What Is Viscosity? Measurement Principle and Inline Viscosity Measurement in Industrial Processes

Whether in coating processes, polymer production, or food manufacturing – viscosity directly influences product quality, process stability, and energy efficiency in many industrial applications. Even small changes in viscosity can indicate formulation deviations, process disturbances, or reaction progress. Continuous monitoring of this parameter is therefore crucial in numerous production processes. Modern inline viscosity sensors make it possible to measure viscosity directly in the ongoing process and under real operating conditions. This enables real-time process monitoring and early optimization of processes.

What is viscosity?

Viscosity describes the internal resistance of a fluid to flow. It is therefore a measure of how easily or difficultly a medium can be deformed under mechanical stress.

A clear everyday example shows the difference:

• Water has a very low viscosity and flows easily.
• Oil or syrup have a significantly higher viscosity.
• Pastes or polymer melts can reach extremely high viscosities.

In industrial applications, viscosity influences, among other things:

• the spray behavior of coatings
• the metering capability of liquids
• the degree of homogenization in mixing processes
• the consistency of food products
• the progress of chemical reactions

A stable viscosity is therefore often a prerequisite for consistent product quality.

Newtonian and non-Newtonian fluids

When considering viscosity, a key distinction must be made between Newtonian and non-Newtonian fluids.

Newtonian fluids

In these media, viscosity remains constant regardless of mechanical stress. Typical examples are water or simple solvents.

Non-Newtonian fluids

In many industrial media, viscosity depends on the shear rate, i.e., the mechanical stress acting on the fluid. Such media are referred to as non-Newtonian fluids.

Typical examples include:

• polymer melts
• paints and coatings
• food products
• cosmetic formulations

Many of these media exhibit a structure-viscous or thixotropic behavior, in which viscosity changes under load.

Since the majority of industrial media belong to this group, in-process measurement is particularly important.

Measurement principle of modern inline viscometers

Many modern inline viscosity sensors, including solutions from Fluid.iO, operate on the principle of oscillating torsion.

A measuring probe is set into a controlled vibration. This vibration typically occurs at a resonance frequency of around 500 Hz with a constant amplitude in the micrometer range.

When the probe is in a fluid, friction occurs between the sensor and the medium, which leads to damping of the vibration.

The sensor electronics detect this damping and automatically compensate for it by increasing the energy supplied to the sensor. The energy required for this is directly related to the viscosity of the medium.

The higher the viscosity of a fluid, the stronger the vibration is damped and the higher the required energy input.

This change is evaluated electronically and output as a viscosity value.

Factors influencing viscosity

The viscosity of a medium is not a fixed value; it can change during a process.

The most important influencing factors include:

Continuous viscosity measurement enables companies to detect process deviations early and control production processes more efficiently.

Since these factors can vary constantly in production, inline measurement under real process conditions often provides significantly more reliable results than laboratory analysis.

Typical applications of inline viscosity measurement

Inline viscosity measurements are now used in many industries.

A common example is coating processes. Here, the viscosity of a coating directly influences spray behavior and the resulting layer thickness. Continuous measurement allows automatic control of viscosity via solvent dosing.

In polymer production, viscosity provides important information about the degree of polymerization and the current process state.

In the food industry, viscosity determines the consistency of many products, such as sauces, creams, or fruit preparations.

In addition, viscosity measurement is frequently used in mixing and reaction processes to monitor process progress or precisely achieve the desired final product state.

Installation of inline viscosity sensors in the process

Inline viscosity sensors can be installed both in pipelines and directly in tanks or reactors.

Important aspects of installation include:

• sufficient immersion depth of the sensor
• suitable flow conditions
• enough installation space
• a position with representative process conditions

In many cases, sensors can also be retrofitted into existing systems without major modifications.

The measured values can then be transmitted to a process control system via various interfaces. Commonly used interfaces include 4–20 mA, Modbus, or other industrial fieldbuses.

Calibration and commissioning

Modern inline viscometers are usually calibrated at the factory. Calibration is performed using several reference calibration fluids, resulting in a precise calibration curve.

During commissioning in the field, only a few steps are typically required:

1. installation of the sensor
2. connection of the electronics
3. performing a zero adjustment

After that, measurement can start immediately.

Due to the low mechanical stress on the sensing element, many applications do not require regular recalibration intervals, which reduces maintenance effort.

Conclusion

Inline viscosity measurement is now a central component of modern process monitoring. It enables continuous quality control, more stable production processes, and more efficient real-time plant operation.

Direct in-process use allows deviations to be detected early and production workflows to be optimized in a targeted manner—an important advantage for modern automated industrial processes.

Companies particularly benefit from:

• more stable production processes
• consistent product quality
• reduced laboratory analyses
• more efficient plants

FAQ – Frequently asked questions about inline viscosity measurement

Would you like to learn more about inline viscosity measurement and industrial process sensing?

Our experts would be happy to assist you in selecting the right sensor solution for your application.