Scuba diving should be in no way competitive. If you are part of a group of divers, you want everyone to have a good time. That said, the human spirit is without doubt naturally competitive and we will find ways to compete, however irrelevant. You’ll often see divers post-dive comparing how much air they used. Somehow it’s often seen as clever to use less than the next person. I was on a dive trip once where my buddy used exactly half the gas I went through. Did this mean he was a better diver? Another friend of mine was supremely fit. When his family came to visit, his wife and daughter would drive over from their home ten miles away but he would always prefer to run. The London Marathon? He completed runs across the Sahara! However, when we went of a dive trip together I was amazed at the rate of knots with which he went through his air supply. I put it down to poor technique at the time. Then I went away diving with a young television Gladiator, Hunter.He was young and fit and made me look totally puny by comparison. It was lucky he could easily manhandle an 18-litre cylinder because that is what we had to get for him to allow him any time underwater on the shallowest and most benign of dives. On the other hand, Umberto Pelizzari, formerly the free-diving world champion, uses yoga to reduce his breathing requirements to negligible levels. One quickly comes to realize that women generally use less air than men on dives. Is it because they are serene and become part of the underwater environment unlike men who tend to be more active? Or is it because they are usually smaller than men and thus have smaller lung capacities? I’ve dived with elderly over-weight ladies who went through less air than me, and a lot of other people, so it’s probably a combination of both. Recently, I was diving with twin-cylinders on a deep dive in Truk Lagoon. A rather large lady of a certain age did the dives with a single 11-litre cylinder of air while her very muscular husband went in with her, armed with twin-cylinders (like me) plus an additional side-slung tank. The dives required considerable decompression stops. I asked if he carried the sling-tank for his lady wife but was told she didn’t need it. He did! What about my lightly breathing friend? Well, he could come back with half a tank of air when I was completely out but he couldn’t get through the night without waking up for a cigarette. He was a very heavy smoker. He also ended every dive with a humdinger of a headache. Am I saying that divers should take up smoking? Definitely not! New Zealander, Professor Simon Mitchell has investigated a subject that could be connected with the smoking phenomenon. He has studied carbon-dioxide retention in divers. Some people are able to retain a lot more carbon dioxide during the act of breathing than others and as you may know, it is the build-up of carbon dioxide that triggers our breathing reflex. It’s not the lack of oxygen but the raised levels of carbon dioxide that makes us have the desire to breathe. As I understand it, carbon-dioxide retainers breathe less. As a complete non-scientist, I wonder if heavy smokers with their constant lung exposure to carbon monoxide get a raised tolerance to carbon dioxide too, so that they can skip-breathe, leaving longer intervals between inhalations. Maybe their lung volumes have simply become very small due to the build-up of tar. Whatever the reason I’ve noticed they are more economic with their air supplies underwater. Cyclist Bradley Wiggins is supremely fit. After winning the exhausting 2172-mile Tour de France, a few weeks later he was competing in the London Olympics road race. How’s that for stamina? I read on the BBC website that he probably has huge left ventricles to his heart, which allows more oxygen-rich blood to be pumped to his muscles than we more ordinary folk. Chris Boardman, the Olympic Gold medalist in the Pursuit at the Barcelona Olympics in 1992, with whom I once dived a lot, had a resting heart-rate at his peak of around 38 bpm which indicates a large heart. I noticed he was an experienced diver who was able to be very relaxed on dives, with good technique and perfect neutral buoyancy, so that he didn’t rush through his air. A person’s endurance can be measured by taking a reading, which indicates how much oxygen is used every minute for each kilogram of body weight. It’s called a VO2 Max reading. A typical young male might have a VO2 Max of around 40 whereas a top cyclist might have a value of twice that, so that not only are they able to use more oxygen, they can also keep their muscles active for longer periods of time. Of course, cyclists have a never-ending air supply whereas for a diver it’s totally finite. I suggest it’s not clever to use less air than anyone else on a dive, although it may under certain circumstances be fortunate. I suggest it’s more intelligent to take sufficient gas with you for the dive in question and to manage that supply in such as way that you get back safely. Unlike running races and competitive cycling, it’s not a competition. A gas-integrated diving computer will help you manage your air supplies.
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!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. 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 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. 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..
The big question that every new diver wants answered is about how long the air in the tank will last. It all depends on how much there is to begin with, how deep it is going to be breathed at, and how much the diver is going to breathe it.The first two parts to this are easily identified. You can read on the shoulder of the tank its fixed volume and you can read from the pressure gauge how much it has been filled. Multiply one by the other. A 10-litre tank filled to 200 bar has 2000 litres of air in it. A 15-litre cylinder filled to 230 bar has 3450 litres of air. It is best to set aside a reserve of air and conventional thinking suggests that quarter of the initial supply is kept aside. This may be over cautious with a large tank but you have to make a judgement based on the circumstances you expect to encounter. Let’s assume that our 10-litre tank has only 1500 litres of air at our disposal with the rest (50 bar) is held in reserve. The next thing to identify is the depth the air is going to be breathed at. At 30m deep, the regulator delivers air at four times the pressure as it would at the surface. Thus if we are diving at this depth, we have only 375 litres of air compressed to four bar to breathe. The final part of the equation has a big question mark hanging over it. How much air do you need? A woman with small lungs will probably breathe a lot less than a male heavy-weight boxer. A man with large lungs will have the ability to pump a huge amount of air through his lungs. A relaxed man might breathe only eight litre every minute but increase his heart-rate by increasing his work load or stress him in some way, and this can easily leap to 30 litres each minute. It has not too much to do with fitness either. An old diver who has smoked all his life may not be very fit at all but if he is relaxed, and often that comes with experience, he will use less gas than a young trained athlete who is working harder than he should underwater. Even thinking hard uses a lot of energy. If you have to swim hard you will consume more than if you are merely hovering in the water. So what figure should we use for a breathing-rate in this calculation? Training agency manuals usually use a figure of 25 litres per minute in their examples of how to calculate air consumption. At 30m, our 10-litre tank (with 50 bar held in reserve) would last only 15 minutes. So what does it feel like to breathe from a regulator underwater? You can easily try this for yourself because it feels exactly the same as it does if you try it on land in a dive shop. You suck, there is a faint resistance and the valve pulls open. The mouthpiece floods with air that you inhale. It stops when you stop. When you exhale, there is also a slight resistance as the exhaust valve opens and allows that air to escape. It bubbles away past your head. Remember, the amount of air in your tank is divided by your respiratory mean volume or breathing-rate multiplied by the absolute pressure in bars at the depth at which it is breathed. If you can get your head around that you are well on your way to understanding your air consumption.