Such classifications exist so that the level can be determined even when opaque or staining liquids are measured, otherwise the liquid will cover the markings and make it impossible to gauge the time the level passes the mark. Reverse-flow viscometers have the reservoir above the markings, and direct-flow are those with the reservoir below the markings. Such viscometers can be classified as direct-flow or reverse-flow. By multiplying the time taken by the factor of the viscometer, the kinematic viscosity is obtained. The time required for the test liquid to flow through a capillary of a known diameter of a certain factor between two marked points is measured. Most commercial units are provided with a conversion factor. The calibration can be done using a fluid of known properties. The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity. Two marks (one above and one below the upper bulb) indicate a known volume. In use, liquid is drawn into the upper bulb by suction, then allowed to flow down through the capillary into the lower bulb. Above there is a bulb, with it is another bulb lower down on the other arm. In one arm of the U is a vertical section of precise narrow bore (the capillary). Another version is the Ubbelohde viscometer, which consists of a U-shaped glass tube held vertically in a controlled temperature bath. These devices are also known as glass capillary viscometers or Ostwald viscometers, named after Wilhelm Ostwald. Ostwald viscometers measure the viscosity of a fluid with a known density. Standard laboratory viscometers for liquids These values are used for calibrating certain types of viscometers.
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