Selecting optimal air diving gradient factors for Belgian military divers: more conservative settings are not necessarily safer

Introduction: In 2018, the Belgian Defence introduced a commercial off-the-shelf dive computer (Shearwater Perdix™) for use by its military divers. There were operational constraints when using its default gradient factors (GF). We aimed to provide guidelines for optimal GF selection. Methods: The Defence and Civil Institute of Environmental Medicine (DCIEM) dive tables and the United States Navy (USN) air decompression tables are considered acceptably safe by the Belgian Navy Diving Unit. The decompression model used in the Shearwater Perdix (Bühlmann ZH-L16C algorithm with GF) was programmed in Python. Using a sequential search of the parameter space, the GF settings were optimised to produce decompression schedules as close as possible to those prescribed by the USN and DCIEM tables. Results: All reference profiles are approached when GFLO is kept equal to 100 and only GFHI is reduced to a minimum of 75 to prolong shallower stop times. Using the Perdix default settings (GFLO = 30 and GFHI = 70) yields deeper initial stops, leading to increased supersaturation of the ‘slower’ tissues, which potentially leads to an increased DCS risk. However, Perdix software does not currently allow for the selection of our calculated optimal settings (by convention GFLO < GFHI). A sub-optimal solution would be a symmetrical GF setting between 75/75 and 95/95. Conclusions: For non-repetitive air dives, the optimal GF setting is GFLO 100, with only the GFHI parameter lowered to increase safety. No evidence was found that using the default GF setting (30/70) would lead to a safer decompression for air dives as deep as 60 metres of seawater; rather the opposite. Belgian Navy divers have been advised against using the default GF settings of the Shearwater Perdix dive computer and instead adopt symmetrical GF settings which is currently the optimal achievable approach considering the software constraints.