OK, so you can see all those calculations looming ahead, but relax - John Liddiard is here to walk you through the process of figuring out your gas requirements, whether you're off for a dive to 18m or 50m. And you might well find that you need more gas than you think

BILL THE PHILOSOPHER HAD A BIT OF A HEAVY BREATHING PROBLEM. No matter how shallow the dive, no matter how large the cylinder, he could suck it down to nothing in 15 minutes flat.
     It was while considering this problem, assisted by several pints of decompression fluid, that he offered one of those profound philosophical statements that in biblical times would have placed him alongside the famous Greeks: "I suppose running out of air is like premature ejaculation - one of the partners remains unsatisfied".
     I was reminded of this gem of modern philosophy on a trip to the Diving Diseases Research Centre a while back. I had asked about causes of diving accidents and was told that running out of decompression gas and subsequently missing stops was one of the most common technical-diving problems.
     It is not just tekkies who make this mistake. Judging from sport-diving accident reports, poor gas-planning seems to be prevalent at all levels of experience, from newly qualified open-water divers upwards.
     We all learn how to plan our gas consumption during training. But let's be honest, how many of us really plan our gas requirements to the same extent as we plan for tides or decompression schedules?
     I know that for the average dive, I don't. I depend on past experience, then just look at my computer and cylinder pressure regularly and let that tell me when it's time to come up. I also normally dive with either a pony cylinder or twin-set. However, for any serious dive, and certainly for technical dives, I calculate my gas requirements fully in advance.
     The starting point for doing this is to know your Respiratory Minute Volume (RMV). RMV is the number of litres of gas you breathe per minute at normal atmospheric pressure (1 bar). For most experienced divers under a light workload such as gentle finning, this will be between 20 and 25 litres per minute.
     If you don't know your RMV, it's time to set up a simple experiment to calculate it. Next time you go diving, choose a point in the dive when your depth is reasonably constant and record both cylinder pressure and time while swimming along. I prefer to do this by using the second hand of my watch to time how long it takes for cylinder pressure to drop by 10 or 20 bar.
     Your measured RMV will be:

    Cylinder size (litres) x Cylinder pressure drop (bar)
    Ambient pressure (bar) x Time interval (minutes)

