Maximum operating depth

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In underwater diving activities such as saturation diving, technical diving and nitrox diving, the maximum operating depth (MOD) of a breathing gas is the depth below which the partial pressure of oxygen (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 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. [1]

Contents

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 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.

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 oxygen toxicity manifests as symptoms such as visual changes (especially tunnel vision), ringing in the ears (tinnitus), nausea, twitching (especially of the face), behavioural changes (irritability, anxiety, confusion), and dizziness. 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 phase). The seizure ends with a period of unconsciousness (the postictal state). [2] [3] The onset of seizure depends upon the partial pressure of oxygen in the breathing 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 of carbon dioxide. [7] [8] [9] Other factors, such as darkness and caffeine, increase tolerance in test animals, but these effects have not been proven in humans. [10] [11]

The maximum single exposure limits recommended in the NOAA 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]

Formula

The formula simply divides the absolute partial pressure of oxygen which can be tolerated (expressed in atm or bar) 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 (msw) by multiplying with the appropriate conversion factor, 33 fsw per atm, or 10 msw per bar.

In feet

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

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.

Tables

Maximum Operating Depth (MOD) in feet sea water (fsw) for pO2 1.2 to 1.6
MOD (fsw) % oxygen
481216202124283236405060708090100
Maximum pO2 (bar)1.6128762740729723121818715613211499735542332620
1.5120558638027621520317314412210591665038292217
1.411225453522561981871601321119583594433251813
1.310405033252351821711461201018674533928211510
1.295746229721516515613210891776646332417117

These depths are rounded to the nearest foot.

Maximum Operating Depth (MOD) in metres sea water (msw) for pO2 1.2 to 1.6
MOD (msw) % oxygen
481216202124283236405060708090100
Maximum pO2 (bar)1.639019012390706657474034302217131086
1.53651781158465615344373228201511975
1.43401651077860574840342925181310864
1.331515398715552443631262316129643
1.229014090655047403328232014107532

These depths are rounded to the nearest metre.

See also

Notes

  1. Feet sea water (fsw) is a unit of pressure. One fsw is equal to the hydrostatic pressure exerted by a standard sea water column of 1 foot height at normal Earth gravity. 33 fsw is approximately equal to one standard atmosphere (atm). A pressure indicated in fsw is gauge pressure (relative to surface pressure) unless specified.

Related Research Articles

Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness.

<span class="mw-page-title-main">Nitrogen narcosis</span> Reversible narcotic effects of respiratory nitrogen at elevated partial pressures

Narcosis while diving is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high partial pressure. The Greek word νάρκωσις (narkōsis), "the act of making numb", is derived from νάρκη (narkē), "numbness, torpor", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness, or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than 30 metres (98 ft).

<span class="mw-page-title-main">Trimix (breathing gas)</span> Breathing gas consisting of oxygen, helium and nitrogen

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

<span class="mw-page-title-main">Partial pressure</span> Pressure of a component gas in a mixture

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 gas mixture is the sum of the partial pressures of the gases in the mixture.

<span class="mw-page-title-main">Breathing gas</span> Gas used for human respiration

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 habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.

<span class="mw-page-title-main">Oxygen toxicity</span> Toxic effects of breathing oxygen at high partial pressures

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, lungs, and eyes. Historically, the central nervous system condition was called the Paul Bert effect, and the pulmonary condition the Lorrain Smith effect, after the researchers who pioneered the discoveries and descriptions in the late 19th century. Oxygen toxicity is a concern for underwater divers, those on high concentrations of supplemental oxygen, and those undergoing hyperbaric oxygen therapy.

<span class="mw-page-title-main">Saturation diving</span> Diving decompression technique

Saturation diving is diving for periods long enough to bring all tissues into equilibrium with the partial pressures of the inert components of the breathing gas used. It is a diving mode that reduces the number of decompressions divers working at great depths must undergo by only decompressing divers once at the end of the diving operation, which may last days to weeks, having them remain under pressure for the whole period. A diver breathing pressurized gas accumulates dissolved inert gas used in the breathing mixture to dilute the oxygen to a non-toxic level in the tissues, which can cause potentially fatal decompression sickness if permitted to come out of solution within the body tissues; hence, returning to the surface safely requires lengthy decompression so that the inert gases can be eliminated via the lungs. Once the dissolved gases in a diver's tissues reach the saturation point, however, decompression time does not increase with further exposure, as no more inert gas is accumulated.

In-water recompression (IWR) or underwater oxygen treatment is the emergency treatment of decompression sickness (DCS) by returning the diver underwater to help the gas bubbles in the tissues, which are causing the symptoms, to resolve. It is a procedure that exposes the diver to significant risk which should be compared with the risk associated with the available options and balanced against the probable benefits. Some authorities recommend that it is only to be used when the time to travel to the nearest recompression chamber is too long to save the victim's life; others take a more pragmatic approach and accept that in some circumstances IWR is the best available option. The risks may not be justified for case of mild symptoms likely to resolve spontaneously, or for cases where the diver is likely to be unsafe in the water, but in-water recompression may be justified in cases where severe outcomes are likely if not recompressed, if conducted by a competent and suitably equipped team.

The equivalent air depth (EAD) is a way of approximating the decompression requirements of breathing gas mixtures that contain nitrogen and oxygen in different proportions to those in air, known as nitrox.

<span class="mw-page-title-main">Gas blending for scuba diving</span> Mixing and filling cylinders with breathing gases for use when scuba diving

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

Hyperoxia occurs when cells, tissues and organs are exposed to an excess supply of oxygen (O2) or higher than normal partial pressure of oxygen.

