Oxygen Partial Pressure Chart (ppO2)
Oxygen partial pressure equals the oxygen fraction multiplied by ambient pressure in atmospheres absolute, where ambient ata equals depth in feet of sea water divided by 33 plus 1. EAN32 at 100 feet is 1.29 ata. The working maximum is 1.4 ata, which NOAA limits to 150 minutes of single exposure, and 1.6 ata is a contingency ceiling limited to 45 minutes, not a planning target.
Oxygen partial pressure is the share of total pressure contributed by the oxygen in your breathing gas, and it is what makes oxygen dangerous at depth. The formula is one multiplication: ppO2 = FO2 x ambient pressure in ata, where ambient pressure in ata is depth in fsw / 33 + 1. The chart below runs that for every common mix across the whole recreational depth range.
Everything here is downstream of one measurement: the oxygen fraction of the gas in the cylinder. A cylinder label records what a fill was intended to be, and an oxygen analyser at the fill station records what it is. Analyse every fill yourself, sign for it, and set the same fraction on your computer before the dive. If the fraction is wrong, every figure on this page is wrong with it, in a direction you will not notice until it matters.
What is the ppO2 of each mix at each depth?
Find your depth in the first column and read across to your mix. Figures shown in red are past the 1.4 ata working limit, and figures in heavy red are past the 1.6 ata contingency ceiling. A red cell is not a warning to be careful. It is a depth that mix should not be taken to.
| Depth | Depth (m) | Ambient | Air | EAN28 | EAN32 | EAN36 | EAN40 |
|---|---|---|---|---|---|---|---|
| 0 ft | 0.0 m | 1.00 ata | 0.21 | 0.28 | 0.32 | 0.36 | 0.40 |
| 10 ft | 3.0 m | 1.30 ata | 0.27 | 0.36 | 0.42 | 0.47 | 0.52 |
| 20 ft | 6.1 m | 1.61 ata | 0.34 | 0.45 | 0.51 | 0.58 | 0.64 |
| 30 ft | 9.1 m | 1.91 ata | 0.40 | 0.53 | 0.61 | 0.69 | 0.76 |
| 40 ft | 12.2 m | 2.21 ata | 0.46 | 0.62 | 0.71 | 0.80 | 0.88 |
| 50 ft | 15.2 m | 2.52 ata | 0.53 | 0.70 | 0.80 | 0.91 | 1.01 |
| 60 ft | 18.3 m | 2.82 ata | 0.59 | 0.79 | 0.90 | 1.01 | 1.13 |
| 70 ft | 21.3 m | 3.12 ata | 0.66 | 0.87 | 1.00 | 1.12 | 1.25 |
| 80 ft | 24.4 m | 3.42 ata | 0.72 | 0.96 | 1.10 | 1.23 | 1.37 |
| 90 ft | 27.4 m | 3.73 ata | 0.78 | 1.04 | 1.19 | 1.34 | 1.49 |
| 100 ft | 30.5 m | 4.03 ata | 0.85 | 1.13 | 1.29 | 1.45 | 1.61 |
| 110 ft | 33.5 m | 4.33 ata | 0.91 | 1.21 | 1.39 | 1.56 | 1.73 |
| 120 ft | 36.6 m | 4.64 ata | 0.97 | 1.30 | 1.48 | 1.67 | 1.85 |
| 130 ft | 39.6 m | 4.94 ata | 1.04 | 1.38 | 1.58 | 1.78 | 1.98 |
Read the EAN32 column downward and the shape of the whole subject appears. At 100 feet it is 1.29 ata, comfortably inside the limit. Ten feet deeper it is still legal by a whisker. Ten feet deeper again it is not. There is no wide margin between fine and past the limit, which is exactly why maximum operating depth is treated as a hard ceiling rather than a soft one. Every mix has its own version of that transition, laid out depth by depth on the nitrox MOD chart.
The air column is worth reading too, because it puts the risk in proportion. Air at 130 feet, the recreational depth limit, is only 1.04 ata, nowhere near a concern. Oxygen toxicity is simply not the limit on air within recreational depths: gas supply, narcosis and no-decompression time all bind first. Oxygen becomes the governing limit only once you enrich the mix, which is the trade nitrox makes and the reason a nitrox certification exists at all. The nitrox versus air comparison goes through what you gain and what you give up.
