Viscosity testing methods, standards, and instrument selection

Viscosity describes how easily a liquid flows. A high-viscosity coating is thick and slow moving; a low-viscosity one is thin and runny. For paints and coatings this single property influences almost every stage of use, from how a product is pumped and dosed in production, to how it atomizes at a spray gun, to how it levels out and resists sagging on a vertical surface. Measuring it accurately, with the right method, is how quality, R&D, and operations experts keep a coating performing the way it was designed to. 

Newtonian, non-Newtonian, and why shear rate matters

Some liquids, like water and solvents, have a viscosity that does not change with how fast they are moved. These are Newtonian. Most paints and coatings are not. Their viscosity changes with shear rate, the speed at which the fluid is deformed, and many are thixotropic, meaning they thin under agitation and recover at rest. This is by design: a paint should be thick in the can so pigment stays suspended, thin under the shear of a brush or spray so it applies evenly, and thick again on the wall so it does not run. Because of this, the shear rate a test applies matters as much as the number it produces, and different methods are built to measure at different shear conditions. 

Method 1: Flow cups

The simplest method is a flow cup. A sample fills a cup with a precision orifice in the base, and the time in seconds for it to drain is recorded, then converted to kinematic viscosity. Flow cups are fast, inexpensive, and ideal on the shop floor for lower-viscosity, near-Newtonian products. Industrial Physics supplies TQC Sheen ISO 2431 viscosity cups and immersion (dip) cups for on-site checks. The two governing standards are ISO 2431 and DIN 53211. Flow cups are less suited to thick or strongly non-Newtonian materials. 

Method 2: Krebs viscometer

The Krebs method is long established for architectural and decorative paints. The TQC Sheen Digital Krebs Viscometer (DV2300) drives a paddle in the sample at a constant 200 rpm and measures the torque, which is proportional to viscosity, reporting the result in Krebs units (KU), centipoise (cP), or grams. It reads across roughly 37 to 141 KU (about 10 to 5,000 cP) with accuracy of 2 percent of full scale, and suits routine mid-range paint QC where results are compared against a familiar KU specification. 

Method 3: Rotational cone-and-plate viscometer

When you need dynamic viscosity at high shear, close to how a coating behaves during application, the cone-and-plate method is the reference. The TQC Sheen Cone and Plate Viscometer (CP1) places the sample between a shallow cone and a flat plate and controls the shear rate tightly to 10,000 s-1 (BS and ISO requirements) or 12,000 s-1 (ASTM). These rates are representative of application by roller or brush, so the reading reflects real-world behavior. It measures to ISO 2884, part one of which covers the cone-and-plate geometry at high shear, along with ASTM D4287 and BS 3900-A7. It is the method of choice for R&D and formulation work on non-Newtonian coatings. 

Method 4: Multi-spindle (Brookfield method)

For the widest range of fluids, a rotating spindle viscometer measures the torque needed to turn a spindle immersed in the sample. The TQC Sheen Multi Spindle Viscometer conforms to the Brookfield method and is used across paints, coatings, inks, adhesives, and many other liquids, offering a broad measurement range through interchangeable spindles. The related Digital Rotothinner (DV2700) is a rotational instrument that also monitors viscosity continuously as solvents and thinners are added, with manual, max-hold, and timed modes across 0.1 to 350 poise (10 to 35,000 cP). 

Choosing the right method

As a rule of thumb: use a flow cup for a quick, low-cost check of thin, near-Newtonian products on the line; use the Krebs viscometer for mid-range decorative and architectural paints measured in KU; use the cone-and-plate viscometer for dynamic, high-shear measurement of non-Newtonian coatings in R&D and formulation; and use a spindle or Rotothinner instrument for broad-range work or for tracking viscosity during thinning. The right choice is driven by the product, the shear condition you care about, and the standard your customer or specification calls for. 

Calibration and common sources of error

Viscosity is temperature sensitive, so the single most important control is measuring at a defined, stable temperature; a few degrees can shift the result noticeably. Other common errors are air bubbles in the sample, worn or damaged flow cup orifices, inconsistent fill or timing, and comparing results from methods that apply different shear rates. Certified viscosity calibration oils, manufactured to ASTM D2162 and traceable to national standards, are used to verify instruments and keep readings accurate and repeatable. 

Why it matters

Viscosity control is quality control. It keeps film build, coverage, gloss, and finish consistent, prevents sagging and running defects, and gives production a fast, objective pass or fail on every batch. Measuring it correctly, to the recognized standards, is part of how Industrial Physics helps quality, R&D, and operations experts protect the integrity of their coatings and their customers’ brands. For more background, see our viscosity testing overview and the guide to types of viscometer.