Hydreliox is an exotic breathing gas mixture of hydrogen, helium, and oxygen. For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.

Argox is the informal name for a scuba diving breathing gas consisting of argon and oxygen. Occasionally the term argonox has been used to mean the same mix. The blend may consist of varying fractions of argon and oxygen, depending on its intended use. The mixture is made with the same gas blending techniques used to make nitrox, except that for argox, the argon is added to the initial pure oxygen partial-fill, instead of air.

<span class="mw-page-title-main">Latent hypoxia</span> Lung gas and blood oxygen concentration sufficient to support consciousness only at depth

Latent hypoxia is a condition where the oxygen content of the lungs and arterial blood is sufficient to maintain consciousness at a raised ambient pressure, but not when the pressure is reduced to normal atmospheric pressure. It usually occurs when a diver at depth has a lung gas and blood oxygen concentration that is sufficient to support consciousness at the pressure at that depth, but would be insufficient at surface pressure. This problem is associated with freediving blackout and the presence of hypoxic breathing gas mixtures in underwater breathing apparatus, particularly in diving rebreathers.

Equivalent narcotic depth (END) (historically also equivalent nitrogen depth) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as nitrox, heliox or trimix. The method is used, for a given breathing gas mix and dive depth, to calculate the equivalent depth which would produce about the same narcotic effect when breathing air.

<span class="mw-page-title-main">Decompression practice</span> Techniques and procedures for safe decompression of divers

To prevent or minimize decompression sickness, divers must properly plan and monitor decompression. Divers follow a decompression model to safely allow the release of excess inert gases dissolved in their body tissues, which accommodated as a result of breathing at ambient pressures greater than surface atmospheric pressure. Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent.

<span class="mw-page-title-main">Hyperbaric treatment schedules</span> Planned hyperbaric exposure using a specified breathing gas as medical treatment

Hyperbaric treatment schedules or hyperbaric treatment tables, are planned sequences of events in chronological order for hyperbaric pressure exposures specifying the pressure profile over time and the breathing gas to be used during specified periods, for medical treatment. Hyperbaric therapy is based on exposure to pressures greater than normal atmospheric pressure, and in many cases the use of breathing gases with oxygen content greater than that of air.

<span class="mw-page-title-main">Decompression equipment</span> Equipment used by divers to facilitate decompression

There are several categories of decompression equipment used to help divers decompress, which is the process required to allow divers to return to the surface safely after spending time underwater at higher ambient pressures.

<span class="mw-page-title-main">Metre sea water</span> Unit of pressure equal to one tenth of a bar

The metresea water (msw) is a metric unit of pressure used in underwater diving. It is defined as one tenth of a bar.

<span class="mw-page-title-main">Built-in breathing system</span> System for supply of breathing gas on demand within a confined space

A built-in breathing system is a source of breathing gas installed in a confined space where an alternative to the ambient gas may be required for medical treatment, emergency use, or to minimise a hazard. They are found in diving chambers, hyperbaric treatment chambers, and submarines.

References

  1. 1 2 3 Lang, M.A. (2001). DAN Nitrox Workshop Proceedings. Durham, NC: Divers Alert Network. p. 52. Archived from the original on October 24, 2008. Retrieved 21 November 2012.{{cite book}}: CS1 maint: unfit URL (link)
  2. 1 2 Clark & Thom 2003, p. 376.
  3. U.S. Navy Diving Manual 2011, p. 44, vol. 1, ch. 3.
  4. U.S. Navy Diving Manual 2011, p. 22, vol. 4, ch. 18.
  5. Bitterman, N (2004). "CNS oxygen toxicity". Undersea and Hyperbaric Medicine. 31 (1): 63–72. PMID   15233161. Archived from the original on August 20, 2008. Retrieved 29 April 2008.{{cite journal}}: CS1 maint: unfit URL (link)
  6. Donald, Part I 1947.
  7. Lang 2001, p. 82.
  8. Richardson, Drew; Menduno, Michael; Shreeves, Karl, eds. (1996). "Proceedings of rebreather forum 2.0". Diving Science and Technology Workshop: 286. Archived from the original on September 15, 2008. Retrieved 20 September 2008.{{cite journal}}: CS1 maint: unfit URL (link)
  9. Richardson, Drew; Shreeves, Karl (1996). "The PADI enriched air diver course and DSAT oxygen exposure limits". South Pacific Underwater Medicine Society Journal. 26 (3). ISSN   0813-1988. OCLC   16986801. Archived from the original on October 24, 2008. Retrieved 2 May 2008.{{cite journal}}: CS1 maint: unfit URL (link)
  10. Bitterman, N; Melamed, Y; Perlman, I (1986). "CNS oxygen toxicity in the rat: role of ambient illumination". Undersea Biomedical Research. 13 (1): 19–25. PMID   3705247. Archived from the original on January 13, 2013. Retrieved 20 September 2008.{{cite journal}}: CS1 maint: unfit URL (link)
  11. Bitterman, N; Schaal, S (1995). "Caffeine attenuates CNS oxygen toxicity in rats". Brain Research. 696 (1–2): 250–3. doi:10.1016/0006-8993(95)00820-G. PMID   8574677. S2CID   9020944.
  12. "Physics of Diving" (PDF). NOAA Diving Manual. National Oceanic and Atmospheric Administration. Archived from the original (PDF) on 31 May 2014. Retrieved 6 September 2013.

Sources