How is the viscosity of a lubricant measured?

The viscosity of a lubricant is measured using an instrument called a viscometer, which determines a fluid's resistance to flow under standard conditions. Because viscosity is highly sensitive to temperature, these measurements are typically conducted in constant-temperature environments to ensure accuracy.12

There are several methods employed by different types of viscometers to calculate this property:

• Flow Rate Measurement: Many devices record the time required for a specific volume of lubricant to flow through an opening or a calibrated glass vessel.32
• Capillary Tubes: These measure the pressure required to force a fluid through a narrow tube at a specified rate.3
• Mechanical Resistance: Some instruments measure the force needed to rotate an object (such as an inner cylinder) within the fluid or the rate at which oscillations of a disk vibrating in the fluid die out.32
• Falling Sphere: This method tracks the time taken for a sphere to fall through the lubricant.3

The resulting measurements are expressed in various units, including centipoises, reyns, or seconds Saybolt universal (SSU), depending on whether metric, English, or commercial standards are used.1