Viscosity and Viscometer Measurement Q&A

Viscosity Defined

1. What is the viscosity of my product?

Kinematic viscosity n (nu), resulting from a flow time measurement, takes account of gravity and concerns rather less viscous fluids and simple rheological behavior: Newtonian fluids. It is expressed in Stokes, cStokes or cm2/s.
Dynamic viscosity h (Eta) qualifies most formulated products; it is free from the effect of density and is most measured with rotary instruments. It can consider the rheological behavior of the product, which gives it an absolute character. Its units are Pa. s and mPa. s (1 mPa.s = 20°C water viscosity) or Poise or cPoise.
In the field of polymers, there are other viscosity values such as specific viscosity, intrinsic viscosity, and relative viscosity, which make it possible to calculate the average viscous molar mass of these polymers in solution

2. Is the measure of the defined viscosity relevant? The answer to this question raises other questions:

Why do I need the viscosity value? Meet a standard, check the stability of the product quality, validate an industrial process, innovate, provide a specification to a customer.
Defining the purpose of this measure is the first step.
Does the temperature at which I am measuring reflect the time I want characterize? it is very important to be as close as possible to what the product undergoes in its life cycle.
Do the quantified results of this measure allow me to identify discrepancies that I judge between two products? The precision of a measurement is not enough to make it relevant.
What information should I communicate around this viscosity value? Specify the standard used where measurement conditions will help to dialogue between services, with your customers and your suppliers.
Why don’t I find the same viscosity value as my supplier? This requires a precise and clear dialogue on the measures established by each other. The more information that accompanies the viscosity value provided, the easier it is to make a measurement in the closest conditions with its own means.

Viscosity Measurement

1. Which parameter is important to perform an accurate viscosity measurement? The most important factors to know or control are the following:

Temperature, because according to the chemical formulation of sample, the temperature could affect the viscosity value. Even the increasing of temperature will give lower viscosity values, it is recommended to compare samples’ viscosities or to be in accordance with standard values.
Speed or shear rate is the major parameter which influence the viscosity of formulated products, that are non-Newtonian fluids. This induces to use defined or standard geometry where this parameter is well-known; like cone-plate, coaxial cylinder systems.
Time is the third variable to control for several types of products, because viscosity has tendency to decrease when the shearing is longer than if it is short. This thixotropic effect is rare, but it could affect drastically process and trouble also accuracy of viscosity control.

2. Viscometer choice, which technology to choose?

Falling Ball Höppler viscometer, standardized into pharmacopeia viscosity controls, is ideal for Newtonian clear syrup and lotions; a set of different size balls enable to measure the range of viscosity and it is possible to control the sample temperature through an external water circulating to connect to a bath.
Standard flow cups, with defined volume and calibrated hole diameter, so present into coating and paints area, easy to use and perfectly adapted to solvent paints.
Rotational viscometer with standard spindle, to answer to ASTM/ISO standard; most popular system to measure relative dynamic viscosity of all type of products; the only one precaution to take is to consider all parameters for an accurate measurement: speed, spindle, volume of sample, time, etc.
Rotational viscometer with shear rate defined geometries (cone-plate, coaxial systems), DIN/ISO standards compatible, recommended for non-Newtonian products into all activity domains. most defined and absolute viscosity values are obtained with those configurations.

Rheology - How Fluids Behave

1. What is the link/difference between Rheology and Viscosity?

Viscosity is the unit element that will be used in rheological studies conducted on a product. It is therefore preponderant and intrinsically linked to the behavior of product rheology.
Rheology, or science of flow, makes it possible to know and understand, how a fluid will behave under the influence of parameters during its manufacture, its packaging, its storage, its transport, and of course its use as a whole.

2. Which instrument to choose to analyze rheology of my product? Often sophisticated, complex, and expensive solutions are recommended, but are in practice not always the best solution. Therefore, we recommend taking a pragmatical approach according to the product to analyze:

What volume?
What order of viscosity?
What is its nature?
Product loaded or not?
What level of expertise is desired (quality control, R&D, basic research)?
What is my budget?
Do I have the in-house skills to properly use such equipment and interpret the data?

The answers to these questions will guide you towards the most convenient choice in terms of material, accessories, and methods according to your needs.

Accurate Measurements with a Rotational Viscometer

Ensure that your viscometer is calibrated properly by testing it with an ISO 17025 certified calibration oil.

