Is the salinity calculated by Sea-Bird valid above 42 psu?

The modern oceanographic definition of salinity is the Practical Salinity Scale of 1978 (PSS-78). By definition, PSS-78 is valid only in the range of 2 to 42 psu. Sea-Bird uses the PSS-78 algorithm in our software, without regard to those limitations on the valid range.

Unesco technical papers in marine science 62 "Salinity and density of seawater: Tables for high salinities (42 to 50)" provides a method for calculating salinity in the higher range (access this paper via Unesco's website).