Viscosity is the measure of a fluid’s internal resistance to flow or its opposition to being sheared. For lubrication oils, this property directly dictates the oil’s ability to form a protective film between moving machine parts. If an oil’s viscosity is too low, the protective film can break, leading to excessive friction and wear. Conversely, if the viscosity is too high, the oil requires more energy to pump and circulate, which reduces efficiency and can cause operating temperatures to rise. Measuring this characteristic is a controlled process involving specialized instruments and established industry standards.
Dynamic and Kinematic Viscosity
Viscosity is quantified in two distinct ways, providing different perspectives on the fluid’s resistance properties. Dynamic viscosity, also called absolute viscosity, measures the fluid’s internal resistance to flow when a force is applied. This measurement expresses the shear stress required to move the fluid at a certain shear rate. The standard units are the Pascal-second (Pa·s), or more commonly in the oil industry, the centipoise (cP).
Kinematic viscosity is the dynamic viscosity of the fluid divided by its density. This ratio is meaningful because it accounts for the fluid’s inertia and how readily it flows under the influence of gravity alone. The standard units are square meters per second (m²/s), but the common unit for lubricants is the centistoke (cSt). Since many practical applications, such as oil draining, are driven by gravity, kinematic viscosity is often the most reported value for engine and industrial oils.
Measurement Methods and Instrumentation
A lubricant’s viscosity is determined using instruments called viscometers or, for complex analysis, rheometers. These devices measure the fluid’s resistance under precisely controlled conditions. Because viscosity is sensitive to temperature, measurements must be taken while the oil sample is held at a constant, standardized temperature, typically within a thermal bath. The technical approach used determines whether the result is dynamic or kinematic viscosity.
Capillary Viscometers (The Flow Method)
Capillary viscometers, such as the Ubbelohde or Ostwald types, are common instruments used to determine kinematic viscosity in a laboratory setting. This method measures the time it takes for a fixed volume of oil to flow through a narrow, calibrated glass tube under gravity. The viscometer is submerged in a temperature-controlled bath, often at 40°C or 100°C, before testing begins.
The flow time, measured in seconds, is recorded as the oil’s meniscus passes between two etched marks. This time is multiplied by a specific calibration constant unique to the tube, yielding the kinematic viscosity value in centistokes (cSt). This method forms the basis for kinematic viscosity standards used in lubricant specifications worldwide. The capillary tube geometry must be selected carefully to ensure the flow remains laminar for an accurate result.
Rotational Viscometers (The Shear Method)
Rotational viscometers measure dynamic viscosity by directly applying an external, measurable force to the fluid. These devices operate by submerging a rotating element, such as a spindle or a cone and plate, into the oil sample. The element is driven at a constant, precisely controlled speed.
The instrument measures the torque the motor must exert to overcome the oil’s resistance. This resistance is directly proportional to the oil’s dynamic viscosity; a thicker oil creates more drag and requires greater torque to maintain speed. The measured torque is converted into dynamic viscosity units, typically centipoise (cP) or Pascal-seconds (Pa·s), using factors related to the spindle geometry and rotational speed. This method is important for simulating the high shear rates experienced by oil in engine bearings.
Falling Element Viscometers
Viscosity measurement can also involve observing the motion of a standardized object through the fluid. Falling ball or falling piston viscometers measure the time it takes for an object of known size and density to fall or be pulled through the oil sample. The speed at which the object descends is inversely proportional to the oil’s viscosity. This method can yield either dynamic or kinematic viscosity depending on the specific design and calculation used.
Standardized Grading Systems
The raw viscosity numbers generated by viscometers are translated into standardized grades used by the industry and consumers to select the appropriate lubricant. Since viscosity changes significantly with temperature, all grading systems are based on measurements taken at specific, standardized temperatures. For industrial oils, the reference temperature is 40°C, while engine oils require testing at both low and high temperatures.
ISO Grading (Industrial Oils)
Industrial lubricants, such as gear and hydraulic oils, are classified using the International Organization for Standardization Viscosity Grade (ISO VG) system. This system is based on the oil’s kinematic viscosity measured at 40°C. The ISO VG number corresponds directly to the midpoint kinematic viscosity in centistokes. For example, ISO VG 32 indicates a midpoint kinematic viscosity of 32 cSt at 40°C, with a permitted tolerance range of plus or minus 10%. This classification provides a simple means of selecting fluids for applications where the operating temperature is relatively consistent.
SAE Grading (Motor Oil)
Engine oils are classified by the Society of Automotive Engineers (SAE) using a system designed to accommodate the wide temperature range of an operating engine. This system uses numbers like 30 or 40 for monograde oils and combinations like 10W-40 for multigrade oils. The number following the “W,” or the single number for monograde oils, is based on the kinematic viscosity measured at 100°C, which approximates the engine’s normal running temperature.
The “W” stands for Winter, and the number preceding it is derived from low-temperature dynamic viscosity tests. These tests, such as the Cold Cranking Simulator, measure the oil’s resistance to flow at temperatures as low as -35°C. This dual-number system ensures the oil is thin enough for cold-start circulation and thick enough to maintain a protective film once the engine reaches operating temperature.