Maximum Operating Death (Part 1)

Cylinder labels – Where and what to write

Scuba diving is still claiming victims. Sometimes, reading newspapers, it is possible to hear about the diver’s oxygen cylinder that ran out. If you read this blog, you are probably a diver and therefore know exactly what I am talking about!
With today’s demand for advanced diver training these journalist could never be so far from the truth. Oxygen cylinders are dangerous, a bit like a loaded weapon. If mishandled, it is not a question of ‘if’ but ‘when’ will it kill!? Unless…

As soon as divers enter in the Technical diving realm, they will carry along a variety of gasses either to extend their bottom time or to help during the decompression or ascent phase of the dive.

All you need is this!

Scuba diving cylinders are to be filled with air most of the time, or and that is becoming more and more popular, anything else than air. Not only it is important to get your ‘air’ fills in reputable filling stations but if the facility is filling also other breathing gasses, such as Nitrox or Trimix, it is important if not vital to analyse the scuba cylinder you are given. Do not assume, guess or gamble!
In Tech dive centers it is a moral obligation as well as a safety guideline to label correctly any cylinders that does contain gas with other content than 21% oxygen and 79% nitrogen.* Pure oxygen must be marked as OXYGEN!!!
In order to warn the novice diver that some cylinders are not for his or her use but are either oxygen rich or lean mixtures. Be aware!! 

So let’s have a look at how and what sort of labels/stickers we’ll use to label our cylinders and what will be written on these ‘reminders’.

For divers using EANx mix in their bottom phase cylinders, it is recommended to write the Maximum Operating Depth (MOD) and a personnal identification to differentiate your cylinders from other team members using the same gas.

It would go as follow:

  • for a Nitrox 32 @1.5P02=37m
  • Diver’s nickname JRx
1.5 PO2 is used for this exemple as an MOD – I would have 1.4 as a TOD

Less is more!

By just giving the maximum depth this cylinder is safe to breath, I limit the confusion and potential danger of breathing the wrong gas at the wrong depth.
If taken to an extrem it can lead to confusion, look at this exemple:


 On the exemple above we must also admit a bit of Photoshop creation and not the use of a permanent marker!
So again, less is more!

We must fear the mishandling of labels and other marking system. More recently a tragic accident occurred during an exploration dive in the famous Wakulla Spring in Florida. A very experience diver and instructor died due to label error and gas switch at the wrong depth. Because this why divers die, switching at the wrong depth with the wrong gas.

Mix Gas Mixing Pannel

Mix Gas Mixing Pannel

If you are diving using trimix blends, you MUST make sure your cylinders are properly and clearly marked. Some trimix blends hold a content of oxygen breathable from the surface to depth with an MOD defined by the PPO2 the diver accepts to expose him or herself. On the other hand, to dive below the 60m range, it will be required to use what is called a hypoxic trimix blend. This denomination simply means that the gas in the cylinder is only breathable at depth and DANGEROUS if breathed at the surface due to a too little oxygen content. Some very deep trimix blends don’t have enough O2 to support life, while at the surface.

In the Part 2 of this article, we will be discussing, where we should place these markings on the cylinders. Stay Tuned!

About Jason

Essential Scuba Training is aimed at divers looking to better their skills in recreational and technical diving while using a holistic configuration. Be it recreational or technical and/or cave diving, Essential Scuba Training will put it's twenty years of experience at your service. View all posts by Jason

7 responses to “Maximum Operating Death (Part 1)

  • Lynne Flaherty

    Are you advocating only labeling cylinders with MOD and initials? That would worry me . . . how would I know that the cylinder had actually been analyzed, and when? I was taught to put the analysis, to one decimal place, AND the MOD, as well as my initials and the date analyzed. That tells me the cylinder WAS analyzed, by me, on a certain date.

