An Ostwald viscometer, also known as a viscosimeter, is a device used to measure fluid viscosity under one type of flow condition. This fluid viscosity represents the fluid’s resistance to flow. It can be thought of as the fluid’s friction, often referred to as internal friction or thickness.
The Ostwald viscometer is also known as a U-tube viscometer or capillary viscometer. The device utilizes direct or reverse flow of the test fluid through a U-shaped tube to measure the fluid’s viscosity. The measurement is determined by noting the time required for the fluid to flow a certain distance through tubing of a specific diameter.
Each side of an Ostwald viscometer consists of different size of tubing. The side with smaller tubing is referred to as the capillary. For a direct flow viscometer, a bulb is located toward the top of the capillary side. On the wider diameter side, a slightly larger bulb is located toward the bottom.
Two marks are placed on the tubing at a known distance apart. These marks are placed above and below the smaller bulb. For reverse flow viscometers, the bulb on the wider side is located above the bulb on the capillary side.
To measure the rate of flow, or Ostwald measurement, suction is used to draw the fluid up the narrow tubing until it rises above the upper mark. The fluid is then released, and the time required for the fluid to pass between the two points is measured. The Ostwald viscometer flow rate is then used to compute the fluid’s viscosity using Poiseuille's equation.
In addition to the Ostwald viscometer, there are several other types of viscometers in use. These include falling sphere viscometers, falling piston viscometers, vibrational viscometers, rotating viscometers and bubble viscometers. Instead of measuring the movement of the fluid, some of these other types measure the movement of an object through the fluid, such as a falling sphere, a falling piston or a rising air bubble. The uses and accuracy of different types of viscometers can vary.