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The Divers Alert Network team dives deep into the myth that divers feel less tired after a dive on nitrox than they would have on air

Nitrox is a great tool for divers to have on hand — it

After more than 25 years of recreational nitrox use, divers and researchers have gained operational experience and significant research-based knowledge of the various nitrox mixes and their affect on dive profiles. The advantages of nitrox are well researched and documented and can be learned quickly by eager divers.

In the nearly three decades of use, however, several misconceptions have surrounded the use of nitrox, most notably that divers report feeling less tired after a dive on nitrox than they would on air. These reports come from divers of all experience levels, and more than one diver has started a heated debate over the topic in postdive conversation. While the reports of this phenomenon are widespread and often fiercely defended, our best understanding is that nitrox can prevent fatigue about as well as a manatee can pass for a mermaid - there might appear to be some similarity on a foggy day, but a close look reveals little resemblance to the truth.

Understanding the relationship between nitrox and fatigue predisposes a basic understanding of fatigue. Physical tiredness is a physiological consequence of exercise. It’s best understood as a subjective feeling of temporarily reduced capacity for work and is frequently associated with lethargy and decreased cognitive performance. It is not an uncommon symptom following an inconsequential dive with no other symptoms of decompression sickness (DCS), but it is also a frequently reported symptom of DCS. Despite divers’ frequent reports to the contrary, objective studies have not found a reliable difference in either physical fatigue or cognitive performance following dives on air compared to identical dives by the same subjects on nitrox.

Given an identical dive, it is generally understood that the higher partial pressure of oxygen in nitrox (and the lower partial pressure of inert gases) will result in less decompression stress. It’s easy to make the logical leap to connect fatigue to minor decompression stress at this point, but it’s important to recognise that a higher partial pressure of oxygen also causes a different type of stress - oxidative stress. Several studies suggest the oxidative stress generated by high partial pressures of oxygen can cause mild endothelial dysfunction, which may actually lead to feelings of fatigue. This challenges the ‘nitrox-as-fatiguebuster’ myth. This oxidative stress is particularly apparent in

hyperbaric treatments, where long exposures to high partial pressures of oxygen result in frequent reports of posttreatment fatigue. Is it likely that nitrox will make you more fatigued in the water? In short — no, but it’s a significant step away from proof that nitrox can combat post-dive fatigue.

Diving is work, no matter how much fun it is. Thermal stress, decompression stress, exercise, prolonged oxygen exposure, anxiety and seasickness can all contribute to leaving you exhausted after a relaxing dive. Being immersed in water even without descending pushes about two-thirds of a litre of blood to the heart during the resting phase of the cardiac cycle and starts a cyclical reaction that releases hormones, eliminates fluids in the body and shunts warm blood to the core. This phenomenon resolves once a diver leaves the water, and the decline in central blood volume and pressure can cause significant fatigue on top of all the other factors in play.

Subclinical DCS (minor or difficult-to-distinguish symptoms of DCS) throw a wrench in what would otherwise be fairly straightforward myth-busting. We have no definitive reason to believe that nitrox can reduce diver fatigue, but unusual fatigue in excess of expected levels or feeling sick after a dive can indicate a more serious issue. Even if you feel less tired after a dive on nitrox, keep in mind that fatigue alone can be a symptom of DCS, and significant fatigue may indicate a need to seek medical evaluation. If you or a buddy feel markedly tired, significantly ill or even just a little funny after a dive make sure to ignore the nitrox-versus-air debate and seek professional help — your choice of diving gas might make a great conversation starter, but it shouldn’t be a reason for symptoms that could potentially indicate a case of DCS. n

For more information on nitrox and diving, visit DAN.org/Health

In the event of a diving incident, we encourage all divers to call the DAN Hotline promptly for advice: • Within Australia: 1800 088 200 • Outside Australia: +1 919 684 9111 • Within Indonesia: 21 5085 8719

In truth, ancient wrecks are located in one of two ways. The first is by accident, which in my experience is the mostcommon method. A fisherman loses his nets on a piece of protruding timber, or a spearfisherman notices an ‘odd’ shape which turns out to be an anchor from a doomed vessel. The second is by thorough research and systematic searching. It is with the latter process that is the subject of this article. The three main problems that face those seeking of ancient vessels are: • The ‘general’ nature of the recorded location of lost ships. • Charts of the time when many vessels foundered were not the hydrographic masterpieces that we enjoy today. Many were, in fact, copies of other cartographer’s work, so there is plenty of room for confusion and incorrect information to slip in. • The depth of the remains. If your vessel is beyond the depth that divers can safely operate at, then you are limited in your approach to what we would call ‘electronic methods’ – some of which might be quite costly. There has been a trend, some might even say an obsession, in recent years to search for deep water sites. Both in terms of diving archaeology and for what we might call ‘prestige’ shipwrecks. The theory is sound. Wrecks that rest in deeper water are less likely to be broken up and therefore more ‘rewarding’ in terms of their preservation and the knowledge, or other values, that they may hold. However, of the three to four million shipwrecks estimated to lie beneath the waves, a large number lie in the shallows. Surely, they deserve some attention! The first key point in the process is to ensure that, whatever search method is employed, a record of the area being covered is made. Many searches omit to do so – which, of course, means that whole exercise has been a waste of time.

Before any type of search is undertaken, a number of factors need to be determined, the most important being: ‘What sort of site you are looking for?’ ‘What is known of the background information to the loss?’ ‘What do you know about the general locality?’ From these it is possible to design a search plan which will define the area to be searched, the width of coverage of the area to be investigated, the type and size of object being sought and the velocity of the search. These factors will indicate how long the search will take.

Let us first deal with what I am going to call ‘visual’ methods. In the main, these involve divers and low-cost equipment in relatively shallow waters but requiring reasonable visibility.

Most divers will be familiar with swim line, grid and circular searches from their training programmes. In the past, to speed things up, divers were pulled along on underwater sledges. The Oliver was located by such a method. These days underwater scooters are used. In shallow water there is no reason why an underwater GPS system could not be employed. When we move beyond the operational depths of divers towed cameras and remotely operated vehicles (ROVs) can be used.

Visual searches work best where you are covering a relatively small area. When you need to cover a large area, it is normal to use what we generally call electronic methods. One of the simplest forms of electronic search equipment is the echo sounder. In 1990, I worked with a team of divers which managed to relocate the wrecked super tanker the Amoco Cadiz using just such a tool. More advanced methods include the use of magnetometers and side-scan sonar. The Mary Rose was located by an early version of the latter. The problem with all these methods is that they have to be towed behind a boat, which makes your search accuracy dependant on surface conditions. So, what do you do when your search area is ‘difficult’ to deal with, due to either sea conditions or other factors? In the next article, we will look at exactly this question. www.wreckhunters.co.uk

As you all know, international COVID travel restrictions have impacted everyone. But the good news is the Wreck Hunters project can finally get underway for an extended season in 2022. This is a unique opportunity to take part in the beginning of a diving archaeology programme on the Caribbean island of Utila. The focus will be getting to the heart of the story of a wreck called ‘The Oliver’, its rich history and the life of 18th Century mariners. Learn all the skills of undersea archaeology, from traditional to cutting edge techniques. If you’re a relatively experienced diver with some skills or experience in drawing (artistic or technical), surveying, photography or in construction work this could be just what you’re looking for (See website for full details). Project Director Mike Haigh will be on hand to explain the details of the course at the Go Diving Show 2022, March 4-6 at the NAEC, Stoneleigh, Kenilworth. Find us near the entrance.

So, if you think this project would help put a smile on your face, why not get in touch to find out more at www.wreckhunters.co.uk