Kinematic viscosity is an important physical property for characterising the flow behaviour of liquids. Its determination is widely used in laboratory testing because it provides quantitative information about how readily a fluid flows under the influence of gravity.
One of the established laboratory approaches is based on measuring the flow time of a liquid through a calibrated glass capillary viscometer under controlled temperature conditions.
In its laboratory equipment catalogue, Pobel includes several solutions for this type of measurement within the Measurement and Apparatus section, including Cannon-Fenske viscometers for transparent and opaque liquids, Ubbelohde viscometers, viscometer supports, thermometers and timing equipment.
What is kinematic viscosity?
Kinematic viscosity describes the resistance of a fluid to flow in relation to its density.
It is related to dynamic viscosity through the expression:
ν = η / ρ
where:
-
ν is kinematic viscosity.
-
η is dynamic viscosity.
-
ρ is fluid density.
The SI unit for kinematic viscosity is square metres per second (m²/s). In laboratory practice, however, values are commonly expressed in square millimetres per second (mm²/s) or centistokes (cSt).
The relationship between these units is:
1 cSt = 1 mm²/s
Kinematic viscosity should not be confused with dynamic viscosity. Although both properties describe fluid flow behaviour, they are not interchangeable because kinematic viscosity also takes density into account.
How is kinematic viscosity measured?
A common laboratory method consists of allowing a liquid to flow through a calibrated glass capillary viscometer.
The sample is introduced into the viscometer and brought to the temperature specified by the applicable test method. Once thermal equilibrium has been reached, the liquid is allowed to flow through the capillary under gravity.
The time required for the liquid meniscus to travel between two reference marks is measured.
For a calibrated capillary viscometer, the basic relationship can be expressed as:
ν = C × t
where:
-
ν is kinematic viscosity.
-
C is the viscometer calibration constant.
-
t is the measured flow time.
Depending on the applicable test method and measurement conditions, additional corrections may be required. The corresponding standard or validated laboratory procedure should therefore always be followed.
The Pobel catalogue specifies approximate constants and working ranges for its Cannon-Fenske viscometers, making it possible to select the appropriate size according to the expected viscosity of the sample.
Why is temperature control so important?
Temperature is one of the most important variables in viscosity measurement.
The viscosity of a liquid can change significantly with temperature. Consequently, a measurement made without adequate thermal control may not be comparable with another measurement performed under different conditions.
For this reason, capillary viscometry generally requires the viscometer and sample to be maintained at the temperature established by the corresponding test method.
Thermal stabilisation
Before measuring the flow time, the sample should be allowed to reach thermal equilibrium.
A thermostatic bath is generally used to maintain the viscometer at a controlled temperature. The exact conditions, stabilisation period and permitted temperature variation depend on the method being applied.
Temperature monitoring should also be carried out using suitable measuring equipment.
The Pobel catalogue includes ASTM thermometers as well as laboratory thermometers. The catalogue indicates that its ASTM thermometers are supplied with a calibration bulletin showing the results of checks at five points on the scale for a specified immersion.
Types of capillary viscometers
Different capillary viscometer designs are available. The appropriate choice depends on factors such as sample transparency, expected viscosity range and the standard or test procedure being followed.
Pobel's catalogue includes Cannon-Fenske transparent, Cannon-Fenske opaque and Ubbelohde viscometers.
Cannon-Fenske viscometers for transparent liquids
For transparent samples, the Pobel catalogue includes Cannon-Fenske Routine viscometers, model 33-C.
According to the catalogue, these viscometers are intended for transparent liquids and are manufactured according to ASTM D445, ASTM D446 and IP 71, with a calibration bulletin.
The catalogue lists different sizes, each with its own approximate constant and viscosity range.
The available ranges extend from 0.4–1.6 cSt for size 25 to 4,000–20,000 cSt for size 600. This wide selection allows the capillary to be chosen according to the expected viscosity of the liquid.
Cannon-Fenske viscometers for opaque liquids
For samples where direct observation through the liquid is difficult, Pobel also lists Cannon-Fenske viscometers for opaque liquids, model 34-C.
According to the catalogue, these are manufactured according to ASTM D445, ASTM D446 and IP 71 and are supplied with a calibration bulletin.
As with the transparent-liquid version, several sizes are available to cover different viscosity ranges.
Selecting the correct size is essential because the expected viscosity of the sample must fall within the working range of the selected viscometer.
