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Viscosity testing is one of the most important methods used to check the quality of materials. Various industries rely on viscosity checks of their products to produce a product with consistent texture. Many important parameters for the production control of materials and also for the development of new products are directly related to the product’s viscosity. In nearly all production stages the viscosity of the material has a great impact, e.g. in the mixing process and while pumping liquids through pipes. Incoming liquid raw materials also have to be checked using viscosity measurements. Rotational viscometers are perfectly suited for the determination of the viscosity of a wide range of different samples. Liquid up to semi-solid samples are suitable for viscosity testing with rotational viscometers. Since there are various rotational viscometers and numerous corresponding spindles on the market, this article should give guidelines for choosing the relevant configuration.
Types of rotational viscometers
Rotational viscometers measure the viscosity of the sample by turning a spindle in a cup. The viscosity is determined through the measurement of the torque on a vertical shaft that rotates a spindle. Two different setups are used when measuring viscosity using a rotational viscometer:
This article only focuses on spring-type instruments. Spring-type rotational viscometers measure as follows: The rotation of the spindle deflects a spring. Optical sensors detect the deflection and the viscosity of the sample is then calculated from it. The test sequence is the following: The user attaches a spindle to the rotational viscometer, sets a speed, and receives the dynamic viscosity and the torque (mostly in %). The resulting driving torque depends on the rotational speed w, the spindle geometry, and the sample viscosity.
In case of low-viscosity substances, the spring needs to be sufficiently sensitive, whereas for samples in the high-viscosity range a more robust spring is required. Consequently, different instrument models are available, with different spring types to cover a broad range of applications, e.g. three different rotational viscometer models can be distinguished:
- Spring type to measure low-viscosity samples. Hereafter called "L-type".
- Spring type to measure medium (regular)-viscosity samples. Hereafter called "R-type”.
- Spring type to measure high-viscosity substances. Hereafter called “H-type”.
Furthermore, spring-type viscometers can be divided into two different types:
- Dial reading: The torque value in % is shown by the pointer on the dial. To convert the torque % reading to viscosity in mPa·s, the dial reading has to be multiplied by the appropriate factor for the spindle and speed in use (Equation 1).
- Digital reading: The viscosity is automatically calculated and displayed on a screen for every spindle/speed combination. There are no further calculations necessary.
Which torque model to choose for which application?
Viscosity measurement can be important for all liquid to semi-solid materials we know from daily life (Figure 1). A spring-type rotational viscometer can be used for countless different applications in various industries. The biggest fields of application for such instruments are the chemical, petrochemical, food, beverage, personal care, and pharmaceutical industries.
Depending on the viscosity of the sample a certain instrument with the accurate torque range has to be used (Table 1). The L-model is suited for the measurement of low-viscosity samples, such as solvents, oils, juices, ink, and mouthwash. The R-model is suited for the measurement of medium-viscosity samples, such as paints, coatings, adhesives, and dairy products. The H-model is suited for the measurement of high-viscosity samples, such as mayonnaise, peanut butter, pastes, and ointments.
Table 1: Overview of torque models and their typical applications
|Chemicals & petrochemicals||Food & beverage||Pharma & cosmetics|
|Oils, lubricating oils||L||Dairy||R||Shower liquid||L|
|Liquid wax||L||Dressing, sauces||R||Shampoo, lotion, cream||R|
|Paints, coatings||R||Blancmange, vanilla sauce||R||Detergents||R|
|Adhesives, epoxies||R||Chocolate and cocoa products||H||Ointments, gels||H|
Which spindle to choose for which application?
For each torque model different spindles exist so that samples with different viscosities can be measured. Usually interchangeable spindles in the form of disks and cylinders are used. They are fixed on the coupling of the instrument. For a given viscosity the flow resistance is related to the spindle’s speed of rotation and its shape and size. The flow resistance increases with the speed and size of the spindle. What does that mean? The lowest viscosity range can be covered by measuring with the biggest spindle at maximum speed. The highest viscosity range can be covered by measuring with the smallest spindle at the lowest speed. For a better reproducibility the same spindle/speed combination should be used for multiple tests.
The maximum measurable viscosity of the spindle at a given speed is called the full scale range (FSR). In other words, the FSR is the maximum viscosity which can be measured with the chosen spindle/speed combination. The minimum viscosity that can be measured is one tenth of the full scale range. By knowing the FSR of the spindle/speed combination, it is possible to determine whether that spindle/speed combination fits to the viscosity of the sample. If the viscosity of the sample is unknown, the viscosity is tested by taking the smallest spindle first and replacing it in ascending order by the next larger spindle until a valid measurement result is achieved. To obtain a valid measurement the torque value must be between 10 % and 100 %. If the torque value is higher than 100 %, a smaller spindle has to be used. If the torque value is lower than 10 %, a bigger spindle has to be used. The higher the torque value the better the accuracy is, since the accuracy of the measuring system depends on the full scale range (usually 1 % of FSR).
