What is Viscosity?

Viscosity is the tendency of a liquid substance to resist any kind of alteration in its shape or movement. The viscosity of a liquid is a percentage of its protection from distorting at a given rate. For fluids, viscosity refers to thickness in simplest terms.

Imagine a plastic cup with a hole in the bottom. If you pour honey into the plastic cup, you will observe that the cup empties slowly. Similarly, if you fill the same cup with water, the plastic cup empties faster as compared to when filled with honey. This suggests that the viscosity of honey is much more than the viscosity of water. In simple terms, the thickness of honey is more than the thickness of water.

Viscosity is the amount of a liquid's resistance to flow. It defines the interior friction of a running liquid. Any fluid that has greater viscosity will oppose movement as its molecular structure provides it with a lot of internal resistance. Any fluid with lower viscosity flows very easily as its molecular structure leads to very less resistance when it moves.

One of the processes is shearing, it occurs when fluid travels or is spread like showering, pouring, blending, spreading, and so on. This property states that the fluids that are high on thickness require more force for movement contrary to the ones with low viscosity.  

A fluid that does not have any guard from shear stress is referred to as an inviscid or an ideal liquid. A liquid with no viscosity is observed rarely at remarkably low levels of temperatures in superfluids.

Units of Viscosity

The most frequently used unit for the dynamic viscosity is the CGS unit centipoise (cP), which is comparable to 0.01 Poise (P). Kinematic viscosity is often assessed in the CGS unit centistokes (CST), which is comparable to 0.01 stokes (St). 

How do we measure Viscosity?

Viscosity is measured using a viscometer. It is an instrument that is used to assess the viscosity of fluids and their properties of the flow. In the further section, we will learn more about the various types of viscometers, their central characteristics, and aspects that are responsible for thorough and repeatable viscosity measurement.

Viscosity has been one of the most important characteristics of base oil, as per tribology and studies of lubrication. Now the question that most often comes to our mind is, why is viscosity so necessary in the lubrication of a machine?

One of the ways in which it can explain is that if the viscosity is very low, it would enable interfacial contact between core components in motion to produce wear. While, if we look at other aspects that is if the viscosity is very high, it would pressurize the machine to make more efforts to overpower the internal opposition of the lubricant. Therefore, for the same reason, it is vital to understand not only the viscosity of the base oil but at the same time how it may vary due to altering operating or environmental conditions.

Viscometers vs. Rheometers

To understand viscosity, it is important to differentiate between the measures of fluids based on their measurement tool. Viscosity refers to the measurement of the liquid’s opposition to flow, shear stress, at a provided temperature. A fluid in which there is a change in viscosity with the changes in flow conditions is known as non-Newtonian fluid. The measurement of such kinds of fluids is done with help of a rheometer. The other kind is Newtonian fluids, their viscosity is measured with the help of a viscometer.

Kinematic vs. Absolute Viscosity

There are two different ways in which viscosity can be extracted amongst a wide range of viscometer methodologies, namely, kinematic viscosity and absolute viscosity. The major distinction between these two kinds of measures is that kinematic viscosity is evaluated by discerning the resistance to flow of the fluid under the gravitational force, whereas absolute viscosity is determined by detecting the resistance to flow of the fluid under an outside and regulated force, either with the help of a capillary or by the movement of a body through the fluid. Kinematic viscosity is conveyed in centistokes (CST), but absolute viscosity is stated in centipoise (cP).

Viscosity Testing Methods

Viscosity can be measured using various methods, and each of them has its own set of advantages and limitations. Below we will look at the most common methods that are used in viscosity testing:

Capillary (Glass) Viscometer Test

A glass tube is the main equipment used in a capillary viscometer test which is usually “U” shaped and therefore, is commonly known as the U-tube. The process for a U-tube entails it to be immersed in a container with a regulated temperature, generally between 40℃ or 100℃ along with an exact time reading, usually in seconds, for the time it requires a predetermined amount of liquid to move within the tube from one distinct point to another by suction or through the pull of gravity. This calculated time is then multiplied by a constant (connected to the specific tube) to estimate the absolute viscosity (suction) or the kinematic viscosity (force of gravity).

Rotational Viscometer Test

As the name suggests, a rotational viscometer uses a rotating apparatus, that is known as the spindle, which is being immersed in the test fluid. The torque on the spinning shaft is then used to calculate the flow resistance of the fluid.

Subsequently, as this estimation is based on the function of the internal shear stress of the fluid and not the force of gravity, the rotational viscometer helps to determine the absolute viscosity of the liquid. A common alternative to this kind of viscometer is known as the Brookfield viscometer.

