Nitrox

Sticker on a bottle with Nitrox mixture

Nitrox or Enriched Air nitrox (EAN or EANx) is a breathing gas mixture of nitrogen (engl. Nitr ogen) and oxygen (engl. Ox yGen) having a higher percentage of oxygen than normal air (usually between 32% and 40% instead of 21%) . It is used when diving to slow down the accumulation of nitrogen in the tissue and thus to extend the no-decompression limit or to reduce the risk of decompression sickness . The oxygen partial pressure , which is higher than that of air and other breathing gases , simultaneously reduces the maximum diving depth in order to avoid oxygen poisoning ( Paul Bert effect ).

Applications

In the early days of technical diving , Nitrox has now established itself as a breathing gas in recreational diving and diving sports associations such as B. PADI , SSI , IANTD or CMAS offer Nitrox courses.

The advantage of diving with Nitrox is that the diver absorbs less of it in his body fluids and tissues due to the lower partial pressure of nitrogen . Characterized the diver a lower N ₂ - narcosis exposed (the total anesthetic effect is contrary to popular belief, however, not reduced, this is because oxygen is classified narcotic similar, such as nitrogen), the zero time is extended and the decompression time or the risk decompression sickness decreases when diving with a diving table for compressed air . The number of decompression sickness accidents is already so low that the use of Nitrox does not lead to a significant increase in safety. While there is no empirical study on this, estimates suggest that using Nitrox within normal compressed air no-decompression limits is only a fraction of a percent, 0.001% less risk. The use of Nitrox offers advantages such. B. for professional divers and diving instructors , where the lower nitrogen content in the breathing gas reduces the burden of multiple ascents, yo-yo dives or repetitive dives . In recreational diving, Nitrox reduces the personal risk on longer dives and with a higher susceptibility to decompression sickness or open foramen ovale .

Diving depths

Maximum diving depth

The maximum operating depth (MOD) is calculated with

{\ displaystyle {\ text {MOD}} = \ left ({\ frac {p _ {{\ text {O}} _ {2}, {\ text {max}}}} {f _ {{\ text {O} } _ {2}}}} - p_ {a} \ right) \ cdot 10 {\ frac {\ text {m}} {\ text {bar}}} = \ left ({\ frac {\ text {Maximum partial pressure of oxygen }} {\ text {oxygen content}}} - {\ text {surface pressure}} \ right) \ cdot 10 {\ frac {\ text {m}} {\ text {bar}}}}

At sea level this corresponds to a depth of

O ₂ content	Max. Depth at O ₂ partial pressure of 1.4 bar	Max. Depth at O ₂ partial pressure of 1.6 bar
21% (normal air)	56.6 m	66.1 m
25%	46.0 m	54.0 m
28% (EAN28)	40.0 m	47.1 m
30%	36.6 m	43.3 m
32% (EAN32)	33.7 m	40.0 m
34%	31.1 m	37.0 m
36% (EAN36)	28.8 m	34.4 m
38%	26.8 m	32.1 m
40% (EAN40)	25.0 m	30.0 m
45%	21.1 m	25.6 m
50% (EAN50)	18.0 m	22.0 m
60%	13.3 m	16.6 m
70%	10.0 m	12.2 m
80%	7.5 m	10.0 m
90%	5.5 m	7.7 m
100% (pure oxygen)	4.0 m	6.0 m

Today it is usually recommended not to exceed a value for the partial pressure of oxygen ( ) of 1.4 bar. It is also recommended never to let the pressure drop below 0.16 bar. ${\ displaystyle p _ {{\ text {O}} _ {2}}}$ ${\ displaystyle p _ {{\ text {O}} _ {2}}}$

The maximum permissible ambient pressure (MOP) for the selected mixture can also be derived from the maximum permissible diving depth.

