What is Viscosity?
Viscosity describes the tickness, stickyness and internal friction of a liquid
Viscometers or viscosimeters are used to measure the resistance to motion of liquids and gases. Several different types of instruments have been designed to measure viscosity, such as the inline falling-cylinder viscometer, the drag-type viscometer, and the Saybolt universal viscometer.
Rotating disc viscometer:
A rotating disc viscometer is a drag-type device. The device consists of two concentric cylinders, with the space between the two cylinders filled with the liquid being measured, as shown in the above figure.
The outer cylinder is driven by an electric motor at a constant speed using a synchronous motor, and the force on the inner cylinder is measured using a torque sensor. The viscosity of the liquid then can be determined. This type of viscometer can be used for viscosities from 50 to 50,000 centipoises, with an accuracy of ±1.0% and repeatability of 0.5% of span. The device can be used for viscosity measurements from −40° to +150°C, and pressures up to 28 MPa(g). In a production environment, the rotating disk viscometer is normally the device chosen.
The falling-cylinder viscometer uses the principle that an object, when dropped into a liquid, will descend to the bottom of the vessel at a fixed rate. The rate of descent is determined by the size, shape, and density of the object, and the density and viscosity of the liquid. The higher the viscosity, the longer the object will take to reach the bottom of the vessel. The falling-cylinder device measures the rate of descent of a cylinder in a liquid, and correlates the rate of descent to the viscosity of the liquid.
The Saybolt instrument measures the time for a given amount of fluid to flow through a standard size orifice, or through a capillary tube with an accurate bore. The time is measured in Saybolt seconds, which is directly related to, and can be easily converted to, other viscosity units.