Multi-level restrictions within the US Navy tables

Abstract

Schemes for multi-level diving are employed in the commercial, scientific, and sport sectors. One approach employs back-to-back repetitive sequencing, assigning groups at the start of each multi-level dive segment based on the total bottom time (actual plus residual nitrogen) of the previous segment. At times, that method allows critical tensions, other than the controlling (repetitive) 120 minute compartment tension, to be exceeded upon surfacing. In the context of the US Navy tables, such a circumstance is suspect. But by tightening the exposure window and accounting for ascent and descent rates, such a multi-level technique can be made consistent with the permissible tension formulation of the US Navy tables. In studying this multi-level technique, we can draw a line (envelope) across the Repetitive Group Table, separating dives violating at least one critical tension at some point in the multi-level sequence from those not violating any critical tensions. Ascent and descent rates of 60 feet (18 m)/min are assumed, and the envelope also maintains tissue tensions below critical values throughout the multi-level dive. Some 16 million multi-level dives were analyzed on a CRAY supercomputer, permitting construction of the dive envelope. The standard US Navy sets of tissue half-lives and critical tensions were employed. The envelope moves non-stop time limits back a group or more in the US Navy tables, restricting the back-to-back repetitive method in the same measure. Restrictions are straightforward and simple for possible wet testing.