For my next example I have selected a much 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 50 metres for 30 minutes. The bottom gas will be air and decompression will be on 80% oxygen. Equipment will be a twin set with independent regulators and a side mounted cylinder for decompression.
A Buehlmann table with decompression using at least 75% oxygen and shallowest stop at 4.5 metres was used for the dive plan. An RMV of 20 litres per minute was used for the bottom gas (air), reducing to 15 litres per minute for the 80% deco gas during the relaxed shallower deco stops. Results have been tabulated rather than showing all the individual calculations.
| Depth | Time | Ambient Pressure |
Gas | Gas Used |
| 50m | 30 mins | 6 bar | Air | 3600 litres |
| Ascend 50m to 18m | 3 mins | 6 bar | Air | 360 litres |
| 18m | 1 mins | 2.8 bar | Air | 56 litres |
| 15m | 3 mins | 2.5 bar | Air | 150 litres |
| 12m | 6 mins | 2.2 bar | Air | 264 litres |
| Total Air (RMV 20 litres per minute) | 4430 litres | |||
| 9m | 8 mins | 1.9 bar | 80% | 228 litres |
| 6m | 4 mins | 1.6 bar | 80% | 96 litres |
| 4.5m | 21 mins | 1.45 bar | 80% | 457 litres |
| Total 80% (RMV 15 litres per minute) | 781 litres | |||
Buehlmann tables actually assume that ascent time is included in the decompression 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% at 9 metres is not required by the decompression schedule, but is included for added safety.
The general rule for such extended range dives is “rule of thirds” for the bottom gas. The rule of thirds comes from cave diving, where gas has been traditionally planned as 1/3 in, 1/3 out, and 1/3 reserve. Whilst this general rule is a good starting point, it is always worth making a risk analysis to check its validity for any particular dive.
Taking the above plan, 4430 litres of air are required for the dive. If this is to be 2/3 of the air carried, then 3/2*4430 = 6645 litres of air needs to be carried. Suitable equipment would therefore be either twin 15 litre 232 bar cylinders (2 * 15 * 232 = 6960 litres).
For decompression gas the general rule is 50% plus 15 bar. This is based on a worst case of the diver’s buddy having loosing his decompression gas so the diver and buddy 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 multiply the anticipated decompression gas usage by 2 and deduct 15 bar from the cylinder pressure before calculating anything, so the minimum size of decompression cylinder required for this dive would be:
781 litres * 2 / (232 litres - 15 litres) = 7.2 litres
A 7-litre cylinder would not quite be enough deco gas for the scenario of the diver and buddy having to decompress of the same cylinder. Options available would be to carry a 10-litre decompression cylinder, carry a second 3-litre decompression cylinder, carry 2 * 5 litre decompression cylinders, make the 9 metre 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, rather than stop at this it is prudent to take into account a few contingencies.
What happens if the bottom time is extended a few minutes longer than planned? (Perhaps a delayed SMB jams and has to be sorted out). What happens if the dive is a few metres deeper than planned? And relating to the comments on decompression gas, what happens if decompression has to be made on air?
| Depth | Time | Ambient Pressure |
Gas | Gas Used |
| 50m | 35 mins | 6 bar | Air | 4200 litres |
| Ascend 50m to 18m | 3 mins | 6 bar | Air | 360 litres |
| 18m | 2 mins | 2.8 bar | Air | 112 litres |
| 15m | 5 mins | 2.5 bar | Air | 250 litres |
| 12m | 7 mins | 2.2 bar | Air | 308 litres |
| Total Air (RMV 20 litres per minute) | 5230 litres | |||
| 9m | 11 mins | 1.9 bar | 80% | 314 litres |
| 6m | 5 mins | 1.6 bar | 80% | 120 litres |
| 4.5m | 26 mins | 1.45 bar | 80% | 566 litres |
| Total 80% (RMV 15 litres per minute) | 999 litres | |||
| Depth | Time | Ambient Pressure |
Gas | Gas Used |
| 53m | 30 mins | 6 bar | Air | 3780 litres |
| Ascend 53m to 18m | 3 mins | 6 bar | Air | 378 litres |
| 18m | 2 mins | 2.8 bar | Air | 112 litres |
| 15m | 4 mins | 2.5 bar | Air | 200 litres |
| 12m | 7 mins | 2.2 bar | Air | 308 litres |
| Total Air (RMV 20 litres per minute) | 4778 litres | |||
| 9m | 9 mins | 1.9 bar | 80% | 257 litres |
| 6m | 4 mins | 1.6 bar | 80% | 96 litres |
| 4.5m | 24 mins | 1.45 bar | 80% | 522 litres |
| Total 80% (RMV 15 litres per minute) | 875 litres | |||
| Depth | Time | Ambient Pressure |
Gas | Gas Used |
| 50m | 30 mins | 6 bar | Air | 3600 litres |
| Ascend 50m to 18m | 3 mins | 6 bar | Air | 360 litres |
| 18m | 1 min | 2.8 bar | Air | 56 litres |
| 15m | 3 mins | 2.5 bar | Air | 150 litres |
| 12m | 6 mins | 2.2 bar | Air | 264 litres |
| Total Air (RMV 20 litres per minute) | 4430 litres | |||
| 9m | 8 mins | 1.9 bar | Air | 228 litres |
| 6m | 6 mins | 1.6 bar | Air | 144 litres |
| 4.5m | 43 mins | 1.45 bar | Air | 935 litres |
| Total Air (RMV 15 litres per minute) | 1307 litres | |||
| Total Air (bottom and deco stops) | 5737 litres | |||
Having calculated the gas consumption for these three 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 is 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 schedule without the 80% decompression 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 53 metres on a “rule of thirds” gas consumption in the first place. The bottom line is that when dive plans get to this level of detail, there is no single correct solution. Much of the plan is a personal judgement 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 real point is to emphasise that at least by calculating gas requirements a diver is in a position to make decisions from a position of knowledge rather than trusting to luck.