Equivalent air depth

Under the equivalent air depth ( Equivalent Air Depth EAD) is defined as the depth at which the nitrogen partial pressure standard air corresponds to that of enriched air mixture. Since the proportion of nitrogen is reduced by the increased oxygen proportion compared to normal compressed air, the EAD (i.e. without Nitrox) is less deep than the diving depth with Nitrox (see example in the next section). The EAD model takes into account the fact that when using Nitrox mixtures, the diver saturates less strongly with nitrogen than during the same dive with compressed air. If the absolute pressure is used instead of the depth, one speaks of the equivalent absolute pressure ( Equivalent Air Pressure , EAP). The EAP is calculated using the formula ${\ displaystyle p _ {{\ text {N}} _ {2}}}$

{\ displaystyle {\ text {EAP}} = {\ frac {f _ {{\ text {N}} _ {2}}} {0 {,} 79}} \ cdot {\ text {p}} = {\ frac {\ text {Nitrogen content}} {0 {,} 79}} \ cdot {\ text {Pressure in the depth}}}

For certain Nitrox mixtures, the EAD can be determined from tables or the EAD is calculated as

{\ displaystyle {\ text {EAD}} = ({\ text {EAP}} - {\ text {1}}) \ cdot {\ text {10}} {\ frac {\ text {m}} {\ text {bar}}} = \ left ({\ frac {f _ {{\ text {N}} _ {2}}} {0 {,} 79}} \ cdot {\ text {p}} - {\ text { 1}} \ right) \ cdot {\ text {10}} {\ frac {\ text {m}} {\ text {bar}}}}

If you do not have an exchange table for a specific Nitrox mixture, you can use the EAD instead of the actual diving depth in an exchange table developed for compressed air according to this table.

The optimal gas mixture (Best Mix)

The maximum proportion of oxygen in the breathing gas for a certain depth is also calculated

{\ displaystyle f _ {{\ text {O}} _ {2}, {\ text {max}}} = {\ frac {p _ {{\ text {O}} _ {2}, {\ text {max} }}} {p}} = {\ frac {\ text {Maximum partial pressure of oxygen}} {\ text {Pressure in the depth}}}}

example

A dive of 40 minutes at 27 meters with EAN38 (i.e. 38% instead of 21% oxygen in the breathing gas) at sea level is planned. Then

${\ displaystyle {\ text {MOD}} = \ left ({\ frac {1 {,} 6 \, {\ text {bar}}} {0 {,} 38}} - 1 \, {\ text {bar }} \ right) \ cdot 10 \, {\ frac {\ text {m}} {\ text {bar}}} = 32 \, {\ text {m}}}$ , therefore the dive can be done at 27 meters.
${\ displaystyle {\ text {EAP}} = {\ frac {1-0 {,} 38} {0 {,} 79}} \ cdot 3 {,} 7 \, {\ text {bar}} = 2 { ,} 9 \, {\ text {bar}}}$ .
${\ displaystyle {\ text {EAD}} = (2 {,} 9 \, {\ text {bar}} - {\ text {1}}) \ cdot {\ text {10}} {\ frac {\ text {m}} {\ text {bar}}} = 19 {\ text {m}}}$ .
${\ displaystyle f _ {{\ text {O}} _ {2}, {\ text {max}}} = {\ frac {1 {,} 6 \, {\ text {bar}}} {3 {,} 7 \, {\ text {bar}}}} = 0 {,} 43}$ .

Deeper diving

Breathing gas mixtures of helium and oxygen are used for diving at greater depths ( trimix , heliox ). In most cases, the oxygen content is then even depleted and several different breathing gases are taken with you to change. See also technical diving .

