Tanking Up: An Inside Look at Scuba Cylinders
Written by Greater Cleveland Aquarium Scuba equipment is absolutely critical to keeping divers protected and alive. Today, I want to talk about the one piece of gear that we divers need and is often overlooked—the scuba cylinder.
Scuba cylinders supply air, enabling us to explore the underwater world that we have come to love. They come in many different shapes and sizes to serve different needs.
Back when the sport of scuba diving started, many innovative divers would actually take, used fire extinguishers and convert them for diving purposes. In fact back in those days a person could just walk into a local department store and buy most of what he or she needed to dive.
Modern scuba cylinders come in two main materials: steel and aluminum. Each of those has their place within the industry. Steel is really the preferred choice of technical divers that go into the darkest reaches of the underwater world. It is more durable, has a better weight balance and can be more easily filled to higher pressures—all the way up to PSI pounds per square inch.
Aluminum on the other hand is lighter and cheaper, so for the recreational diver or for the dive operation that needs a large supply of them, this becomes the better choice. Aluminum is not as forgiving when it comes to its durability and cannot be filled to as high a pressure.
Aluminum cylinders can be bought in almost any color your heart desires. Cylinder capacity is dictated by several factors. First, divers have to consider just how long you intend to stay underwater and the depth, since the deeper you are the more air you will use.
Your breathing rate is the last piece of information to consider. Cylinder capacity is measured in cubic feet, and they can range from 3 cubic feet to more than cubic feet. An average recreational aluminum scuba cylinder has a capacity of 80 cubic feet and generally weighs about 35 pounds and more than 42 pounds when full.
What do we put in scuba cylinders? Many people think that cylinders are full of oxygen but that is relatively unusual, most cylinders have air. What is air? It is very important for a diver to know what is in his or her scuba cylinder because diving with a gas mixture different from air can have consequences. Diving pure oxygen at depth can be a fatal mistake! One alternative gas some divers use is nitrox, which is similar to air but with less nitrogen and more oxygen.
Using nitrox in our cylinders allows diver to stay at depth for longer and we divers love that. You'll find out when you feed one.
Lightweight SCUBA Tanks
Valleys and Folds? A good candidate for a thread microscope and NDT inspection to check for cracks. Note:- The Luxfer Visual Inspection Guide calls for 8 defect free threads starting from the top for a 3,psi cylinder. How Bad can corrosion pits get? Presenting below — A scuba tank that not even a Yemeni Bedouin compressor operator straight off the camel would fill… Internal corrosion pits — somewhere in a dive center near you in Asia under those corrosion bi-product mounds will be pitting deep enough to cause the cylinder to fail inspection.
For most dive operators this is of cause unavoidable; just be aware that storing cylinders exposed to salt spray will quickly cause external corrosion that will cause it to fail visual inspection due to external corrosion pits relatively early in its design life. See below for examples. The relevant CGA copyrighted specifications are listed below and are available for download here from the CGA web site. Test stations applying for DOT approval must have their own individually licensed copies of the CGA documents needed to carry out the cylinder inspections the test station offers.
Note:- That the CGA charges fees for each downloaded document. Also note:- That these older cylinders are made of the troublesome T aluminum alloy. The alloyed used in these cylinders has well known susceptibility to Static Load Cracks SLC They have caused several catastrophic explosions and the deaths of many cylinder fillers.
This marking should make you instinctively reach for your NDT tester if it ever graces your test bench. Extract from the latest CGA C
Cylinders & Valves
The relevant CGA copyrighted specifications are listed below and are available for download here from the CGA web site. Test stations applying for DOT approval must have their own individually licensed copies of the CGA documents needed to carry out the cylinder inspections the test station offers. Note:- That the CGA charges fees for each downloaded document.
Also note:- That these older cylinders are made of the troublesome T aluminum alloy. The alloyed used in these cylinders has well known susceptibility to Static Load Cracks SLC They have caused several catastrophic explosions and the deaths of many cylinder fillers.
This marking should make you instinctively reach for your NDT tester if it ever graces your test bench. Extract from the latest CGA C Steel cylinders tend to be physically smaller for the same capacity, and are less buoyant when submerged.
For these reasons, steel cylinders are often the favorite of cold-water divers, and those who dive in the close quarters of overhead environments.
Luxfer now offers a high-tech scuba cylinder made of a composite material designed for high pressure and low weight. These are basically aluminum cubic-foot cylinders hoop-wrapped with fiberglass to increase their strength. Essentially the same size and weight as a typical aluminum 80, these 4,psi pressure vessels pack a proud cubic feet of breathing gas.
Most recreational divers choose something in the middle — cubic feet — for their main cylinder, and those at the short end of the spectrum for redundant air supply needs.
Just as cylinders come in a variety of sizes and air capacities, the service pressure for cylinders can also vary dramatically.
The older carbon steel cylinders have a service pressure of 2, psi, while newer chrome-molybdenum steel cylinders have a service pressure of 2, psi. High-pressure steel cylinders are also available with pressure ratings as high as 3, psi. Most aluminum cylinders are rated for 3, psi, but some are rated to 3, Advertisement Cylinder Valves Although they are sometimes sold separately, the valve is often considered an integral part of the cylinder.
