Notes about things regarding scuba diving that I have come to know.
The Evil Aluminum 80
In considering a scuba cylinder there are several properties to consider other than how many cubic feet of air will it hold. Weight as in how heavy is it when you are walking across the beach is one as is buoyancy as in how will it effect what you carry for weight? Those are 2 very different questions as are the concepts of buoyancy and weight. Diving is supposed to be fun and not reserved for the body building crowd. As you will see, by understanding cylinder properties you can make choices to allow your diving to be easier and more comfortable.
The most common scuba cylinder found in North America and the Caribbean is the Aluminum - 80 cubic foot cylinder. For striding off of a boat or getting into the water in a thin wetsuit needing minimal dive weight it is a tolerable product. However it is relatively heavy for the air it contains and since it becomes positively buoyant by about 4 pounds by the end of a dive you need to wear weight just to hold the cylinder down to allow for a proper safety stop and orderly ascent.
When you take this cylinder to a more challenging environment it becomes a true liability. When diving in cold water divers need more protection. This takes the form of a heavy neoprene wetsuit or a dry suit with the appropriate undergarment. In either case insulation increases a diver's buoyancy and more weight must be worn. As mentioned the Aluminum 80 becomes positive requiring the dive to wear 4 pounds of weight just to neutralize the tank at the end of the dive. This is like adding insult to injury.
Most steel cylinders on the other hand remain negatively buoyant or neutral at all times. They often weigh less on land for a given air storage capacity especially in the high pressure models. Since they may remain negatively buoyant not only do you not need to wear weight to sink them, they can help sink your exposure protection letting you take a few more pounds of lead off of your belt or out of your BC.
Sometimes these properties can make the difference between diving and not. Smaller divers, especially women cite the heavy gear as a reason not to take up or continue diving. You will see that cylinder selection can do a lot to make diving comfortable.
High pressure steel cylinders are the most efficient compressed air package available to divers. We have a collection of PST (Pressed Steel Tank) cylinders but with their absence in the marketplace I have reworked this page to illustrate the Worthington / XS Scuba X-Series 80 cubic foot cylinder. The original PST based page can be seen here. What follows compares and contrasts this cylinder to the Aluminum 80. Using the reference sites mentioned below or other sources you can get specifications and repeat these calculations for any cylinder combination.
As mentioned, an AL-80 has 4.4 pounds of positive buoyancy when empty. Meanwhile a X-80 Cylinder will be 3 pounds negative when empty.
4.4+3 = 7.3 pounds of difference in buoyancy.
The aluminum tank is a liability since you need to add weight to hold it down at the end of the dive. The steel tank contributes to keeping you and your exposure suit down. The 7.3 pounds represents 7.3 pounds of weight you do not need to wear when diving a X-80. This comes right off your belt.
Now when you go to walk across the beach...
A Luxfur 80 weighs 31.4 empty less the valve so add about 2 LB for a typical valve = 33.4 pounds
A X-80 weighs 27.7 pounds empty including the valve.
Again both are filled with approximately the same air volume.
So since 33.4 - 27.7 = 5.7 the aluminum 80 is 5.7 pounds heavier on land.
Also remember the AL-80 diver gets short sheeted with only 77.4 CF in a nominal fill but that's trivial in terms of weight. The X-80 filled to 3442 PSI hold 81 cubic feet of breathing gas. That difference in capacity approaches 5% !
So now the AL-80 diver has a tank that's 5.7 pounds heavier and he's (or she's) wearing 7.3 pounds of extra weight making him 13 pounds heavier hiking across the beach. He also has that much more mass he's pushing through the water, Newton's Laws do apply to divers. Challenging Newtons' Laws consumes energy and that serves to deplete your air supply faster, shortening your dive.
References:
Using the tables linked to above or a number of other online specification tables you can run the numbers to evaluate your prospective cylinder purchase.
As long as you stay with 80CF the cost difference is not that dramatic. Look at in terms of the investment you have made in this sport as well as the enjoyment of your lesiure time.
So there you have it, in my eyes the great AL-80 hoax, a cruel joke the industry played on the diving community.
I recognize that there are some applications like doubles and stages where these properties can work as an advantage. When used to inflate lift bags for recovery operations they can be sent up a line when empty since they become positively buoyant. For the single tank cold water shore diver I think it's a shame.