VISCOMETERS

Laboratory viscometers

Laboratory viscometers are measuring instruments used to determine the viscosity of liquids under controlled conditions. Viscosity is an important physical property in quality control, research and industrial testing because it influences how a liquid flows, mixes, pumps and behaves during processing and use.

Pobel offers a range of viscometers for professional laboratory applications, including different glass and capillary designs suitable for transparent and opaque liquids, different viscosity ranges and standardised test methods.

The most appropriate viscometer depends on factors such as the type of liquid, expected viscosity, temperature, required measurement method and whether calibration documentation is needed.

What does a laboratory viscometer measure?

A viscometer measures a fluid's resistance to flow. However, it is important to distinguish between different types of viscosity measurement.

Dynamic viscosity

Dynamic viscosity describes the internal resistance of a fluid to flow when a force is applied. It is commonly expressed in units such as pascal-seconds (Pa·s), millipascal-seconds (mPa·s) or centipoise (cP).

Kinematic viscosity

Kinematic viscosity relates dynamic viscosity to the density of the fluid and is commonly expressed in square millimetres per second (mm²/s) or centistokes (cSt).

Many of the glass capillary viscometers used in laboratory testing determine kinematic viscosity by measuring the time required for a liquid to flow through a calibrated capillary under gravity.

For a broader explanation of applications and measurement principles, you can also read Pobel's guide on what a viscometer is used for.

How do capillary viscometers work?

Capillary viscometers determine viscosity by measuring the flow time of a known volume of liquid through a narrow calibrated tube.

The basic procedure generally involves:

• introducing the sample into the viscometer;
• bringing the liquid to the required test temperature;
• allowing the sample to flow through the capillary under controlled conditions;
• measuring the time required for the meniscus to pass between reference marks;
• and applying the instrument constant according to the specified test method.

The exact procedure varies according to the viscometer design and applicable standard, so the method specified for each instrument should always be followed.

Types of laboratory viscometers

Different viscometer designs are available because not every liquid or test method has the same requirements. Pobel's range includes Ostwald, Cannon-Fenske, Ubbelohde, Engler, capillary and other glass viscometers.

Viscometers for transparent liquids

When the sample is transparent, the movement of the meniscus through the calibrated sections of the instrument can be observed directly.

Different capillary designs are available depending on the viscosity range and the standardised method being followed. Pobel's Cannon-Fenske models for transparent liquids, for example, are available across multiple measurement ranges in centistokes.

Viscometers for opaque liquids

Opaque or highly coloured liquids require instruments specifically designed for samples where the meniscus cannot be observed in the same way as in transparent fluids.

Selecting the correct design is particularly important when testing oils, petroleum products or other dark liquids. Pobel offers Cannon-Fenske models specifically intended for opaque liquids across different kinematic viscosity ranges.

Glass capillary viscometers

Glass viscometers are widely used for kinematic viscosity testing because their precisely manufactured capillary tubes provide a simple and repeatable measurement principle.

Borosilicate glass is particularly suitable for this type of laboratory equipment because it provides good chemical resistance, transparency and dimensional stability under normal testing conditions.

Why temperature control is essential

Temperature has a major effect on viscosity. For most liquids, viscosity decreases as temperature increases, meaning even relatively small temperature variations can significantly affect the measured flow time.

For this reason, viscosity measurements should be performed at the temperature specified by the applicable method and under stable thermal conditions.

The viscometer must be allowed to reach thermal equilibrium before the measurement is taken. Consistent temperature control is particularly important when results need to be compared between samples, batches or laboratories.

How to choose the right viscometer

Choosing the correct laboratory viscometer requires more than simply selecting a particular design. The instrument must match both the characteristics of the sample and the test method being used.

Type of liquid

The first consideration is whether the sample is transparent or opaque. Certain viscometer configurations are specifically designed for one or the other.

Expected viscosity range

Each capillary viscometer has a specific measurement range. An instrument should be selected so that the expected viscosity of the sample falls within the recommended range.

Pobel's Cannon-Fenske range, for example, includes different sizes and constants covering kinematic viscosity ranges from low-viscosity liquids to substantially more viscous samples.

Test method and standard

The applicable analytical method may specify a particular type of viscometer, test temperature, flow-time requirement or calculation procedure.

For standardised laboratory testing, the viscometer should therefore be selected according to the method required by the laboratory rather than only according to its general design.

Calibration requirements

Some applications require greater metrological traceability. In these cases, a viscometer supplied with the appropriate calibration documentation may be necessary.

Calibration information allows the laboratory to use the instrument constant required to calculate viscosity from the measured flow time and can also support internal quality procedures and standardised testing.

Applications of laboratory viscometers

Viscosity measurement is important in many scientific and industrial sectors because changes in viscosity can indicate variations in composition, temperature, formulation or product quality.

Common applications include:

• lubricating oils and hydraulic fluids;
• fuels and petroleum products;
• solvents and chemical solutions;
• pharmaceutical products;
• food and beverage products;
• cosmetics and personal care formulations;
• polymers and chemical materials;
• research and product development;
• industrial and laboratory quality control.

The appropriate measurement technique depends on the nature of the sample. The glass and capillary instruments in this category are particularly relevant where viscosity is determined from liquid flow through a calibrated tube.

Good practices for accurate viscosity measurement

Reliable results depend not only on selecting the correct viscometer but also on using it under appropriate laboratory conditions.

• Maintain a stable temperature: viscosity is highly temperature-dependent, so the test temperature must be controlled carefully.
• Use the correct measuring range: select a viscometer whose calibrated range is appropriate for the expected viscosity of the sample.
• Avoid air bubbles: bubbles can interfere with liquid flow and affect measurement accuracy.
• Keep the capillary clean: residues or contamination inside the viscometer can alter the effective capillary diameter and affect flow time.
• Follow the specified test method: sample preparation, temperature, timing and calculation procedures should comply with the applicable analytical method.
• Handle glass viscometers carefully: damage to the capillary or calibrated sections can invalidate the instrument.

Calibration and measurement traceability

For quantitative viscosity measurements, the instrument constant is a fundamental parameter. In capillary viscometry, the measured flow time is used together with the viscometer constant to calculate kinematic viscosity according to the applicable procedure.

Pobel offers models for which constant values and calibration information are provided. Certain Cannon-Fenske instruments are also manufactured for use according to recognised test methods such as ASTM D445 and specifications such as ASTM D446.

The level of calibration and documentation required should be determined according to the laboratory's quality system, analytical method and regulatory requirements.

Laboratory viscometers from Pobel

Pobel offers a range of laboratory viscometers designed for different viscosity measurement requirements, including instruments for transparent and opaque liquids and different capillary configurations and measurement ranges.

The category includes Ostwald, Cannon-Fenske, Ubbelohde, Engler, capillary and glass viscometer families, allowing laboratories to select equipment according to the sample, method and required measurement range.

If you need help choosing the most suitable viscometer for your sample, viscosity range or test method, contact us for technical advice.