Correction for adiabatic effects in the calculated instantaneous gas consumption of scuba dives

In scuba-diving practice, instantaneous gas consumption is generally calculated from the fall in cylinder pressure without considering the effects of water temperature (heat transfer) and adiabatic processes. We aimed to develop a simple but precise method for calculating the instantaneous gas consumption during a dive.

With gas thermodynamics and water/gas heat transfer, the instantaneous released gas mass was modelled. In addition, five subjects made an open-water, air, open-circuit scuba dive to 32 metres' sea water. Depth, cylinder pressure and water temperature were recorded with a dive computer and gas consumption was calculated and compared using different methods.

After descent in open-water dives, the calculated gas mass in the cylinder was the same as calculated from cylinder data, suggesting that the model is adequate. Modelled dives showed that adiabatic effects can result in considerable overestimate of the gas consumption, depending on the dive profile, exercise-dependent pulmonary ventilation and the cylinder volume. On descending, gas thermodynamics are predominantly adiabatic, and the adiabatic correction of ventilation is substantial. During the dive, the adiabatic process (at the start 100%) decreases steadily until the end of the dive. Adiabatic phenomena are substantially different between square and saw-tooth profiles. In the emergency situation of a nearly empty cylinder after a square-wave dive involving heavy physical exertion, the adiabatic effect on the cylinder pressure is generally > 20%. Then, with a strongly reduced consumption at the start of the ascent, heat inflow produces an increase of cylinder pressure and so more gas becomes available for an emergency ascent.

Adiabatic effects, being indirectly dependent on exercise, the profile and other conditions, can be substantial. The developed method seems sufficiently accurate for research and possibly for reconstruction of fatalities and is implementable in dive computers.