Nitrox

  • An Intro to Diving Computers

    When we go under pressure, our bodies start to absorb the inert part of the air we breathe, the nitrogen. At normal atmospheric pressure we are saturated with nitrogen but by going underwater breathing compressed air, we allow our bodies to soak up more.

    Provided we stay no longer at depth than a slow ascent to the surface can give time for our bodies to off gas, we experience no problems. If we exceed these ‘no-stop’ times, we need to make stops at points during the ascent to allow our bodies to ‘catch up’ with this process.

    An advanced computer watch A typical computer watch

    All certified divers should know this and nowadays most wear a computer to monitor the potential state of decompression during and after each dive. I say ‘most’ because I have recently heard of cases of individual divers who eschew a computer saying ‘they know how deep and for how long they can stay’. This is very dangerous thinking.

    Of course, many years ago when computers were in their infancy, conservative divers refused to use them, believing a watch and depth-gauge combined with a decompression table was safer. They might have been safe if their watches and depth-gauges were accurate and they were disciplined in their use.

    One way to make leisure diving ‘safer’ in this regard is to breathe a gas with less nitrogen in the mix – nitrox – but it is only safer if you don’t take advantage of the longer no-stop times available. Stay longer and you still soak up just as much nitrogen as you would breathing air for the shorter no-stop time mandated.

    Computer manufacturers try to make things as safe as possible to keep them away from possible litigation. That’s why they build in a few precautions that sometimes casual users fail to comprehend.

    suuntodx_elastomerIf you dive with your computer set in ‘air’ mode, it will not allow you to switch to another mode such as ‘nitrox’ until a sufficient period has passed – usually twenty-four hours. If you want to switch between air and nitrox, it’s important to start off in nitrox mode, setting air as nitrox 21 (which it is).

    For the same reason, if you want to use your computer simply in ‘gauge’ mode, reading only depth and time, it will not be able to calculate your residual nitrogen levels should you wish to then switch to a nitrox or air diving mode, so it will lock you out for a period, up to forty-eight hours if you have been diving deep.

    Diving computers have a sampling rate typically of every 10, 20 or 40 seconds. Normally the 20-second setting is the default setting. During a leisurely dive this is entirely practical but it is not suitable for free-diving.

    Some computers have a mode specifically for free-diving when the sampling rate is much more often, even every second. This is because if you swim down to, say, 20-metres deep, a less frequent sampling rate might make a sample point at ten-metres on the way down and the next at 15-metres on the way up, totally missing the fact that you went to 20-metres in between. So gauge mode is unsuitable for use by free divers. You may need a computer with a ‘free-diving’ mode. Choose a computer that has the modes you require.

    Some of our customers tell us they want to free-dive between scuba dives. Current medical thinking believes this to add a degree of hazard to the activities because the scuba diver’s body will still be loaded with residual nitrogen at this time and that will be recompressed during a breath-hold dive. No computer can calculate for these short bounces while in diving mode because of the aforementioned sampling rates. For this reason, no computer should allow you to switch to free-diving mode while it is still calculating nitrogen levels during a surface interval.

    Some foolish divers will leave their computer to ‘off-gas’ at the surface while they go for a swim, ever tempted to duck-dive below the surface. That is a silly as leaving a computer tied off to a rope at the last decompression stop to ‘offgas’ while the diver climbs back on board. We positively do not recommend this. Nor do we suggest you buy a second instrument and switch between them during a day’s diving. That is the road to decompression illness.

    A diving computer can only monitor the nitrogen loading of your body if it is attached to you while you on-gas and off-gas. Use it properly and it will keep you safe – although, since everyone is physiologically different and the computer’s algorithm was written for a theoretically typical person, no computer manufacturer can guarantee this.

    Always read the instruction manual and be familiar with what you computer displays. Too often people go into decompression status during dives, especially where the water is warm and clear, and fail to understand that this is what their computer is telling them.

