Nitrox MOD, EAD and Best Mix Calculations: Worked Examples

How to calculate nitrox MOD at 1.4 and 1.6 bar

Divide the ppO₂ limit by the oxygen fraction, subtract 1 and multiply by 10. The result is the depth in metres where the mix reaches that limit. Our nitrox calculator rounds it down to 0.1 m, so the number it shows is never deeper than the true limit. Worked through for EAN32:

• Limit 1.4 bar, FO₂ 0.32: 1.4 ÷ 0.32 = 4.375 bar absolute pressure.
• Remove the atmosphere: 4.375 − 1 = 3.375 bar of water pressure.
• Convert to depth: 3.375 × 10 = 33.75 m, rounded down to 33.7 m (about 110 ft).
• At 1.6 bar: 1.6 ÷ 0.32 = 5.0, minus 1 is 4.0, times 10 is 40.0 m.

Why 1.4 bar for the dive and 1.6 bar only at rest?

Because the risk of central nervous system oxygen toxicity climbs with ppO₂ and with how hard you are working. DAN describes 1.4 bar as the widely accepted limit for the working or bottom phase of a dive, with 1.6 bar acceptable at rest during decompression. Its oxygen toxicity page treats 1.4-1.6 as a caution zone, keeps 1.5-1.6 for rest such as decompression, and says recreational divers should not go above 1.6.

The working phase gets the lower number because swimming, current and cold all push risk up. DAN notes that divers who retain carbon dioxide during exercise may be at higher risk, and lists unplanned depth excursions and hard exertion in an emergency. NOAA's scientific diving manual caps nitrox at 1.6 and tells supervisors to lower the limit when cold, strenuous work or long exposures are expected.

In practice: plan the dive to the 1.4 bar MOD and treat the 1.6 bar depth as a line you never reach. The table above shows the catch. The richer the mix, the smaller the gap between the two depths, so EAN36 leaves 5.6 m for a surge or a buoyancy slip where air leaves 9.5 m. Time counts too, because the allowed exposure gets shorter as ppO₂ rises; the CNS calculator shows how much of the oxygen budget a dive uses.

How to calculate EAD and use it with an air table

EAD is the air depth with the same nitrogen pressure as your nitrox depth. Take the nitrogen fraction, divide by 0.79 (the nitrogen in air), multiply by the absolute pressure, subtract 1 and multiply by 10. The calculator rounds up to 0.1 m, the direction that gives a shorter no-stop time. For 24 m on EAN32:

• Absolute pressure: 24 ÷ 10 + 1 = 3.4 bar.
• Nitrogen ratio: (1 − 0.32) ÷ 0.79 = 0.68 ÷ 0.79 = 0.8608.
• 0.8608 × 3.4 = 2.927; minus 1 is 1.927; times 10 is 19.27 m, rounded up to 19.3 m.
• On NOAA's air table, 19.3 m falls between the 18.3 m (60 ft) and 21.3 m (70 ft) rows. Use the deeper row: 48 minutes of no-stop time.

Checking the EAD answer against NOAA's nitrox tables

The EAD method and NOAA's own EAN32 table agree. NOAA's EAN32 table gives 48 minutes at 24.5 m (80 ft), the same as the air-table route above, while the same dive on air would be limited to 39 minutes. The US Navy's EAD table, reprinted in NOAA's diving medical technician reference, uses the same rule of rounding to the next greater table depth.

Second example, 30 m on EAN32: the EAD is 24.5 m. That is 0.1 m deeper than the 24.4 m (80 ft) row, so you drop to the 27.4 m (90 ft) row and get 30 minutes. NOAA's EAN32 table gives 30 minutes at 30.6 m (100 ft); air at 30.5 m (100 ft) allows 25. A tenth of a metre past a row boundary still means the next row down.

In feet, NOAA's reference writes the same formula as (1 − FO₂) ÷ 0.79 × (D + 33) − 33, with D in feet of seawater. A dive computer set to the right mix accounts for the lower nitrogen continuously, so on the boat you need EAD for tables, for comparing mixes, or for sanity-checking what the computer tells you.