EAN36 shows the trade at its sharpest. At 100 feet it reaches 1.45 ata, past the working limit, so the mix that buys the most bottom time in the 60 to 90 foot band is the mix that cannot go to 100 feet at all. Richer is not better. Richer is shallower, and choosing a mix is choosing which limit you would rather be constrained by.
Why is 1.4 the working limit and 1.6 only a ceiling?
Because central nervous system oxygen toxicity gives no reliable warning, and its most serious presentation is a convulsion. Underwater, a convulsion means the regulator leaves the mouth. In open water at depth that is usually fatal regardless of how attentive a buddy is, which puts it in a different category from almost every other risk in recreational diving.
Susceptibility also moves around in ways nobody can measure at depth. It differs between people, it differs in the same person from one day to the next, and it rises with exertion, with cold and with carbon dioxide retention. The 1.4 figure is a working margin against that variability rather than a threshold where something switches on. Nothing dramatic happens at 1.41 and nothing is guaranteed at 1.39.
The 1.6 figure exists for one narrow case: a diver at rest, on a decompression stop, breathing a rich gas, doing no work at all. The exposure limits below show exactly how much margin the extra 0.2 ata costs. Plan on 1.4, and if a dive needs more depth than 1.4 allows on your mix, change the mix rather than the limit. The nitrox MOD calculator will show you what a leaner mix buys.
What are the NOAA oxygen exposure limits?
A partial pressure figure describes an instant. The exposure limits describe how long you can sustain it. Both constrain a dive, and the second one is the constraint most divers forget, because it accumulates across a day rather than resetting between dives.
| ppO2 | Single exposure limit | Percent per minute | What it is for |
|---|---|---|---|
| 1.6 ata | 45 min | 2.22% | Contingency ceiling only, a resting diver on a decompression stop |
| 1.4 ata | 150 min | 0.67% | The working maximum for the bottom phase of a dive |
| 1.3 ata | 180 min | 0.56% | A sensible planning limit for long or repetitive nitrox days |
| 1.2 ata | 210 min | 0.48% | Conservative, and worth using when the diving is cold or hard |
| 1.1 ata | 240 min | 0.42% | Very long exposures |
| 1.0 ata | 300 min | 0.33% | Effectively unconstrained for recreational profiles |
The jump from 1.4 to 1.6 is the row to stare at. Two tenths of an atmosphere takes the allowed single exposure from 150 minutes to 45, a reduction of more than two thirds. That is the clearest available statement of how much margin the 1.4 working limit is carrying, and it is a better argument against planning to 1.6 than any amount of prose.
The percent per minute column is what your computer is actually counting. Your CNS clock is a running percentage of the allowance, accumulated across the whole day and only slowly released during surface intervals. At 1.4 ata you spend it at about two thirds of a percent every minute. Three dives in a day, each near the deep end of a rich mix, can push that percentage uncomfortably high even though every individual dive respected its maximum operating depth. This is precisely why divers doing repetitive nitrox days often plan against 1.3 or 1.2 rather than 1.4.
A practical note: not every computer displays the CNS percentage on the main screen, and some entry level units do not track it at all. A Shearwater Peregrine shows it alongside depth and no-stop time, while a simpler watch style unit such as the Cressi Neon handles nitrox to 99 percent but presents less of the underlying exposure data. Know which yours does before you plan a week of repetitive nitrox diving. The dive computer roundup covers what each unit tracks.
What about the nitrogen half of the mix?
Oxygen is the limit that ends dives suddenly, but nitrogen is the limit that shapes almost every recreational dive plan, and the two move in opposite directions when you enrich a mix. Partial pressure of nitrogen is calculated exactly the same way: the nitrogen fraction multiplied by ambient pressure.
| Depth | Ambient | Air ppO2 | Air ppN2 | EAN32 ppO2 | EAN32 ppN2 | Nitrogen saved |
|---|---|---|---|---|---|---|
| 30 ft | 1.91 ata | 0.40 | 1.51 | 0.61 | 1.30 | 0.21 |
| 60 ft | 2.82 ata | 0.59 | 2.23 | 0.90 | 1.92 | 0.31 |
| 80 ft | 3.42 ata | 0.72 | 2.71 | 1.10 | 2.33 | 0.38 |
| 100 ft | 4.03 ata | 0.85 | 3.18 | 1.29 | 2.74 | 0.44 |
| 130 ft | 4.94 ata | 1.04 | 3.90 | 1.58 | 3.36 | 0.54 |
That nitrogen saved column is the entire commercial case for nitrox, expressed in the only units that matter. Lower nitrogen partial pressure means slower tissue loading, which means longer no-decompression time at the same depth, which the equivalent air depth calculator converts into a depth you can read straight off the no-decompression limits table.
