A gas plan answers 3 questions before the dive: how much gas the dive uses, how much you keep back for an emergency, and at what gauge reading you stop going further and start the way back.
During the dive you compare your gauge with the plan. The numbers you plan with are the reserve, the turn pressure and the minimum gas at each waypoint.
- Profile (air)
- Minimum gas
- Turn pressure
Gas use over a dive
The figure shows a dive planned by the DiveLogic engine: 10 minutes at 30 m, then 8 minutes at 21 m and 6 minutes at 15 m on the way back. The diver breathes air from a single 12 L cylinder filled to 232 bar.
The bar under the profile is the cylinder gauge. It falls throughout the dive, faster at some points than at others.
Consumption and depth
Your RMV (respiratory minute volume) is the volume of gas you breathe in a minute, measured at the surface. A relaxed diver typically breathes 12-20 L/min. Some divers call this SAC, and some use SAC for the same rate in bar per minute on a particular cylinder. DiveLogic uses litres per minute.
At depth each breath holds the same volume of gas at a higher pressure, so it takes more gas from the cylinder (see the gas laws). At 30 m the pressure is about 4 bar, so an RMV of 15 L/min uses about 60 L of surface gas every minute. At 15 m it is about 2.5 bar and 38 L/min.
Reserve and minimum gas
The reserve is gas you do not plan to breathe. It is there for a problem, most often a buddy who is out of gas.
Minimum gas, also called rock bottom, sizes the reserve from that problem. It is the gas 2 divers need to reach the surface from the deepest point of the dive, both breathing from one cylinder at a stressed rate. DiveLogic’s minimum gas tool, used here, allows 1 minute at depth to sort out the problem, an ascent at 9 m/min and a 3-minute stop, with both divers at twice their normal RMV. The engine gives the result rounded up to the next 5 bar.
Turn pressure
On a dive where you return the way you came, the turn pressure is the gauge reading at which you end the outbound leg. From then on you are on your way back.
A common convention is the rule of thirds: use a third of the gas going out, a third coming back, and keep a third as the reserve. DiveLogic generalises this for any reserve. It takes the reserve off the start pressure and allows half of what remains for the outbound leg. You turn at the end of the planned leg, or when you have used that half, whichever comes first. The turn pressure is rounded up to the whole bar, so the turn never comes later than the rule allows.
Turning early
If the outbound leg uses more gas than planned, for example because you swim against a current, the gauge reaches the turn pressure before the planned time. You turn then, not at the planned time.
The figure shows the same dive at an RMV of 25 L/min. The planned leg would use more than half of the gas above the reserve, so the engine limits the turn by gas instead of by the plan.
When the return costs more than the way out
The equal split assumes the return uses as much gas as the outbound leg. That is not always true.
- A current. If the current carries you out, you swim back against it, and the return uses more gas than the outbound leg. Plan to turn earlier, or with a larger reserve. If you swim out against the current, the return is easier.
- Penetration. In a cave or inside a wreck you cannot ascend directly: the reserve has to cover the whole exit, not only an ascent. The exit can also take longer than the way in if the visibility drops or a diver has a problem far from the entrance. The rule of thirds comes from cave diving, and many overhead teams keep more than a third in reserve.
- OK
- Behind plan
- End dive (below minimum)
Waypoints
A waypoint is a planned point on the dive, for example each level change and the start of the ascent.
For each waypoint the plan gives a minimum gas figure: the gas to finish the dive from that point, plus the reserve. If you have at least that much when you reach the waypoint, you can finish the dive with the reserve intact. The figure shows this dive’s 3 waypoints with their minimum gas in bar, rounded up.
Checking the gauge
Write the waypoints on your slate or wrist notes. At each one, read your gauge and compare it with the minimum.
The plan also gives the reading you should expect at each waypoint. On this dive, at the planned RMV of 15 L/min, every expected reading is above its minimum.
Below the minimum
If you breathe harder than planned, the gauge falls faster. Here the same dive is breathed at 20 L/min.
At waypoint 1 the gauge is behind the plan but still above the minimum, so the dive can continue with closer checks. At waypoint 2 it is below the minimum. The remaining dive can no longer be finished with the reserve intact, so the diver ends the dive at that point and ascends.
Try it
Set the cylinder, the start pressure, your RMV, and the depth and time of the outbound leg. The engine gives the turn pressure and the minimum gas for a pair ascending from that depth. Choose the reserve: minimum gas, or a third of the start pressure. Deeper legs and higher RMVs increase both the gas used on the way out and the minimum gas, and a small cylinder can leave no gas for the dive at all.
- Outbound
- Return
- Reserve
What to remember
- Gas use rises with depth: at 30 m each minute uses about 4 times the gas of a minute at the surface.
- Minimum gas is the reserve 2 divers need to reach the surface from the deepest point, sharing one cylinder at a stressed rate.
- The turn pressure keeps an equal return plus the reserve. DiveLogic turns at the end of the planned leg or after half of the gas above the reserve, whichever comes first. With thirds that half is a third of the start pressure.
- A current or an overhead can make the return cost more than the way out. Plan a larger margin for it.
- At each waypoint, compare the gauge with that waypoint’s minimum gas. Below it, end the dive.
Gas planning in DiveLogic
The turn pressure and minimum gas tools use the same engine as the figures above. Every DiveLogic plan also has a gas ledger with the gauge reading and the minimum gas at each level, which you can copy to your slate.
Sources
- Exley S. (1979). Basic Cave Diving: A Blueprint for Survival. National Speleological Society, Cave Diving Section. (The rule of thirds.)
- DiveLogic Handbook. Turn pressure and Minimum gas tool pages (method and defaults of the calculations used here).