Turn pressure answers at what pressure do I turn the dive? It finds the gauge reading that still holds enough gas for a return as long as the outbound leg, plus the reserve you decide to keep.
Turn pressure
At what pressure do I turn the dive?
155bar
Turn at 155 bar. A 20-minute leg would use 87 bar, more than the 77 bar allowed out, so the gauge reaches the turn first.
- Leg would use
- 87bar20 min at 25 m
- Allowed out
- 77barLeaves an equal return
- Reserve
- 77bar
The turn keeps gas for an equal return plus the reserve: the outbound leg may use at most half of what is left after the reserve. With thirds that is a third of the start pressure. The turn figure rounds up.
What you enter
- Outbound leg: Depth (m) and Time (min) of the leg before you turn, at one constant depth.
- Rule: Thirds keeps a third of your start pressure as the reserve. Custom opens Keep (% of start), the reserve as a share of the start pressure.
- RMV (L/min) and Water: shared with the other consumption tools.
- Size, each (L), Start (bar) and Cylinders: your cylinder, also shared.
How the turn is chosen
The reserve comes off the start pressure first. Half of what remains may be used going out, because the other half has to bring you back. That half is Allowed out. With thirds it is a third of the start pressure.
The planner then compares Allowed out with the gas your planned leg would use, and you turn at whichever limit you reach first:
- The leg ends first. Your planned time uses less than Allowed out. Turn at the planned time; Turn at is the reading you should see then.
- The gas runs out first. The planned leg would use more than Allowed out. Turn at is the reading that leaves an equal return and the reserve, and you reach it before the planned time. The figure shows in amber and the verdict says so. The screen above is this case: the 20-minute leg would use 87 bar against 77 bar allowed out, so the turn comes at 155 bar.
Turn at rounds up to the whole bar, so the turn never comes later than the rule allows.
Reading the result
- Turn at (bar): the main figure. Its rail runs up to your start pressure, with a solid mark at the reserve.
- Leg would use (bar): the gas the planned leg uses at your RMV.
- Allowed out (bar): the most the outbound leg may use.
- Reserve (bar): the pressure you hold back under the rule.
What the figure does not cover
- A return that differs from the way out. The return is assumed to use as much gas as the outbound leg. A current, a deeper return or a slower swim back needs more.
- A change of depth. The leg is one depth for its whole time; a leg that climbs or drops uses a different amount.
- Whether the reserve is enough. Thirds is a rule, not a calculation. Work out what two divers need to reach the surface with Minimum gas, and compare it with the Reserve.
Next
- Minimum gas sizes the reserve from the ascent itself.
- RMV from a dive gives the RMV these figures rest on.