Prepare your sample in accordance with a standard test method, such as ASTM D2196-10 Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer, or ISO 2555, ISO 1652 [1].
Use the same viscometer, spindle, rotational speed(s), test time, container shape, size and placement, and sample size for repetitive and QC testing. Always use the spindle guard leg.
Control the ambient temperature as well as the temperature of your sample, spindle, and spindle guard with a temperature bath or various accessories to ensure accurate and repeatable results. Allow everything to equilibrate for at least one hour before measuring. Use an accessory sample temperature probe with your instrument during measurement to ensure that you maintain a constant sample temperature.
Ensure that your spindle is clean, shaft is not bent and has not any dings or dents.
Ensure that you do not introduce air bubbles into your sample during preparation.
Be careful not to shear the sample while preparing through shaking, stirring or mixing as some materials (shear thinning / shear thickening) take time to recover to their resting viscosity.
Ensure that your sample container is clean without any reside from prior tests.
Ensure that the spindle is immersed in the sample up to the middle of the line on the shaft. Overfilling or underfilling can result in erroneous results.
Avoid turbulence, normally caused by higher speeds, which can alter results. This is especially important with lower viscosity samples.
Ensure to use the same test time as many fluids change viscosity over time.
In most cases ensure that the spindle rotates at least 5 times before you record a value. This may need to be adjusted with some non-Newtonian fluids. Allow the displayed viscosity to stabilize before measurement.
Prevent the sample from drying or evaporating, as it will lead to higher viscosity.

Choose the Correct Viscometer Range and Spindle

Rotational viscometers are manufactured in three different viscosity ranges to enable a broad range of viscosity measurement.

Determine the viscosity range that is close to the viscosity of the products that you will be measuring - either low, medium, or high viscosity.

Low Viscosity	Medium Viscosity	High Viscosity
Adhesives (solvent base)	Adhesives (hot melt)	Asphalt
Biological Fluids	Ceramic Slurries	Caulking Compounds
Chemicals	Creams	Chocolate
Coatings, Water-Based	Dairy Products	Epoxies
Edible Oils	Detergents	Gels
Inks, Water-Based	Dressings & Sauces	Grease
Juices	Gums	Inks (ballpoint, offset)
Liquid Soap	Inks (screen printing)	Mayonnaise
Liquid Wax	Lotions & Cremes	Molasses
Mouthwash	Paints	Ointments
Paints, Latex	Paints & Coatings	Pastes
Photo Resist	Paper Coatings and Pulp	Peanut Butter
Soda	Plastisols	Putty
Solvents	Shampoo & Conditioner	Roofing Compounds

Select Spindle and rotational speed. There are several factors that you should consider before selecting a spindle and rotational speed:

To duplicate a method or result, use the same spindle, rotational speed, container, and sample size used in the method to be duplicated.
To approximate a particular shear rate, for example the shear that will be created when your product is applied, you should choose a rotational speed that will approximate that shear rate.
You know the viscosity of the sample to be tested, use the appropriate setting on your viscometer. Input the spindle code and RPM setting. The instrument will display the viscosity range of that combination. Try different combinations to select the appropriate spindle.
You do not know the viscosity of the fluid to be tested, your goal is to find a speed and spindle combination that will give you a torque reading between 10% and 100%. For accuracy look for the highest possible torque reading below 100%. Measure your sample with the chosen spindle at various speeds. If you can’t obtain a reading between 10% and 100% try a different spindle. If your reading is above 100% at the slowest speed, choose the next smallest spindle. If your reading is below 10% at the highest speed, choose the next largest spindle.
You need to test at multiple speeds, choose a spindle that will yield readings between 10% and 100% for at least three speed settings.

The lowest viscosity range can be measured with the biggest spindle at maximum speed. The highest viscosity range can be measured with the smallest spindle at the lowest speed.

Evaluation of Viscosity Flow Curves

Newtonian Differential viscosity and viscosity coefficient are constant with shear rate.
Shear-thickening Differential viscosity and coefficient of viscosity increase continuously with shear rate.
Shear-thinning (pseudoplastic) Differential viscosity and coefficient of viscosity decrease continuously with shear rate. No yield value.
Shear thinning (pseudoplastic) with yield response Differential viscosity and coefficient of viscosity decrease continuously with shear rate once the apparent yield stress has been exceeded.
Bingham plastic (ideal) Obeys the Bingham relation ideally. Above the Bingham yield stress, the differential viscosity is constant and is called the plastic viscosity, while the coefficient of viscosity decreases continuously to some limiting value at infinite shear rate.
Bingham plastic (non-ideal) Above the apparent yield stress the coefficient of viscosity decreases continuously, while the differential viscosity approaches a constant value with increasing shear rate. Extrapolation of the flow curve from the linear, high shear rate region (plastic region) to the stress axis gives the apparent Bingham yield stress. The differential viscosity in the linear region is termed the plastic viscosity.

