VISCOMETER (from the late Latin word "viscosus", which means "viscous", and Greek word "metreo", which means "measure" * EN: viscosimeter; DE: Viskosimeter, Zahigkeitsmesser; FR: viscosimetre; ES: viscosimetro; RU: вискозиметр) is the instrument for the determination of the viscosity of the gases and liquids. Within the petroleum industry, the viscometer is used for the determination of the viscosity of the drilling and tamping fluids, of the petroleums, and of the petroleum products. The rotary and ball viscometers have obtained the greatest distribution within the drilling.

Within the rotary viscometer, the medium, which is researched, is placed into the gap between the two coaxial rotating bodies, for example the cylinders, one of which (usually internal) is stationary, and another cylinder may rotate with the determined angular speed. They determine the viscosity according to the magnitude of the torque, which is acting from the liquid, which is researched, onto the stationary cylinder, in case of the specified angular speed of the rotation of the movable cylinder. For the measurement of the torque, there serves the dynamic system with the interchangeable measuring springs and scale, which is linked with the string, onto which there is suspended the stationary cylinder. The limits for the measurement using the rotary viscometer are from 1 to 10^5 pascal-seconds, and the relative error is 3-5%.

Within the ball viscometers, they measure the viscosity, while determining the speed of the rolling of the ball within the calibrated tube, which is filled with the liquid or gas, which are researched. According to this principle, there is constructed, for example, the viscometer for the measurement of the viscosity of the liquids and gases under the layer conditions. At the extreme position, the ball is retained by the electric magnet. After the switching of the electric magnet off, the electric timer is activated automatically. Upon the reaching of the inductive sensor by the ball, the electric timer is switched off. The limits for the measurement using the viscometer with the rolling ball are from 10^-4 to 5•10^2 pascal-seconds, the relative error is approximately 0.5%.

They determine the viscosity of the drilling fluids also in terms of the conventional units, namely, seconds, according to the time of the outflow of the determined volume of the fluid from the SPV-5 funnel through the thin pipe with the hole with the diameter of 5 millimetres.