Nitrox MOD Chart: EAN22 to EAN40
A 32 percent nitrox mix has a maximum operating depth of 111 feet, or 33.8 metres, at the 1.4 ata working oxygen limit, and 132 feet, or 40 metres, at the 1.6 ata contingency ceiling. EAN36 reaches 1.4 ata at 95 feet. The formula is MOD in feet of sea water = 33 x (maximum ppO2 / oxygen fraction - 1).
Maximum operating depth is the deepest a breathing gas can be taken before its oxygen partial pressure passes the limit you have chosen. It is the one number attached to a nitrox fill that behaves like a hard ceiling rather than a guideline, because the failure mode is a convulsion underwater rather than a shortened dive. The chart below runs every mix from EAN22 to EAN40 through the same formula at both the 1.4 ata working limit and the 1.6 ata contingency ceiling.
A chart is only useful if the oxygen fraction going into it is real. The label on a cylinder records an intention; an oxygen analyser at the fill station records a fact, and the fact is what you plan with. Shops keep an analyser on the bench, so owning one matters most when you travel or dive off a boat that does not have one to hand. Whatever you read there also has to be typed into your computer before the dive: a Shearwater Peregrine , or any other nitrox capable unit, will run air calculations on a nitrox fill quite happily if nobody tells it otherwise.
What does maximum operating depth actually mean?
Maximum operating depth is the depth at which a given gas reaches a chosen oxygen partial pressure limit. Oxygen is toxic to the central nervous system when its partial pressure is raised, and because pressure climbs with depth, a mix that is entirely benign at 60 feet becomes dangerous at 150. Nothing about the gas changes. Only the pressure does.
The mechanism is the gas laws and nothing more exotic. Oxygen partial pressure equals the oxygen fraction multiplied by ambient pressure in atmospheres absolute. Air at the surface is 0.21 times 1, so 0.21 ata. The same air at 132 feet is 0.21 times 5, so 1.05 ata. EAN36 at that same 132 feet is 0.36 times 5, which is 1.8 ata, well past any acceptable limit and roughly 37 feet below where that mix should have stopped. The chart below is that arithmetic, run once for every mix so you do not have to run it on a boat deck.
What is the MOD for every nitrox mix?
MOD (fsw) = 33 x ((ppO2 limit / FO2) - 1)
MOD (m) = 10 x ((ppO2 limit / FO2) - 1)
Sea water is 33 feet per atmosphere and 10 metres per atmosphere. Fresh water is 34 feet per atmosphere, which makes the fresh water MOD about 3 percent deeper for the same mix, a difference small enough that most divers plan on the salt water figure everywhere and take the extra as margin. Every row below is salt water.
| Mix | FO2 | MOD at 1.2 | MOD at 1.3 | MOD at 1.4 | MOD at 1.4 | Ceiling at 1.6 | Ceiling at 1.6 |
|---|---|---|---|---|---|---|---|
| Air | 0.21 | 156 ft | 171 ft | 187 ft | 56.7 m | 218 ft | 66.2 m |
| EAN22 | 0.22 | 147 ft | 162 ft | 177 ft | 53.6 m | 207 ft | 62.7 m |
| EAN23 | 0.23 | 139 ft | 154 ft | 168 ft | 50.9 m | 197 ft | 59.6 m |
| EAN24 | 0.24 | 132 ft | 146 ft | 160 ft | 48.3 m | 187 ft | 56.7 m |
| EAN25 | 0.25 | 125 ft | 139 ft | 152 ft | 46.0 m | 178 ft | 54.0 m |
| EAN26 | 0.26 | 119 ft | 132 ft | 145 ft | 43.8 m | 170 ft | 51.5 m |
| EAN27 | 0.27 | 114 ft | 126 ft | 138 ft | 41.9 m | 163 ft | 49.3 m |