Ubbelohde viscometers
Another option included in the Pobel catalogue is the Ubbelohde viscometer.
The catalogue states that these viscometers are manufactured according to ASTM D445, ASTM D446, IP 71, ISO 3104, ISO 3105 and DIN 51562, and are supplied with a calibration bulletin.
Different types are available for different viscosity ranges. The catalogue lists options from type 0c, with a range of 0.6–3 cSt, up to type IV, with a range of 2,000–10,000 cSt.
How to choose a suitable viscometer
Choosing the correct viscometer is a fundamental step in obtaining reliable measurements.
Expected viscosity range
The first consideration is the approximate viscosity of the sample.
The selected capillary must provide a working range appropriate for that liquid. Choosing a viscometer solely because it is available in the laboratory, without considering its specified range, can lead to unsuitable flow times or results outside the intended operating conditions.
Pobel's catalogue provides viscosity ranges and approximate constants for its Cannon-Fenske models, as well as working ranges for the different Ubbelohde types.
Sample transparency
The optical characteristics of the sample are also relevant.
For transparent liquids, a Cannon-Fenske design intended for transparent samples may be suitable. For opaque liquids, the corresponding Cannon-Fenske opaque model should be considered.
The selection should always be consistent with the validated analytical method.
Applicable test method
The test standard or internal laboratory procedure may specify the type of viscometer, measurement temperature, preparation conditions and calculation method.
For this reason, equipment selection should not be based exclusively on viscosity range. The requirements of the applicable method must also be considered.
General procedure for measuring kinematic viscosity
Although the exact procedure depends on the viscometer and standard being used, a typical laboratory workflow includes the following stages.
1. Prepare the viscometer
The capillary viscometer should be clean, dry and free from residues that could affect the flow of the sample.
It should also be visually inspected before use. Deposits, contamination or damage to the glass can compromise the measurement.
2. Prepare the sample
The sample should be representative and prepared according to the applicable method.
Where required by the procedure, appropriate precautions should be taken to avoid contamination, particles or other factors that could interfere with capillary flow.
3. Fill the viscometer
Introduce the required amount of sample according to the instructions for the selected viscometer.
Particular attention should be paid to the presence of air bubbles, as they can interfere with the movement of the liquid and affect timing.
4. Position the viscometer correctly
Correct positioning helps ensure that the measurement is carried out under reproducible conditions.
The Pobel catalogue includes a universal support for different types of viscometers, manufactured from PP and identified as reference SOUV 0001. It also includes an acrylic rack for four viscometers, reference SOMV 0004.
5. Thermostat the sample
Place the viscometer under the temperature conditions required by the method and allow sufficient time for thermal equilibrium.
The measurement should not begin until the required temperature conditions have been reached.
6. Measure the flow time
Allow the sample to flow through the capillary according to the procedure established for the viscometer.
Measure the time required for the meniscus to pass between the specified timing marks.
For timing operations, the Pobel catalogue includes the CRNM 0001 digital stopwatch, which provides a resolution of 0.01 seconds for the first 30 minutes.
7. Repeat the measurement
Where required by the method, repeat the determination and check that the measurements comply with the applicable repeatability criteria.
Large differences between repeated measurements may indicate problems such as temperature instability, contamination, bubbles or timing errors.
8. Calculate the kinematic viscosity
For a calibrated capillary viscometer, the basic calculation uses the viscometer constant and the measured flow time:
ν = C × t
The calibration constant corresponding to the specific instrument should be used.
If the applicable method requires additional corrections or calculations, these should be applied as specified in that method.
Good practices for reproducible results
Reliable viscosity measurements depend not only on the viscometer itself but also on correct laboratory technique.
Useful practices include:
-
Selecting a viscometer with a suitable range for the expected viscosity.
-
Maintaining the temperature specified by the test method.
-
Ensuring that the capillary is clean before measurement.
-
Avoiding bubbles in the sample.
-
Keeping the viscometer correctly positioned.
-
Using the calibration constant corresponding to the specific viscometer.
-
Repeating measurements when required by the method.
-
Recording the temperature, flow times and any relevant observations.
-
Keeping calibration information traceable to the instrument being used.
The Pobel catalogue indicates that its Cannon-Fenske and Ubbelohde viscometers are supplied with a calibration bulletin, an important document when identifying the characteristics associated with the individual measuring instrument.