It also quietly settles the narcosis argument. The difference in nitrogen partial pressure at 100 feet between air and EAN32 is real but modest, and narcosis is driven by that partial pressure. Anyone who reports feeling clearer at depth on nitrox is describing better hydration, lower carbon dioxide from a calmer breathing pattern, or a shorter and shallower profile. Treating nitrox as an anti-narcosis measure is a mistake, and a consequential one if it leads a diver to plan a deeper dive than they otherwise would.
How do you use this chart on an actual dive day?
In four steps, all of which happen before you get in the water. Analyse the fill and write the fraction on the tag. Look up the maximum operating depth for that fraction and agree it with your buddy out loud. Set the same fraction on your computer, at the same moment, rather than later when someone is handing you a fin. And write the mix, the MOD and the turn pressure on a wrist slate so that the plan exists somewhere other than in two people's memories.
The failure that this sequence prevents is not exotic. It is a cylinder labelled 32 that analyses at 36, dived by a diver whose computer says 32, to a depth that was fine for the mix on the label. Every link in that chain is ordinary. The whole chain is dangerous, and it breaks the moment someone reads an analyser and writes the number down. The nitrox diving guide covers the full procedure from fill station to entry.
One last habit worth building: know your numbers well enough to catch a wrong one. A diver who knows that EAN32 tops out at 111 feet, that EAN36 tops out at 95, and that air at recreational depths never approaches an oxygen limit at all, will notice when a briefing, a fill or a computer setting does not fit. That intuition is what these charts are actually for.
Related charts and calculators
- Nitrox MOD chart, the same arithmetic solved for depth
- Nitrox MOD calculator, for any mix and limit
- Equivalent air depth calculator, the nitrogen side
- Nitrox diving guide, the procedures behind the numbers
- Nitrox versus air, what the mix actually buys
Frequently asked questions
How do you calculate oxygen partial pressure?
Multiply the oxygen fraction of the gas by ambient pressure in atmospheres absolute, and ambient pressure is depth in feet of sea water divided by 33, plus 1. EAN32 at 100 feet is 0.32 multiplied by 4.03, which is 1.29 ata. The same calculation on air at 100 feet gives 0.85 ata. Nothing about the gas changes with depth. Only the pressure it is delivered at changes.
What is the maximum safe ppO2 for scuba diving?
The working maximum for the bottom phase of a dive is 1.4 ata. The 1.6 ata figure is a contingency ceiling for a resting diver on a decompression stop and is treated as an absolute limit rather than a target. NOAA single-exposure limits reflect the difference sharply: 150 minutes is allowed at 1.4 ata but only 45 minutes at 1.6, which tells you how much margin the higher figure spends.
What are the NOAA oxygen exposure limits?
The NOAA single-exposure limits are 45 minutes at 1.6 ata, 150 minutes at 1.4, 180 minutes at 1.3, 210 minutes at 1.2, 240 minutes at 1.1 and 300 minutes at 1.0. They cap total time at a partial pressure rather than the instantaneous value, which is why a mix that is legal at a depth can still be limited by how long you plan to stay there or how many dives you make that day.
What is the CNS clock on a dive computer?
It is a running percentage of your allowed oxygen exposure, calculated from the NOAA limits and accumulated across the whole day rather than reset at the surface. At 1.4 ata you use the allowance at roughly two thirds of a percent per minute. Repetitive nitrox diving near the deep end of a mix can approach the limit even though every individual dive stayed under the maximum operating depth.
Does the partial pressure of nitrogen matter too?
Yes, and it drives both narcosis and decompression. On air at 100 feet the nitrogen partial pressure is about 3.18 ata; on EAN32 it is about 2.74. That reduction is the entire reason nitrox extends no-stop time. It is also small in narcosis terms, which is why nitrox does not meaningfully clear your head at depth despite the persistent claim that it does.
What happens if oxygen partial pressure goes too high?
The risk of central nervous system oxygen toxicity rises, and its most serious presentation is a convulsion. Underwater that usually means losing the regulator, which in open water at depth is frequently fatal. There is no dependable warning: the classic signs of visual disturbance, ear ringing, nausea and twitching are inconsistent, and susceptibility varies between people and between days for the same person.
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.