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Gear Meister answers your gear questions...
"Tanks a lot!"
This month's topic is your Scuba Cylinder, also known as your “tank.” Or that big heavy thing on your back...
You probably have a pretty good idea of your cylinder’s primary function – it holds your breathing gas. But there’s also a lot more to know about your cylinder that can make your diving easier and more enjoyable.
Sizes and materials
Compressed gas cylinders for scuba come in various sizes and materials, each with it’s own advantages and disadvantages. The most common cylinder is the ubiquitous "aluminum-80." This is a cylinder made of an aluminum alloy that holds 80 cubic feet of air at 3000 psi (or 2265 liters at 20 bar). The next most common cylinder in the U.S. is probably an "aluminum-63." This is the same aluminum alloy, but since it holds only 63 cubic feet of air, it’s much lighter than the larger "80" tank. (27 lbs /12.5 kg as opposed to 32 lbs /14.5 kg.) Most aluminum cylinders are filled to 3000 psi (20 bar), although there are some with a lower or higher fill rating.
Heavy metal
The other widely available cylinders are made of steel. Steel is generally stronger than aluminum and therefore the walls of the cylinder can be thinner. If you have cylinders with the same outside dimensions, a steel cylinder will have more internal room for your breathing gas than an aluminum one. The added strength of steel is useful in a couple of other ways. Comparing equal sized steel and aluminum tanks: 1) you can get the same amount of gas into a steel tank using a lower fill pressure than needed for aluminum one or 2) we can use a high fill pressure on the steel tank and really cram a lot of breathing gas into the cylinder. For example, some relatively small (by outside dimensions) steel cylinders can hold up to 120 cubic feet of air at 4500 psi (3400 liters at 30 bar).More air sounds pretty good - especially if you dive deep or are a "heavy breather!
Another thing that a lot of divers like about steel cylinders is that they are heavier than the same size aluminum cylinder. So, the typical diver can use about 5 pounds (2.2 kilos) less weight on their weight belt when using a steel cylinder than they would need if they were using an aluminum cylinder.Steel vs. Aluminum
Both materials have additional advantages and disadvantages. As already mentioned, aluminum cylinders hold less air than the same size steel cylinders. Steel cylinders usually cost a good bit more than a similar size aluminum cylinder.Corrosion
Both types will hold up fine if you take care of them. But what if - ooops! - water or moisture accidentally gets into your cylinder? Aluminum has the advantage here. Water in an aluminum cylinder will cause corrosion, but aluminum corrosion is somewhat self-sealing and it usually does not result in damage that renders the cylinder unusable. Water in a steel cylinder is another matter. Corrosion in a steel cylinder tends to continue, causing deepening pits and a cylinder that becomes unusable in a fairly short time.Rolling Stones
Minor surface corrosion in either type of cylinder (steel or aluminum) can be treated by “tumbling”. A bunch of ball bearings or special stones are placed inside the cylinder and a special rack rotates the cylinder, allowing the stones to gently abrade the corrosion out.The weight of air
All cylinders become more buoyant as the breathing gas inside is depleted. (Yes, compressed gas actually has weight!) This is why we were taught to do our buoyancy check - making sure we're wearing the right amount of weight- at the end of the dive with a near-empty tank, so that we could still stay down at the safety stop. What if you want to do a buoyancy check with a full tank? You can mathematically calculate the proper weighting based on a full aluminum-80 holding about 4 lbs (1.8 kg) of air.The type of gas
Steel or aluminum cylinders can hold air, EAN/NITROX, or another gas for breathing. It’s just a matter of how clean the cylinder is and making sure that it's properly marked. All cylinders must be clean, but EAN/NITROX cylinders must be especially clean and use non-petroleum based o-rings. The reason for this is that they may come into contact with 100% oxygen which can be flammable/explosive.Getting attached to your regulator - DIN or yoke
Cylinders have either a yoke-style valve or a DIN valve. Yoke valves are very common in the U.S. while DIN valves are more common in Europe. The difference is that the regulator screws onto the yoke cylinder valve whereas a DIN screws into the cylinder valve.
Safety inspections - "VIPs and hydros"
Cylinders hold compressed gas under very high pressure. To do this safely, they must be structurally sound. Two separate inspection processes - visual and hydrostatic testing - are carried out on all cylinders to ensure their structural integrity and safety. You, the diver, should check to make sure that both inspections are current before using the cylinder.A visual inspection (VIP) checks the outside of the cylinder for damage and then looks on the inside for damage, corrosion, or other problems.
Particular attention is placed on the threaded neck area as this is the weakest area of the cylinder and cracks are often found here. In many places, an electronic machine called a visual eddy is used to help find very small cracks.
A visual inspection is good through the last day of the month one year later than the month and year stamped on the sticker. For instance, if the sticker is punched on June 2008, the visual inspection expires on June 30, 2009.
A hydrostatic test, or “hydro”, is used to check the elasticity of the cylinder. Whenever a cylinder is filled with compressed gas, it expands. As the gas is used up, the cylinder shrinks back to its original size. But, like the elastic in your waistband, after so many stretches, it doesn’t bounce back to its original shape. The loss of elasticity can make the cylinder unsafe for continued use. In a hydrostatic test, the cylinder is placed in a pressurized water container and both the cylinder and the container are filled with water. The cylinder is filled to 5/3 of the rated fill pressure. As long as the cylinder returns to its original shape when the pressure is removed, the cylinder is good. A hydro is valid through the last day of the month five years after the month and year stamped on the cylinder bell.
It's all Greek to me...
Which leads us to all those hieroglyphics stamped on the cylinder bell. Here’s the short translation:
That’s about it for cylinders. Which one is right for you depends on how much breathing gas you need for your typical dives, what kind of regulator you want to use (yoke or DIN), how well you take care of your stuff, and how much you’re willing to spend. After that, go dive!
