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
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
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
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?
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
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
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
(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).
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.
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.
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.
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
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:
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.
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
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.
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
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.
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.
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
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
factors that can impact viscosity measurements include the appropriate sample
handling, timing device’s technique, precision, and in-depth and constant
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.