Understanding your Viscosity Measurements

When the method reveals the rheological behavior of your products

In numerous industrial sectors such as paints, coatings, cosmetics, agri-food, chemicals, and pharmaceuticals, understanding how products behave under stress is critical. This is where rheology plays a vital role: it is the science that examines the flow and deformation behavior of fluids. Rheological analysis helps optimize formulations and manufacturing processes, while also ensuring the quality and consistency of final products.

However, in everyday practice, the instruments used to evaluate rheological properties – flow viscosity cups, rotational viscometers, cone-plate viscometers or rheometers are often perceived as simple measuring tools, capable of delivering a viscosity value or a flow time.

This simplified vision masks a more complex reality: these instruments do not just "measure", they reveal the intimate behavior of materials subjected to various constraints. Each method applies a particular type of stress to the product, and it is this mode of application that will condition the result obtained and its interpretation. The same product can thus appear fluid with one method and pasty with another, simply because the underlying rheological principles (pseudoplasticity, thixotropy, flow threshold, etc.) have not been considered.

In this article, we invite you to go beyond just reading numbers: by understanding how your instruments act on your samples you will discover their real rheological behavior. You will then be able to better choose your measurement method, better understand what happens during the tests, and above all, better interpret the data obtained.

Measuring is not simply quantifying, it is analyzing, understanding and anticipating.

Why does the choice of method influence your results?

When you measure a fluid or paste-like product, you evaluate its response to stress: this is the basis of rheology. But not all instruments measure equally. The type of stress applied therefore conditions the behavior you will observe and the results you will record. This has to do with a fluids different responses to stress. In general, liquids can be categorized in two different ways:

Newtonian and Non-Newtonian Fluids

Newtonian fluids are fluids, whose shear stress is proportional to the shear rate. In a Newtonian fluid, viscosity depends only on conditions like temperature and pressure, not on the applied shear rate (how fast adjacent fluid layers move past each other). That means if you double the shear rate, the shear stress doubles, giving a linear relationship between shear stress and shear rate. An ideal Newtonian fluid will have a constant viscosity at varying shear rates.
Non-Newtonian fluids exhibit viscoelastic tendencies, and their viscosity will vary depending on the shear rate or shear stress as well as possibly time. In practice, every fluid will have some degree of Non-Newtonian behavior, and it is the task of the rheologist to determine the influence of this behavior for the specific test environment. Non-Newtonian fluids are non-linear requiring more than one measurement point to determine the viscosity.

The most important takeaway from this, is that the viscosity is not an absolute value, but a function of the shear rate. It is also common for substrates to exhibit a yield value. This means that the fluid will only flow when a certain amount of force is applied to it.

Each instrument applies a different type of effort

Gravity driven flow: The consistometric cup (or flow cup) is based on a very simple principle: let the product flow by gravity through an orifice. It is the mass of the product and the earth's gravity that cause the flow. No precise or uniform constraints are applied. The result: this method reflects the overall behavior of the product under its own weight, without controlling the actual shear experienced by the sample.
Single shear: Rotational viscometers with fixed geometries or cone-plate viscometers impose a uniform shear between two surfaces (a disc or cylinder for rotational viscometers or a cone and a plate for cone-plate viscometer). The fluid is stretched and slides between these two surfaces at a well-defined speed. This allows for the precise measurement of apparent viscosity under conditions of constant shear. This is ideal for studying non-Newtonian fluids, whose viscosities vary with shear rate.
Progressive and continuous shear: The rheometer offers more finesse: it can control either the shear rate (like an advanced viscometer) or the applied stress. This makes it possible to carry out ramps (gradual shear rise), rest tests, flow threshold measurements or to characterize thixotropic or rheopectic behaviors. The rheometer explores the behavior of the product over a range of mechanical conditions.

Illustration of mechanical stresses and flow conditions

Each method therefore subjects the product to a very different stress profile:

Diagram explaining how a product flows through

Figure 1: A flow cup (a). Here the product flows under its own weight through an orifice. The shear rate is based on the density of the liquid and gravity. A rotational viscometer (b) where the viscosity is measured via the resistance of a spindle inside of a container. Here the shear rate is not constant, as the gaps are not defined. A cone and plate viscometer (c). Here we apply a uniform shear rate between a rotating cone and a stationary plate. The shear rate is constant and depends on the geometry of the cone. An oscillating rheometer (d). Here, shear stress or a shear rate is applied progressively which enables the testing of thixotropic behaviors, flow curves and yield values.

