Controlled emergency swimming ascent

Further information: Emergency ascent (diving)

Controlled Emergency Swimming Ascent (CESA) is a technique used by scuba divers as an emergency procedure when a diver has run out of breathing gas in shallow water and must return to the surface.

The technique involves simply ascending at a controlled pace, typically about 18 metres (60 feet) per minute, while exhaling slowly. As the diver ascends, the air in the lungs expands as surrounding water pressure decreases. Exhaling allows excess volume to escape from the lungs, and by exhaling at a suitable rate the diver can continue exhaling throughout the ascent and still have air in his or her lungs at the surface. If the diver fails to exhale during the ascent, lung over expansion injury is likely to occur. If exhalation is limited to relaxing and allowing the expanding gas to escape without effort, there should not be a feeling of running out of breath, as the air inhaled at depth expands during the ascent and the lung volume should remain nearly constant.

While in a practical sense there is little difference between a CESA and a "free ascent" (aka Emergency Swimming Ascent or ESA), the technical difference between the two is that in a CESA the regulator second stage is retained in the mouth and the diver exhales through it (in case gas becomes available due to the drop in ambient pressure) while in free ascent, the regulator is not retained or there is no regulator available, and the diver exhales directly into the water.