Tech Tidbits
Article 02
by Arnie Warshawsky
(This is a reprint of a previous series, which will be released on a regular basis. Enjoy!)
Lets Talk About Equipment: SCUBA Cylinders
Preface
In this series of articles with a technical diving theme, I do not intend to teach anything. I will share history, debunk urban legends, and offer my insights, suggestions, and tips gleaned from over a decade of technical diving and as a technical diver instructor. If you decide to seek technical dive training, the important thing is to find an experienced instructor who you trust. Your instructor is by far more important than the training agency. Aqua Tutus Dive Club is blessed to have Dennis Hocker, an accomplished technical instructor, in its ranks. If you read my first article (“How It All Began,” Aqua Tooter, May 2019) you know that Dennis was my technical diving instructor. Over the years we continue to go technical diving together. But if learning in the cold waters of Monterey is not your cup of tea, come see me in sunny Hawaii. Better yet, bring Dennis with you!
“My dive equipment is in pretty good shape, so I can just use what I have when I begin learning how to do technical dives. I don’t need to buy anything else. Right?”
Not necessarily! You may recall from last month’s article, I mentioned the importance to minimize single-point failure in your dive kit. Why this is so has a lot to do with philosophy. Recreational dive training emphasizes buddy diving. An unstated presumption is that if you have a problem, you can usually rely on your nearby buddy to help you out of the jam. If not, you can ascend directly to the surface with no ill effects. Not necessarily so with technical diving where you might have a decompression obligation that precludes a direct ascent to the surface. By contrast, technical dive training emphasizes self-reliance. You bring what you need and the know-how to use it on every dive. Since you can’t anticipate every possible problem it is critical to develop strong problem-solving skills and always stay flexible. Never panic. Panic kills. While I don’t discount the help a dive buddy can provide, nor the fun from sharing a dive with a friend, I believe the emphasis on buddy diving subconsciously establishes a dependency mindset that does a disservice to the diver.
Acquiring the gear suitable for technical diving can be expensive. From the equipment perspective, self-reliance translates to using very reliable gear supplemented with judicious redundancy. It is up to the individual diver to decide what works for her. You have to use common sense and knowledge here. You don’t want to look like the fellow in figure 1. Most technical divers start out buying what their instructors recommend then slowly change their kit as they develop their own diving style. With respect to redundancy, it does not mean bring two or three of everything. In this article, we launch into the discussion of technical diving equipment by tackling SCUBA cylinders.
Some historical background. Sport diving began in the US several years after the end of WWII; but no training and scant equipment was available. In 1946, the year I was born, the French company Air Liquide formed La Spirotechnique led by Jacques Cousteau, Emile Gagnan, and Jean Delorme. (Cousteau coined the name Aqua Lung for English-speaking countries, but in France it is still La Spirotechnique.) They began manufacturing and selling regulators. In 1950, René Bussoz, of René Sports in Los Angeles, became the exclusive US retailer for La Spirotechnique products and created US Divers to market them (Air Liquide bought US Divers from Bussoz in 1956).
La Spirotechnique did not manufacture cylinders. Most of the pioneer divers bought WWII military surplus cylinders (typically 38-cf steel cylinders), industrial cylinders (including some previously used in fire extinguisher systems), or medical cylinders and figured out how to valve them, connect them, and use them for diving. At this time, all cylinders were made from steel. What about aluminum?
