s not like it happens all of a sudden when you get down to 500 PSI. Using an AL80 for example (80 CF at 3000 PSI) you’ll become one pound more buoyant with every 500 PSI (1/6 of the cylinder) that you breathe. Note that if you had a steel 80 you’d STILL become 1 pound buoyant for every 1/6th of the cylinder you breathe.

So it’s the endpoints, not the swing that differs between identical sized cylinders made of different materials and/or using different pressures. Up to a point steels make a more desirable cylinder because you can take some weight off your belt.

Roak

 

[SM Diver]

Roak: Thank's for taking the time to educate me on that. It now makes a lot more sense to me.

 

[jonnythan]

tampascott once bubbled...
Of course, you can plan ahead for that buoyancy change by adding more weight, but wouldn't it be nicer to carry less weight.

That's exactly why a lot of people use steel tanks, and why I plan on getting some next summer when I can afford it. I might be able to get my weight pouches down to 30 or so from 35

 

[redacted]

Warm water diver may need (or at least can better tolerate) the bouyancy of aluminum over steel. In warm fresh water, with no exposure protection needed, I can get by with no added weight. So a steel tank would be undesirable due to its negative bouyancy. In warm salt water, I use 8 to 10 pounds with a .5 mil suit so steel is not as attractive as it is to the cold water divers. And you are not really getting rid of weight. You are moving it from a belt or pouch to the tank on your back.

 

[Jarhead]

awap once bubbled...
Warm water diver may need (or at least can better tolerate) the bouyancy of aluminum over steel. In warm fresh water, with no exposure protection needed, I can get by with no added weight. So a steel tank would be undesirable due to its negative bouyancy. In warm salt water, I use 8 to 10 pounds with a .5 mil suit so steel is not as attractive as it is to the cold water divers. And you are not really getting rid of weight. You are moving it from a belt or pouch to the tank on your back.

Reducing the amount of ditchable weight.
Example: If your only diving 8-10 lbs on your weight belt/pouches. And you replace 6 lbs of that with a steel tank. You now only have 2-4 lbs. of ditchable weight.

BUT... how ditchable weight do you really need?

Jarhead