Common errors in kinematic viscosity measurement
Using an unsuitable capillary range
Selecting a viscometer whose working range does not match the expected viscosity can produce unsuitable measurement conditions.
The expected sample viscosity should therefore be estimated before selecting the viscometer size.
Insufficient temperature control
Starting the measurement before the sample reaches the specified temperature can affect the result.
Temperature should be treated as a controlled measurement parameter rather than simply an environmental condition.
Incorrect timing
Human reaction time or incorrect identification of the timing marks can introduce variability.
A clear observation of the meniscus and appropriate timing equipment help improve repeatability.
Air bubbles
Bubbles can disturb the normal movement of the liquid through the capillary.
The filled viscometer should therefore be inspected before the measurement begins.
Contaminated capillary
Residues from previous samples or cleaning agents may alter the flow conditions.
Proper cleaning and drying procedures are therefore essential.
Confusing dynamic and kinematic viscosity
Although both quantities are related, they represent different properties and use different units.
Laboratory reports should clearly state which type of viscosity has been determined and under which measurement conditions.
Safety during viscosity measurement
Laboratory viscosity measurements often involve glassware, controlled-temperature baths, samples with different chemical properties and cleaning agents.
Appropriate laboratory safety procedures should therefore be followed.
Before use, glass viscometers should be checked for visible damage. Suitable personal protective equipment should be selected according to the sample, cleaning procedure and working temperature.
When thermostatic baths or heat-transfer fluids are used, their operating and safety instructions should also be followed.
The specific hazards depend on the sample and laboratory procedure, so the corresponding safety data and internal laboratory protocols remain essential.
Pobel equipment for viscosity measurements
For laboratories carrying out capillary viscosity measurements, the Pobel catalogue includes several complementary product categories:
-
Cannon-Fenske 33-C viscometers for transparent liquids, available in different viscosity ranges.
-
Cannon-Fenske 34-C viscometers for opaque liquids, also available in several ranges.
-
Ubbelohde viscometers, with different types according to the required viscosity range.
-
SOUV 0001 universal viscometer support and SOMV 0004 four-viscometer rack.
-
Laboratory and ASTM thermometers for temperature monitoring.
-
CRNM 0001 digital stopwatch for time measurements.
At Pobel, we offer laboratory equipment that can be integrated into the different stages involved in capillary viscosity measurements, from sample flow and temperature monitoring to viscometer positioning and timing.
Frequently asked questions about kinematic viscosity
What units are used for kinematic viscosity?
The SI unit is m²/s, although laboratory measurements commonly use mm²/s or centistokes (cSt).
One centistoke is equivalent to one square millimetre per second.
Why must the measurement temperature be reported?
Because viscosity is temperature-dependent. A viscosity value without information about the measurement temperature may be insufficient for meaningful comparison with other results.
Which viscometer can be used for transparent liquids?
The Pobel catalogue includes Cannon-Fenske Routine model 33-C viscometers specifically described for transparent liquids.
Which viscometer can be used for opaque liquids?
Pobel lists the Cannon-Fenske model 34-C for opaque liquids.
What is the difference between Cannon-Fenske and Ubbelohde viscometers?
They are different capillary viscometer designs. Their suitability depends on the applicable test method, sample characteristics and required viscosity range.
The Pobel catalogue lists both Cannon-Fenske and Ubbelohde models with their respective standards and working ranges.
How is kinematic viscosity calculated from flow time?
In its basic form, the calculation for a calibrated capillary viscometer is:
ν = C × t
where C is the calibration constant and t is the measured flow time.
The applicable standard should be consulted to determine whether additional corrections are required.
Should the measurement be repeated?
Repeated measurements are commonly used to assess the consistency of the result. The number of determinations and acceptance criteria should follow the applicable standard or validated laboratory procedure.
Conclusion
Measuring kinematic viscosity in the laboratory requires more than simply timing the flow of a liquid through a capillary. Temperature control, cleanliness, correct viscometer selection, absence of bubbles, stable positioning, accurate timing and the appropriate calibration constant all contribute to reliable results.
The first step is to identify the characteristics of the sample and the requirements of the applicable test method. From there, the laboratory can select a capillary viscometer with an appropriate working range and establish controlled measurement conditions.
Pobel's laboratory catalogue includes Cannon-Fenske viscometers for transparent and opaque liquids, Ubbelohde viscometers, viscometer supports, thermometers and timing equipment, providing complementary solutions for laboratory kinematic viscosity measurements.
Related products