And just as there are a variety of cylinders, valves, too, can be categorized according to their performance, features and capabilities. The basic distinction among cylinder valves is between the standard yoke K-style valves with which most divers are familiar, and the DIN valves for Deutsches Institut fuer Normung, a European association of engineers and manufacturers that sets standards for compressed gas cylinders and valves.
Proposed changes to UK cylinder testing
The DIN fitting of the regulator first stage actually screws into the female threaded DIN valve body, capturing the O-ring and making it virtually impossible for the O-ring to fail and cause a loss of breathing gas. Technical divers often prefer DIN valves for precisely this reason. One feature common to all cylinder valves is a pressure relief device PRDalso known as a burst disk. This tiny, frangible disk is designed to break and release the pressure within the cylinder when it reaches a critical pressure close to the hydrostatic test pressure of the cylinder.
This can happen as a result of overfilling, or due to an increase in the temperature of the cylinder. Bottle Buoyancy One fact that divers sometimes overlook is the changing buoyancy of their cylinders. Regardless of whether the cylinder is made of steel, aluminum, or a composite, the air which we compress into that cylinder has mass and weight.
However, its weight does. The greater the capacity of a cylinder, the greater the weight difference between empty and full. A standard cubic foot of air weighs about 0.
A cylinder that carries cubic feet will weigh about 10 pounds more when full than it does when empty. Although the change in buoyancy for a given air capacity is the same regardless what the cylinder is made of, the actual buoyancy figures are different depending on the material and cylinder design.
A typical aluminum 80 might be a pound and a half negatively buoyant when full, with a positive buoyancy of about 4. A steel cylinder of similar capacity might be around 6 pounds negative when full, and neutral when empty. Consequently, a diver using an aluminum cylinder would generally need to carry more weight on his belt to avoid becoming positively buoyant at the end of a dive than one wearing a steel tank of the same capacity.
By adding additional material about 4 more pounds of aluminum primarily to the bottom and barrel, the cylinder is neutrally buoyant in seawater when empty and floats in a more horizontal orientation than typical aluminum 80s.
Choosing the Right Cylinder Divers and cylinder manufacturers have long debated the pros and cons of aluminum vs. Without a doubt, both are rugged and reliable, and will safely meet the needs of some — if not all — divers. The best choice for a diver depends on a number of variables, including his physical makeup and the environment in which he dives.
Obviously, buoyancy characteristics are a major consideration when choosing a cylinder. Since aluminum cylinders are more buoyant, they may be a better choice when diving in warm water where a diver wears less buoyant exposure protection, and thus needs less weight to achieve the correct buoyancy.
For a diver in cold water wearing a thick wet suit or dry suit, the steel cylinder would lighten the load on the weight belt. Also important are the physical dimensions of the cylinder. Capacity is also critical. A heavy breather, the buddy of a heavy breather, or a cold-water diver might require more air to meet his diving needs than one who consumes air more efficiently or dives in warmer water.
Needless to say, those who dive deeper, or venture into caverns, caves and wrecks may also require considerably more air capacity than their shallow, open-water counterparts.
If you intend to travel to remote areas with a high-pressure cylinder, keep in mind that not all air stations can fill to higher pressures. Check to see that high-pressure fills are available at your destination. Safe Handling Techniques Scuba cylinders are heavy, and if not handled properly, can easily damage other gear or injure a diver. Rule No.
Never leave a cylinder standing in an upright attitude, since it can easily be knocked or tipped over. Merely laying the cylinder on its side is inadequate, especially in a van, car, truck or boat where accelerations, decelerations, turns and bumps can cause a cylinder to roll.
When it comes to carrying or moving a cylinder, even the professionals have differing opinions. Some recommend lifting cylinders with the buoyancy compensator BC attached, and wearing the BC as a means of carrying the cylinder, as doing so will help avoid injury.
Others think that lifting or carrying a cylinder by the BC straps makes dropping the cylinder a more likely outcome. A variety of devices are available that are designed to make scuba cylinders safer and easier to carry. However you decide to carry the cylinder, remember to do so carefully, and always lift with your legs rather than your back to avoid injuries. The first thing to make certain of is that the technician filling your cylinder knows the correct cylinder pressure.
The service pressure for the cylinder assumes a temperature of 70 degrees Fahrenheit 21 degrees Celsiusso even a properly filled cylinder will show higher if hot or lower if colddepending on the temperature.
Make certain your cylinder is being filled with the right stuff. Remember that aside from air, many air stations now provide nitrox, oxygen and argon to meet the needs of nitrox and technical divers. Air quality is always a potential concern, especially when diving in out-of-the way places. Before connecting the fill line, the connector should be either blown or wiped dry.
NOAA recommends a fill rate of psi per minute to prevent excessive heating during the fill. Once your fill is complete, double-check that your cylinder is securely fastened to your BC. Since nylon stretches when wet, some divers wet the nylon straps of their cylinder harnesses before mounting to ensure they are securely fastened. A loose cylinder can be a safety hazard for those wearing them, as well as anyone below them in the water.
First, make certain your cylinder is visually inspected and hydrostatically tested on a regular basis. An annual visual inspection is recommended, but if a cylinder is used frequently in warm, humid climates, the inspections should be performed more frequently every three to six months.
Hydrostatic tests are required every five years, or any time a cylinder is damaged.