Online Viscometers: Your Questions Answered – Hydramotion

What is an online viscometer?

An online viscometer is an instrument that continuously measures the viscosity of a fluid during an industrial process. It provides real-time viscosity measurements without requiring the fluid to be removed from the process for laboratory testing. Online viscometers are also commonly referred to as 'inline' or 'process' viscometers.

These sensors are typically used for production environments where the probes are inserted through a port in the pipe or tank wall or fitted into a bypass loop, meaning measurement takes place under actual process conditions. This live, uninterrupted measurement makes online viscometers particularly valuable in industries where fluid consistency is critical to product quality or process efficiency, or when the measurement lag from laboratory sampling is inadequate for controlling the process.

What is viscosity?

Viscosity is a measure of a fluid's resistance to flow. This resistance arises from the attractive forces between the molecules of the fluid. A fluid will only flow if enough energy is supplied to overcome these forces.

For a body to be able to move through a fluid, the fluid has to flow around or across it. Therefore, the energy required to move a body through a fluid is directly related to the degree to which that fluid resists flow, i.e. its viscosity.

Learn more: what is viscosity? >

How does an online viscometer work?

Most online viscometers operate using one of several physical measurement principles. The most common are vibrational, rotational and oscillating piston designs.

For a vibrational online viscometer, a probe is driven to oscillate at a known frequency inside the fluid. As the surface of the sensor shears through the fluid, energy is lost due to the viscosity. Higher viscosity causes a greater loss of energy and hence a higher reading. Determining the amount of energy lost to the fluid is ideal for process viscosity measurement, since it is practically independent of all other factors such as flow rate or pressure.

Rotational online sensors rely on the same founding principles of the laboratory viscometer - measuring the drag on a plate caused by fluid viscosity. Drag measurements are particularly vulnerable in a flowing fluid and are typically better suited to laboratory conditions.

Oscillating piston viscometers move a piston back and forth through a small chamber filled with the process fluid. The time or force required for this motion is related to the fluid viscosity. Given the reliance on a moving piston and a defined measurement chamber, they can suffer in a volatile process environment.

Learn more: drag vs resonance >

What is the difference between an online viscometer and a lab viscometer?

The fundamental distinction is where and how measurement occurs. A lab device such as a rotational viscometer, U-tube capillary viscometer, or cone-and-plate instrument, is a bench-top device designed for precise, controlled measurement of a discrete sample under tightly defined conditions.

Lab instruments can cover a wide range of viscosities and shear rates, and are capable of characterising non-Newtonian behaviour. They are often the gold standard for regulatory and compliance frameworks in industries such as pharmaceuticals and food. However, they inherently rely on sample collection and laboratory controlled conditions, which introduces time delays and often has limited value for process measurement.

An online viscometer is designed primarily for process control rather than fluid characterisation. It measures the process fluid directly, under real operating conditions, without any sample handling. This means the data reflects what is actually happening inside the process in real-time, something a lab instrument cannot offer. For quality control decisions that need to happen in seconds or minutes rather than hours, inline measurement is indispensable.

When should you use an online viscometer?

An online viscometer is often essential when viscosity is deemed a key process variable and when real-time control or monitoring is operationally or commercially important. Viscosity is a very sensitive indicator of change, so its measurement can be hugely insightful in most industrial production processes.

Consider inline viscometry if any of the following apply:

viscosity varies during batch production and needs to be controlled within tight limits
changes in raw material quality or concentration need to be detected quickly
the fluid is hazardous, volatile, or difficult to handle as a sample
production rates make frequent lab sampling impractical
end-product quality specification is directly linked to viscosity

Wherever lab sampling creates an unacceptable lag between process change and corrective action, an inline viscometer provides a compelling advantage. Common trigger points include polymerisation reactions where viscosity tracks reaction progress and determines the endpoint, polymer compounding lines where melt viscosity determines mechanical properties, food manufacturing where texture or pourability must be consistent, and pharmaceutical processing where formulation viscosity directly dictates API release and bioavailability.

What is the best online viscometer for process control?

The right instrument largely depends on the application requirements. Key factors to evaluate include the measurement principle, viscosity range of the fluid, the operating temperature and pressure, the chemical compatibility of wetted materials, the required measurement accuracy and repeatability, whether the fluid is Newtonian or non-Newtonian, and the hygienic requirements of the process (for food, beverage, or pharmaceutical applications, sanitary-rated sensors with CIP compatibility are essential).

