How to Mark Deco and Stage Bottles: MOD Labels and Gas Analysis

Twenty feet. You're holding your stop, ready to switch to oxygen.

You take the regulator off the bottle. Your teammate looks at the bottle. Then at your depth. Then at you. A nod.

That's the whole point of the marking. Two people confirm the gas before anyone breathes it.

Why mark bottles this big?

Because the gas switch is where a simple mistake turns serious. Breathe a high-oxygen gas too deep and you risk an oxygen toxicity seizure underwater. There's no small version of that problem.

So the marking isn't for the shop. It isn't for the boat. It's for the moment you're about to breathe it, in low light, maybe in green water, with your teammate a few feet away.

If they can't read it from there, it's not marked.

What are the UTD marking rules?

UTD's cylinder marking standards come down to five rules:

1. MOD on every deco cylinder, in markings about 3 inches (7.5 cm) high, easily read by both you and your teammate.
2. Use local units. In the US, feet: nitrox 50 is marked 70 ft. In metric countries, meters with an "M": the same bottle is marked 21 M.
3. Oxygen bottles say OXYGEN down the cylinder, in letters about 3 inches high, in addition to the MOD.
4. No extra stickers. No nitrox stickers, no "not air" labels. Only required stickers, placed so they don't hide the MOD.
5. Every cylinder used in training gets an analysis label near the neck.

Put the MOD on both sides of the bottle. Whichever way it hangs, someone can read it.

What goes on the analysis label?

The analysis label sits near the neck. It lists:

• The gas. Oxygen to one decimal place, then helium to one decimal place if there is any. A trimix 21/35 that analyzes slightly off might read 21.2/34.8. A nitrox mix lists oxygen only.
• The date it was analyzed.
• The initials of the person who analyzed it.
• The MOD, if the bottle has one.
• The pressure, in psi or bar.

Your initials go near the bottom of the bottle too. Then everyone on the boat knows whose it is.

Who should analyze the gas?

Here's the reality check.

The comfortable belief: the fill station analyzed it. There's a sticker.

The truth: a sticker you didn't write is a promise you didn't check.

Analyze your own gas. Write your own label. Then show it to your team. The bottle is going to keep you alive at the end of the dive. It deserves sixty seconds on the dock.

Why don't stage bottles carry an MOD?

A stage bottle carries bottom gas, the gas you start the dive on. Deco bottles carry the gases you switch to on the way up, and those are the switches where a wrong bottle at the wrong depth hurts you. So deco bottles get the MOD. Stage bottles get an analysis label and your initials.

Standard gases make this simpler

With standard gases, the numbers never change. Nitrox 50 is always 70 ft. Oxygen is always 20 ft. You see the bottle, you know the gas, you know the depth. No arithmetic at the stop.

That's one of the quiet benefits of standard gases. The bottles look the same on every dive, on every diver.

Can your teammate read yours?

Go look at your bottles. Could your teammate read the MOD from six feet away? In green water? At night? Through a scratched mask?

If you're not sure, make the numbers bigger.

Marking, analyzing and switching gases are all part of the Stage and/or Deco Bottle Extension. For the rest of the bottle, read How to Rig a Stage Bottle.