Measuring viscosity

    Viscosity is measured in devices called viscometers.

    There are three main types of viscometers:

    • Capillary viscometers
    • Ball drop viscometers and
    • Rotation viscometer

    In addition, there are two viscosity gauges:

    • kinematic viscosity in mm²/s (formerly cSt = centistokes),
    • dynamic viscosity in mPa·s (mPa·s = millipascal second, formerly cP = centipoise).


     

    Capillary viscometer

    At test temperature, a certain quantity of oil flows through a given section of a capillary tube (a long and very thin tube). Viscosity is calculated based on the flow time. There are two possible methods to perform the measurement:

    • Pure gravity flow of the oil for low shear rate to measure kinematic viscosity,
    • Pressurised flow of the oil in a viscometer, which results in a significant increase of the shear rate (more real-life situation) to measure dynamic viscosity.

       

    Schematic diagram of a capillary viscometer

    Enlarge image

    Ball drop viscometer

    A ball is dropped along a certain distance in a slanted glass tube filled with the tested oil at testing temperature. Dynamic viscosity is calculated based on the measured time and the device constant.

     

    Schematic diagram of a ball drop viscometer

    Enlarge image

    Rotation viscometer

    At test temperature, the tested oil is filled into a model bearing with its rotor driven by an electric motor. Dynamic viscosity is calculated based on the torque required for driving the rotor. 

    Schematic diagram of a rotation viscometer

    Enlarge image