How should stop times be calculated if loss occurs at 30'?

The correct approach to calculating stop times when a loss occurs at a depth of 30 feet involves the air/O2 trading ratio. This method takes into consideration the amount of oxygen consumed versus the amount of air available, which is critical during dive emergencies.

Using the air/O2 trading ratio allows for a more precise calculation of how much time should be spent on decompression stops. This calculation is vital to ensure that divers safely off-gas nitrogen accumulated during the dive and that they do not ascend too quickly, which could lead to decompression sickness.

The other methods suggested do not incorporate the necessary physiological considerations. Halving the current depth lacks the specificity needed to address the complexities of nitrogen absorption and off-gassing related to time spent at depth. Rounding down to the nearest minute fails to account for the exact requirements of decompression schedules, which are often precise in their recommendations. Multiplying by current O2 levels could give a misleading calculation without taking into account the diver's overall exposure to pressure and time, which are more critical factors in determining safe stop times.

Utilizing the air/O2 trading ratio provides a systematic and scientifically grounded approach to ensuring diver safety during decompression, making it the most effective method in this scenario.