    A range of different computer displays at depth A range of different computer displays at depth
  • Nitrox - All You Needed To Know

    Know what you breathe before you take that plunge! Know what you breathe before you take that plunge!
    What is Nitrox? Air is mainly made up of two gases – 21% oxygen and 78% nitrogen (1% other gases). We metabolise some oxygen in the air we breathe but the greater part, the nitrogen, is inert. When we put ourselves under pressure, as we do when we go under water, our bodies absorb some of this inert nitrogen. As we go deeper and stay longer we absorb more. The time that we spend underwater is limited by the amount of that nitrogen we absorb. That is why we used to use tables or, more recently, a diving-computer. So why not breathe an air that has less inert nitrogen gas and so reduce the problem? If you’ve been breathing air, you’ve already been breathing nitrox – nitrox21. Other nitrox mixes have the percentage of oxygen increased and therefore the percentage of nitrogen is decreased. Nitrox 32 has 32% oxygen and nitrox36 has 36% oxygen.Nitrox Diving Breathing a richer nitrox mix instead of plain old air reduces the chance of decompression illness due to a diver staying down too long or coming up too quickly - providing no-stop times and ascent-rates for air are adhered to. If you want to continue with conventional levels of caution you can simply adjust your decompression requirement by adjusting your computer to match the nitrox mix you use and in that way get more time underwater.Scuba Jump However oxygen too has its problems too. Pure oxygen becomes poisonous at quite low pressures. It is currently thought unsafe to breathe pure oxygen at a greater pressure than 1.6 bars underwater and that occurs at only 6m deep. Therefore each specific nitrox mix has its own maximum operating depth and nitrox training agencies are unanimous in limiting the use of oxygen to 1.4bars of partial pressure within a mix with nitrogen._DSC0019 What you need to know is that the oxygen in air (nitrox21) can become hazardous at 54m deep. That does not affect leisure divers limited to an absolute maximum depth of 40m. A standard mix of nitrox32 should not be breathed deeper than 32m. Some training agencies tells new divers that this limit is 30m. Most popular sites for diving in the world now adhere to a 30m limit for leisure diving anyway. PADI Open Water Divers with Level One training are still limited to a maximum depth of 18m during training as before but suitably qualified divers can use nitrox32 to its full maximum operating depth (MOD).
    No additional equipment is need in the water. No additional equipment is need in the water.
    So for nitrox mixes up to 40% oxygen, no additional diving equipment is needed, only the knowledge of how to analyse the contents of a tank before diving, using the analyser supplied by the dive-centre and knowing how to set your computer to match. The day is foreseen when all new divers will start off breathing nitrox and air for diving will only be for specialised uses. More advanced divers that have been deeper than normal leisure diving depths use nitrox to speed up their decompression.
    Using a richer Nitrox in an additional tank to speed up decompression after a deep dive. Using a richer Nitrox in an additional tank to speed up decompression after a deep dive.
    They take additional tanks of rich nitrox with them and swap to these once they have ascended shallow enough for it to be safe do so. There are more advanced diving computers that allow you to set different levels of nitrox and to switch to the one that matches the actual mix the diver is currently breathing, and in that way track both decompression requirements and oxygen exposure accurately. All the diving computers sold at Ocean Leisure are nitrox compatible. If you want to know how to take high-speed sequences of pictures or successfully take photographs underwater, ask the guys at Ocean Leisure Cameras.

  • With Respect to my Bête Noire.

