Marking our cylinders -

Marking our cylinders

Article by José Pablo Mir

The incident

When Antonio took the deco cylinder with Nitrox 37 I handed him, he carefully read its labels and executed a gas switch process quite similar to the one I use and teach. Moments before starting the ascent, the valve on his Nitrox 50 cylinder had been damaged, and he feared that his back gas (Trimix) would be insufficient to manage all the decompression ahead until he could safely switch to his Oxygen cylinder at the six-meter (20 foot) mark.

I quickly realized that Antonio had been well-trained to remain calm and act decisively in unfamiliar and challenging situations. I later learned that he is primarily a freshwater diver with minimal open-sea experience and he faced a scenario that could easily unsettle such a diver (the Atlantic Ocean). While being a freshwater diver is not a disadvantage, it often heightens anxiety when one suddenly finds oneself in the open blue, without sight of the bottom, a ceiling, or any wall, while having to cope with a potentially problematic situation like the one Antonio was enduring.

At that moment, I didn’t know his name or his level of training and experience, so I closely followed each of his movements until he completed the gas switch, which included defining on his computer this new mix I provided. During the return trip to the port, as we discussed what had happened, he admitted with humor and pride that this had been his first time programming mixes during a dive, having previously only read about it. He did it well. He was entirely in control of his situation.

My partner and I stayed with him until the end. Although the ascent wasn’t as smooth as usual, since we both had to share her lean mix (Nitrox 36) until the 30-foot (10 meter) stop, where we each switched to our rich mix (Nitrox 80), it remained within the expected and desirable parameters. It’s also worth noting that Antonio had his own ascent profile, configured with his own conservative settings and had a different rich mix than ours (Oxygen).

Diana, originally from Cali, Colombia, had traveled to South Florida for a weekend of technical diving on some of the region’s remarkable wrecks. She wasn’t expecting to be part of a real out-of-deco-gas scenario, but it ended up being an enriching experience. Although we had meticulously planned the dive and reviewed numerous emergency procedures, this was both her first local dive and our first dive together. She executed her role with remarkable composure and precision, as though we had rehearsed it beforehand.

That day’s conditions were undoubtedly favorable: the surface was exceptionally calm, virtually no waves, and the visibility was outstanding. However, a strong current at 160 feet (50 meters) was crucial in separating Antonio from his buddy.

The catastrophic failure of the valve knob on his Nitrox 50 cylinder occurred just as he began his ascent. The current on top of the wreck was powerful. Antonio and his buddy didn’t correctly anticipate its possible consequences. While his buddy was immediately blown away from the wreck, Antonio hit part of the structure, breaking his lean gas cylinder valve knob and stem. He grabbed the structure to assess the situation and to decide on the best course of action, losing precious seconds, while his buddy drifted away, unable to return. It was a free ascent dive; there was no mooring line, and his buddy, already clear of the wreck, had no choice but to begin the ascent. Although they could still see each other at a distance, which helped Antonio remain calm, they both knew that reuniting during the ascent would not be a viable option.

Diana and I were just a few meters from Antonio, deploying our Delayed Surface Marker Buoy while the current carried us toward his position. His decision to approach us was the best choice he could have made.

Units

I typically use the US customary unit system when diving, so I express my cylinder’s maximum operating depth (MOD) in feet (ft). However, Diana is more familiar with the metric system, so we labeled our cylinders with both units. For the lean, low-oxygen mix (Nitrox 37), the MOD was marked as 33 m / 110 ft. The rich blend (Nitrox 80), was labeled 10 m / 33 ft. These labels, placed on the body of the deco cylinders, ensured that Antonio could easily understand the MOD of the new cylinder (my Nitrox 37), donated by an unknown diver (me), without any confusion.

This experience led me to include both feet and meters on all my deco and travel cylinders moving forward. While it’s unlikely that a similar situation will arise again, it’s always better to be prepared.

After all, divers carry seven or eight-foot (2 or 2.4-meter) hoses on their regulators in case they need to donate gas during an out-of-air emergency. Many of us may never need to use them in a real situation, but we are still prepared for the possibility. By the same logic and considering the increasing frequency of dive travel, it seems prudent to internationalize our markings, ensuring any diver quickly understands them. Planning on assisting only those familiar with our strategies, procedures, and nomenclature may no longer be the most responsible or noble approach.