A closer look into the rheological principles

The flow behavior of your samples depends on the external stresses they are under. The samples can be influenced in different ways:

Shear-thinning Behavior: This behavior corresponds to a decreasing viscosity as the shear speed increases (e.g. water-based coatings, thick sauces, cosmetic gels).
During a flow cup measurement, the product flows quickly after a latency time, giving the impression of a low viscosity.
With a rotational viscometer: the measured viscosity will be higher at low speed and will decrease as the speed increases, revealing the shear thinning character.
Thixotropy: The product becomes more fluid under prolonged stress, then remains fluid at rest for a set amount of time until returning to the viscosity before shearing (e.g. paint). Thixotropic behavior can be measured by a rotational viscometer by utilization of a Time to stop measurement. This is not possible using a Flow cup.
With a rheometer, it is possible to measure an up and down ramp, which will result in a hysteresis cycle that allows for clear visualization of this behavior.
Rheopexy (less common): Rheopexy is the opposite of thixotropy: the product thickens when subjected to prolonged stress or shear.
Flow Threshold: The flow threshold is when a product only begins to flow after a minimum of stress has been applied (e.g. mortar, chocolate paste).
The Flow cup will be of little relevance because the product may not flow at all.
A rotational or cone-plate viscometer is only suited to analyzing this behavior in a limited capacity because only the speed of the rotation and not the applied shear stress can be controlled.
Only a rheometer can precisely measure the yield stress thanks to controlled stress ramps.

By choosing an instrument without knowing these principles, you can under or overestimate the actual viscosity of your products, and therefore misinterpret the pumping, spreading or storage behavior of your products.

How your measurements reflect the actual behavior of the product

To further illustrate these principles, we have chosen three different substrates that each were measured using a flow cup, a rotational viscometer and a rheometer.

Example 1: Paint-varnish

Paint-varnish is a non-Newtonian, pseudoplastic Liquid. It exhibits shear thinning behavior and therefore gives different results depending on the method.

Flow Cup: The product appears to be "stuck" at first (at rest, high viscosity), then begins to flow after a short delay. The shear that is being applied to the liquid depends on the size of the orifice of the cup and the weight of the sample. As more sample flows out of the orifice, less pressure is being applied to the fluid and therefore the shear decreases. This means that the measurement will be inaccurate, as the viscosity first decreases after opening the orifice, and then increases as the fluid levels decreases.
Rotational viscometer or cone-plate viscometer: At slow speeds, the product resists (high apparent viscosity). By accelerating the rotation, it becomes more fluid: the measured viscosity decreases. Shear thinning behavior is clearly observed.
Rheometer: The viscosity drop can be precisely characterized over a wide shear range. If the product is also thixotropic, the rheometer will also reveal its recovery time.

Possible misinterpretation: By using only a Flow cup, one could believe that this product is more viscous than it really is under industrial conditions of implementation.

Example 2: Thixotropic coating

Flow cup: The product flows but with a variable time, because the viscosity of the product decreases over time
Rotational viscometer (byko-visc RT, DS) or Cone and Plate viscometer: Thixotropic effect can be detected using a Time-to-Stop measurement.
Rheometer: Thixotropic behavior can be measured through a shear rise/fall loop or a time steps test.

Risk: Poorly adjusting the formulation by bad evaluation of the viscosity, which changes through time.

Example 3: Cosmetic gel with flow threshold

Flow cup: The gel will not be able to flow out because of its flow threshold or with a very long time and by closing the flow hole of the cup.
Rotational viscometer (byko-visc RT) or cone-plate viscometer: the viscosity measured at a point will not allow knowing the flow threshold. The ramp mode with curve fitting will give access to this structure data.
Rheometer: The threshold measurement by a stress ramp or an oscillation measurement

Risk: Interpret as a high viscosity the fact that the product does not flow by gravity.

Interpret better to produce better

It is therefore obvious that rheological measuring instruments provide numerical data — flow time, apparent viscosity, flow curve, stress, elastic modulus, etc. But this data is only valuable if it is correctly interpreted. For this, an understanding, even an elementary one, of rheology is essential.

A raw reading is not enough!

Without this theoretical basis, it is tempting to compare the results as simple numerical values: "product A = 4500 mPa·s; product B = 3000 mPa·s however, B is smoother.

The raw reading does not take into account the following:

The type of stress applied
The internal structure of the product
Thixotropy
Flow threshold

As a result, erroneous interpretations can lead to poor technical or industrial decisions. The right choice of method directly improves industrial performance.

