The starting point for working out gas requirements 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 own RMV, I suggest it is time to set up a simple experiment to calculate it. Next time you go diving pick a point in the dive when your depth is reasonably constant and record 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.
RMV will then be:
Cylinder Size (litres) * Cylinder Pressure Drop (bar)
Ambient Pressure (bar) * Time Interval (minutes)
If like me you record time using the second hand of your watch, you will need to convert minutes and seconds into a decimal value.
To put this into context, suppose a diver is at 30 metres and is breathing from a single 10-litre cylinder. Over a period of 2 minutes and 30 seconds gentle swimming cylinder pressure drops by 20 bar.
Ambient pressure is:
30 metres /10 + 1 = 4 bar
RMV is:
(10 litres * 20 bar)/(4 bar * 2.5 minutes) = 20 litres per minute
If you want to be more thorough, measure your RMV under different work loads, from hanging neutrally buoyant on a decompression stop to swimming as hard as you can against the current. You may be surprised at just how much it can vary.
Once you know your RMV, it is a simple matter 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 (in litres) = Ambient Pressure * RMV * Time
Suppose our example diver with an RMV of 20 litres per minute was to make a dive to 18 metres for 30 minutes.
Ambient pressure is:
18 metres /10 + 1 = 2.8 bar
Gas requirement is:
2.8 bar * 20 litres per minute * 30 minutes = 1680 litres
How big a cylinder would be needed for this dive? The total gas capacity of a cylinder is calculated by multiplying the cylinder size by the pressure it has been filled to. So if our diver is again using a 10-litre cylinder, with the usual 232 bar fill:
Gas available is:
10 litres * 232 bar = 2320 litres
On a first glance this seems adequate for the planned dive. The diver would only use 1680/2320 = 72% of the available gas, leaving 28% 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 a lower than allowed gas consumption during the descent at the start of the dive. After all, it takes a minute or so to descend to 18 metres. Maybe this is where so many out of air accidents happen. When swimming to descend divers can easily have a higher than average RMV. Similarly when ascending, if buoyancy control is less than perfect, a divers RMV may be higher than allowed. Personally I like to play it safe by allowing for the ascent time at the bottom pressure.
Continuing with the example 18-metre dive, this would be approximately another 2 minutes at 2.8 bar.
Ascent gas requirement is:
2.8 bar * 20 litres per minute * 2 minutes = 112 litres
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 hard. So hard that the buddy’s RMV shoots up to 50 litres per minute.
Ascent gas requirement is:
(2.8 bar * 20 litres per minute * 3 minutes) + (2.8 bar * 50 litres per minute * 3 minutes) = 588 litres
If we add this to the diver’s original gas requirement the overall gas used is now 1680 + 588 = 2268 litres or 98% of the diver’s 10 litre cylinder.
If this still sounds a safe margin then think again and be concerned. How many times have you started a dive with a cylinder that is a few bar down? If it were only pumped to 210 bar our divers would have run out before reaching the surface. If the bottom time were just a couple of minutes longer our divers would also have run out 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 18-metre dive. 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.