Labels

The following label, intended for the body of a cylinder, speaks for itself:

33m/110ft.

No one, for any reason, who needs to use that deco cylinder should have any doubt about what the label is trying to communicate: the maximum operating depth of the mixture it contains. This is true even if the cylinder is handed over underwater without further explanation, as was the case with Antonio.

If preferred, the label could begin with the acronym MOD, which derived from English, is widely recognized in diving literature across multiple languages (If traveling, familiarize yourself with the local terminology for MOD). However, including this term might result in an overly large label and difficulty positioning amid bungees and attachment hardware. If you believe the term is necessary, go ahead; don’t be intimidated.

Another key aspect of labeling that I want to highlight is the importance of including the percentage (fraction) on the neck label of the cylinder:

37%

110ft.

That 37% mark allowed Antonio to identify the mix in the cylinder instantly. He used this value to define the mix on his dive computer and indicate the gas switch. With this input, his computer adjusted the decompression profile for the new gas. The % symbol adds clarity. It is not true that a label without this symbol is clearer or easier to read. It simply isn’t, regardless of who claims otherwise.

Meanwhile, the 110 ft. is also self-explanatory. Including the unit (in this case, feet) contributes to clarity and ease of interpretation. It is obviously the MOD; no rational decompression diver would think otherwise.

While I didn’t include the depth in meters on the neck label to allow the use of more prominent characters in an already limited space, the body labels [1] provide the internationalization I previously mentioned. Antonio, a Neapolitan diver who doesn’t speak a word of Spanish (my first language) and very little English (my second language and the official language here in the USA), had no trouble understanding what the cylinder contained or what he needed to do with it.

Now, I don’t include the percentage symbol for a Trimix blend. Instead, I use the typical O2/He notation. For instance, for a mix with 18% oxygen and 45% helium, the label would read 18/45.

Based on my experience, though there may be exceptions, cylinders containing Trimix blends, whether used as deco or travel cylinders, tend not to be the smallest ones; they are usually 80 cubic feet or 10 liters, or larger. The greater circumference at the neck provides more space, so I often (though not always) explicitly indicate that it’s Trimix by placing a Tx before the fractions or adding another label saying Trimix.

18/45

210ft.

Both labels, the previous one and the following one, should be equally clear and evident to any Trimix diver.

Tx18/45

210ft.

Nevertheless, I completely understand those who argue that this is unnecessary. They claim that anyone who doesn’t understand what 18/45 means wouldn’t grasp the meaning or implications of a label with Tx18/45 or even the word Trimix itself. I believe they’re right. Still, if I have the space, I include it, imagining that it might help a diver suffering from nitrogen narcosis or CO2 retention, whom I might need to assist at some point.

Thinkers, not Followers!

Divers have various ways of doing whatever they do; cylinder marking is a perfect example. There isn’t a specific standard, and I don’t believe there should be one [2], only a set of practical recommendations.

SDI/TDI, true to their philosophy of training thinking divers rather than mere followers, do precisely that: suggest marking cylinders in a way that allows identification of the gas fractions they contain [3][4][5][6] and the maximum operating depth of the mix. How to achieve this, beyond a few good guidelines, is left to each of us to decide.

The reality is that every diver marks their cylinders in some way. We often consider our chosen method the most efficient or logical because it is the one we are most familiar with. It’s also natural to assume that our approach is the most widely used and accepted, as our dive buddies typically adopt similar practices for consistency. However, the more narrowly tailored our method is to our specific group, the less effective it becomes when applied beyond our immediate circle.

Optional Labels

Depending on the dive and, more importantly, the type and number of divers I expect to encounter, adding a label with our name to each cylinder can be a good idea.

Given the dive sites I frequent and the boats I typically use, I only label cylinders with my name if they might be left unattended on the boat, for instance, while underwater using other cylinders.

José Pablo Mir

For cylinders containing hypoxic mixes (low oxygen content), and depending on the dive conditions, it may be prudent to include a label that warns those unaware. For example:

Hypoxic Mix

Low Oxygen content

Do not breathe at the surface

When carrying deco (or travel, or stage) cylinders on both sides of your body, the Right Rich (RR) and Left Lean (LL) trick provides an extra half a point of safety when performing gas switches [7].

