Reading Gas Labels: EAN, Trimix, and Heliox

Walk into any technical diving shop and you will see tanks labeled with cryptic shorthand: EAN32, Tx 18/45, Heliox 10/90. These labels follow consistent conventions, and once you know the system, they are easy to read.

Air

Standard air is approximately 21% oxygen and 79% nitrogen. No special label is needed, but tanks are often simply labeled “Air” or left unlabeled (depending on local practice).

Nitrox (EAN)

Nitrox — also called Enriched Air Nitrox — is any nitrogen-oxygen mix with more than 21% oxygen. The standard notation is:

• EAN followed by the oxygen percentage: EAN32 means 32% oxygen, 68% nitrogen
• Sometimes written as Nx32 or Nitrox 32
• Common recreational blends: EAN28, EAN32, EAN36, EAN40

When you see a nitrox label, the number always refers to the oxygen percentage. The remainder is nitrogen (since there are only two gases in the mix).

Reading a nitrox tank label

A properly labeled nitrox tank should show:

1. Oxygen percentage: The analyzed O2 content (e.g., “O2: 32%”)
2. MOD: The maximum operating depth for the stated ppO2 limit
3. Date: When the analysis was performed
4. Analyzer’s initials: Who verified the mix
5. ppO2 limit used: Whether the MOD was calculated at 1.4 or 1.6 bar

Example label: O2: 32% | MOD: 33m @ 1.4 | 2026-02-14 | JD

Trimix (Tx)

Trimix is a three-gas blend of oxygen, helium, and nitrogen. The notation uses two numbers:

• Tx O2/He: The first number is oxygen percentage, the second is helium percentage. Nitrogen is the remainder
• Tx 18/45 means 18% oxygen, 45% helium, 37% nitrogen
• Sometimes written as Trimix 18/45 or TMx 18/45

Common trimix blends for various depth ranges:

Normoxic vs hypoxic

• Normoxic trimix: 18% oxygen or more. Safe to breathe at the surface (ppO2 of at least 0.18 bar)
• Hypoxic trimix: Less than 18% oxygen (some definitions use 16%). Not safe to breathe at the surface. These mixes have a Minimum Operating Depth that must be respected

Heliox

Heliox is a two-gas blend of oxygen and helium with no nitrogen. The notation is:

• Heliox O2/He: Same as trimix notation but the two numbers always sum to 100
• Heliox 10/90 means 10% oxygen, 90% helium, 0% nitrogen

Heliox eliminates nitrogen narcosis entirely and is used in very deep commercial and military diving. It is less common in recreational technical diving because of the high cost of helium.

Oxygen

Pure oxygen (100% O2) is used for shallow decompression stops and surface decompression:

• Labeled simply as O2 or 100% O2
• MOD at 1.6 bar ppO2: 6 meters
• Never to be breathed deeper than 6 meters

Deco mixes

Decompression mixes are richer blends used at shallow stops to accelerate off-gassing:

• EAN50 (50% oxygen): Common intermediate deco gas, MOD 22m at 1.6 bar
• EAN80 (80% oxygen): Shallow deco gas, MOD 10m at 1.6 bar
• O2 (100%): Shallowest deco gas, MOD 6m at 1.6 bar

Color coding

Tank color conventions vary by region but some common standards:

• Nitrox: Green and yellow band around the shoulder (international standard)
• Trimix: No universal standard — some use red and green bands
• Oxygen: Green body (US convention) or white shoulder (international)

Color coding alone should never be trusted. Always read the label and analyze the gas yourself.

Key takeaways

1. EAN + one number = nitrox (the number is O2%)
2. Tx + two numbers = trimix (first is O2%, second is He%)
3. Nitrogen is always the remainder — it is never written explicitly
4. Always verify — analyze every tank, every time, regardless of what the label says
5. Know your limits — every mix has both a maximum and potentially a minimum operating depth

Sources

• NOAA Diving Manual, 6th Edition
• TDI Standards and Procedures
• GUE General Standards