Scuba Cylinders 101

Image: Emy Stratton
L: Steel 30, 46 and 72s,
C: Steel 27s and 112s
R: Aluminum 9, 14, 40s, 50s, 63s and 80s

By Fred Stratton
Instructor & Repair Technician
fred@bubblesornot.com

Image: Tara McNaylor
Aluminum 50, 67 and 80 cubic foot cylinders

Most of us take excellent care of our scuba regulators; they enable us to breathe underwater, thus they are keenly appreciated even by newly certified divers. Cylinders, however, wish they would receive half the love we give our regulators. We give them a quick post-dive rinse (sometimes), fill them for the next dive and otherwise neglect them until our local dive shop professional tells us that they need attention.

This is understandable... to a point. Modern cylinders are manufactured to be pressurized to their rated pressure an astounding 100,000 times. Cylinder valves are simple, well-built devices that perform for years under a wide range of conditions and treatment. In short, cylinders are so reliable that we take them for granted. However, they are an investment and an indispensible part of your life support system, so let's look at how providing some TLC - Total Love for your Cylinders - pays dividends.

Cylinder Material

Image: Fred Stratton
Steel 72 made in January 1965 still
passes VIP and hydrostatic retesting!

Scuba cylinders generally are made from steel, aluminum or composite materials. (Stainless steel and titanium have seen limited production for research and military applications.) Composite cylinders see less use largely due to cost. A Luxfer fiber reinforced plastic (FRP) cylinder containing 106 cubic feet (CF) of gas at 4350 PSI costs around $700. You can buy two or three steel or four aluminum cylinders for that amount so we'll focus on steel and aluminum cylinders.

Steel Cylinders 

Steel has been used for scuba in the US since 1949. Steel cylinders are less than 1/5" thick (.190") yet they are incredibly strong and durable. My oldest steel 72CF cylinder was manufactured by Pressed Steel Corporation (PST) in January 1965! Reputable manufacturers include Faber and Worthington.

Sizes: Range from 15CF (for rebreathers) up to 149CF behemoths for extended diving and -- ahem -- gas-thirsty divers. Popular sizes for cave, technical, and sidemount diving include 95s and 108s.

Attributes of Steel: Denser and displacing less water than aluminum cylinders, steel cylinders are preferred by divers who want to carry less lead and deal with a more optimal range of buoyancy  during the dive.  For example a Faber steel HP 100 is 8 pounds negative at service pressure (3442 PSI)  and 0.5 pounds negative when empty. Steel cylinders are built like Abrahms tanks; they can last your entire diving lifetime with proper TLC. This makes their higher cost seem reasonable when   amortized over 30 years or more.

Care: Steel is a ferrous metal, or a metal that includes iron, which can rust. Thoroughly rinse your steel cylinder from valve to boot. Remove the boot regularly to clean salt and debris. Never fully drain your steel cylinder, as this could let in moisture. Oxidation is a process that can only occur with a catalyst, in this case, any moisture that is able to enter a completely empty tank. The rusting process will reduce the O2 content in a steel cylinder, which could reduce the lifespan and efficiency of your tank. Manufacturers utilize zinc to provide a protective coating over the steel that reduces corrosion risk as long as the coating isn't compromised.

Addressing Corrosion: BoN can remove corrosion inside cylinders if the corrosion and pitting haven't exceeded Department of Transportation (DoT) limits. The process involves tumbling a mildly abrasive material inside the cylinder followed by specific cleaning and drying procedures.

Aluminum Cylinders

Image: Tara McNaylor

According to Mr. Bill High, founder of Professional Scuba Inspectors (PSI), in a A Brief Scuba Cylinder History, aluminum cylinders were produced for the US Navy in the mid 1950s. Luxfer Gas Cylinders offered the first commercially available aluminum cylinders in 1971 using 6351-T6 alloy (to be discussed in a future article). Catalina Cylinders began producing aluminum cylinders from 6061 alloy in 1986 with Luxfer switching to 6061 by June 1988. Luxfer and Catalina are the only aluminum scuba cylinder manufacturers in the US. Their products are exported around the world. As of 2018, Luxfer has produced over 50 million scuba cylinders!

Image: Fred Stratton
DAN O2 kit with Luxfer M9
 aluminum cylinder.

Sizes: Range from 4 cf (for drysuit inflation) up to 100 cf at service pressures of 3000 and 3300. Popular sizes for DAN O2 kits include the M9 (9CF) and the Jumbo D (23CF).

Attributes of Aluminum: Aluminum cylinders are thicker and displace more water than steel, making them more buoyant. A Luxfer 80 (3000 PSI) is 1.7 pounds negative when filled to working pressure and 4.2 pounds positive when empty. Aluminum cylinders generally cost 25% to 50% less than steel cylinders, so active divers will rapidly recoup their investment in an aluminum cylinder.  (And there's just something about owning ALL your own scuba gear that makes you feel good.) Although it is possible to dent or gouge aluminum cylinders more easily than steels, TLC will return years of dutiful service. (My oldest aluminum cylinder was born in May of 1980 and she still passes inspection!)