The mix with the highest oxygen content (Rich) is mounted on the right side (Right), and the leaner oxygen mix (Lean) goes on the left side (Left). Some divers mark their cylinders as RR and LL at the bottom of the cylinder so it doesn’t interfere with other markings and labels, but it can be done with the words Rich and Lean wherever preferred. These markings are for our own use when clipping the cylinders to our body [8]; they don’t need to be visible or validated by teammates during the dive (but again, if they are, it could be an additional benefit).

Of course, following this strategy is possible without marking the cylinders as LL and RR. Still, it requires discipline and attention, so I always recommend doing the marking when possible.

Using the same principle, when I carry a pony (escape or bailout) bottle, whether with air or Nitrox, I use one marked as LL or without LL/RR markings, and of course, I clip it on my left side. In this case, I specifically avoid the use of a bottle marked as RR as a way of fostering good habits.

Handmade

Some divers prefer high-quality, durable, preprinted labels. This could be a practical option if you frequently dive in locations where the necessary gas mixes are consistently available. However, many of us prefer to create our own labels for each dive. This approach stems from the reality that preprinted labels rely on the accuracy and availability of gases at the fill station we use, which isn’t always guaranteed.

Adapting to the conditions at hand by recalculating profiles and modifying plans to suit the available mixes is far more preferable to missing a dive or resorting to the worst kind of label possible: the narrow, poorly sticking ones that the fill station may indeed have. This dangerous label will have to cover the original preprinted one, whose values are no longer accurate.

Regardless of the method, all labels should be appropriately sized with large, legible lettering and contrasting colors for optimal readability. Painter’s tape and duct tape are reliable options that remain secure when wet. However, painter’s tape is easier to remove after the dive, while duct tape becomes challenging to peel off if left in place for several days. But be careful, on specific cylinder paints and in some brushed finishes, painter’s tape may not adhere well and can easily peel off underwater. Experiment with both and choose the best one for your specific cylinders.

We must acknowledge that it’s impossible to satisfy everyone’s preferences, so embracing variability (diversity is a buzzword these days, but variability seems more fitting in this context) is key. Strive to adapt to different scenarios while prioritizing clarity and avoiding ambiguity. As one of my most respected instructors often said, in diving, we are constantly evaluating and mitigating.

[1] In this case, I’ve used two labels on the body of each deco cylinder, making it easier for my dive buddies to validate the mixtures from any position.

[2] We are discussing advanced diving. None of us are beginners needing help to cross the street. Our students and teammates must be capable of developing their own strategies and procedures. Sticking to pre-established rules should be an option, not the only path, especially when the goal may degrade into just having to please peers, instructors, or agencies instead of prioritizing better and safer ways.

[3] For Nitrox divers, SDI recommends indicating the oxygen fraction (FO2) and the Maximum Operating Depth (MOD), for example, EAN28, MOD 132 ft.

[4] TDI recommends deco cylinders be labeled in two locations: on the neck, allowing us to read the label while the cylinder is attached to our BCD, and on the body, enabling our dive buddy to validate during a gas switch, that the gas is breathable at our current depth.

[5] For mixes including helium (Trimix and Helitrox), TDI notes the helium fraction should also be displayed along with oxygen. For example, 20/35 indicates 20% oxygen and 35% helium. The nitrogen fraction could also be shown (e.g., 20/35/45 for Oxygen/Helium/Nitrogen), but I’ve never seen a diver use this method.

[6] These practices are quite common across the industry beyond SDI/TDI.

[7] It’s true that carrying all cylinders on the left side frees up the right side and ensures the long hose on our regulator setup remains unobstructed, greatly facilitating an emergency gas donation. However, many prefer to distribute cylinders on both sides for balance and operational simplicity. These personal preferences depend heavily on the environment and diving style. Find what works best for you.

[8] Placing labels on the bottoms of cylinders is a good idea unless they’re steel cylinders with convex bottoms or have protective boots. On some cylinders, I’ve added large vertical labels in a subtle color (in my case, green) that look like manufacturer marks. These don’t interfere with or get confused with the content and MOD labels I’ve discussed in this document.

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