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SCUBA CYLINDERS
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MAINTENANCE
Before purchasing a cylinder keep in mind that every cylinder needs to be visually inspected every year and hydrostatically tested every 5 years. Prices vary for these procedures depending upon the location where you have your cylinders serviced. During a visual inspection your favorite dive shop personal will be looking for different things depending upon what type of cylinder you are diving with. These visual inspections serve to test the wear and tear occurring to your cylinder from the wear and tear of your use. If you own a steal cylinder the person visualizing your cylinder will largely be looking for rust spots and pitting (more on this later). For aluminum cylinders the person inspecting the cylinder will be keeping tabs on the amount of rust on your cylinder, but will mostly be checking your cylinder for neck cracks during a visual plus inspection. Hydrostatic tests check the wear and tear on your cylinder due to fills, neglect, and mechanical damage to your cylinder that may occur from rolling in the back of your car or being dropped during transport. In order to pass a hydrostatic test your cylinder will be over pressurized beyond its filling capacity. The cylinder is then checked to measure its ability to return to its original volume. If the cylinder is unable to closely return to its original shape the cylinder will fail the hydrotest. Since a hydrostatic test is a destructive test it is recommended that you do not hydrostatic test your cylinder every year.
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ALUMINUM OR STEAL
Aluminum cylinders have the obvious advantage of being cheaper than steal cylinders. The difference in cost can be as much as 300 dollars depending upon location when comparing cylinders of the same volume. Aluminum cylinders have another advantage over steal cylinders in that when aluminum cylinders rust they produce aluminum oxide which does not spread throughout the cylinder or bore through the cylinder walls to cause pitting; as is the case with steal cylinders. Affordability and lack of corrosion are the key plusses for aluminum cylinders, however just like everything else in life these cylinders do have their drawbacks. Aluminum cylinders become positively buoyant as the air pressure inside the tank decreases throughout the dive. As a result an aluminum cylinder with less than a thousand psi may be 4 pounds of positive buoyancy. To compensate for this divers add weight to their weight system so they will not float to the surface at the end of the dive. The other side affect of the change in buoyancy properties of these cylinders is that when positively buoyant the cylinders tend to alter the divers profile in the water and thus increases drag and therefore decreases efficiency underwater.
Aluminum cylinders come in a variety of sizes from as small as 3 cubic feet to the very common aluminum 80’s and somewhat more exotic aluminum 95’s. Most divers will start their dive careers on either an aluminum 65 or 80 cubic foot cylinder. If you are unsre if the cylinder you are using is aluminum take a look next to the DOT stamp (Department Of Transportation) for the 3AL designation which is the symbol for aluminum cylinders.
Steal cylinder have much thinner walls than aluminum cylinders. Steal cylinders are much smaller than aluminum cylinders with the same volume when filled to capacity. Steal cylinders may also be filled to higher pressure than aluminum cylinders. As a result many divers enjoy diving with a cylinder of air much smaller than an aluminum 80 which actually holds 20 more cubic feet of air. For people with smaller builds, people who dive with excessive amounts of weight, or people who just want to dive with a lesser load; steal cylinders offer extended bottom time and resolve the weight and buoyancy issues many divers experience. Depending on which size steal cylinder you go with you may take 4-10 pounds of weight off of your weight belt and still have more bottom time. The draw backs of steal cylinders are two fold. One, steal cylinders rust when not stored properly and therefore should always have at least 500 psi in them after every dive. Two, steal cylinders as mentioned before can be significantly more expensive than aluminum cylinders.
There are three filling pressures to
steal cylinders that are commonly available in any local dive shop. The first
is the low pressure cylinder which may be rated between 2250 and 2400 psi. For
the first five years of the life of this cylinder the cylinder has a plus
rating. The plus rating allows the person filling the cylinder to overfill
the cylinder up to ten percent above the working pressure. Once the cylinder
has been hydrostatically tested it may no longer be filled with the plus
fill. As a result most cylinders such as a steal low pressure 72 may no
longer hold 72 cubic feet of air since many steal
manufacturers measure their
fill capacity and include the plus fill when determining the over all volume
of the cylinder. The second fill capacity is the 3000 psi working pressure
cylinder which may also have a plus designation. The final group is the high
pressure or HP cylinders. These cylinders may be filled to 3,500 psi and do
not have the plus designation. In fact for those with the backs for it, there
are steal high pressure cylinders that can be filled with up to 120 cubic feet
of air! Now that will get you some serious bottom time!
Ultimately whether you go steal or aluminum is your decision. As a diver I prefer steal cylinders. For me the ability to take 6 pounds out of my BC and reduce the strain on my back from carrying the extra length of an aluminum 80 far out weighs the difference in price. In addition the benefit I gain from being able to “trim” my BC coupled with the buoyancy properties of a steal cylinder allow me to greatly extend my bottom time and comfort level when diving. Aluminum cylinders are a great way to get in the water, but for my money steal cylinders are far superior to aluminum cylinders in the long run.
Have any questions or comments? Post a reply in the input section or drop us a line at info@calcoastdivers.com! We’ll get back to you in 24 hours or less!
Copyrighted by Ryan Jaeger & Matthew J. Wahlrab 2002 - 20032