Better Product Optimization: By knowing the actual rheological behavior of your formulas, you can:

Adjust the texture or viscosity agents more precisely
Formulate products that perform better in application (e.g., spreading, pumping, flowing, holding)
Anticipating the effects of ageing or storage

Example: The measurement of a shear thinning paint must reflect the ease of application on the substrate and its consistency at rest to avoid sagging. A high shear (cone-plane viscometer) combined with a low shear (rotational viscometer) measurement, or a rheometer can allow the complete characterization of the flow behavior throughout the lifecycle of the coating.

Chart showing viscosity decreasing as shear rate increases.

Better quality control: By choosing the right method for the targeted behavior of the product, you:

Detect production deviations faster
Target the right critical parameters (temperature, agitation, batch)
Reduce false rejections or incorrectly calibrated tolerances

Example: A food gel subjected to a Flow cup will appear to be "compliant", but a viscometer test will reveal a variation in structure if the gums have been poorly hydrated.

Improved reproducibility: A well-chosen, well-interpreted and well-understood measurement:

Is reproducible from one operator to another
Allows you to define robust tolerance ranges
Ensures the stability of the product in the logistics chain (filling, transport, end-use)

Example: If the viscosity is measured at the same speed but the product is thixotropic, the results will fluctuate with each measurement. Only a method that integrates a rest time and a stress history will ensure repeatability.

How to choose the right method? Three key criteria

1.Simple or in-depth measurement:

For rapid screening, production, inlet control — Flow cup or rotary viscometer are sufficient. They give a clear idea of the overall flow of the product.
To understand the structure, detect complex phenomena — Rheometer or cone-plane viscometer: they make it possible to draw complete curves and reveal subtle effects.

Tip: Start simple if you run out of data but switch to a finer method as soon as the product behaves unexpectedly.

2.Development (R&D) or production phase:

In R&D: You are looking to understand, optimize, predict. You need curves, profiles and process modeling. A rheometer is best suited for these applications as you will be able to identify and quantify the critical formulation parameters of the sample.
In production/quality control: Fast, robust and repeatable measurement methods are required. A fixed speed rotational viscometer or a Cone and Plate viscometer is often ideal, provided that the parameters have been well defined upstream.

Tip: Validate parameters in R&D with a rheometer, then translate them into simple protocols in production.

3. Stable or complex product:

Simple, Newtonian, homogeneous products (oils, syrups, single-component resins): Fixed-speed or cutting measurement: little variation, easy measurement.
Complex, unstable, time, shear or temperature sensitive products (gels, emulsions, suspensions, creams, sealants): Require an adapted test: rise-fall, cycles, thresholds — rheometer or advanced methods.

Tip: As soon as abnormal behavior or lot-to-lot variation appears, switch to a method to qualify the phenomenon (structure, recovery time, forced flow, etc.).

In summary – the right method is one that:

Reflects the actual conditions of use of the product
Allows you to clearly distinguish between a compliant product and a non-compliant product
Is adapted to your level of expertise, your resources and your field constraints

Comparison Table – Instruments, Detectable Phenomena and Product Types

Instrument	Detectable phenomena	Typical products	Recommended use
Flow Cup	Overall apparent viscosity (free flow)	Paints, sauces, glues, homogeneous semi-liquid products	Simple quality control, quick screening
Rotational Viscometer	Partial shear thinning, behavior at fixed speeds	Gels, creams, food products, polymers	Quality control, stability monitoring
Cone-plate Viscometer	Fine shear thinning, low-shear behavior	Clear liquids, concentrated solutions, fine cosmetics	Viscosity study in detail with small sample sizes

Conclusion

Measuring the viscosity of a product is not simply a matter of obtaining a numerical value: it is above all an attempt to understand its real behavior under constraint. Each measurement method, each instrument, highlights a specific aspect of the rheological behavior of your formulations. Thus, the choice of tool is never neutral: it conditions the nature of the information you will obtain... and therefore the quality of your decisions.

Choosing the right instrument means starting to analyze your product with relevance. Whether in R&D to optimize a formulation, in production to guarantee reproducibility, or in quality control to detect deviations, the right measurement protocol becomes a real lever for industrial performance.

Our team supports you in this process with instrumental solutions adapted to your products and your challenges, as well as personalized technical support to deepen the interpretation of your measurements and fully value your rheological data.

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