Abstract: An apparatus and process for the continuous manufacture of high stress, high denier industrial yarn spun from polyethylene terephthalate polyester. A control system continuously measures melt viscosity of the polymer both at the outlet from a polymerization zone and at a downstream location adjacent a manifold immediately preceding a plurality of spinnerettes through which the polymer is drawn into filamentary material. Adjustments are continuously made to a meaningful operating condition within the polymerization zone, such as the magnitude of the vacuum therein, whenever IV inferred from operating data strays from a pre-established level. Adjustments are also regularly made at discrete short term intervals, for example, minute by minute, whenever viscosity at the outlet from the polymerization zone strays from the pre-established level.