Oxygen toxicosis

As the oxygen content in the breathing gas increases, the maximum possible diving depth ( maximum operating depth , MOD ) decreases . The reason lies in the toxicity of oxygen above a certain partial pressure ( CNS poisoning ). The diving organizations recommend that you do not exceed the limit of 1.2–1.6 bar oxygen partial pressure ( ) for recreational diving . Under optimal conditions (warm water, no stress factors, little exertion, low complexity of the dive etc.) 1.4 bar is often recommended , otherwise 1.2 or 1.3 bar is recommended as the maximum . The tolerance to oxygen poisoning is individual and depends on the daily form. Therefore, exceeding these recommendations does not necessarily lead to accidents. However, the diving sports associations recommend using the limits with caution and not always exhausting them. ${\ displaystyle p _ {{\ text {O}} _ {2}}}$ ${\ displaystyle p _ {{\ text {O}} _ {2}}}$

If the partial pressure of oxygen is too high ( > 1.7 bar), seizures can occur suddenly and without warning , which resemble epileptic seizures . These seizures generally do not cause any direct consequential damage and end spontaneously within 1–5 minutes after the partial pressure of oxygen has normalized ( = 0.21 bar). This is usually followed by an unconsciousness of 5–10 minutes. Occasionally, uncontrolled bowel movements occur . Great tiredness , nausea , vomiting and headache are to be expected. Lung damage can occur as a long-term consequence. ${\ displaystyle p _ {{\ text {O}} _ {2}}}$ ${\ displaystyle p _ {{\ text {O}} _ {2}}}$

If an oxygen spasm occurs during a dive, there is a risk of losing the regulator from your mouth and drowning in the subsequent unconsciousness.

Oxygen tolerance

The tolerance of a human to an increased oxygen partial pressure is influenced by several factors: ${\ displaystyle p _ {{\ text {O}} _ {2}}}$

Water / air change
Ambient temperature in the water (faster cooling )
Increased work performance (higher air consumption)
Increased in the air ${\ displaystyle p _ {{\ text {CO}} _ {2}}}$
Repetitive O ₂ load on the body
Overactive thyroid gland ( hyperthyroidism )
Low blood sugar ( hypoglycaemia )
fever
Drugs , ethanol (alcohol)
Active ingredients such as B. ASA , steroids ( cortisone ), insulin , acetazolamide , catecholamines ( adrenaline ), high-dose penicillin , high-dose vitamin C.

During the diving fitness test , the doctor must clarify the effects of drugs that have been prescribed for the diver.

Diving equipment

Due to the high reactivity of oxygen, specially designed diving equipment is recommended for Nitrox dives. In Germany, a standard regulates that any breathing mixture with an increased proportion of oxygen (compared to normal breathing air) is to be treated like pure oxygen. Therefore, divers in Germany often require special equipment. In many other countries, on the other hand, you can carry out nitrox dives with a mixture of up to 40% O ₂ with compressed air equipment.

Nitrox equipment, like other diving equipment, should only be serviced by trained specialist dealers . They should be familiar with the maintenance of oxygen-free equipment and have the appropriate equipment.

The equipment is dismantled, cleaned in an ultrasonic bath and then treated with special chemicals . Lubricated parts must be greased with special, oxygen-resistant grease.

Regulator

Regulators (first and second stage), which are used with breathing gases with an oxygen content of more than 21%, must in some countries (e.g. Germany) have been made oxygen-free or have already been manufactured in this way. This means the use of special lubricants (colloquially oxygen grease ) as well as special O-rings suitable for the increased oxygen content. In other countries, breathing gas with an oxygen content above 21% can be used with conventional regulators, e.g. B. up to 40% O ₂ in Switzerland.

In addition, regulators in the EU should be equipped with a special nitrox thread (M26x2).

The most common manufacturers of regulators either offer nitrox versions of some models directly ex works or conversion kits that contain suitable O-rings and lubricants.