Viscosity methods and influence of shear rate

Most paints are non-Newtonian liquids, their viscosity depends on the applied shear rate, Shear rate is a measure for how the paint is sheared or worked during a flow. Many paints have a lower viscosity when high shear rates are applied.

Different viscosity measuring methods apply different shear rates during measurement. It is important to choose a method that provides a reproducible shear rate when checking the quality of paint. The image gives an overview over shear rates applied at certain applications as well as measuring methods.

1. Rheometers are the instruments of choice to study the entire rheological behavior. While these instruments will give you the full picture, they are complicated in usage and data interpretation requiring highly trained personnel and are most expensive.

2. Rotational Viscometer are easy to operate and most often used for QC applications. Like rheometers they give repeatable and reliable results and operate at shear rates most suited for the respective application

Basic rotational viscometers a very versatile and cover a broad range of shear rates and viscosities.
Krebs or Stormer viscometers are best used with paints that will be spread with a paint brush or roller, that is paints that will be applied at a medium shear rate from 10 to 100 s-1.
Cone and plate viscometers are often used at high shear rates and therefore allow control over the paint’s viscosity during application but can measure also at shear rates down to 20 sec-1 as well.

3. Flow Cups are available at a low cost and offer a quick way to check a viscosity. Their primary result is the efflux time that can be calculated into kinematic viscosity. They should only be used for Newtonian liquids for accuracy. See the Insta Visc Viscosity Calculator app to help to calculate the viscosity from measurements with flow cups

Viscometers for special applications

Cone and Plate Viscometer used to evaluate dynamic viscosity measurement as described in DIN ISO 2884-1 and ASTM D4287.Â Cone & plate viscometers are a practical tool for any QC or R&D lab requiring quick and easy testing of materials, regardless of application. They are suitable for Newtonian or non-Newtonian materials with viscosities up to 15,000 poise and shear rates from as low as 25s-1 up to 13000 s-1. Available with either fixed or set speeds and built in, heating and cooling to allow testing from 5°C to 235°C.
Krebs Stormer Viscometer The most popular method to determine viscosity of architectural paint uses a Krebs Stormer viscometer as described in ASTM D562. The Stormer viscometer uses a paddle that rotates through the paint at 200 rpms in a standardized container. The resistance created by the paint is measured and expressed in Krebs units, or KUs. The higher the KU number, the more viscous the paint. Modern Krebs Stormer viscometers such as the BYK byko-visc DS also simultaneously display viscosity in centipoise (cP) and grams (gm). The BYK unit is useable with viscosities in the 40-141 KU range, which is equivalent to 27-5274 centipoise (cP) per ASTM D562. They are simple, easy to use, and yield operator independent results with no calculations needed.
Measurement of Creams, Gels and Ointments Flow Characteristics are extremely important in the Cosmetic industry. Cosmetics have a shear-thinning behavior meaning the viscosity decreases when the shear strain we impose on it increases. Moreover creams, ointments and gels have a plastic behavior, that means they don’t flow only on gravity effects. Sometimes they also add some visco-elastic properties meaning they can range from solid-like products to liquid-like products.
Rotational Viscometer You can apply a defined speed of rotation (ISO 2555) or obviously a shear rate or shear rates ramp to a sample (ISO 3219). This helps formulators understand what force is required to enable the products to start flowing, The amount of force required to start the cream or gel to flow is called the yield stress. When the sample starts to flow it takes on a shear-thinning behavior. Shear thinning - is the non-Newtonian behavior whose viscosity decreases under shear strain. Using the rotational viscometer allows formulators to define the shear thinning curve based on defined shear rates (ISO 3219). Thus, a better understanding of how the product will flow or be easy to apply to the skin, is achieved.

Verification & Calibration of Viscometers with 17025 Certified Viscosity Standard

When using a viscometer to test substances of varying viscosities it is important to ensure that your viscometer is calibrated in the viscosity range of the products that you are testing. Modern rotational viscometers like the BYK byko-visc RT series enable the user to verify the calibration, as well as to calibrate their instrument on site using ISO 17025 certified standard oils.