| EAN28 | 0.28 | 108 ft | 120 ft | 132 ft | 40.0 m | 156 ft | 47.1 m |
| EAN29 | 0.29 | 104 ft | 115 ft | 126 ft | 38.3 m | 149 ft | 45.2 m |
| EAN30 | 0.30 | 99 ft | 110 ft | 121 ft | 36.7 m | 143 ft | 43.3 m |
| EAN31 | 0.31 | 95 ft | 105 ft | 116 ft | 35.2 m | 137 ft | 41.6 m |
| EAN32 | 0.32 | 91 ft | 101 ft | 111 ft | 33.8 m | 132 ft | 40.0 m |
| EAN33 | 0.33 | 87 ft | 97 ft | 107 ft | 32.4 m | 127 ft | 38.5 m |
| EAN34 | 0.34 | 83 ft | 93 ft | 103 ft | 31.2 m | 122 ft | 37.1 m |
| EAN35 | 0.35 | 80 ft | 90 ft | 99 ft | 30.0 m | 118 ft | 35.7 m |
| EAN36 | 0.36 | 77 ft | 86 ft | 95 ft | 28.9 m | 114 ft | 34.4 m |
| EAN37 | 0.37 | 74 ft | 83 ft | 92 ft | 27.8 m | 110 ft | 33.2 m |
| EAN38 | 0.38 | 71 ft | 80 ft | 89 ft | 26.8 m | 106 ft | 32.1 m |
| EAN39 | 0.39 | 69 ft | 77 ft | 85 ft | 25.9 m | 102 ft | 31.0 m |
| EAN40 | 0.40 | 66 ft | 74 ft | 82 ft | 25.0 m | 99 ft | 30.0 m |
Three rows are worth memorising because they cover most of the diving anyone actually does. EAN32 is 111 feet at 1.4, which is 33.8 metres. EAN36 is 95 feet, or 28.9 metres. EAN40 is 82 feet. Those are the three fills most shops offer, and knowing them without looking is the difference between catching a bad plan on the boat and discovering it at depth.
The lean end of the chart is less familiar and worth a moment. EAN22 reaches 1.4 ata at 177 feet, barely shallower than air, which tells you something useful: a very lean nitrox mix buys almost nothing in depth terms and correspondingly little in nitrogen terms either. Mixes below about EAN28 are usually the result of a fill station blending down rather than a deliberate choice, and if you analyse a cylinder you expected to be EAN32 and read 26 percent, the correct response is to replan the dive around the number you actually have.
Why is 1.4 ata the working limit rather than 1.6?
Central nervous system oxygen toxicity gives no dependable warning. The textbook signs, which include visual disturbance, ringing in the ears, nausea, twitching, irritability and dizziness, are unreliable in the specific sense that a convulsion can arrive without any of them appearing first. A convulsion underwater means the regulator leaves the mouth, and in open water at depth that is usually fatal no matter how attentive the buddy is.
Susceptibility also moves around. It varies between people, it varies in the same person from one day to the next, and it rises with exertion, with cold and with carbon dioxide retention. None of those are things a diver can measure at depth. The 1.4 figure is a working margin against that uncertainty. It is not a line where something switches on at 1.41 and stays off at 1.39.
The 1.6 column exists for one narrow purpose: a diver at rest on a decompression stop, breathing a rich gas, doing no work. Using it as a bottom phase planning target removes the whole margin that 1.4 was there to provide. The 1.2 and 1.3 columns run the other way and are worth using when a dive is long, cold, physically hard, or repeated several times a day, because total oxygen exposure accumulates across a day in a way a single MOD figure does not capture. Full exposure limits are laid out on the partial pressure chart.
How do you pick the best mix for a planned depth?
Run the formula backwards. The richest mix that a depth allows is the partial pressure limit divided by the ambient pressure at that depth.
