Breathing Gases in Cave Diving: Why Every Cylinder Is Analysed and Labelled

All breathing gases look the same. A cylinder of air, one of oxygen-enriched nitrox and one of pure oxygen are indistinguishable by sight or smell, yet breathing the wrong one at the wrong depth can be fatal within minutes. Cave divers frequently carry more than one gas, and some dive with three or four. That makes gas identification, through analysis and labelling, as much a part of the equipment system as regulators and lights. This article explains the concepts. It does not explain how to blend gases or how to plan which gas to use where; both are taught in formal courses with qualified instructors.

What cave divers breathe, in broad terms

Air (about 21 percent oxygen, the rest mostly nitrogen) is the starting point. Nitrox, or enriched air, contains more oxygen and less nitrogen. The NSS Cave Diving Section's Nitrox Diver course, for example, covers nitrox up to 40 percent oxygen breathed as a single gas without required decompression stops, and treats multi-gas diving and rebreathers as beyond its scope ^[3].

For deeper cave dives, divers use mixtures containing helium. The NSS-CDS describes helium-based mixtures as essential for deeper cave diving and notes that its entry-level cave training is limited to 40 m ^[4]. Global Underwater Explorers (GUE) sets a maximum equivalent narcotic depth in its general standards and calculates it on the assumption that helium is the only non-narcotic component ^[1], which reflects why helium is used: it reduces the narcotic effect of the gas at depth. Many cave divers also carry gases richer in oxygen, up to pure oxygen, for decompression; the NSS-CDS trimix course, for instance, asks for oxygen cylinders that are properly rigged and cleaned ^[4].

Every gas has a range of depths where it is suitable. Too much oxygen at depth risks oxygen toxicity, which can cause convulsions; too little oxygen, a particular risk in shallow water and with rebreathers, can cause loss of consciousness ^[9]. Training agencies therefore publish limits on oxygen exposure and narcotic depth, and GUE's general diving standards include both ^[1]. Exley's third recommendation for safe cave diving, written in 1979, was simply to avoid deep diving in caves ^[5], and later training literature frames the same idea as diving no deeper than is safe for the gas being breathed ^[2].

Why gas is analysed

What is in a cylinder is not always what was ordered. Fill stations can make mistakes, cylinders can be swapped or mislabelled, and a cylinder can be refilled without its old label being changed. Analysis means measuring the gas in each cylinder before it is used, normally with an oxygen analyser, plus a helium analyser for mixtures containing helium. NSS-CDS course equipment lists include an oxygen analyser for nitrox training and both oxygen and helium analysers for its trimix cave course ^[3][4]. GUE lists gas analysis among the land drills in its entry-level Performance Diver course and requires that filled cylinders be analysed and labelled in accordance with its standard operating procedure ^[1][6].

Incident research shows why this matters. In a review of 67 trained cave divers who died in the United States between 1985 and 2015, Potts, Buzzacott and Denoble found that seven had witnessed convulsions at depth; in at least four, the diver had breathed from the wrong cylinder, a decompression gas that was toxic at the depth of the dive ^[2]. The authors recommended analysing all breathing gases before every dive, clearly marking cylinders, and double- and triple-checking that the gas being breathed suits the depth ^[2].

Oxygen is not the only concern. Writing in Diving and Hyperbaric Medicine, Millar and Mouldey described carbon monoxide poisoning from contaminated compressor intake air and depletion of oxygen inside corroded steel cylinders as known, potentially fatal hazards, and suggested that volatile contaminants generated inside overheated compressors may explain some unexplained incidents ^[7]. They noted that air quality is tested infrequently and often inadequately ^[7].

Why gas is labelled

Analysis is only useful if its result stays attached to the cylinder. Labelling and marking turn a measurement into something every team member can read underwater. GUE's general standards are a clear example of the principle: cylinders must be free of unnecessary stickers; each stage cylinder (apart from oxygen) must carry its maximum operating depth (MOD) in large numerals about 7.5 cm high; oxygen cylinders must be marked with the word OXYGEN and their MOD; markings must use the unit system of the country where the diving is done and be oriented so that teammates as well as the wearer can read them ^[1]. The NSS-CDS trimix cave course likewise requires decompression cylinders to be properly rigged and labelled ^[4].

The maximum operating depth is the deepest point at which a gas keeps the diver within the planned oxygen limit. Displaying it in large numbers on the cylinder lets the diver and the team confirm, before switching to a gas, that the depth is right for it. The arithmetic behind MOD is part of nitrox and technical training and is deliberately not reproduced here.

Analysers and sensors age

Oxygen-measuring sensors, whether in a handheld analyser or a rebreather, do not last forever. In rebreather diving this is a recognised safety issue: GUE requires analog oxygen sensors in rebreathers used on its courses to be less than one year from their manufacturing date ^[1], and the 2023 Rebreather Forum 4 consensus statements asked manufacturers to consider warnings based on elapsed time since sensor manufacture ^[8]. Potts and colleagues described one fatality in which a rebreather with out-of-date oxygen sensors and a malfunctioning solenoid may have raised the oxygen level without triggering an alarm ^[2].

Gas identification as a team habit

Cave diving culture treats gas identity as a shared responsibility. Exley's 1979 booklet described a pre-dive routine in which team members check each other's equipment item by item ^[5], and modern standards from GUE emphasise team awareness and shared protocols ^[1]. Team members who can read each other's cylinder markings provide a second set of eyes at the moment a diver is about to change gas.

The lesson from the research is not that mixed gases are unusually dangerous in themselves, but that their risks are invisible. Analysis, clear labels and consistent team habits make an invisible property visible.