How Nitrox Diving works?

What is Nitrox?

Nitrox is a gas that is generally used for recreational diving. The gas is simply a mixture of oxygen and nitrogen, which in recreational diving terms, is sometimes referred to as ‘enriched air nitrox (EAN)’. The oxygen concentration in the air is normally around 21%, however the oxygen concentration in nitrox is higher. Nitrox refers to any combination with levels of oxygen between 21 and 40 percent- 32% is a common oxygen content used.

The well-used term ‘decompression sickness’ (DCS), also known as ‘the bends’ starts when nitrogen enters the divers bloodstream and as they dive deeper, the pressure will also increase.

When the nitrogen builds up and the diver comes to the surface too quickly (without performing decompression stops), the diver’s body will need time to remove the absorbed oxygen and may experience decompression sickness as a result. DCS is extremely unpleasant and in some cases it can be deadly, which is why so much research has been driven into how we can prevent it from happening.

Instead of using a standard mix, divers started using nitrox as it helps increase your allowed diving time due to offering less nitrogen and more oxygen. This means that less nitrogen can be absorbed into the bloodstream which therefore extends the ‘no decompression limit’, or in other words, you get longer bottom times.

While this does not completely prevent DCS, it reduces the risk of it occurring.

However, divers must also remember not to exceed maximum depth or bottom time!

Another benefit of using nitrox is shorter decompression times, this is because there is less nitrogen to be absorbed in the body, meaning that there is less nitrogen to expel from the body when resurfacing.

While oxygen is a key element to human life, oxygen at higher levels can be harmful to us too, this is called oxygen toxicity. If the oxygen concentration in the nitrox mix is too high, the diver may experience seizures, unconsciousness, breathing difficulty, damage to the lungs and eyes and in worst cases death.

Oxygen toxicity gives no or little warning before you experience these severe symptoms, which is what makes it so dangerous. It is safer to check your gas mix before you dive in order to eliminate risk of oxygen toxicity.

Deep Stops

World-recognised decompression physiologist and cave explorer David Doolette explains the new evidence-based findings on “deep stops,” and shares how and why he sets his own gradient factors. His recommendations may give you pause to stop (shallower).

by Associate Professor David J. Doolette

Gradient factors are mechanisms which modify the decompression stops prescribed by the Buhlmann ZH-L16 decompression algorithm. ZH-L16 is a “gas content” algorithm, which tracks the uptake and elimination of inert gas in notional tissue compartments and schedules decompression stops to not exceed specified maximum permissible inert gas partial pressures in the compartments. When such maximum permissible inert gas partial pressures are specified for decompression stop depths, they are referred to as M-values.

Gradient factors (GF) modify M-values (and consequently allowed gas supersaturation) to a fraction of the difference between ambient pressure and the original M-value. Thus, GF 80 modifies the M-value to 80% of the difference between ambient pressure and the original M-value. Typical proprietary implementations of the GF method require the diver to select two gradient factors: GF low modifies the M-values for the deepest decompression stop, and GF high modifies the M-value for surfacing (often designated as GF low/high, e.g. GF 20/80). The algorithm then interpolates a series of modified M-values in between these two user-specified points. If the GF low is set less than 100%, this forces deeper stops to limit supersaturation in the fast tissues early in the ascent, and setting the GF high to less than 100% will produce longer, shallower stops to reduce supersaturation in the slower tissues in the latter phase of the ascent.

In contrast to gas content decompression algorithms, bubble decompression algorithms (VPM-B is one such algorithm familiar to GUE divers) characteristically prescribe deeper decompression stops. In simple terms, bubble decompression algorithms favour deeper stops to limit supersaturation and thereby bubble formation early in the decompression, whereas traditional gas content decompression algorithms favour a more rapid ascent to maximize the inspired–tissue gradient of inert gas partial pressures to maximize tissue inert gas washout.

New Findings on Deep Stops

Deep stops came to the attention of early technical divers in the form of empirical “Pyle stops,” a practice serendipitously developed by ichthyologist and technical diving pioneer Richard Pyle, arising from a requirement to vent the swim bladders of fish specimens collected at great depth before arriving at his first decompression stop. There followed a strong trend toward the adoption of bubble algorithms, and also for the use of gradient factors to force gas content algorithms to impose deep stops (for instance, using GF low values of 30% or less). Based largely on supportive anecdotes, a widespread belief emerged among technical divers that deep-stop decompression schedules are more efficient than shallow-stop schedules. Efficiency, in this context, means that a schedule of the same or even shorter duration has a lower risk of DCS than some alternative schedule.