    • Jason

      Hi Lynne, pleased to see I have a reader! 🙂
      I am not advocating anything. This my point of view, I do not pretend to have The answer, although… If you think carefully about it, what does the MOD says to whom is looking at taking a particular cylinder? If you don’t know anything about it, then it is clearly not for you! If, on the other hand, you know what it stands for, you are good to go. Are you? The initials on the label would confirm that this is YOURS and you’ve analysed it personally. A date? What for? IS that going to help me switch to the right gas at the right depth just by knowing when this blend was made? I don’t think so. Bear with me here, this is only my opinion.
      Now, you also were taught to put the analysis, very precisely, which is great of course. It is important to have it right! I just spent 5 months blending, nitrox, lean or rich and also some trimix. Let me tell you, to reach the mix you desire for the depth you are trained to switch is not that easy! I had to re-do some mixes for some GUE divers who would not accept to switch gas at different depth that they were trained to… For me, it’s just a question of adaptation but that’s another debate. So my idea for this is to simplify it to the point where, you just need to know the MOD of the gas in this cylinder plus a personal identification mark.
      What I plan to discuss in the Part 2, is how and where we label. I am sure it’s going to raise more questions and I like that. So, I hope my answer helps you understand where I am coming from. Remember, I am not a GUE diver.
      Glad to see you here Lynne.

  • Aaron Wootton

    I’d have to agree Jason, less is more. If you know the MOD then all else is secondary, and in many cases, possibly dangerous. I plug my analysed gases into the ‘puter and the deco-plan goes on the slate along with gas mixes etc. I never saw the relevance of putting the date on the mix, I don’t leave cylinders lying too long after a mix anyway!. I actually go a step further and have dog tags with the MOD on the second stages of the deco regs (in addition to the cylinder markings). With sidemount fonfiguration, I have 4 regs within 12″ of each other so I get a bit fussy. Probably excessive but long deco stops are good for coming up with daft ideas!


  • Lynne Flaherty

    Well, if you don’t put the date on the sticker, and you only put the MOD, it seems to me it would be all too easy to accidentally leave a previous analysis tape on the tank and not realize it. I sometimes pick up a half dozen tanks from the fill station, and analyze and check pressure on all of them, so I could forget whether I had done a particular tank, unless the sticker had a date on it. Similarly, someone else might pick up on my incorrect calculation of MOD, if I have the actual analysis on the sticker. If I just put the MOD, there’s no crosscheck.

    If I go to use a tank and the sticker is very old, I’ll reanalyze on the day of diving, just to be safe. Analysis is easy, and toxing is not something I ever want to do!

  • Jason

    You’ve got a point Lynne, don’t trust a label/marking if you didn’t just analyse and wrote the label yourself. As you speak of tanks being filled for you to pick up later, it is your responsibility to analyse and mark them, even if the blender has done it. Trust no one!
    The blender has a lot of liability regarding tank analysis but the final word comes down to the person using the tank.
    Blend once and analyse twice.
    So a date on a blend is irrelevant mainly if you go diving straight ahead with the tank you just analysed. I don’t think so. That said, if you come to a filling station to analyse in order to save time on your next trip, then a date is a good idea…Although, wouldn’t you re-analyse your tanks prior to do the label…and dive..??
    Never guess, assume or gamble! Ever heard of the gag reflex?
    Thanks for your comment Lynne.

  • Aaron Wootton

    Thanks Lynne, you make a valid point in relation to the crosscheck, I think doing so within a team environment is a good idea. I’m not so convinced about accidentally leaving a previous analysis tape on the tank without realising it. Not saying that it doesn’t happen, but serious retraining is in order if a diver makes that error (not you Lynne, you are famous even this side of the pond, LOL!)
    I can’t speak for the way others do things but when I blend, I analyse the mix and again a couple of hours later. I always check the mix again just prior to the dive so I do my own cross check. I just don’t feel comfortable with the possibility that I may have made a mistake and relying on someone else to pick up on it (or not!) Post dive, MOD duct tape comes straight off, the only other permanent markers on my tanks are my initials on the side.

    And yeah, toxing is not something I ever want to do either!


  • Kalle Selin

    My preference and the way we are teaching it in SwedTech Diving is similar to your opinion Jason. What’s really important is the MOD and noting else. A name could be useful, but not necessary. What you do after filling is analyse, mark and attach your regs. I agree with the problem of a person taking your cylinders by mistake and dive with a gas not suitable, but there’s really no way to avoid that. Say that you delivered your twins to a dive shop and ordered a 12/70, they gets filled and put in a corner for you to analyse and pick up. If anyone takes them by mistake there’s no marking in the world that can help because they haven’t been analyzed yet… And I agree with you on the date too Jason. If you are unsure on what gas you’re having in your cylinders, reanalyse, it’s as simple as that! And for that sake, what would your rather trust, a 6 month old sticker that has a name, mod and date, or a fresh analyse?

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