Filling systems

Nitrox breathing gas mixtures can be prepared by a number of methods. In the membrane process (see also membrane technology ), the air is pressed through an N ₂ filter, in which part of the nitrogen remains. With this method the oxygen content of the mixture can be increased up to 40%. The systems are easy to use, the mixture can be used immediately and there is no need to deal with pure oxygen. In the enrichment process , the oxygen is constantly added during filling. The mixture can be used immediately and mixtures with an oxygen content of over 40% can be produced. With the partial pressure method, pure oxygen is first filled into the bottle up to a certain pressure and then the target pressure of the bottle is established with normal compressed air (usually 200 bar, topped ).

bottle

A Nitrox cylinder is usually specially cleaned and marked with a yellow-green color and the imprint "NITROX". The oxygen content in the bottle is checked by the diver himself after filling and noted on the bottle as a percentage. The diver calculates the maximum diving depth (MOD) - at which the mixture can be breathed - and also notes this on the bottle. If different mixtures are used during a dive, this is to ensure that the correct bottle is always used for breathing. A check of the diving partner should also be made easier in this way.

Nitrox cylinders should only be filled at Nitrox filling systems and not at conventional compressed air filling stations. In Nitrox filling stations, it should be ensured that the breathing mixture itself meets the requirements by using oxygen-free elements in the system and that it does not contain any impurities. If oil or other dirt particles get into the high-purity Nitrox bottles through a contaminated filling system, they should not be refilled with NITROX until they have been properly cleaned, in order to prevent self-ignition inside.

Depending on the filling process, there are different requirements for the bottle. The highest requirements arise when, as with the partial pressure method, pure oxygen is used. There is a risk of explosion! If, as in the membrane process, an already finished mixture is filled into the bottle, the bottle "only" has to be suitable for this mixture and the risk of explosion is lower. The regulations and safety rules differ from country to country.

According to the old regulations (valid until June 1, 2006), oxygen bottles were entirely blue, and the designation "oxygen" had to be stamped. According to the new standard (according to DIN EN 1089-3) the container is white throughout; During the changeover it is marked with two "N" ( diametrically offset). In contrast to compressed air cylinders , Nitrox cylinders are subject to the regulations for oxygen cylinders because they are partially filled with pure oxygen during the preparation of the mixture.

Cylinder valves

For cylinder valves in Switzerland a thread G ³ / ₄ inches required, which should make it impossible to combine the first steps for normal air with a Nitrox bottle.

Within the EU , M26x2 valves (EN144-3, so-called Euro-Nitrox) are recommended for cylinders with increased oxygen content.

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o Lothar Becker: Nitrox manual . 2nd Edition. Delius Klasing Verlag, Bielefeld 2007, ISBN 978-3-7688-2420-0
↑ ^a ^b ^c ^d ^e ^f Des Gorman: Enriched Air Diver Manual , Revision 1/09. PADI, Rancho Santa Margrita 2009.
↑ Helge Weber: Production of a NITROX mixture according to the partial pressure method. (PDF) teaching sample TL **. Deutsche Lebens-Rettungs-Gesellschaft , September 30, 2005, p. 4 , accessed on May 15, 2017 .
↑ Beat A. Müller: Diving and standardization with spec. Consideration of EN144-3 (Nitrox thread M26 x 2). (PDF) V2.6. Swiss Cave Diving, April 15, 2008, pp. 33-36 , accessed on May 15, 2017 .

[Becker-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o Lothar Becker: Nitrox manual . 2nd Edition. Delius Klasing Verlag, Bielefeld 2007, ISBN 978-3-7688-2420-0

[Gorman-2] ↑ ^a ^b ^c ^d ^e ^f Des Gorman: Enriched Air Diver Manual , Revision 1/09. PADI, Rancho Santa Margrita 2009.

[3] Helge Weber: Production of a NITROX mixture according to the partial pressure method. (PDF) teaching sample TL **. Deutsche Lebens-Rettungs-Gesellschaft , September 30, 2005, p. 4 , accessed on May 15, 2017 .

[4] Beat A. Müller: Diving and standardization with spec. Consideration of EN144-3 (Nitrox thread M26 x 2). (PDF) V2.6. Swiss Cave Diving, April 15, 2008, pp. 33-36 , accessed on May 15, 2017 .