However, since about 2005, evidence has been accumulating from comparative decompression trials that shows deep stops are not more efficient, and possibly less efficient, than shallow stops.

However, since about 2005, evidence has been accumulating from comparative decompression trials that shows deep stops are not more efficient, and possibly less efficient, than shallow stops. Most studies have used venous gas emboli (bubbles) as an indicator of comparative risk of decompression sickness (DCS). Blatteau and colleagues compared dives using French Navy air and trimix decompression tables (relatively shallow stop schedules) to experimental schedules with added deep stops and longer total decompression time (similar to Pyle stops). Despite longer total decompression time, the deep stops schedules resulted in either the same or more VGE than the shallow stops schedules, and some cases of DCS. (1)

Spisni and colleagues compared trimix dives conducted using a deep stops schedule (ZH-L16 with GF 30/85) to an even deeper stops schedule with longer total decompression time (a UDT version of ratio deco) and found no difference in VGE.(2)

An as-yet-unpublished study compared trimix dives using a DCAP shallow stops schedule to a ZH-L16 GF 20/80 deep stops schedule with similar total decompression time, and the deep stops schedule resulted in significantly more VGE.(3)

A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives. (4)

What To Do About Gradient Factors

The emerging body of evidence against deep stops suggest common gradient factor setting should be modified to de-emphasize deep stops. Fraedrich validated dive computer algorithms by comparing them to well-tested U.S. Navy decompression schedules, including the schedules from the deep stop study outlined above. For that dive, ZH-L16 with a GF low >55% (e.g. GF 55/70) produced a first decompression stop between 70 and 40 fsw.(5)

Tyler Coen at Shearwater Research Inc. noted that GF settings recommended by Fraedrich modify ZH-L16 M-values so that approximately the same level supersaturation is allowed at all stop depths. To understand this requires delving a little further into M-values.

The emerging body of evidence against deep stops suggest common gradient factor setting should be modified to de-emphasize deep stops.

M-values are typically a linear function of stop depth. In older algorithms such as ZH-L16, the M-value generating functions have a slope greater than one (in ZH-L16, the slopes are the reciprocals of the “b” parameters), resulting in increasing supersaturation allowed with increasing stop depth. In more modern algorithms developed by the U.S. Navy since the 1980s, including the one used to produce the shallow stops schedule in the study outlined above, the slope of the M-value generating functions are generally equal to one, so that the same level of supersaturation is allowed at all stop depths. This results in modestly deeper stops than older algorithms, but still relatively shallow stops compared to bubble models.
With this information in mind, I set my GF low to roughly counteract the ZH-L16 “b” parameters (I have been using Shearwater dive computers with ZH-L16 GF in conjunction with my tried and true decompression tables for about three years). In ZH-L16, the average of “b” parameters is 0.83. I choose my GF low to be about 83% of the GF high, for instance GF 70/85. Although the algebra is not exact, this roughly counteracts the slope of the “b” values. This approach allows me to believe I have chosen my GF rationally, is not so large a GF low as I am unable to convince my buddies to use it, and satisfies my preference to follow a relatively shallow stops schedule.

This article was prepared by Assoc. Professor Doolette in his personal capacity. The opinions expressed in this article are the author’s own and do not reflect the view of the Department of the Navy or the United States government.

1. Blatteau JE, Hugon M, Gardette B. Deeps stops during decompression from 50 to 100 msw didn’t reduce bubble formation in man. In: Bennett PB, Wienke BR, Mitchell SJ, editors. Decompression and the deep stop. Undersea and Hyperbaric Medical Society Workshop; 2008 Jun 24-25; Salt Lake City (UT). Durham (NC): Undersea and Hyperbaric Medical Society; 2009. p. 195-206.

2. Spisni E, Marabotti C, De FL, Valerii MC, Cavazza E, Brambilla S et al. A comparative evaluation of two decompression procedures for technical diving using inflammatory responses: compartmental versus ratio deco. Diving Hyperb Med 2017;47:9-16.

3. Gennser M. Use of bubble detection to develop trimix tables for Swedish mine-clearance divers and evaluating trimix decompressions. Presented at: Ultrasound 2015 – International meeting on ultrasound for diving research; 2015 Aug 25-26; Karlskrona (Sweden).

4. Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. Technical Report. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. 53 p. Report No.: NEDU TR 11-06.

5. Fraedrich D. Validation of algorithms used in commercial off-the-shelf dive computers. Diving Hyperb Med 2018;48:252-8.

Additional Resources:

PADI recently published an excellent post, “Evolving Thought on Deep Decompression Stops,” by John Adsit, on the subject of Deep Stops.
Alert Diver magazine published a profile and interview with Doolette in the Fall of 2016.