Image: Fred Stratton
Poorly maintained Catalina K valve requires
ultrasonic cleaning and new parts. 

Care: The same care as steel cylinders applies here; post-dive freshwater rinse from valve to boot and keep at least 50 to 100 PSI in your cylinder when not in use to prevent moisture intrusion.

Corrosion Remedy: It is a myth that aluminum cylinders are corrosion proof. Aluminum does corrode into aluminum hydroxide (a gel-like substance) and aluminum oxide (dry white powder).  However, both Luxfer and Catalina utilize a manufacturing process that induces a corrosion-limiting quality to its 6061 alloy cylinders.

Upon discovering corrosion your scuba technician will remove it using the least intrusive means possible, clean and dry the cylinder, and then verify that the cylinder meets DOT inspection criteria before returning it to service. Corroded valves are disassembled, ultrasonically cleaned and reassembled using new parts.

Cylinder Inspection & Service Requirements

Informal Inspection: BoN conducts an informal inspection every time you bring your cylinders to be filled. The fill station operator:

   - checks for visible external damage (dents, dings, gouges)

   - verifies that the evidence of inspection (EOI) or VIP sticker is current

   - verifies that the cylinder has been hydrostatically tested (or retested) within the past five years

   - notes the service pressure (maximum fill pressure)

   - confirms that the cylinder isn't empty which would require a VIP to be performed

   - taps the side of steel cylinders with a hammer listening for a clear, bell-like tone
  - ensures the cylinder is properly labeled according to its designated usage (air, EANx, oxygen,       argon or trimix)

Image: Tara McNaylor
BoN Evidence of Inspection (EOI) aka
VIP sticker

  

Visual Inspection by a Professional (VIP): BoN cylinder inspectors are trained by Professional Scuba Inspectors - Professional Cylinder Inspectors (PSI-PCI) and follow PSI protocols which adhere to US DoT standards and procedures developed with cylinder manufacturers. This annual inspection determines the cylinder's suitability for continued service which includes detecting issues in their early stages before it becomes necessary to tumble the cylinder or even condemn it. Think of it like the annual wellness exam with your doctor. 

A good time for getting your cylinders inspected is during your annual regulator servicing. The Visual Cylinder Inspector will:

   - remove all accessories: boots, nets, etc

   - inspect all external surfaces and valve for corrosion and damage

   - check inside for liquids or solids

   - inspect interior walls and threads

   - inspect Pressure Relief Device (PRD aka burst disc) for proper rating and integrity

   - remove old evidence of inspection (EOI aka VIP sticker), apply new sticker and log it

   - fill cylinder to service pressure and check for leaks

Hydrostatic Retesting: The DoT requires scuba cylinders to be hydrostatically tested at the time of manufacture and retested at regular intervals thereafter. The test measures the expansion of the metal to ensure it falls within limits determined to be safe by the DoT which oversees manufacture, testing, filling and transport of compressed gas cylinders in the US. (It is the Transport Canada (TC) for our neighbors to the north.) This interval is five years for steel and aluminum cylinders, so a cylinder stamped 02-18 would require retesting in February 2023. 

But what exactly is a "hydro"? This test involves filling the cylinder with water, then immersing the cylinder in a closed water-filled container. The cylinder is pressurized to 5/3 of service pressure (5000 PSI for a 3000 aluminum cylinder, 3750 for a 2250 steel.) The pressure is held for a specific period of time to measure how much water is displaced from the enclosed container. The technician determines if the displacement falls within DoT limits and records the results.  

If the cylinder passes it is emptied, dried, stamped with the current month, year and retesting facility's identification number (RIN), and prepared for filling. If the cylinder fails retesting the technician is required to condemn it and discuss options for disposition of the cylinder with the customer.

Image: Pixabay

How are Scuba Cylinders Made?   

These five-minute clips explain how manufacturers transform aluminum slugs and chromoly steel blanks into compressed gas cylinders and then hydrostatically test them. 

Aluminum cylinder manufacturing from the TV show How It's Made

Steel cylinder manufacturing video from Worthington Cylinders

Image: Rose Bennett
Cylinders with air, 32% nitrox and 50%
oxygen for Tec 40 Course students.

Final Thought: Aluminum 80s for all my Friends!

Not so fast. My wife Emy and I usually surface from recreational dives with nearly identical readings on our SPGs even though she weighs 93 pounds and I weigh 160.  Why doesn't she have WAY more gas than I?  She dives 53s while I use an 80. She simply has less tissue to oxygenate than I do...and an 80 would hang down past her butt! 

What's your point, Fred? My point is that if you are unsure about your options, let BoN's dive professionals help you choose cylinders appropriate for your physique and diving interests. You may determine together that aluminum 67s suit your current needs and that you'll invest in steel 85s when you enroll in a PADI or IANTD sidemount course.

* My gratitude to Odessa Stratton for editing and improving this article. *

Coming In June

Scuba Cylinders: Valves, Markings & Other Fun Stuff