\rē-▐a-mә-tәr\ n [ISV] (ca. 1859) (Plastometer) An instrument for measuring the flow behavior of high-viscosity materials such as molten thermoplastics, rubbers, pastes, and cements. The most widely used principle is that of the Capillary Rheometer of which a variety of makes and models are in daily use. Instruments for measuring the flow properties of less viscous fluids, e.g., dilute polymer solutions, are called Viscometers but the terms rheometer and viscometer are often used interchangeably. Currently (1993), computerized, on-line capillary rheometers linked to a single control station can simultaneously monitor melt viscosity in ten or more extruders in a resin-finishing plant.