Standard Gases: Why DIR Teams Use the Same Mixes

Standard gases are a short list of mixes every diver on a team uses for a given depth range: nitrox 32, trimix 25/25 and 21/35, and nitrox 50 and oxygen for decompression. UTD's rule is the right oxygen level for the average depth, an equivalent narcotic depth of 100 feet or less, and no deep air.

By Jeremiah Gardner · Last reviewed October 27, 2026

Ask a DIR diver what's in their cylinder and the answer is usually one of a handful of mixes. That's not a lack of imagination. It's one of UTD's ten covenants, and it does a lot of quiet work for the team.

What is the standard gas rule?

UTD's fourth covenant: dive the desired oxygen partial pressure at the target average depth, and keep the equivalent narcotic depth to 100 feet or less. No deep air.

Two limits sit behind it:

Oxygen. UTD's standard bottom mixes are chosen for an average working oxygen partial pressure of 1.2 and a maximum of 1.4. Decompression mixes are used up to 1.6.
Narcosis. The equivalent narcotic depth stays at 100 feet or less. UTD's calculation treats oxygen as narcotic, so only helium reduces it.

UTD also caps gas density for training dives at 5.7 grams per liter, because dense gas is harder to breathe at depth.

What are UTD's standard gases?

Bottom mixes

Mix	Working depth	Maximum operating depth
Nitrox 32	0 to 100 ft	111 ft
Trimix 25/25	100 to 130 ft	151 ft
Trimix 21/35	130 to 160 ft	190 ft
Trimix 18/45	160 to 200 ft	220 ft
Trimix 15/55	200 to 240 ft	275 ft
Trimix 12/60	250 to 300 ft	352 ft
Trimix 10/70	300 to 400 ft	429 ft

Notice the gap between the working depth and the MOD. That's on purpose. The working depth is where you dive the mix. The MOD is the oxygen limit, not a target. Nitrox 32 is a 100-foot gas, full stop.

Decompression mixes

Mix	Maximum depth
Oxygen (100%)	20 ft
Nitrox 50	70 ft
Trimix 35/25	120 ft
Trimix 21/35	190 ft

Trimix numbers give oxygen first, then helium: 21/35 is 21 percent oxygen, 35 percent helium and the rest nitrogen.

Why does the team use the same gas?

Gas sharing is predictable. If a teammate donates, you're breathing the same mix you planned for, at the depth you planned for it.
Decompression plans line up. Same gas, same profile, same plan. The whole team ascends on one schedule.
Planning is simpler. With a short list of mixes, maximum depths and switch depths become familiar. Fewer numbers means fewer mistakes.
Logistics are simpler. UTD's bottom mixes are built by adding helium and topping with nitrox 32, so fills are easy. You still analyze every cylinder before you dive it.
No deep air. Keeping narcosis in check means the team can think clearly when something goes wrong, which is the whole point of a thinking team.

Which gases does each course use?

Course	Gases
Recreational 2 / Advanced	Nitrox 32 to 100 ft
Recreational 3	Trimix 25/25 to 130 ft, within no-decompression limits
Essentials courses	Air or nitrox 32 to 60 ft (100 ft for nitrox-certified divers in Essentials of Rec)
Techreational	Trimix 25/25 to 130 ft, one oxygen deco bottle, limited to one oxygen cycle
Tech 1	Nitrox 32, trimix 25/25, 21/35 or 18/45 to 160 ft; one deco bottle of oxygen (one oxygen cycle) or nitrox 50 (30 min)

In UTD's recreational Min Deco table, the entries between 110 and 130 feet apply to trimix 25/25 only. They don't authorize air or nitrox 32 at those depths.

Does helium make a dive technical?

No. UTD's recreational training goes to 130 feet on 25/25 within no-decompression limits. What moves a dive into technical training is planned decompression, and overhead environments are their own category. The gas follows the depth; it doesn't define the dive.

To check any mix's MOD, END and oxygen clock, use the nitrox and trimix calculator.

Learn to plan with them

Tech 1 builds decompression diving to 160 feet on standard bottom gases with one deco bottle, oxygen or nitrox 50. For the ascent side of the plan, see Gradient Factors Explained, and for the whole path, How to Become a Technical Diver.

Frequently asked questions

What does 21/35 mean?

A trimix with 21 percent oxygen and 35 percent helium, with the rest nitrogen. In UTD's system, it's the standard bottom mix for roughly 130 to 160 feet.

Why not just use the best mix for each dive?

Because a team works better when everyone breathes the same gas. Teammates can share gas without surprises, decompression plans line up, and blending is simple. The small gain from a custom mix rarely beats that.

What is equivalent narcotic depth?

An estimate of how narcotic a gas feels, expressed as the depth at which air would feel the same. UTD keeps it at 100 feet or less and treats oxygen as narcotic in the calculation.

Does diving with helium make a dive technical?

No. UTD's recreational training uses trimix 25/25 to 130 feet within no-decompression limits. Planned decompression or an overhead environment, not the gas alone, is what moves a dive into technical or overhead training.