One third in, one third out, one third shared reserve. Guidelines with primary reel. Arrow markers point to the exit. Distance knots every 30 m. NSS-CDS and TDI: three independent light sources.
Calculating the turnaround pressure (rule of thirds) and which system to use for measurement generates constant debate. The placement and management of the reel in the water, individual versus team marking, and navigation in environments without direct visibility are critical skills. Incidents in well-known caves (Pozo Azul, Cueva del Agua, Font d'Estramar) are documented as warnings.
The rule of thirds means that when one third of the total gas available between both divers has been consumed, the team must begin the return without exception. Navigation is based on laying a guideline from the entrance using a primary reel. Protocols include arrow markers pointing toward the exit, distance knots every 30 meters, and personal markers. According to NSS-CDS and TDI standards, no diver should enter a complete darkness zone without three independent light sources and having completed cavern and cave diver training.