  We present the master (i.e. unique) behavior of the correlation length, as a
function of the thermal field along the critical isochore, asymptotically close
to the gas-liquid critical point of xenon, krypton, argon, helium 3, sulfur
hexafluoride, carbon dioxide and heavy water. It is remarkable that this
unicity extends to the correction-to-scaling terms. The critical parameter set
which contains all the needed information to reveal the master behavior, is
composed of four thermodynamic coordinates of the critical point and one
adjustable parameter which accounts for quantum effects in the helium 3 case.
We use a scale dilatation method applied to the relevant physical variables of
the onecomponent fluid subclass, in analogy with the basic hypothesis of the
renormalization theory. This master behavior for the correlation length
satisfies hyperscaling. We finally estimate the thermal field extent, where the
critical crossover of the singular thermodynamic and correlation functions
deviate from the theoretical crossover function obtained from field theory.
