Cylinder Marking & Gas Switches

Cylinder Marking: The Fine Points

Cylinders should be stripped of all unnecessary stickers. Place the VIP label beneath the handle, below the valve opening, and remove Nitrox banners or any other non-essential markings, as multiple markings can create confusion. Each cylinder should be marked horizontally on two sides with large, three-inch-high numbers identifying its MOD (Maximum Operating Depth). Each cylinder should be marked on the bottom with large numbers identifying its MOD (Maximum Operating Depth). Bottom marking is IN ADDITION TO and NOT A SUBSTITUTE FOR the MOD marking on the side of the cylinder. Bottom MOD greatly enhances visibility when diving a single file and allows a quick inventory of all tanks the diver is carrying or breathing. Bottle marking is never a substitute for proper gas switching protocols and verification. Oxygen cylinders should be marked “20” with the word OXYGEN written horizontally along the cylinder, to prevent divers from mistaking “20” for “70” or “120”. “20” can be used on the bottom of AL oxygen cylinders, and steel oxygen cylinders do not require the MOD on the bottom.

Argon cylinders should be labeled ARGON.

The diver’s name should also be marked on the cylinders to simplify identification. Last name should be used, if possible, to avoid confusion. No gas percentages should be placed on the cylinder for identification purposes, as these require divers to make underwater calculations. Any content information is the result of analysis and should be placed near the neck of the bottle. For example, when analyzed, an 18.2% oxygen and 45% helium mixture would read 18.2/45; when analyzed, a 32% Nitrox mixture would read 32.0%. The date analyzed and the tester’s initials should be included with the percentage marking.

Gas Switching Procedures

Any truly reliable tank identification system must work in conjunction with a dependable gas switching procedure. Divers who are inattentive or fail to exercise caution when using different gas mixtures will inevitably use the wrong mixture at depth, possibly resulting in a fatality. The following gas switching procedures should always be followed:

  • Divers should operate as a team, verifying proper mixture and depth. Arrive at the desired switching depth, retrieve and attach the cylinder if required (i.e., in a cave dive where bottles are left behind). Locate the properly marked cylinder by checking the MOD marking and deploy its second stage (the regulator is stored in a retaining band on the cylinder).
  • Open and close the valve, confirm the pressure drop while looking at the SPG. Repeat. Before wrapping the hose around your head and putting the regulator in your mouth, wait for your buddy’s “OK” signal to confirm you are about to breathe from the correct bottle. Once complete, you may then watch your buddy conduct the same procedure.
  • If beginning decompression, start decompression time.