Steel vs. Aluminum Tanks (Part 2)
Now we continue the holy war of Steel vs. Aluminum. In the previous post, we talked about tank weight, corrosion, working pressure, and cost. Here we get into a biggie—the weight of the tank in the water.
Buoyancy
Before we get into all that, let’s talk a little bit about buoyancy. Simply put, negative buoyancy: bad, positive buoyancy: bad, neutral buoyancy: good, right? Well kind of. Positive buoyancy basically means we float, negative buoyancy basically means we sink, and neutral buoyancy is what we strive for, the ability to hover. So how does my tank figure into all this? Good question. Remember when we said an Aluminum 80 tank weighs about 35 pounds while we are carrying it down to the boat? Well, in the water it weighs much less. In fact, it weighs about 3.6 pounds. That’s right—it’s like wearing a 3.6 pound weight belt on your back. Let’s see: a steel 80 would weigh about 32.4 pounds out of the water (remember it’s lighter than aluminum out of the water) and about 13.2 pounds in the water. Holy Cow—that’s like having an 13.2 pound weight belt on your back!
Advantage: Aluminum
…Not So Fast
Let’s look at the big picture. Let’s take a 5'11, 185 pound diver wearing a 5 mm wetsuit and see how much weight he will need to go into the ocean. He will need approximately 22 pounds of weight to offset his suit. Now the diver must wear a tank right? So let’s put on a Steel 80 cubic foot tank with a working pressure of 3500 psi. We said that tank weighs 13.2 pounds when we put it in the water. Well, if the diver needs 22 pounds to be neutrally buoyant and the tank weighs 13.2 pounds, can’t we take 13.2 pounds off of our weight belt? We have to wear a tank anyway.
What does air weigh?
The approximate weight of air is 0.073 lbs/cu.ft. So the air in our Steel 80 cu.ft. tank would have about 5.9 pounds (80 x 0.073 = 5.9 lbs). When the tank is completely empty, how much does it weigh? Well if it weighs 13.2 pounds full of air, then it would weigh about 7.3 pounds when it is empty (13.2 lbs - 5.9 lbs = 7.3 lbs). Well that’s all great, but won’t our diver be a little light at the end of his dive? Yup. But remember the golden rule, you should be on the surface with no less than 500 pounds of air in your tank as a safety precaution. So the truth is, our diver may be a little light but we want him to be at the end of the dive. It’s better that you can’t sink when you only have 500 psi in your tank. Okay so what about an Aluminum 80? Well let's figure it out. 80 cu.ft. x 0.073 = 5.9 pounds of air. Now an Aluminum tanks weighs about 3.6 pounds when it is full of air and submerged in the water? Wait how can that be? Don't we have 5.9 pounds of air in the tank when it is full? Yes we do, the problem is that an aluminum tank when it is empty actually floats. That's right—it is positively buoyant when empty. How buoyant? Well lets take its full weight 3.6 pounds and subtract the weight of air 5.9 pounds and we get -2.3 pounds. So at the end of our dive, we are being pulled right out of the water. But remember the Steel, when we wore that we were able to take 13.2 pounds off of our weight belt. With the Aluminum, we can’t drop anything off of our belt. In fact, we need to add a little weight so that halfway through the dive we can still stay neutrally buoyant. Hmm let’s see…we need to wear a tank anyway and a Steel tank will take 13.2 pounds off of our belt while an Aluminum tank will add 3 pounds to our belts. What can I say, I’d rather wear a 10 pound belt than a 25 pound belt.
Advantage: Steel
So you still don't believe me huh? Well try it out. Below is a chart so that you can see the differences between steel and aluminum tanks.
Comparison chart of steel vs. aluminum dive tanks.