Viscosity is the measure of a liquid’s resistance to flow and can be measured using a viscometer. Falling sphere and Ostwald viscometers are simple options, while more precise measurements can be obtained using electronic devices. Gravity-based devices, such as the Zahn and Ford cups, can also be used to measure kinematic viscosity.
In many scientific and industrial situations it is necessary to know the viscosity of liquids. Viscosity is the measure of the liquid’s resistance to flow. High viscosity liquids have higher resistance to flow and are not easily deformed by physical stresses, while low viscosity liquids are “thin” and flow easily. The viscosity of liquids can be measured using an instrument known as a viscometer, of which there are many different types. Where less precise measurements are acceptable, viscosity can also be measured using simple gravity-based devices.
One of the most common types of viscometers is the falling sphere viscometer. This setup measures the viscosity of liquids by timing how long it takes for a small sphere of known density and size to fall a certain distance through a liquid. The sphere is placed in a vertical tube filled with the liquid and allowed to reach its terminal velocity as it falls. At terminal velocity, the drag force pulling the sphere up equals the force of gravity pulling it down, and the sphere ceases to accelerate, maintaining a constant velocity as it falls. Once the terminal velocity, the density of the liquid and the sphere, and the size of the sphere are known, a formula, Stokes’ law, can be used to calculate the viscosity of the liquid.
Another fairly simple viscometer that is used in laboratory settings is the Ostwald viscometer, also known as a glass capillary viscometer or U-tube viscometer. This U-shaped glass tube device consists of two bulbs, one in the lower left arm of the U and the other in the upper right. It is held vertically while the liquid is drawn into the upper bulb and then allowed to flow back into the lower bulb, past two marks on the tube. The viscosity of liquids can be deduced by taking into account the diameter of the glass tube, the time it takes for a liquid to flow past the two marks, and the density of that liquid.
Laboratories requiring precision measurements can use more elaborate viscometers that incorporate electronics and measure viscosity using an oscillating piston or vibrating resonator immersed in liquid. In other contexts, such as the paint industry, simpler physical principles can be used to infer the approximate viscosity of liquids. These measurements are often based on a measure known as kinematic viscosity, the resistance of a liquid to flow under gravity.
The Zahn cup and Ford viscosity cup are examples of gravity-based devices used to measure kinematic viscosity. In these devices, the liquid—paint, in the case of the Zahn cup, or motor oil for the Ford cup—drains through a small hole in the bottom of a cup while being held aloft. The liquid flows in a smooth stream until a certain point where it breaks into drops. Depending on the viscosity of the liquid, the breakdown will occur at different times. A measure of kinematic viscosity can be found by multiplying this time in seconds by the specification number of the cup, which is calibrated for the appropriate liquid.
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