Abstract: Based on the theory of viscous fluid mechanics, the formula regarding the radial velocity gradient of rotating liquids in the measuring device is obtained, and the more accurately calculation formula of viscosity coeffi- cient for rotating cylinder viseometer is further worked out, the errors of approximate formula used in the derivation in textbooks and documents nationwide are put forward, and the torque of frictional drag in the measuring device is calculated by using the parameter estimation method, which makes the measuring results calibrated be accurately. The experimental results show that the difference of the radial velocity gradient and the liquid viscous coefficient calculated by the approximate and exact formula are 13.1% and 28.6% , respectively; the torque of frictional drag in the measuring device is 9. 482×10-5N · m when temperature of the castor oil is 25.5 ℃ , which is equivalent to the torque caused by 0. 540 grams of weight.

Key words: rotating cylinder viscometer, coefficient of viscosity, velocity gradient, frictional torque