Isolated in 1807, aluminum, the most abundant metal in the earth, cost more than gold until 1888 when a practical extraction technique was invented. Availability increased and prices dropped steadily finally leveling off in the mid-1950s. Just at that time the US Navy, which was interested in SCUBA cylinders that did not have a magnetic signature, contracted with a few companies to produce SCUBA cylinders from aluminum. These cylinders were manufactured with a very different process than is used today, they had round bottoms, like steel cylinders. The diameter was sized so that the new cylinder would fit into the harnesses already in use. Note: these cylinders are no longer legal for use in the US
The Navy project provoked a commercial market for aluminum cylinders despite being much more expensive than steel cylinders. Dennis notes that these aluminum SCUBA cylinders were made using the least possible amount of aluminum, possibly to keep material costs down, but the resultant cylinders were very buoyant, which did not go over well with divers. In early 1971, Luxfer USA, Ltd. was formed to manufacture aluminum cylinders for SCUBA diving. It was joined shortly by Walter Kidde, Norris Industries, Cliff Impact, and Kaiser Aluminum (marketed as AMF). Catalina Tank Co. entered the market much later in 1986 (it subsequently absorbed Cliff Impact as a division). The manufacturing process had been changed from the Navy contract days to cold extrusion from an aluminum billet producing a seamless cylinder, preferred by USDOT for a pressure vessel. One result is the flat bottom you see today. Another is that the buoyancy of the original design was lowered. As availability increased, the price for aluminum cylinders began to drop eventually becoming cheaper than steel cylinders. Currently, a 3,000-psi aluminum 80 costs $175 and a 3,442-psi steel 80 costs $300. The 3,000-psi, 80-cf aluminum cylinder reigns as the worldwide king of cylinders.
The aluminum alloy used for most of these early cylinders, called 6351-T6, was implicated in what is called sustained load cracks (SLC). Tiny cracks were first observed in cylinder necks in 1983. As of 2006, only 29 cylinders (12 in the US and includes SCUBA and SCBA cylinders) using this alloy ruptured explosively out of an estimated 25.4 million manufactured. It was never established that SLCs were responsible for any of these ruptures. The more common problem is leaking gas from the cracks in the threads. Nevertheless, near hysteria set in. A non-destructive testing technique was developed to assist finding small cracks in the neck thread region—the so-called eddy-current test. Several companies stopped manufacturing aluminum SCUBA cylinders—only Luxfer and Catalina remain—and by 1990 no one used 6351-T6. 6061-T6 aluminum is now used.
SLCs have never been observed in cylinders manufactured from 6061-T6 but many dive shops still insist on eddy-current testing to pass their visual inspection. On its website, Luxfer states, “Luxfer does not require or recommend eddy-current testing of Luxfer SCUBA cylinders made from aluminum alloy 6061.” Some dive shops even insist on eddy-current testing for steel cylinders not understanding that that makes no physical sense at all. USDOT requires eddy-current testing for any cylinder manufactured from 6351-T6 at requalification, the official term for the hydrostatic pressure test (hydro). In Safety Advisory No. 94-7, USDOT assumes that any cylinder manufactured before 1990 (with a few exceptions noted) is made from 6351-T6 unless determined otherwise. The Advisory specifically excludes Catalina cylinders, since they were always made from 6061-T6. In my opinion, eddy-current testing is a lucrative solution still searching for a problem.
Markings: Markings on the cylinder crown provide a wealth of information, much of which is only of interest to cylinder inspectors. (See figure 2.) The three critical items you should be aware of is the serial number of the particular cylinder, the service pressure, and the date of the most recent hydro. USDOT requires a hydro plus visual inspection every five years. It recommends (but does not require) an annual visual inspection. The only limitation on the number of times a cylinder can be hydo’ed is that the mark, which shows the hydro month, year, and the requalifier identification number (RIN—the facility that does the hydro test), must be stamped onto the cylinder’s crown. So there has to be room on the crown to stamp the mark. My oldest cylinder, a steel-72, was manufactured in April 1960 by Pressed Steel Tank, Co. (PST). It continues to pass hydros. In fact, older cylinders tend to pass hydro with better values than new cylinders—a testimony to their rock solid manufacture.
