EFX
Contributor
Actually both gasses are measured in atm pp (partial pressure). Exceeding the O2 limit is, relative to N2 loading, more or less instantaneous. Hence, the limit is expressed as a pp without time constraints. The N2 pp in the tissues loads and unloads in a time dependent fashion also based on depth (the ambient pressure). Your computer is always calculating the pp of N2 in each tissue compartment. Since the onset of bends is time delayed the instantaneous display of ppN2 is not as important as the NDL time in keeping you out of DCS. More importantly, offgassing of N2 to prevent DCS is rate limited which implicitly calls for a time measurement.