A high temperature and shear rate capillary viscometer has been designed, constructed and recently commissioned. This device will be used to measure the viscosity of semi-solid metals under the high temperature and shear rate conditions, similar to those found in industry. Design criteria for the device included a requirement for a highly controllable temperature (±1ºC) up to 650ºC, capability for injection shear rates above 10,000s−1 and controllable injection profiles. The design of this viscometer was aided with the use of numerical modelling methods based on a power law thixotropic fluid flow relation. This analysis allowed calculation of required injection speeds and expected system forces. Computational modelling work, based on current power law fluid models, was also performed in order to investigate how the viscosity would be expected to fluctuate with shear rate and fraction solid. This data could then be used to compare with experimental work. The computational model was a 2D two-phase theoretical unsteady state model. This was used to evaluate the viscosity of semi-solid metals passing through the designed capillary viscometer at injection speeds of 0.075, 0.5 and 1 m/sec. The effects of fractions solid (fs) of the metal from 0.25, 0.3, 0.33 and 0.50 were also investigated. Strong correlations between these parameters and the resulting viscosity were noted.