A complete guide to the types of laboratory viscometers
— Equipo JP Selecta
Viscosity is one of the most important physical and rheological properties in materials analysis. In highly regulated sectors such as pharmaceuticals, food, cosmetics or chemicals, a fluid's resistance to deforming or flowing directly conditions the final quality of the product, its stability over time and its behaviour during industrial processing.
To characterise this property precisely, laboratories turn to different types of viscometers, analytical instruments designed to measure the internal friction of a fluid under controlled thermal and mechanical conditions. Understanding how each technology works is essential in order to select the right equipment for the nature of the product being analysed.
What is viscosity?
Physically, viscosity measures the internal resistance of a fluid to flowing when a shear force is applied to it. Depending on the test technique and the physical variable obtained, we distinguish two main quantities:
- Dynamic or absolute viscosity (ƞ): represents the force needed to overcome the internal friction of a liquid. It is usually expressed in millipascal seconds (mPa · s) or centipoise (cP), where 1 mPa · s = 1 cP
- Kinematic viscosity (ν): is the ratio between dynamic viscosity and the density of the fluid at the same temperature (ν = ƞ/ρ). It is measured under the action of gravity and expressed in square millimetres per second (mm2/s) or centistokes (cSt).
Viscosity is a critical quality control parameter. In pharmaceutical formulation it determines the dosing of syrups or the injectability of suspensions; in the food sector it conditions the texture of sauces and dairy products; and in the chemical industry it is essential for adjusting the rheology of paints, coatings and adhesives.
Types of laboratory viscometers
The wide variety of fluids in industry calls for different measurement principles. The four main families of viscometer types are classified according to the physical phenomenon used to evaluate flow: rotational, capillary, falling ball and vibrational.
| Type | Measurement principle | Typical range (mPa·s) | Ideal fluids |
|---|---|---|---|
| Rotational | Torque on a rotating spindle | 1 – 320,000,000 | Non-Newtonian fluids (gels, creams, polymers) |
| Capillary | Gravity flow time through a capillary tube | 0.4 – 20,000 | Newtonian fluids (oils, solvents, water) |
| Falling ball (Höppler) | Fall time of a sphere in an inclined tube | 0.5 – 100,000 | Transparent Newtonian fluids |
| Vibration (oscillation) | Damping of the resonance frequency of a probe | 0.1 – 10,000 | In-line measurement and process control |
Rotational viscometer
This type of rotational viscometer determines viscosity by measuring the torque (resistance force) needed to rotate a moving element (spindle) at a constant rotation speed within the fluid. The value is obtained directly in mPa · s or cP.
It is the most versatile in the laboratory because it makes it possible to characterise non-Newtonian fluids. These are fluids whose viscosity varies as the shear rate changes, such as creams, suspensions, gels, paints or polymers.
- Limitation: it requires selecting the appropriate spindle and test speed in order to work within the optimum scale range.
- Viscometer equipment such as the STS-2025 series from J.P. Selecta allows multiple speeds to be configured (0.01 to 200 rpm), the torque force to be measured and rheological parameters such as shear rate and shear stress to be evaluated.
Capillary viscometer (glass)
The capillary viscometer evaluates kinematic viscosity (mm2/s or cSt) by measuring the time it takes a fixed volume of liquid to flow by gravity through a borosilicate glass tube of known capillary diameter. The transit time between two calibrated marks is multiplied by the nominal constant of the tube.
It is the technique of choice for low or medium viscosity Newtonian fluids, such as mineral oils, solvents, fuels or dilute aqueous solutions. The J.P. Selecta catalogue includes standardised models such as the Ubbelohde, Cannon-Fenske (both for transparent liquids and for opaque ones by reverse flow), Micro-Ubbelohde viscometer types and U-tubes.
- Limitation: it is not suitable for non-Newtonian fluids (since it relies solely on gravity) or for liquids loaded with suspended particles that could block the capillary.
Falling ball viscometer (Höppler)
Based on Stokes' Law, this method determines dynamic viscosity by measuring the time it takes a sphere of known mass, density and diameter to travel a fixed distance by gravity through an inclined glass tube filled with the sample liquid.
It is a highly reproducible alternative for evaluating transparent Newtonian fluids of medium or low viscosity, such as sugar solutions, light oils or beverages.
- Limitation: it can only be used with Newtonian fluids and completely transparent samples that allow visual observation of the sphere passing through.
Vibrational / oscillation viscometer
These types of viscometers immerse a sensor in the fluid (such as a rod or disc) that vibrates at a constant ultrasonic frequency. The viscosity of the liquid generates physical damping of the vibration; the equipment measures the energy required to keep the oscillation constant.
Its main advantage is the absence of moving mechanical parts in rotation, which allows continuous measurements without maintenance and integration into industrial production lines (in-line).
- Limitation: It provides a single spot value and does not allow complete rheological curves to be analysed, so it is not the ideal tool for R&D formulation.
How to choose the right viscometer for your laboratory?
Selecting the right type of viscometer requires carefully assessing the properties of the product and the regulatory requirements of the test. To get the decision right, we recommend following these six key criteria:
- Nature of the fluid. Determine whether your sample is Newtonian (its viscosity is constant regardless of the stirring speed) or non-Newtonian (pseudoplastic, thixotropic or dilatant). Non-Newtonian fluids necessarily require a rotational viscometer.
- Estimated viscosity range. Define the minimum and maximum values expected in mPa · s or cSt. For example, low viscosity samples (such as solvents or juices) require special spindles such as the LCP adapter or fine capillary tubes, while resins or thick creams call for high torque models (R or H series).
- Test temperature control. Viscosity is extremely sensitive to thermal changes (an increase of 1°C can reduce viscosity by up to 10%). Consider whether you need accessories with an external circulation jacket connected to a precision thermostatic bath (such as the VB-1423 or TV-1452 models).
- Applicable regulations. Make sure the method complies with the international standards required in your sector, such as ASTM D445 and ISO 3104 (for glass capillary viscometers), ASTM D2196 (for rotational ones) or the applicable pharmacopoeias (USP/Ph. Eur.) in the pharmaceutical environment.
- Sample type and volume. Assess the transparency of the sample, its chemical compatibility with the materials of the equipment (stainless steel, PTFE, glass) and the volume available. If you have little volume (between 3 and 16 mL), use small volume adapters such as the APM or microcapillaries.
- Traceability and data integrity requirements. In laboratories under GMP or ISO 17025 standards, the viscometer must provide traceable calibration certificates and have digital outputs (such as USB ports or dataloggers) to export tamper-proof reports.
At J.P. Selecta we design and manufacture scientific instrumentation to the highest standards of precision and traceability. If you need advice on selecting the most suitable laboratory viscometer for your application, our technical team can help you configure the right equipment and measurement probes.