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Highlights • The circular Couette flow of Herschel–Bulkley fluids is analysed. • The conditions under which the rates of strain share a common point are specified. • The errors when using apparent instead of true shear rates are highlighted. • The errors in the calculated material parameters are discussed. • The findings of the present work are demonstrated on experimental data. Abstract We analyse the circular Couette flow of Herschel–Bulkley fluids to investigate the validity of the assumption that the rate of strain distributions across the gap share a common point. It is demonstrated that this is true only with fully-yielded Bingham -plastic flow. In other cases, e.g., in partially-yielded Bingham-plastic flow or fully-yielded Herschel–Bulkley flow, the common point for the fully-yielded Bingham case provides a good approximation for determining material constants only if the gap is sufficiently small. We also revisit the important issue of determining material properties of viscoplastic fluids by using "true values" for the rate of strain and demonstrate that the material properties can be very different from those obtained using "apparent' values for the rate of strain. [ABSTRACT FROM AUTHOR]

DuPont's perfluoropolyether oil Krytox $$^{\textregistered }$$ GPL 102 is a promising candidate for the high-temperature, high-pressure Deepwater viscosity standard (DVS). The preferred DVS is a thermally stable liquid that exhibits a viscosity of roughly 20 $$\hbox {mPa} \cdot \hbox {s}$$ at 533 K and 241 MPa; a viscosity value representative of light oils found in ultra-deep formations beneath the deep waters of the Gulf of Mexico. A windowed rolling-ball viscometer designed by our team is used to determine the Krytox $$^{\textregistered }$$ GPL 102 viscosity at pressures to 245 MPa and temperatures of 311 K, 372 K, and 533 K. At 533 K and 243 MPa, the Krytox $$^{\textregistered }$$ GPL 102 viscosity is $$(27.2 \pm 1.3)\,\hbox {mPa} \cdot \hbox {s}$$ . The rolling-ball viscometer viscosity results for Krytox $$^{\textregistered }$$ GPL 102 are correlated with an empirical 10-parameter surface fitting function that yields an MAPD of 3.9 %. A Couette rheometer is also used to measure the Krytox $$^{\textregistered }$$ GPL 102 viscosity, yielding a value of $$(26.2 \pm 1)\,\hbox {mPa} \cdot \hbox {s}$$ at 533 K and 241 MPa. The results of this exploratory study suggest that Krytox $$^{\textregistered }\, \hbox {GPL}$$ 102 is a promising candidate for the DVS, primarily because this fluoroether oil is thermally stable and exhibits a viscosity closer to the targeted value of 20 mPa $$\cdot $$ s at 533 K and 241 MPa than any other fluid reported to date. Nonetheless, further studies must be conducted by other researcher groups using various types of viscometers and rheometers on samples of Krytox GPL $$^{\textregistered }$$ 102 from the same lot to further establish the properties of Krytox GPL $$^{\textregistered }$$ 102. [ABSTRACT FROM AUTHOR]