This part of ISO 1628 specifies a method for the determination of the viscosity number (also referred toas “reduced viscosity”) of dilute solutions of thermoplastic polyesters (TPs) in certain specified solvents.The method is applicable to poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT),poly- (cyclohexylenedimethylene terephthalate) (PCT), and poly(ethylene naphthalate) (PEN), as well asto copolyesters and other polyesters, defined in ISO 7792-1, that are soluble in one of the specifiedsolvents under the specified conditions.The viscosity number is determined by the general procedure specified in ISO 1628-1, observing theparticular conditions specified in this part of ISO 1628.The determination of the viscosity number of a thermoplastic polyester provides a measure of therelative molecular mass of the polymer.