Inunderwater diving activities such assaturation diving,technical diving and nitrox diving, themaximum operating depth (MOD) of abreathing gas is the depth below which thepartial pressure ofoxygen (pO₂) of the gas mix exceeds an acceptable limit. This limit is based on risk ofcentral nervous system oxygen toxicity, and is somewhat arbitrary, and varies depending on thediver training agency or Code of Practice, the level of underwater exertion expected and the planned duration of the dive, but is normally in the range of 1.2 to 1.6bar.^[1]

The MOD is significant when planning dives using gases such asheliox,nitrox andtrimix because the proportion of oxygen in the mix determines a maximum depth for breathing that gas at an acceptable risk. There is a risk of acuteoxygen toxicity if the MOD is exceeded.^[1] The tables below show MODs for a selection of oxygen mixes. Atmospheric air contains approximately 21% oxygen, and has an MOD calculated by the same method.

Acute, or central nervous system oxygen toxicity is a time variable response to the partial pressure exposure history of the diver and is both complex and not fully understood.

Central nervous system oxygen toxicity manifests as symptoms such as visual changes (especiallytunnel vision), ringing in the ears (tinnitus),nausea, twitching (especially of the face), behavioural changes (irritability,anxiety, confusion), anddizziness. This may be followed by atonic–clonic seizure consisting of two phases: intense muscle contraction occurs for several seconds (tonic phase); followed by rapid spasms of alternate muscle relaxation and contraction producing convulsive jerking (clonic phase). The seizure ends with a period of unconsciousness (thepostictal state).^[2][3] The onset of seizure depends upon the partial pressure of oxygen in thebreathing gas and exposure duration. However, exposure time before onset is unpredictable, as tests have shown a wide variation, both amongst individuals, and in the same individual from day to day.^[2][4][5] In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms.^[6] Decrease of tolerance is closely linked to retention ofcarbon dioxide.^[7][8][9] Other factors, such as darkness andcaffeine, increase tolerance in test animals, but these effects have not been proven in humans.^[10][11]

The maximum single exposure limits recommended in theNOAA Diving Manual are 45 minutes at 1.6 bar, 120 minutes at 1.5 bar, 150 minutes at 1.4 bar, 180 minutes at 1.3 bar and 210 minutes at 1.2 bar.^[1]

The formula simply divides the absolute partial pressure of oxygen which can be tolerated (expressed inatm orbar) by the fraction of oxygen in the breathing gas, to calculate the absolute pressure at which the mix can be breathed. (for example, 50% nitrox can be breathed at twice the pressure of 100% oxygen, so divide by 0.5, etc.). Of this total pressure which can be tolerated by the diver, 1 atmosphere is due to surface pressure of the Earth's air, and the rest isdue to the depth in water. So the 1 atmosphere or bar contributed by the air is subtracted to give the pressure due to the depth of water. The pressure produced by depth in water, is converted to pressure infeet sea water (fsw) ormetres sea water (msw) by multiplying with the appropriate conversion factor, 33 fsw per atm, or 10 msw per bar.

In feet

${\displaystyle MOD(fsw)=33\mathrm{f}\mathrm{s}\mathrm{w}/\mathrm{a}\mathrm{t}\mathrm{m}\times \left[\left(\frac{p{O}_2\mathrm{a}\mathrm{t}\mathrm{a}}{F{O}_2}\right)-1\right]}$

In which pO₂ is the chosen maximum partial pressure of oxygen in atmospheres absolute and the FO₂ is the fraction of oxygen in the mixture. For example, if a gas contains 36% oxygen (FO₂ = 0.36) and the limiting maximum pO₂ is chosen at 1.4 atmospheres absolute, the MOD in feet of seawater (fsw)^{[Notes 1]} is 33 fsw/atm x [(1.4 ata / 0.36) − 1] = 95.3 fsw.^[12]

In metres

${\displaystyle MOD(msw)=10\mathrm{m}\mathrm{s}\mathrm{w}/\mathrm{b}\mathrm{a}\mathrm{r}\times \left[\left(\frac{p{O}_2\mathrm{b}\mathrm{a}\mathrm{r}}{F{O}_2}\right)-1\right]}$

In which pO₂ is the chosen maximum partial pressure in oxygen in bar and the FO₂ is the fraction of oxygen in the mixture. For example, if a gas contains 36% oxygen and the maximum pO₂ is 1.4 bar, the MOD (msw) is 10 msw/bar x [(1.4 bar / 0.36) − 1] = 28.9 msw.

These depths are rounded to the nearest foot.

These depths are rounded to the nearest metre.

• Oxygen toxicity – Toxic effects of breathing oxygen at high partial pressures
• Minimum operating depth – Depth above which the oxygen concentration is too low to breathe safely

• Clark, James M; Thom, Stephen R (2003). "Oxygen under pressure". In Brubakk, Alf O; Neuman, Tom S (eds.).Bennett and Elliott's physiology and medicine of diving (5th ed.). United States: Saunders. pp. 358–418.ISBN 978-0-7020-2571-6.OCLC 51607923.

• Donald, Kenneth W (1947)."Oxygen Poisoning in Man: Part I".British Medical Journal.1 (4506):667–672.doi:10.1136/bmj.1.4506.667.PMC 2053251.PMID 20248086.

• Lang, Michael A, ed. (2001).DAN nitrox workshop proceedings. Durham, NC: Divers Alert Network, 197 pages. Archived from the original on October 24, 2008. Retrieved20 September 2008.

• U.S. Navy Supervisor of Diving (2011).U.S. Navy Diving Manual(PDF). SS521-AG-PRO-010 0910-LP-106-0957, revision 6 with Change A entered. U.S. Naval Sea Systems Command. Archived fromthe original(PDF) on 2014-12-10. Retrieved29 Jan 2015.

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• Underwater diving safety
• Breathing gases

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