underwater diving Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on contex ...

activities such as saturation diving,

technical diving Technical diving (also referred to as tec diving or tech diving) is scuba diving that exceeds the agency-specified limits of recreational diving for non- professional purposes. Technical diving may expose the diver to hazards beyond those normal ...

and nitrox diving, the maximum operating depth (MOD) of a

breathing gas A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed ...

is the depth below which the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

(pO₂) of the gas mix exceeds an acceptable limit. This limit is based on risk of central nervous system oxygen toxicity, and is somewhat arbitrary, and varies depending on the

diver training Diver training is the set of processes through which a person learns the necessary and desirable skills to safely dive underwater within the scope of the diver training standard relevant to the specific training programme. Most diver 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 1.6

bar Bar or BAR may refer to: Food and drink * Bar (establishment), selling alcoholic beverages * Candy bar * Chocolate bar Science and technology * Bar (river morphology), a deposit of sediment * Bar (tropical cyclone), a layer of cloud * Bar ( ...

. The MOD is significant when planning dives using gases such as heliox, nitrox and trimix 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 acute

oxygen toxicity Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, l ...

if the MOD is exceeded. 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.

Safe limit of partial pressure of oxygen

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 The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

oxygen toxicity manifests as symptoms such as visual changes (especially tunnel vision), ringing in the ears (

tinnitus Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...

nausea Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of th ...

, twitching (especially of the face), behavioural changes (irritability,

anxiety Anxiety is an emotion which is characterized by an unpleasant state of inner turmoil and includes feelings of dread over anticipated events. Anxiety is different than fear in that the former is defined as the anticipation of a future threat wh ...

, confusion), and

dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a common medical c ...

. This may be followed by a tonic–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 Clonus is a set of involuntary and rhythmic muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many ca ...

phase). The seizure ends with a period of unconsciousness (the

postictal state The postictal state is the altered state of consciousness after an epileptic seizure. It usually lasts between 5 and 30 minutes, but sometimes longer in the case of larger or more severe seizures, and is characterized by drowsiness, confusion, ...

). The onset of seizure depends upon the partial pressure of oxygen in the

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. 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. Decrease of tolerance is closely linked to retention of

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

. Other factors, such as darkness and

caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is mainly recreational drug use, used recreationally as a Nootropic, cognitive enhancer, increasing alertness and attentional perfor ...

, increase tolerance in test animals, but these effects have not been proven in humans. The maximum single exposure limits recommended in the

NOAA The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...

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.

Formula

The formula simply divides the absolute partial pressure of oxygen which can be tolerated (expressed in atm or

) 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 is due 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 in feet sea water (fsw) or

metres sea water The metre (or meter) sea water (msw) is a metric unit of pressure used in underwater diving. It is defined as one tenth of a bar. The unit used in the US is the foot sea water (fsw), based on standard gravity and a sea-water density of 64&nb ...

(msw) by multiplying with the appropriate conversion factor, 33 fsw per atm, or 10 msw per bar. In feet

MOD (fsw) = 33\mathrm \times    \left left ( \right ) - 1\right /math>

In which pO

₂ 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) is 33 fsw/atm x 1.4 ata / 0.36) − 1= 95.3 fsw. In metres

MOD (msw) =10\mathrm \times    \left left ( \right ) - 1\right /math>

In which pO

₂ 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.

Tables

These depths are rounded to the nearest foot. These depths are rounded to the nearest metre.

Notes

References

Sources