    If you use the second hand of your watch, you will need to convert minutes and seconds into a decimal value. Ambient pressure is calculated as Depth (m)/10 + 1 bar.
    To put this into context, suppose a diver is at 30m and breathing from a single 10 litre cylinder.
     Over a period of 2 minutes 30 seconds' gentle swimming, cylinder pressure drops by 20 bar. Ambient pressure is (30m/10) +1 = 4 bar.
    RMV is (10 l x 20 bar)/(4 bar x 2.5min) = 20 l/min
     If you want to be more thorough, measure your RMV under different workloads, from hanging neutrally buoyant on a decompression stop to swimming as hard as you can against the current. You might be surprised at how much it can vary.
     Once you know your RMV, it's simple to work out the air requirements for a dive. At a specified depth, multiplying RMV by the ambient pressure and the time at the depth will give the volume of gas required.
     Gas requirement is: Ambient Pressure x RMV x Time
     Suppose our diver with an RMV of 20 litres per minute is diving to 18m for 30min.
     Ambient pressure is: (18m /10) + 1 = 2.8 bar
     Gas requirement is: 2.8 bar x 20 l/min x 30min = 1680 l
     How big a cylinder would be needed for this dive? The total gas capacity of a cylinder is calculated by multiplying its size by the pressure to which it has been filled. If our diver is using a 10 litre cylinder with the usual 232 bar, the total gas available is:
     10 l x 232 bar = 2320 l
     At first glance this seems adequate for the planned dive. The diver would use only 1680/2320 l = 72 per cent of the available gas, leaving 28 per cent in reserve.
     But if the diver left the bottom at 30 minutes, additional gas would be needed to reach the surface.
     It is sometimes argued that this gas does not need to be accounted for, because it is cancelled out by lower-than-allowed-for gas consumption during the initial descent. Perhaps this is where so many out-of-air accidents happen. After all, it takes a minute or so to descend to 18m, and divers can easily have a higher-than-average RMV during this part of the dive.
     Similarly, if buoyancy control is less than perfect, a diver's RMV can be higher than allowed on the ascent. I play it safe by allowing for the ascent time at the bottom pressure. Continuing with our 18m dive, this would add another 2min at 2.8 bar.
     Ascent gas requirement is 2.8 bar x 20 l/min x 2min = 112 l
     Now, suppose it is at the start of the ascent that the diver's buddy runs out of gas or suffers a regulator failure. It takes a minute or so to sort out an octopus regulator and begin an ascent which takes 2 minutes.
     The buddy is panicked, and breathing so hard that his RMV shoots up to 50 l/min.
     Ascent gas requirement is (2.8 bar x 20 l/min x 3min) +
     (2.8 bar x 40 l/min x 3min) = 588 l
     If we add this to our diver's original gas requirement, the overall gas used is now 1680 + 588 = 2268 l - or 98 per cent of that 10 litre cylinder.
     If this still sounds a safe margin, think again and be concerned. How many times have you started a dive with a cylinder that's a few bar down? If it were pumped to only 210 bar, our diver would have run out before reaching the surface.
     If the dive were just a couple of minutes longer on the bottom, our diver would not have been able to rescue his buddy without running out of air before reaching the surface.
     Shaving reserve gas margins places a diver at the top of the incident pit before anything else has happened. Most of the time all will be well, but when something else goes wrong, the consequences could easily be much more serious than they would be with adequate reserves.
     And remember, this is just an 18m dive. So it's little surprise that divers making deeper dives, decompression dives and technical dives carry pony cylinders or twin-sets with independent regulators. They also plan for much greater reserves of gas.
     For the next example I have selected a far more serious dive, on the basis that most dives are somewhere between my two examples and anyone who follows this refresher will be able to apply the principles to such dives.
     The plan is for a dive to 50m for 30 minutes. The bottom gas will be air and decompression will be on 80 per cent oxygen. Equipment will be a twin-set with independent regulators and a side-mounted cylinder for decompression.
     An IANTD Buehlmann table with decompression using at least 75 per cent oxygen and shallowest stop at 4.5m was used for the dive plan.
     An RMV of 20 l/min was used for the bottom gas, reducing to 15 l/min for the 80 per cent deco gas during the relaxed shallower deco stops. Results have been tabulated rather than showing all the individual calculations (Table A).

     Buehlmann tables assume that ascent time is included in the deco stop. Users of Buehlmann-based dive computers will have seen this happening as their deeper stops disappear from the display before they get to them.
     In practice, most users of tables prefer the additional margin gained by omitting this feature of the algorithm. The switch to 80 per cent at 9m is not required by the decompression schedule, but is included for added safety.
     The general rule for such extended-range dives is the "rule-of -thirds" for the bottom gas. This comes from cave-diving, where gas has been traditionally planned as 1/3 in and 1/3 out with 1/3 in reserve.
     This makes a good starting point, although it's always worth making a risk analysis to check its validity for any particular dive. Taking the plan in (Table A).
, 4430 litres of air are required for the dive. If this is to be 2/3 of the air carried, 3/2 x 4430 = 6645 litres. Suitable equipment would therefore be either twin 15 litre 232 bar cylinders (2 x 15 x 232 = 6960 l) or twin 12 litre 300 bar cylinders (2 x 12 x 300 = 7200 l).
     For decompression gas, the general rule is 50 per cent plus 15 bar. This is based on a worst case of the diver's buddy losing his decompression gas so that both divers end up decompressing on the same cylinder, and leaves a 15 bar margin for gauge error and interstage pressure.
     The easiest way to work this out is to double the anticipated deco gas usage and deduct 15 bar from the cylinder pressure before calculating anything.
     As pure oxygen is available only at 200 bar, filling a 300 bar cylinder with an 80 per cent nitrox mix is not practical. The decompression cylinder is limited to 232 bar. So the minimum size of decompression cylinder required for this dive would be:
     781 l x 2 /(232 l - 15 l) = 7.2 l
     A 7 litre cylinder would not provide quite enough deco gas should both divers have to decompress off the same cylinder.
     Options available would be to carry a 10 litre decompression cylinder, a second 3 litre decompression cylinder or two 5 litre decompression cylinders; make the 9m stop on air; or to have a contingency plan for decompressing on air.
     The basic gas-requirement plan is now complete (though a real plan would also take into account oxygen toxicity).
     However, it's prudent to take into account a few contingencies. What happens if bottom time is extended a few minutes longer than planned, if perhaps a delayed SMB jams and has to be sorted out? ( Table B).