Exotics: In 1990, Russia produced a SCUBA cylinder from titanium; it has a service pressure in excess of 6,000 psi; getting it filled would be challenging. Since it was a welded cylinder, USDOT was not enamored with it. Luxfer produces a hoop-wrapped composite SCUBA cylinder that has a 4,350-psi service pressure and holds 105-cf of gas. Currently, it is the only composite cylinder with USDOT certification for in-water use. This cylinder uses a standard 300-bar DIN valve. (Luxfer uses a series 7 aluminum alloy, 7075, for the pressure liner. Series 7 aluminum alloys, very high strength materials, are primarily used in the aircraft industry.) Unfortunately, as a composite, it has a legal service life of only 15 years, after which it may no longer be used, or more precisely, cannot be given a hydro.
Service Life: People are used to commodities having a clearly stated service life. Even my coffee creamer has a “use by” date. Not so for SCUBA cylinders. As long as a cylinder passes visual inspections and the five-year hydro test, it is approved for use. Cylinders are designed to undergo tens of thousands of pressurization cycles and to have significant safety margins. By design, the cylinders are supposed to stretch and return to the original shape so that the stresses of pressurization are distributed safely. Steel does this better than aluminum. Once the metal can no longer flex, the cylinder is at risk of bursting. Hydro tests are conducted specifically to identify cylinders far in advance of reaching this condition. Many of my steel cylinders are older than my students. With just a little bit of care, steel cylinders will last more than a hundred years—just keep water out of the inside and fire away from the outside. Steel cylinders are likely to outlast aluminum cylinders because steel, so much harder than aluminum, will hold up better to the rough and tumble of scuba diving. Dennis’ oldest is a steel cylinder manufactured in 1953. Yet, if you routinely overfill your cylinders you will markedly lower the service life (more on overfilling later).
Technical divers typically carry multiple cylinders to satisfy two or three different purposes. One purpose is to hold your primary gas supply—back gas, so-called because this supply is usually carried against your back. Other purposes are to hold a decompression gas supply or a secondary gas supply. Cylinders for these are often referred to as deco or pony bottles, respectively. These cylinders are usually smaller than back cylinders—I find 40-cf is a good size. They are usually carried slung under your arms close to your hips. If you wear a dry suit on your diving adventures, you might also carry a small bottle of argon to inflate your dry suit. One of the technical diving skills you learn during your training is how to carry all of these cylinders and their associated regulators during a dive.
During your initial diver training class you learned that the deeper you dive and, of course, the longer you dive the more gas you use. Technical divers learn how to calculate how much gas they will need for any intended dive plus a safety margin to placate Mr. Murphy. Obviously, your cylinders must have enough capacity to hold that amount. Your choices for cylinders abound. You can buy big cylinders, small cylinders, steel cylinders, aluminum cylinders, high-pressure (HP) cylinders or low-pressure (LP) cylinders. So many choices. How to decide? We’ll begin by looking at the main attributes of the various cylinders.
Capacity: First, I’ll address how cylinders capacities are described. In Europe, cylinder capacity is given by “water volume.” This measure is good for comparing physical sizes but not very good for comparing how much gas the pressurized cylinder will hold. Depending on the service pressures, a physically smaller cylinder could hold more gas. In the US, cylinder capacity is given in nominal gas volume at the service pressure. Marketing exuberance tends to slightly overstate capacities. For example, a Luxfer S80 cylinder actually only holds 77.6-cf of gas at its 3,000-psi service pressure; in Europe this cylinder is called 11.1 liters. It can be more complicated for a steel cylinder depending on if it has a “+” stamped after the hydro mark. The plus means that you are permitted to overfill the cylinder by 10% for as long as that hydro is valid. Aluminum cylinders can never have a plus stamp. Many modern steel cylinders also do not have the plus stamp. This is because to avoid being classified as a HP cylinder by USDOT, the operating pressure must stay below 3,500 psi, so the highest possible service pressure where the cylinder can get a plus stamp is 3,180 psi. Old PST steel 72s, my favorite cylinders, have a 2,250-psi service pressure, but because they also had the plus stamp they could be filled to 2,475 psi during the first hydro period. At the rated service pressure, the cylinder holds 64.7 cf; at the 10% overfill pressure (2,475 psi) the same cylinder holds 71.2 cf. The 71.2-cf capacity was rounded up to 72 for marketing. (For those who care, the water volume never changes; it is 731 cubic inches.)