Torsionally oscillating sensors such as those from Hydramotion, are widely used in process environments because they provide an optimal combination of process robustness and fluid sensitivity, and can be configured to suit the full range of process and application requirements outlined above.

The best approach is to specify the application in detail and select a sensor rated for the process conditions, with appropriate process connections and output signals for your control system.

Which industries use online viscometers?

Inline viscometers are used across a broad range of process industries. In the petroleum and petrochemicals sector, they are utilised to monitor fuel blending, lubricant formulation, and crude oil pipeline transport.

In polymers and plastics, inline viscometers measure melt viscosity on extruders and compounding lines to ensure consistent molecular weight and flow properties.

The food and beverage industry uses them to control the consistency of sauces, dairy products, chocolate, and beverages during manufacture.

Pharmaceutical and biotechnology manufacturers use inline viscometry to monitor fermentation broths, coating solutions, and injectable formulations.

Pulp and paper producers use them to control pulp slurry and coating viscosity.

Paint, ink, and adhesive manufacturers rely on inline measurement to maintain application consistency.

The common denominator across all these sectors is the need for reliable, real-time viscosity data to keep the process and products within specification.

What are non-Newtonian fluids?

The English physicist Sir Isaac Newton knew that viscosity changed with temperature. He also assumed that viscosity was always independent of shear rate. Fluids which exhibit this type of behaviour are called “Newtonian”. If we measure the viscosity at a number of shear rates for such a fluid (water, alcohol, motor oil) and plot the results on a graph, we will see a straight line.

However, many fluids are “non-Newtonian”. Non-Newtonian fluids have a viscosity that is dependent on shear rate. These fluids are loosely classified as “shear-thinning” and “shear thickening”. It is inaccurate to talk of a 'viscosity' figure for non-Newtonian fluids - all that can be said is that such fluids have an “apparent viscosity” at a particular shear rate.

Learn more: non-Newtonian fluids >

How accurate are online viscometers?

When discussing inline viscometer accuracy, it is important to distinguish between two related but separate concepts: calibration accuracy and process repeatability.

Because most real-world process fluids exhibit non-Newtonian behaviour, the concept of an absolute viscosity does not apply. The majority of industrial fluids; polymer melts, food products, coatings, slurries have no single "true" viscosity value to be accurate to; the reading is inherently a function of the shear conditions inside the sensor.

Calibration accuracy refers to how closely the instrument agrees with a traceable reference standard. Hydramotion inline viscometers are calibrated to within 1% accuracy of certified calibration oils, whose viscosity values are traceable to NIST (National Institute of Standards and Technology). This tells you that the sensor performs correctly against a known, internationally recognised standard under controlled conditions.

In the context of the process environment, what matters in practice is repeatability: the instrument's ability to return a consistent reading under the same process conditions of temperature, pressure, and shear rate. This repeatability is the foundation of process control, enabling operators to detect drift, maintain consistency, and compare batch to batch with confidence.

Can online viscometers replace lab viscosity testing?

A lab viscometer and an inline viscometer don't do the same job. Problems arise when plants rely on one to do the work of both. In most production environments, inline viscometers complement rather than fully replace laboratory testing.

For process control, lab sampling is often inadequate. The most significant limitation is measurement lag; the delay between sampling and receiving results back to the process. For example, in a high-stakes polymerisation reaction, this delay means critical moments can be missed, often with irreversible consequences. Inline sensors provide continuous process feedback, allowing operators to intervene immediately if viscosity drifts. The lab test then serves as a periodic verification and compliance record rather than the source of process control information.

For applications without formal regulatory requirements, inline viscometry can be the sole measurement method.

What is the difference between an online viscometer and a process viscometer?

All online viscometers are process viscometers, but not all process viscometers are truly 'in-line'. The distinction is subtle and the two terms are often used interchangeably. An online viscometer specifically refers to a sensor installed directly in the process line - usually a pipe or tank - where the sensing element is in continuous contact with the flowing fluid.

A 'process viscometer' is a broader term that can encompass any viscosity measurement instrument designed for industrial process environments, as opposed to laboratory use.

This would then include bypass viscometers (where a slip-stream of fluid is diverted through a measurement cell before returning to the main line), at-line instruments (bench-top or portable instruments used near, or next to the process for rapid sampling), and flow-through cells connected to sampling systems.