Best mix FO2 = ppO2 limit / ((depth in fsw / 33) + 1)
| Planned depth | Depth (m) | Ambient | Richest mix at 1.4 | Common fill | ppO2 on that fill | Depth margin |
|---|---|---|---|---|---|---|
| 50 ft | 15.2 m | 2.52 ata | 55% | EAN40 | 1.01 ata | 32 ft |
| 60 ft | 18.3 m | 2.82 ata | 49% | EAN40 | 1.13 ata | 22 ft |
| 70 ft | 21.3 m | 3.12 ata | 44% | EAN40 | 1.25 ata | 12 ft |
| 80 ft | 24.4 m | 3.42 ata | 40% | EAN40 | 1.37 ata | 2 ft |
| 90 ft | 27.4 m | 3.73 ata | 37% | EAN36 | 1.34 ata | 5 ft |
| 100 ft | 30.5 m | 4.03 ata | 34% | EAN34 | 1.37 ata | 3 ft |
| 110 ft | 33.5 m | 4.33 ata | 32% | EAN32 | 1.39 ata | 1 ft |
| 120 ft | 36.6 m | 4.64 ata | 30% | EAN30 | 1.39 ata | 1 ft |
| 130 ft | 39.6 m | 4.94 ata | 28% | EAN28 | 1.38 ata | 2 ft |
The depth margin column is the part worth internalising. Choosing the common fill rather than the theoretically ideal one leaves you a buffer of real feet, and real feet are what a dive consumes. Your actual maximum depth is rarely exactly what you planned: a photographer follows a subject over a ledge, a current pushes you down a wall, a sloping bottom is misjudged in poor visibility. Any of those can put you 10 or 15 feet deeper than intended. A mix chosen with margin absorbs that. A mix chosen at exactly the planned depth does not.
Rounding down is always the correct direction. It costs a little no-decompression time and buys back oxygen margin, and of the two, oxygen margin is the one whose absence has no warning and no recovery. There is a matching rule at the shallow end: on a dive planned to 60 feet the chart says a 49 percent mix is legal, but nobody carries EAN49 on a recreational reef dive, because a mix that rich has a MOD shallower than many boat descents and turns a routine overshoot into an emergency. The nitrox MOD calculator will run any mix and any depth you like, including the ones you should not dive.
What does a MOD not tell you?
A great deal, and this is where nitrox divers get caught out. Maximum operating depth is a pure oxygen calculation. Three other limits run in parallel, and any of them can bind first.
- Nitrogen loading. Nitrox reduces it but does not remove it. The equivalent air depth calculator shows what a mix does to your no-stop time, and the no-decompression limits table shows how little time air buys you at the deep end of the recreational range.
- Gas supply. At 99 feet you consume gas four times as fast as at the surface. A longer no-decompression limit very often just means you run out of gas before you run out of table, which the air consumption chart lays out depth by depth.
- Cumulative oxygen exposure. MOD governs the instantaneous partial pressure. Total exposure across a dive, and across a day of repetitive dives, is tracked separately as the CNS clock, and your computer counts it. NOAA single-exposure limits run 45 minutes at 1.6, 150 minutes at 1.4 and 180 minutes at 1.3, which is exactly why repetitive nitrox diving near the deep end of a mix is more constrained than one dive to the same depth.
Narcosis deserves a line of its own because the myth is persistent. Nitrox does not meaningfully reduce it. The narcotic effect at depth is driven by nitrogen, and trading 4 to 15 percent of the nitrogen for oxygen makes no practical difference to how a diver feels at 100 feet. Any claim that nitrox produces a clearer head at depth is describing better hydration, lower carbon dioxide from a relaxed breathing pattern, or a shorter and shallower profile, not the mix itself. The nitrox versus air comparison goes through what the mix does and does not buy.
How do you keep a MOD from being exceeded on a real dive?
Three habits do almost all the work, and none of them happen underwater.
First, analyse and record. You read the fraction yourself, you write it on the cylinder tag, and you sign the fill log. Second, set the computer at the same moment, not later on the boat when somebody is handing you a fin. A computer set to air on a nitrox fill is conservative and safe; a computer set to EAN36 on an air fill is neither, and both errors come from the same lapse of sequence. Third, write the plan down. A folding wrist slate carrying the mix, the MOD and the turn pressure removes most of the ways a MOD gets exceeded, and it costs less than one boat dive.
It is also worth knowing your computer's own nitrox ceiling before you rely on it. Some entry level units cap out well below the range in this chart: the Cressi Leonardo handles air and nitrox to 50 percent, which covers every recreational mix, while a few very simple computers stop at 40 percent. That ceiling is a specification to check before a trip rather than a surprise to discover at a fill station. The dive computer roundup lists what each unit supports.
