Rationale
Rotational viscometers determine viscosity by measuring the torque subjected to a rotor immersed in a sample at a constant rotational speed. Its basic principle is based on the laminar flow shear of Newtonian fluids between concentric cylinders, and the viscosity value can usually be obtained by the formula η = K · (M/ω) calculation, where η is the dynamic viscosity, K is the instrument constant, M is the torque, and ω is the angular velocity. The instrument provides continuous data on shear rate and viscosity relationship, making it suitable for analyzing the rheological properties of fluids.
A Stormer viscometer is a load-based viscosity measurement device that obtains viscosity values by measuring the gravitational load required to rotate a particular paddle in a sample at a fixed speed (typically 200 rpm), often expressed in Krebs units (KUs). Its working principle is closer to simulating the shear conditions in actual construction, focusing on obtaining a single-point viscosity value at a specific high shear rate to evaluate the application performance of the coating.
Comparison of application scopes
The two instruments differ significantly in terms of measurement range, data output, and application scenarios. The following comparison summarizes its core features.
| Rotary viscometer | Stormer viscometer |
| The measurement principle is rotational torque | The measurement principle is the load weight |
| Viscosity-shear rate curves can be output | Typically outputs a single-point KU value |
| Suitable for Newtonian and non-Newtonian fluids | Mainly suitable for non-Newtonian coatings |
| Wide measurement range and high accuracy | Narrow measurement range for medium to high viscosity |
| It is commonly used in R&D and quality control | It is often used for production site control |
| Thixotropy, yield stress, etc. can be analyzed | Focus on the evaluation of construction viscosity consistency |
Select the instrument according to the type of coating
The rheological characteristics of the coating system are the primary basis for selecting instruments. For systems close to Newtonian fluids, such as varnishes and low-viscosity solvent-based coatings, their viscosity is less affected by the shear rate, and the rotational viscometer can provide accurate absolute viscosity values to meet the requirements of data accuracy in R&D and quality control.
For most architectural coatings (such as latex paints), high-viscosity industrial coatings and other non-Newtonian fluids with obvious shear thinning behavior, the stormer viscometer is widely used for consistency control of production batches because its measurement conditions (high shear) are close to brush or roller coating construction, and the KU value obtained has a good empirical correlation with the construction feel.
When comprehensive rheological characterization of coatings is required, such as to study thixotropic recovery, yield values, or to establish complete flow profiles, rotational viscometers are a necessary tool. It helps formulators optimize anti-settling, anti-sagging, and more.
Practical application
In addition to the type of coating, the purpose and scenario of the test also need to be considered. In the laboratory research and development stage, it is necessary to deeply understand the relationship between material structure and performance, and the multi-rate test mode of rotary viscometer is more advantageous. When performing quick quality compliance checks on the production line or on the paint shop, the ease of operation and intuitive readings (KU values) of the Stormer viscometer make it a preferred choice.
Compliance with standards is also an important consideration. Many industry standards specify specific testing methods. For example, some architectural coatings standards specify the use of a stormer viscometer to report KU values, while some industrial coating standards may require the use of a rotational viscometer to report viscosity in pascal seconds (Pa·s) or millipascal seconds (mPa·s). Users should prioritize compliance with the standard requirements applicable to their products.
On the whole, the choice is not exclusive. Many professional coating laboratories will be equipped with both instruments at the same time, using the stormer viscometer for rapid production monitoring and rotary viscometer for in-depth formulation analysis and problem diagnosis, so as to achieve complete quality control from R&D to production.
References
ASTM D2196-20, Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer.
ASTM D562-10(2021), Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer.
ISO 3219:1993, Plastics — Polymers/resins in the liquid state or as emulsions or dispersions — Determination of viscosity using a rotational viscometer with defined shear rate.