For the same internal volume, steel cylinders are smaller than aluminum ones because the walls of steel cylinders are much thinner. For the same gas capacity, HP cylinders are smaller than LP ones. Manufacturers don’t make it easy for us to compare alternatives; but the internet has readily available tables that list all of the important physical characteristics of SCUBA cylinders that have entered the market. As for size, SCUBA cylinders span the range from the tiny 1.7-cf (Spare Air) to the very heavy 190-cf (Heiser) behemoth. While capacity is a critical factor, there are other important considerations:
• Buoyancy, which depends on weight and displacement.
• Filling a HP cylinder to its rated service pressure at your local dive shop or on a dive boat can be difficult.
• Steel is more prone to corrosion than aluminum, but it is a sturdier metal.
• Purchased new, steel cylinders are more expensive than aluminum cylinders.
You can save a lot of money by buying used cylinders. Other than my first two cylinders, which I bought new from Bamboo Reef in Monterey, all of my nearly 60 cylinders were acquired used. I use Craig’s List rather than internet sellers because I can examine the cylinder before forking over my money plus I avoid shipping charges. Here are some tips for buying used cylinders:
• Pay attention to the outside condition. If the outside shows signs of poor maintenance, the inside likely will, too.
• Be sure you can read the serial number and that there are no X’s stamped over it.
• Check the most recent hydro date stamp—five years from that date you will have to get another hydro—especially if you plan to have your cylinders filled at a dive shop.
• Look at the valve. Is it free of corrosion? Does the handle turn easily?
• If the cylinder is empty, ask the seller to remove the valve so that you can check inside (bring a pencil flashlight). Don’t buy cylinders with severe internal corrosion (rust for steel, white powder for aluminum).
• For aluminum cylinders, you might want to avoid Luxfer cylinders manufactured before July 1988 (check the first hydro) to avoid the maligned 6351-T6 alloy.
• Don’t buy any cylinder manufactured by Walter Kidde, Norris Industries, Kaiser (AMF), or Cliff Impact. Their USDOT authorizations expired in the 1990s and can no longer be hydro tested.
• Don’t buy any aluminum cylinders with a round bottom; they are no longer legal for use.
• Don’t buy any old steel cylinders with a concave bottom; these are no longer approved for in-water use.
Triples (see figure 3) were used during the early days of sport diving, probably because available cylinders had limited capacities by today’s standards. Triples are rarely used now except by divers who enjoy using vintage gear. Today, for technical diving, the traditional configuration for primary cylinders is a set of doubles. Doubles are two cylinders held together with a set of steel bands and ganged together with a manifold to create, in effect, one large gas reservoir. It’s not hard to set up doubles, but it is tedious. This configuration not only increases how much gas you can carry, but the two valves, one on each cylinder, allow you to use two regulators. Redundant regulators increase the likelihood that you will be able to breathe even if one of them fails. Alas, if you use doubles you may find that you cannot mount them onto your BCD and must do something else. We’ll discuss the options available to you in a later article. Another configuration, often called independent twins, is to band two cylinders together as for normal doubles, but without the connecting manifold. This configuration adds insurance against losing all of your back gas in the event of a cylinder neck o-ring failure or a blown burst disc. However, the “cost” of this option is that you must adopt a rigid breathing strategy alternately breathing from one cylinder then from the other in stages (typically, 500 psi at a time) throughout the dive. A related twin cylinder configuration, called sidemount, was pioneered by cave divers who often must swim through very constricted spaces. Mounting their “back gas” cylinders next to their hips instead of on their back makes it easier to negotiate constricted spaces and simplifies access to the cylinder valves. Over the past decade, it has become increasingly popular for non-technical divers, especially those who have difficulty walking with a cylinder on their back, to embrace the sidemount approach using a pair of smaller cylinders, often donned in the water.
