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This part of ISO 1628 specifies a method for the determination of the viscosity number (also referred to as "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 as to copolyesters and other polyesters, referred to in iso:proj:76396ISO 20028-1:2019, that are soluble in one of the specified solvents under the specified conditions. The viscosity number is determined by the general procedure specified in iso:proj:86451ISO 1628-1:2024, observing the particular conditions specified in this part of ISO 1628. The determination of the viscosity number of a thermoplastic polyester provides a measure of the relative molecular mass of the polymer.