What’s an Ostwald viscometer?

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An Ostwald viscometer, also known as a viscometer, is a device used to measure the viscosity of fluid under one type of flow condition. This fluid viscosity represents the fluid’s resistance to flow. It can be thought of as the friction of the fluid, often referred to as internal friction or thickness.
The Ostwald viscometer is also known as a U-tube viscometer or a capillary viscometer. The device uses the forward or reverse flow of the test fluid through a U-shaped tube to measure the viscosity of the fluid. The measurement is determined by noting the time it takes for fluid to flow a certain distance through a pipe of a specific diameter.

Each side of an Ostwald viscometer consists of tubes of different sizes. The side with the smaller tube is referred to as the capillary. For a direct flow viscometer, a bulb is towards the top of the capillary side. On the wider diameter side, a slightly larger bulb sits towards the bottom.

Two marks are placed on the pipe at a known distance from each other. These marks are placed above and below the smaller bulb. For reverse flow viscometers, the bulb on the wider side is above the bulb on the capillary side.
To measure the flow rate, or Ostwald measurement, suction is used to draw the fluid up the narrow tube until it passes the upper mark. The fluid is then released and the time it takes for the fluid to pass between the two points is measured. The flow rate of the Ostwald viscometer is then used to calculate the viscosity of the fluid using the Poiseuille equation.

In addition to the Ostwald viscometer, there are many other types of viscometers in use. These include falling ball viscometers, falling piston viscometers, vibrational viscometers, rotary viscometers and bubble viscometers. Instead of measuring fluid motion, some of these other types measure the motion of an object through fluid, such as a falling ball, falling piston, or rising air bubble. The uses and accuracy of different types of viscometers vary.

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