    I have been diving for a very long time indeed. Many years ago, before the term ‘Technical Diving’ was invented, I made a friend of cave diver Rob Palmer and we went diving together. He had some ideas on how diving could be made more adventurous without making it any more hazardous. We used ideas first put forward by Dick Rutkowski of NOAA. This included mixing extra oxygen with our air and even adding helium to it when we thought it gave us an advantage. Of course at that time we had to keep what might have been considered witchcraft strictly secret. The training agencies of the day would have pilloried us. Today those mixtures are called 'Nitrox' and 'Trimix'. We often took a pair of cylinders with us (a twin-set) filled with air but side-slung an extra tank that was as much as fifty percent oxygen so that we could cut short the otherwise onerous decompression-stop times that we needed thanks to the extra depth we went to. Sometimes, toting a massive camera, I preferred to avoid the extra encumbrance of the sling tank and made do with one cylinder of air alongside one of what we know today as a Nitrox mix of, say, thirty-six percent oxygen, both on my back. Rob introduced me to a giant of the diving world, Bret Gilliam. An American, Bret had worked for years as a US Navy diver, photographing nuclear submarines at depth when the only gas available was air. He held a record for the deepest dive on air at 150-metres deep and was and to this day remains one of the most competent divers I have ever known. Rob died later in an unfortunate diving accident (not related to the subject of this story) but Bret and I are close friends to this day, even if he does insist on calling me Mick Fleetwood!

    DSCN3899 Independent twin cylinders.
    Bret was a technical diving pioneer with IANTD and then broke away from it to start TDI with Rob Palmer. Together, they wrote the first books on technical diving. If I am away in a comfortable diving destination such as Bikini Atoll or Truk Lagoon and want to dive a wreck that is just a little deeper than is suitable to use nitrox32 for, I still tend to dive with air and take a second cylinder with a richer Nitrox mix so that, in conjunction with a Nitrox gas-switching computer, I can reduce the mandated decompression-stop time I might otherwise need to endure if I only used air. This is where one gentleman in particular and a few other divers come into the story. My application of a simple rig of two independent cylinders on my back, swapping regulators when I got shallow enough not to exceed the maximum operating depth of the Nitrox, upsets some people because it was never written up in a textbook. Rob Palmer might well have done that had he lived and Bret has no problem with me using it. (I hasten to add it would be foolhardy to do this with a manifolded twin-set.) In some circles it has become known as ‘the Bantin rig’ and not always with polite intentions. My critics questioned what I would do if my air regulator stopped working (an unlikely occurrence) beyond the MOD of the Nitrox in the other tank, to which I answer that I would put the Nitrox regulator in my mouth and breath off that, making my way at a safe ascent rate up to where there would be no danger of oxygen toxicity. Oxygen toxicity is subject to total exposure that is both a combination of time and percentage. I have needed to rescue divers from more than fifty-metres deep more often than I would like whilst equipped only with a single tank of nitrox32 and although I wouldn’t ever recommend it, I am here to tell the tale.
    _EBC4349 Japanese tank on the deck of the San Francisco Maru.
    My critics seem to think it is OK to take a tank of Nitrox side-slung but not conveniently on my back. I hasten to point out that I am often doing these dives alongside those equipped only with a single tank. They wonder if I might put the ‘wrong’ regulator in my mouth by mistake. Well, anyone who does that probably should not be underwater in the first place. I go in with one regulator in my mouth and the other one is for the ascent. There is no confusion. A number of my friends including my wife have dived this way when it’s a dive that is suitable for this approach. The wreck of the San Francisco Maru in Truk
    Staff car deep in the bowels of the San Francisco Maru Staff car deep in the bowels of the San Francisco Maru
    and the wrecks in Bikini Atoll are good examples. It’s a way of having plenty of gas for what otherwise might be a deeper than usual dive and at the same time cutting long waits on the decompression bar.
    Truk4389 My wife reading a book whilst enjoying an accelerated decompression stop with the right (red hose) regulator in her mouth.
    That one particular gentleman decided to become my bête noire on the Internet during the time I wrote regularly for Diver Magazine. It is ironic that he actually worked for Ocean Leisure in those days. Although I’m very happy to take four or more tanks with me if the dive requires it, as Bret Gilliam says, “If it works for you, that’s what matters.” Choose an appropriate solution and dive safely within your abilities..    

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