An improved alternative to the rotational viscometer is the Stabinger viscometer. It is based on the application of an improved separate floating spindle that is regulated by electromagnetic forces to produce rotation within the fluid. This is beneficial because it eradicates the complex task of factoring in the bearing resistance of a connected motor to the spindle.

Falling Ball and Falling Piston Viscometer Tests

Falling ball and falling piston viscometer tests are amongst the least common options for methods to test viscosity. A ball or piston is enabled to fall into the liquid in these tests, and the time is noted between the passing from one distinct point to a second distinct point. In order to compute the viscosity in line with Stokes’ law, the terminal velocity, size, and density of the ball or piston must be identified.

Other Test Methods

In a few cases, a bubble technique may be used that is unusual in oil viscosity testing. This test typically includes measurement of the time it requires for a bubble to rise a definite distance. This calculated time can be then proportionally related to the viscosity of the fluid. Another alternative consists of measuring the opposition to vibration from a probe.

Types of Viscometers

Nonetheless, there are numerous techniques by which viscosity can be assessed and most of the viscometers that are sold commercially are based on their use as intended. Following is a list of available viscometers:

Portable Viscometers

As the name suggests, portable viscometers are determined for use anywhere a laboratory location is not practical. In various on-the-spot and in-the-field functions, viscometers are required to arrange for a rapid, estimated viscosity reading typically for the intention of necessitating added analysis.

Small Onsite Viscometers

For more precise viscosity analyses, small onsite viscometers are promoted for laboratory locations but at a fair price. Such viscometers comprise those of the U-tube ranges or the Brookfield. While there are numerous variants of the U-tube viscometer, three of the most well-known are the Ostwald, Ubbelohde, and Cannon-Fenske.

Full-service Viscometers

In order to find the maximum degree of precision in viscosity testing, a full-service viscometer must be used. These units normally have the ability to automatically test kinematic viscosity and absolute viscosity in a varied range of temperatures as well as offer density and certain gravity computations. One normal difficulty oil analysis laboratory go through is with samples demanding multiple tests, is the ability to achieve these tests with the as minor sample as possible.

More than a few of the more refined full-service viscometers can make precise evaluations within 0.1 percent duplication using a sample size of less than 0.1 milliliter. In large laboratories that examine hundreds of samples every day, this is an essential advantage of full-service viscometers.

Inline/Online Viscometers

While most oil viscosity analysis is carried out after the collection of a controlled sample, a few viscometers can give viscosity readings uninterruptedly inactive flow lines. This frequently is achieved with the help of a sensor or transducer that is directly inline, portion of the pipeline, or online, circumvent stream or transducer. Viscosity measurements are derived and then directed by electronic means as signals to a computer or added monitoring equipment.

Factors that Affect Viscosity Measurements

Common U-tube Viscometer Variations

Even though the viscosity of a liquid may look very easy to determine, carrying out accurate measurements on such fluids can be a complex task to accomplish. Various factors must be recognized and kept constant in order for an assessment to be successful, including the following:


A container with a regulated temperature has the sole charge is to make sure that the entire viscosity measurement experiment stays at an accurate temperature usually at 40o C or 100o C and is regulated within 0.02 degrees C. Handling the container to this degree can be demanding, therefore a more developed viscometer comprises the temperature-controlled bath system as part of the unit.

Viscometer Glass

Capillary viscometers not only be influenced by temperature for accurateness but also by the exact internal diameter within the tube. For the same, these glass tubes are shaped with a completely galvanized, low-expansion borosilicate glass and frequently come with the tube’s related viscometer constant as the correction feature. Recalibrating the viscometer’s constant on a yearly basis and in accord with the temperature at which it will be operated is normal practice to make sure as little error as possible.

Viscometer Size

Viscometers differ in size due to the extensive diversity of viscosities that may be assessed. It is suggested that a viscometer takes a minimum of 200 seconds for the test liquid to pass from one distinct point to the other in order to prevent human error.

Rinsing Between Measurements

One more vital viscometer test factor is the quality of the rinse inside the tube amongst measurements. Usually, rinsing, cleaning, and drying stages are accomplished with the help of a wide range of residue-free solvents. In labs, where samples are run uninterruptedly on high-volume viscometers, incorporated cleaning systems make sure cleanliness efficacy. However, a visual check is yet essential to ascertain if a second rinse cycle is required, particularly for used oils.

Other significant factors that can impact viscosity measurements include the appropriate sample handling, timing device’s technique, precision, and in-depth and constant quality regulator.

After all, with viscosity being the most significant physical property of base oil, the attempts to achieve viscosity measurements through cautious approaches and rigorous standards can compare to more consistent machinery lubrication and ultimately to longer-lasting machinery.


This guide helps you to understand what is viscosity, units of viscosity, and in detail about viscometer, its types, and factors that can affect viscosity measurement.