AbstractThe paper describes the design and operation of an instrument for the measurement of the viscosity of liquids in the temperature range 10–150°C and the pressure range 0,1–30 MPa. Measurements of the viscosity of liquid water under these conditions are reported. The instrument is of the oscillating‐disk type and its design has been carried out with due regard to the theory of its operation and other, physical, constraints so as to optimize the precision of the viscosity measurements for water and aqueous solutions of inorganic salts. In addition, the experimental measurements of the logarithmic decrement and period of the disk oscillations have been reduced to the electronic measurement of time so that the experimentally realizable precision is made commensurate with the theoretical expectation. — The physical limitations imposed by size, measurement capability and corrosion resistance have precluded the possibility of absolute measurements of viscosity with the present instrument. Nevertheless, the availability of accurate values for the viscosity of water in the temperature range 0–100°C at a pressure of 0.1 MPa and in the range 100–150°C at its saturation vapor pressure, has allowed an accurate determination of the edge correction factor for the oscillating‐disk assembly to be made. In turn, precise determinations of the viscosity of liquid water over the entire temperature range 10–150°C and pressure range 0,1–30 MPa relative to this calibration have been performed. It is estimated that the precision of the present measurements is one of ±0.2%, which is consistent with their reproducibility, and that the accuracy of the final reported viscosities is ±0.3% up to 40°C deteriorating to ±0.5% at 150°C. — The experimental data are compared with the correlation promulgated by the Eighth International Conference on the Properties of Steam. The standard deviation is one of only ±0.3%, which is within the error band of the international correlation. Owing to the high precision of the present experimental results, they are to be preferred. — A correlation of the present experimental data is provided which reproduces the tabulated viscosities to within their estimated uncertainty. The correlation has been developed in the form η = η (P, T) wherein pressure and temperature are the independent variables. The choice is preferable to a correlation in terms of density and temperature from the viewpoint of practical application.