Failure to Label Cylinders Correctly
Opinions vary as to how one “must” label Nitrox, O2 and Trimix cylinders. Nevertheless, there is general agreement that, as a minimum, markings should include:
- Fraction of Oxygen (FO2).
- Fraction of Helium (FHe), if any.
- Maximum Operating Depth (MOD).
- Analysis date.
- The name or initials of the person who did the analysis.
Additionally, the MOD needs to be displayed in such a way that it is readily visible to the user and his or her team members. This way, should your buddy accidentally switch to the wrong deco mix, you have a better chance of catching it before he toxes or gets bent.
On Nitrox mixes, O2 content is generally displayed as either a percent (i.e., 32%) or an EANx value (i.e., EAN32). Combined O2/He values are generally displayed as whole numbers divided by a slash (i.e., 10/50 or 21/33).
You are expected to correctly label your cylinders for a variety of reasons, including:
- It warns you and others not to assume the cylinder contains air or something other than what it really is.
- It lets you know what value(s) to enter into your dive computer or deco software.
- It tells the blender the content of any gas remaining in your cylinder, so that he or she does not necessarily have to dump valuable oxygen or helium in order to re-fill your tank.
- If you manage to kill yourself, it at least helps others understand why you died. While this may do you no good, at least it will provide an object lesson to those who would play fast and loose with proper analysis procedures.
Training agencies and others sell ready made decals with places to record all this data. These are better than having no information at all. The only problem with such decals is that, by trying to cram all of this information on to one small decal, the data becomes vanishingly small. The two most vital pieces of data, FO2 and MOD, are seldom more prominent than less consequential data such as analysis date, analyzer initials or fill pressure. Additionally, the MOD cannot be readily seen by other team members.
A more viable alternative may simply be to use a piece of brightly colored duct tape. On this, you can write the FO2 large enough to be easily seen and make less consequential data smaller.
MOD markings are available from a variety of sources. For maximum safety, MOD numbers should appear prominently on both sides of the cylinder (as it will be seen by your buddies), as well as on the bottom. The best ones use large, easy-to-read numbers (i.e., MOD20 or, simply, 20), and avoid extraneous information such as agency logos and contact info, which only serves to make the markings harder to read.
The one danger in relying on ready-made MOD markings comes when your mix turns out to be something other than what you wanted (or you want to use a gas mix different from the one the cylinder normally contains on a one-time basis). For example, lets say you have a dedicated EAN50 deco cylinder. On its sides or bottom are decals that loudly proclaim MOD70. Unfortunately, what the blender ends up giving you analyzes out at EAN54. (Yeah, we know it’s supposed to be ±1 percent — but don’t bitch until you’ve tried to blend an aluminum 30 or 40 yourself.)
In this case, your actual 1.6 MOD is 65 feet — noticeably shallower than the MOD 70 that appears on your tank. What to do?
Duct tape to the rescue again. In this case, you can temporarily cover up the “permanent” MOD markings with a couple strips of brightly colored duct tape containing the real MOD. In fact, if you are regularly exposed to the vagaries of partial-pressure blending, you may want to dispense with the “permanent” MOD wraps or decals, and simply use duct tape to mark the real MOD every time.
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