     What happens if the dive is a few metres deeper than planned? (Table C).
And what happens if decompression has to be made on air? (Table D).
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     Having calculated the gas consumption for these contingencies, it can be compared against the gas available.
     In all cases the 6960 litres of air carried in twin 15 litre cylinders is sufficient to complete the dive.
     With this in mind, it's reasonable to cut the margin on the decompression gas to use a 7 litre cylinder, as both the diver and buddy will have enough air to complete the decompression without the 80 per cent oxygen mix, should that be necessary. A more contentious point is contingency 2 - straying deeper than planned. It can be argued that in this case a diver should either abort or plan 53m on a rule-of-thirds gas consumption in the first place.
     The bottom line is that when dive plans get down to this level of detail, there is no single correct solution.
     Much of the plan is based on a personal judgment of likely scenarios and the level of acceptable risk.
     Whether you agree with the assessment of risks and contingencies I have made or not, my aim is to emphasise that by calculating gas requirements, a diver can at least make decisions from a position of knowledge - rather than trusting to luck.

WHY DO I BREATHE SO HEAVILY? Factors governing a diver's air consumption fall into three categories: Physical: Basic diver training teaches us Boyle's law, that the volume of a gas is inversely proportional to pressure. The deeper we go, the more gas each breath will take from a diving cylinder. Physiological: If we swim hard or work under water, we breathe more, because our bodies metabolise food and oxygen to produce energy. The harder we work, the more energy we need, the more oxygen we need to provide that energy, and the more air we need to breathe. We also require energy to keep our bodies warm, so a cold diver will breathe harder. Psychological: Worried or nervous divers get through their air more rapidly because they breathe faster than necessary.

HOW TO LOWER YOUR RMV Whatever you do, don't try to save air by breath-holding or deliberately breathing shallowly. At best you will surface with a headache, but you could end up with a burst lung or worse. The way to use less air is to tackle the problem, not its symptoms. Buoyancy control: This might seem like a topic for novices, but the buoyancy control of many "experienced" divers is less than perfect. A buoyant diver will be swimming down all the time. A heavy diver will have to swim upwards or bounce along the bottom, reducing the visibility for everyone else or damaging the marine life. Maintaining neutral buoyancy throughout a dive avoids unnecessary effort, improves comfort, and reduces RMV. Over-weighting: An overweighted diver will have to put extra air into a BC or drysuit to maintain neutral buoyancy. By itself, this is insignificant compared to the amount of gas a diver breathes, but it will have a bad effect on your position in the water, tending to turn you upright. Swimming requires greater effort due to the unnatural angle in the water and increased water resistance, with a consequent increase in RMV. Finning: Poor finning technique such as a bicycle kick wastes effort and increases RMV. It can be difficult to spot problems in your own finning technique, so ask your buddy to keep an eye on it and give you some feedback; you might be surprised at the results. Efficient finning uses strong, gentle strokes of the whole leg, taking each kick through to completion. Equipment: Think about the gear you carry and how it is adjusted. A poorly adjusted regulator with a high breathing resistance will require extra effort just to breathe, again increasing RMV. A loose weightbelt will often rotate round a diver's waist, pulling you off balance, making the dive uncomfortable and causing you to expend effort to compensate. It can also slip downwards, pulling you upright and making swimming more difficult. In both cases the consequence will be increased RMV, not just from the effort, but because a diver will feel generally awkward and uncomfortable. Any loose, badly sized or poorly adjusted equipment can have a similar effect, pulling a diver off balance or making control difficult. And having the right diving suit for the water temperature will also help to reduce RMV, because a cold diver breathes more. Overall, be comfortable and happy in the water. Become familiar with your equipment. Learn to move gracefully and economically, with good buoyancy control and finning technique. Once these factors are under control your RMV will take care of itself. A low RMV is a Zen thing.