Metal Matters — Are Your Tanks Aluminum or Steel?

At the huge international dive show in Dusseldorf, Germany, in January, I asked many traveling divers what irked them most about diving in an unfamiliar location. Besides the obvious things like local conditions and currents, the one subject that cropped up time and time again was the need to do a buoyancy check, despite using familiar equipment, because of using unfamiliar tanks.

When we dive, we work to be neutrally buoyant despite some pieces of our equipment being positively buoyant and others not. We wear lead weights to counteract the excessive buoyancy created by our combined gear, including the effect of the thickness of the suit and the amount of water we and our gear individually displace.

Since we generally dive with standard aluminum 80s, most of us know how much lead we need, given the suit we are wearing. And then we arrive at a destination that uses different tanks, our regular weight requirement no longer works, and we have to experiment.

The bigger the tank, the more water it displaces, so more lead is needed. Well, not quite. The in-water weight of the tank also needs to be taken into consideration. It's not just a matter of aluminum or steel. Some steel tanks are manufactured in such a way that their bases contain a lot of metal, making them much heavier than other steel tanks of apparently the same size.

Aluminum tanks tend to be bigger than steel tanks because their walls are much thicker than those of a steel tank. So, they are 'lighter" when immersed. On the other hand, they can be heavier out of the water and on your back. Steel tanks can be lighter to carry but displace less water for their mass, so you don't need to wear so much lead. In fact, a diver with steel doubles might not wear much extra lead at all, whereas a diver with aluminum doubles will wear more than he would with a single tank.

Some say it's safer to use an aluminum cylinder with the extra weight needed because in an emergency, you can jettison the lead. The jury's out on that one.

A checkout dive lets you determine the amount of weight you need when you're using an unfamiliar tank. In some parts of the world, Aluminum 80s (11 liters) are standard, but if you decide you need a bigger tank, it's likely you'll be given a steel 15-liter tank (looks like a 105 cu ft at 3000 psi), and you'll need less lead in the water. Air has a weight too: 80 cu. ft. of air weighs about 5.5 pounds. Remember, you need less lead in fresh water than you do in the ocean.

- John Bantin