Whatever configuration of cylinders you decide upon, the total capacity must be sufficient for the dive. But for which dive? Your choice here is either buy something that will work for all conceivable dives that you might make (overkill) or buy several differently sized sets (expensive). Alternatively, you might consider renting or borrowing until you decide what you really need. To have the flexibility I want, I keep several sets of doubles dedicated for my technical diving. But that’s me. You might be much more sensible. You should work closely with your instructor to determine your best option.
Labeliing: Because the gas mixture you use on one dive may be different from what you use on another dive, it is critical that each cylinder be conspicuously labeled. From a safety perspective, it is important that you and other divers in your group be able to readily see the maximum operating depth (MOD) of each cylinder’s contents. This helps you avoid accidentally switching to an inappropriate gas mixture during the dive. Remember, you—and only you—are responsible for analyzing your cylinder contents. You need to know what you will be breathing.
Like the rest of your gear, cylinders need to be maintained. Fortunately, this is very easy to do. A fresh water rinse after each day’s use will keep corrosion at bay. Especially if you throw away the boot. Don’t let water get inside the cylinder. Two steps to help in this regard are to never let your cylinder go completely empty and after a rinse, open the valve to blow out any water droplets that have lodged inside the valve orifice. Industry convention (not USDOT rules) calls for a formal visual inspection annually. All dive shops will insist on seeing a current evidence of inspection sticker on your cylinder before they will fill it. Some shops only honor their own inspections. Many shops will also insist on doing a visual inspection if you bring in a completely empty cylinder.
Treat with respect: If a cylinder is not properly maintained, especially if allowed to develop internal corrosion, or is significantly overfilled it could fail explosively. As a point of reference, the energy stored in a standard 80-cf cylinder pressurized to 3,000 psi is 1.22 megajoules, which is the same energy released by the detonation of 0.29-kg of TNT. Figure 4 shows three cylinders that have ruptured: Two aluminum and one steel. This style of rupture is usually called a banana peel; the cylinders have split open along the side, most likely with a lot of noise, excitement, and possible injuries. Ruptures typically happen during the filling process, so your local dive shop has a vested interest in being sure that your cylinders are in pretty good shape. They are the ones most at risk.
This is also why a good dive shop will not overfill a cylinder. Overfilling a cylinder is a thorny issue for technical divers. The folklore says: Never overfill an aluminum cylinder, be cautious overfilling a HP steel cylinder, and it is fine to overfill a LP steel cylinder. The folklore is mostly wrong: Never overfill any cylinder. Besides being illegal, overfilling a cylinder stretches the metal beyond design parameters, and according to PST, permanently deforms the cylinder. At a minimum, it will reduce the lifetime of the cylinder by reducing the metal’s elasticity. Worse is that the likelihood of an explosive rupture is increased. While there is a metallurgical reason for why steel is more resilient than aluminum (has to do with the material fatigue limit) the risk is not worth the gain. If you need a larger capacity cylinder for your diving, buy a bigger cylinder.
Storing cylinders between dives is simplest if you rent them. Give them back after your dive. Otherwise, you need to take precautions to be sure your cylinders won’t fall, possibly damaging a valve or a toe. If you store your cylinders upright, a retaining chain to prevent accidental falls is a good idea. I prefer to store my cylinders outside my home; I haven’t lost any yet. Though I did excite my neighbor when a burst disc blew. Lastly, I recommend storing aluminum cylinders either completely full or nearly empty—nothing in between.
Over the years, better alloys have been developed both for steel and aluminum cylinders. Manufacturing techniques have changed. Sometimes the improved alloys allow greater service pressure or less metal, etc. New concepts, e.g., composites, have come to the fore. As valves evolve, so too must cylinders, if for no better reason than to keep the valve/cylinder threads compatible. The industry continues to move forward.
Next month: Scuba Valves.