How to pick the best nitrox mix for a planned depth

Divide the ppO₂ limit by the absolute pressure at the deepest point you plan to reach, then round down to the whole percent. For 28 m at 1.4 bar: 1.4 ÷ 3.8 = 0.368, so 36%. EAN36 then reaches 1.4 bar at 28.8 m, only 0.8 m below the plan. The Best Mix tab of the nitrox calculator does the same sum.

The best mix is the richest gas that still works at that depth. It is not automatically the one to take. Three things push you leaner:

• Margin. If the reef base is at 28 m but the sand keeps sloping, plan on the deepest depth you could plausibly reach. EAN32 (MOD 33.7 m) gives the same 28 m dive 5.7 m of room.
• Equipment. For 18 m the formula says 50%, but NOAA requires oxygen-clean service for equipment exposed to more than 40% oxygen, and PADI names 32% and 36% as the common recreational mixes.
• Availability. Some shops fill pre-blended EAN32 into any tank but want an oxygen-cleaned tank and a day's notice for richer custom blends. For a 30 m wreck the formula gives 35%; if the choice is 32% or 36%, EAN36 is out (MOD 28.8 m) and EAN32 works at 1.28 bar on the bottom with 3.7 m to spare.

How to analyse and label a nitrox tank

Analyse it yourself, write the result on the tank, and dive the number you measured rather than the number you ordered. NOAA's scientific diving rules make each diver analyse their own cylinder before the dive and sign for five items: oxygen fraction, MOD, cylinder pressure, date of analysis and name. DAN gives recreational divers the same rule: analyse each cylinder and label it with your name, the mix, the date and the MOD. A typical sequence:

• Switch the analyser on, let it settle, and calibrate it on air as the manufacturer instructs.
• Fit the flow limiter to the valve and open the valve slowly so the flow stays gentle and steady.
• Wait until the reading stops moving, then record it.
• Work out the MOD for the measured percentage at 1.4 bar.
• Write the oxygen percentage, MOD, date and your initials on the tank label, and sign the fill log.
• Enter the measured percentage in your dive computer, confirm the ppO₂ limit, and read the label once more before you get in.

What if the analysis doesn't match what you ordered?

Plan with the measured number. A tank ordered as EAN32 that analyses at 34% has a MOD of 31.1 m instead of 33.7 m, so two percentage points cost 2.6 m of depth. On a 30 m wreck, EAN34 still works at 1.36 bar, with 1.1 m of margin instead of 3.7 m. On a 32 m dive the same tank puts you at 1.43 bar, over the working limit, where real EAN32 would sit at 1.34.

A leaner reading moves the MOD deeper and shortens the no-stop time, so set the real number in your computer and let it credit you with the nitrogen you are actually breathing. NOAA prints its nitrox tables per mix, each marked for use only with that oxygen percentage, and the US Navy's EAD table builds in an allowable ±0.5 percent error in gas analysis. NOAA also sets a floor for the analyser itself: it must read 0-100% oxygen to within 1% accuracy. A tank that misses the order by a full point or more is a different gas; plan it as one.

Common nitrox calculation mistakes

The formulas are short. The mistakes below come from feeding them the wrong number or leaving a setting behind.

• Treating absolute pressure as depth. Skip the minus 1 and EAN32's MOD comes out at 43.7 m instead of 33.7 m.
• Planning the bottom phase to 1.6 bar. EAN32 would then be 'fine' to 40.0 m with no margin at all; 1.6 is for resting deco stops.
• Rounding the wrong way. MOD goes down (33.75 m becomes 33.7 m) and EAD goes up (19.27 m becomes 19.3 m).
• Leaving the computer on yesterday's mix. On Suunto's Zoop Novo, a mix set to 22% or more stays set until you change it and does not revert to 21%. Dive air with the computer still on 32% and it assumes less nitrogen than you are breathing.
• Setting the computer to air for extra margin and forgetting the oxygen side. Suunto's documentation says both the oxygen percentage and the ppO₂ limit must be entered for correct nitrogen and oxygen calculations; on an air setting the 1.4 bar alarm sits at 56.6 m, not 33.7 m.
• Checking settings once a day. NOAA's manual has divers check the computer's oxygen and ppO₂ settings before each dive.
• Forgetting which tanks hold nitrox. The one oxygen toxicity death in DAN's 10-year review of US recreational fatalities involved EAN32 doubles filled for a cancelled dive and used weeks later at about 160 ft (49 m).