Diving guide
Turn Pressure and the Rule of Thirds
Calculate a usable turn pressure, distinguish thirds from minimum gas, and choose the earlier limit for direct-ascent and return-to-exit dives.
A reserve says what must still be available for trouble. A turn pressure says when normal exploration ends so the team can finish the route without spending that reserve. They are connected, but they are not interchangeable.
Reserve comes before turn pressure
First calculate or select the reserve required by the actual exit: direct ascent, a swim back to the anchor, an overhead, or decompression. Gas above that reserve is the usable planning budget. The rock-bottom guide calculates a two-diver emergency ascent and explains why a fixed 50 bar / 700 psi can be too small.
Then divide usable gas according to the route. A direct-ascent drift dive, an out-and-back reef, and a cave penetration do not justify the same split. The gas planner models the planned profile; it does not grant overhead-diving training.
The classical rule of thirds
For a return-to-exit profile, classical thirds allocates one third out, one third back, and one third in reserve. With 210 bar at the start, one third is 70 bar: the outbound leg turns at 140 bar, the return consumes the second 70 bar, and 70 bar remains.
Thirds is not universal
A worked minimum-gas turn
Suppose an out-and-back open-water dive begins at 200 bar and the calculated minimum gas is 85 bar. There are 115 bar above the emergency floor. If the outbound and return legs are expected to cost the same, allocate half—57.5 bar—to each. The computed turn is 200 − 57.5 = 142.5 bar, rounded conservatively to 145 bar.
A classical thirds calculation from 200 bar would turn near 135 bar and leave about 65 bar. Because that leaves less than the 85 bar calculated minimum, it is not conservative enough for this example; the 145 bar turn controls. In a different profile, classical thirds may be the earlier limit.
In imperial units, a 3,000 psi start with a 1,300 psi minimum leaves 1,700 psi above reserve. Half for the outbound leg is 850 psi, producing a 2,150 psi turn, rounded upward to a gauge-readable team value.
The earliest team limit controls
- Agree starting pressure, minimum gas, turn pressure, route, and who leads before descent.
- Compare actual gas volume when team cylinders differ; use the RMV converter rather than comparing gauge fractions blindly.
- The first diver to reach their planned limit calls the turn; the other diver's surplus does not cancel it.
- Recompute after a deeper profile, delay, current change, gas loss, or route change.
Sources
References and further reading
These official and specialist sources support the concepts in this guide. Always use current training materials and operational procedures for dive planning.