Each instrument model typically has a set of disk spindles (Figure 2). The set depends on the torque range of the instrument. An L-instrument, such as ViscoQC™ 100-L , usually contains four spindles and an R/H-instrument usually contains six spindles. Disk spindles produce accurate and reproducible results. Nevertheless, it is crucial that these spindles are relative systems because the shear gap is not defined (so-called “infinite gap” systems) . Disk spindles are used in 600 mL beakers with a minimum inner diameter of 83 mm. They measure the viscosity according to ISO 2555. The viscosity value is only comparable to results of the same instrument type with the same setup. An accessory used for disk spindles is the spindle protector. It consists of a metal plate that has a “U” form. It is recommended to always use the spindle protector for measurements on both the L-model and the R-model. While protecting the spindle against e.g. bending, the spindle protector also influences the measurement results, causing a different flow behavior of the sample in the beaker. For the H-model, no spindle protector is needed for viscosity measurement. To get reliable results it is necessary to always use the spindle protector when it’s mandatory and to use a 600 mL beaker with a defined geometry (approx. 83 mm inner diameter).
Cylindrical spindles (Figure 3) when used with the corresponding cup are called concentric cylinder systems (Figure 4). Concentric cylinder systems are absolute measuring systems. Due to the defined spindle geometry it is possible to calculate shear rate values. This means the concentric cylinder systems measure the viscosity according to ISO 3219 because of the defined shear gap. Concentric cylinder systems are useful for the measurement of non-Newtonian samples. There are several reasons to prefer absolute measuring systems:
- Defined gap of the measuring system: Provides constant shear conditions so that measurements are independent of the instrument and measuring system.
- Mathematical models to analyze flow/viscosity curves: Mathematical models such as the “Bingham yield point calculation” can be applied by using such measurement systems.
- Low sample volume: The sample volume needed for the measurement is relatively low (approx. 2 mL to 20 mL) in comparison to the volume needed for disk spindles.
Double-gap measuring systems (Figure 5) are especially suited for measuring low-viscosity samples (≥1 mPa·s) according to DIN 54453.
Tips for a successful rotational viscosity measurement
- If available, the sample should be prepared according to a suitable standard test method, guide, or practice.
- The sample preparation can have a considerable effect on the measurement results. Thixotropic materials in particular are affected by stirring/mixing/pouring procedures. Thixotropic behavior of materials is characterized by a decrease in viscosity at constant shear conditions over time.
- For a good reproducibility the appropriate sample container has to be used for the measurement and placed concentric to the spindle.
- A sufficient sample filling height is important because the tip of the spindle should be at least ten mm above the vessel’s base. If used, a spindle should be immersed as far as the mark on its shaft.
- To avoid changing the rheological structure of the sample, spindles should be slowly dipped into the sample vessel. Incline disk spindles to avoid trapping air bubbles at the base of the spindle.
- In order to get reliable measurement results, the spindle should have completed at least five full turns before a value is taken. If not applicable the readings should be taken after a specified period of time . In particular for non-Newtonian samples viscosity/torque % readings may not stabilize after five full turns because of their thixotropic behavior. In that case the readings should be taken after a defined period of time. Rule of thumb: 20 seconds for speed >5 rpm, 60 seconds at least for speeds <5 rpm
- An error source during measurements might be turbulences which occur at high speeds. Turbulences (Eddy currents) can cause higher viscosity readings.
- In the case of high-viscosity substances (>30,000 mPa·s) shear heating is a potential error source during measurement. It is not recommended to set a speed higher than 100 rpm.
- As viscosity is strongly temperature-dependent a constant temperature of the liquid during the whole measurement is very important. Tracing the sample temperature with a Pt100 temperature sensor is recommended.
The market offers many different rotational viscometers for analyzing the viscosity of liquids for quality control purposes. To find the instrument model that suits a certain application best, some factors have to be taken into account first. First of all, the viscosity range of a sample has to be considered (low, medium, or high). Furthermore, the correct choice depends on the speed required for analyzing the liquid sample. The available sample volume also has a great impact on the choice of the measuring system. If only a small sample amount is available for the measurement, concentric cylinders in combination with the right torque model have to be chosen. The flow behavior of the sample – for example shear-thinning or shear-thickening behavior – or thixotropy narrows the selection. In the majority of cases more than one instrument model is suitable for measuring a sample’s viscosity in rotation.
- Mezger, T. (2011). The Rheology Handbook. 3rd revised ed. Hanover: Vincentz Network
- ISO2555:2017: Plastics – Resins in the liquid state or as emulsions or dispersions – Determination of apparent viscosity using a single cylinder type rotational viscometer method
- ASTM E2975-15: Standard Test Method for Calibration of Concentric Cylinder Rotational Viscometers
- ISO 3219:1994-10: Plastics – Polymers/resins in the liquid state or as emulsions or dispersions – Determination of viscosity using a rotational viscometer with defined shear rate
- DIN 54453:1982-01: Testing of adhesives for metals and of bonded metal joints; dynamic viscosity determination of anaerobic adhesives by rotation viscometer
- ASTM D2196-10: Standard Test Methods for Rheological Properties of Non-Newtonian Materials by Rotational (Brookfield type) Viscometer