The Math behind the ZH-L16 Model: Bühlmann established, by means of many hyperbaric chamber experiments with volunteers, how much supersaturation the individual tissue compartments can tolerate without injury. He expressed the relationship through the following equation:
pamb. tol. = (pt. i.g. – a) ·b
or
pt. tol. i.g. = (pamb / b) + a
pamb. tol. – the ambient pressure tolerated by the tissue
pt. i.g. – the pressure of the inert gas in the tissue
pt. tol. i.g. – tolerated (excess)pressure of the inert gases in the tissues
pamb – current ambient pressure
a, b – parameters of the model ZH-L16 for each tissue. “a” depends on the measure unit of pressure used, while “b” represents the steepness of the relationship between the ambient pressure pamb. and the pressure of inert gas in the tissue pt. i.g. The first equation shows which lower ambient pressure pamb. tol. will still be tolerated at the actual pressure of inert gas in the tissues pt. i.g. The lower equation shows which level of supersaturation pt. tol. i.g. can be tolerated at a given ambient pressure pamb for a given tissue.

Dr. David Doolette began scuba diving in 1979 and was introduced to the sinkholes and caves of Australia in 1984. Around this time, he alternated between studying for his B.Sc. (Hons.) and working as a dive instructor, when he developed an interest in diving physiology. He planned and conducted some of the first technical dives in Australia in 1993. Since being awarded his Ph.D. in 1995, he has conducted full time research into decompression physiology, first at the University of Adelaide, and since 2005 at the U.S. Navy Experimental Diving Unit in Panama City, Florida.
He has been a member of the Undersea Hyperbaric Medical Society since 1987, received their 2003 Oceaneering International Award, and is a member of their Diving Committee. He has also been a member of the South Pacific Underwater Medicine Society since 1990 and served as the Education Officer for five years. He is a member of the Cave Diving Association of Australia, the Australian Speleological Federation Cave Diving Group, Global Underwater Explorers, and the Woodville Karst Plain Project. He remains an avid underwater cave explorer, both near his home in Florida and abroad

Swanage Dive Weekend 30 August – 1 September 2019

Swanage

Salt, salt, salt and more salt – if you want to expand your diving from the fresh water lakes and quarries we typically dive through out the year, this is the trip for you. The trip is for all qualified divers, with something for everyone.

With spaces booked on the excellent shuttle services run by Swanage Boat Charters, four current bookings are in place for two dives on Saturday and two dives on Sunday, the first dive on the Saturday is on the Valentine Tanks – max depth 15m and the second shuttle dive on the Carantan – max depth 30m. The Sunday sees us diving the Carantan first and then the Valentine Tanks second.

Swanage also has a pier, and it is possible to dive this – tides allowing, and other shuttle services can be booked, if the demand exists for drift or scenic dives.

We will be staying in shared rooms at the YHA on Friday and Saturday night, spaces and booking are limited, to secure your place or for further information email: training@rec2tecdiving.co.uk

Carantan Class

About the Carantan

Seized by the Royal Navy from occupied France in 1940, this 400-tonne submarine chaser was 120ft long with a narrow beam of just 20ft. By December of 1943 she was operating under the control of the Free French and was escorting a submarine towards Portsmouth when a fierce storm caused her to capsize. This tragic outcome may have been speeded along by a large Boer War-era gun bolted to her foredeck and top-heavy brass plating used for her superstructure.
Only 6 of the crew of 23 were saved, making the Carantan a war grave. Lying on her side, what is left of the Carantan juts up around ten feet off the seabed at the highest point, although much of it is smashed and broken. Where plating had come away from the skeletal frame of the superstructure in places.

Valentine Tanks

The Valentine Tanks

These tanks were in fact sea-going vessels in their own right. Before their demise, they were taking part in Exercise Smash around Poole Bay in preparation for the forthcoming D-Day landings of 6 June, 1944. The idea behind the top-secret design was that a canvas frame fitted around the tank would, when raised, displace enough water to enable them to float. They were known as “DD” or “Duplex Drive” Valentine tanks, and a propeller was also mounted at the rear to move the vehicle forward and to within beach range, where the tank’s tracks would take over the job. Today, divers usually visit only two examples of these tanks because, conveniently, one pair has been roped together to help divers to locate both on a single dive. They lie upright only 70m apart.

PADI Sidemount – Vobster – 30/31/03/19

Over the weekend of the 30-31/03/19, Rec2Tec Diving was at Vobster Quay in Somerset for a weekend of sidemount diving, with two students working towards their PADI Sidemount qualification. The weather was kind, the water cool and the viz fab - the students did really well too.

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