What should you do if you go past your MOD?
Ascend, immediately and under control, until the partial pressure is back inside your limit, then either continue conservatively or end the dive. There is no procedure to run at depth and no way to tell whether you had a close call, because the absence of symptoms is not evidence of safety. Do not attempt to make up the lost bottom time later in the day.
The useful work happens afterwards. An exceeded MOD is almost never one mistake. It is a planning chain that broke somewhere, and the chain has identifiable links: was the fill analysed, was the analysis written down, was the computer set to the analysed fraction, was the maximum depth agreed with the buddy before entry, was it agreed out loud or assumed. Walking back through those five questions after the dive is worth more than any amount of resolving to be more careful. The nitrox diving guide covers the full sequence from fill station to entry.
Related charts and calculators
- Oxygen partial pressure chart, the same arithmetic read the other way
- Nitrox MOD calculator, for mixes and depths not on this chart
- Equivalent air depth calculator, the nitrogen half of the nitrox question
- Air consumption by depth, because gas usually binds before oxygen does
- Nitrox diving guide, the procedures the chart assumes
Frequently asked questions
What is the maximum operating depth of EAN32?
EAN32 has a maximum operating depth of 111 feet, or 33.8 metres, at the 1.4 ata working oxygen limit. At the 1.6 ata contingency ceiling the same mix reaches 132 feet, or 40 metres. Plan every recreational dive against the 111 foot figure. The 132 foot figure exists as an absolute ceiling for a resting diver on a decompression stop and is not a depth to swim to deliberately.
How do you calculate MOD for any nitrox mix?
Divide your oxygen partial pressure limit by the oxygen fraction, subtract one, then multiply by 33 for feet of sea water or by 10 for metres. EAN36 at a 1.4 limit is 1.4 divided by 0.36, which is 3.889, minus one is 2.889, times 33 is 95 feet. The subtraction of one is the step people forget, and forgetting it produces an answer 33 feet too deep.
Why does this chart use 1.4 rather than 1.6?
Because 1.4 ata is the working limit for the bottom phase of a dive and 1.6 ata is a contingency ceiling. Central nervous system oxygen toxicity risk rises with partial pressure, with exertion, with cold and with carbon dioxide retention, none of which you can measure at depth. The 1.4 figure is the margin against that uncertainty. Planning to 1.6 spends the entire margin before the dive starts.
Does air have a maximum operating depth?
Yes. Air is a nitrox mix with a 21 percent oxygen fraction, so it reaches 1.4 ata of oxygen at 187 feet and 1.6 ata at 218 feet. Neither number is a recreational depth. Recreational training limits stop at 130 feet, and nitrogen narcosis, no-decompression time and gas supply all bind long before the oxygen ceiling on air does.
What mix should I use for a 100 foot dive?
At 100 feet ambient pressure is 4.03 ata, so the richest mix that stays at or under 1.4 ata is 34 percent oxygen. In practice you round down to the fill you can actually get, which usually means EAN32 with about 11 feet of depth margin in hand. Rounding down costs a small amount of no-decompression time and buys back oxygen margin, which is always the right trade.
Do I have to analyse the mix myself?
Yes, every fill, every time, and you sign the log for it. The label on a cylinder records what the fill was meant to be, not what it is. Analysing takes under a minute at the fill station, and the fraction you read off the analyser is the number you use in this chart and the number you set on your computer. A mislabelled cylinder puts a diver below a MOD they believe they are respecting.
How we choose: we compare published manufacturer specifications, published training agency standards, and verified owner reviews across retailers. We do not test gear in person, and every depth rating, cylinder capacity and algorithm name quoted here is the manufacturer's published figure unless we say otherwise, so confirm it on the current spec sheet before you buy. Nothing here is dive instruction, and no calculator output on this site is a dive plan. Scuba diving carries a real risk of decompression sickness, oxygen toxicity, barotrauma and drowning. Dive within the limits of your certification, verify every plan with your own computer, and buy the training before the gear that assumes it.
Logging your own SAC rate and gas plans? The Dive Kit & Air Planning Workbook is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.