You intend to buy a viscometer?
What basic questions should you ask yourself before buying – which points should you discuss with the seller? Below you may find a short list of possible questions and answers.
What kind of viscometer (handheld-, table- or built-in device) do you need?
Do you need a table - viscometer, which you might like to use once as a mobile device – or should it be really a portable (handheld) instrument?
If you would like to use the viscometer in the process for continuous measurement of the viscous liquid, then there are a few more questions on this topic for you to answer, besides these initial ones. In this case, it makes sense to talk to one of the technicians of PCE Instruments on the telephone or to arrange an appointment with our sales representative.
What is the application area / What type of liquid do you measure (Newtonian, non-Newtonian, dilatant, thixotropic or plastic)?
As you can see from the question, you want to measure viscosity in liquids and not in gases. When it goes about measurement in liquids, first, a distinction between Newtonian liquids (linear relationship) or non-Newtonian liquids (time- and shear velocity-dependent flow process) must be made. The shear velocity-dependent flow process is called dilatancy and the time-dependent flow process is referred to as thixotropy. Please, think at once what type of the fluid you are testing. In the case of a plastic flow, a limit value of the shear stress must first be exceeded, in order to start the flow process.
What measuring range / viscosity range do the liquids to be tested have?
You can assess the viscosity range of the liquid you are testing. There are tables in the literature which approximately depict the expected viscosities of different fluids. For example, further you can see the classification of some known substances:
Substance: Viscosity η (mPa•s)
Water (20 °C): 1
Engine oil (150 °C): 3
Blood (37 °C): 4 - 25
Engine oil (25 °C): 100
Earth’s mantle: 1021 – 1023
Is a temperature measurement / temperature control on the viscometer necessary?
The dynamic viscosity generally decreases with the fluid’s temperature increase. Therefore, in order to obtain always comparable measured values, it is recommended to use a tempering bath. In this way, the same general conditions always prevail and you can expect a high comparability of the measured values. However, it also depends on the type of the fluid to be measured, whether a temperature control is needed or not. Also, the surrounding pressure is also a critical parameter, which influences the comparability of the measured values.
Which reproducibility of the results and / or which measuring accuracy should be achieved?
Often the viscometers with an accuracy of ± 3 % can be found in the production laboratories. In the chemical laboratory, however, viscometers with at least ± 2 % should be used. Most devices from PCE Instruments offer accuracies of ± 1 %.
Is service offered? What about the provision of spare parts, e.g. spindles?
Please, make sure that the supplier / manufacturer offers sufficient amount of spare parts for the viscometer, especially it applies to measuring spindles.
Are the viscometers calibratable? Is a recalibration offered?
When ordered for the first time, all PCE Instruments viscometers are adjusted and calibrated. In the case of a recalibration, calibration oils (silicone oils) are usually used. Then, the temperature dependence matters again. Therefore, the respective temperatures are also noted on the ISO calibration certificates. The calibration oils themselves must be stored according to the manufacturer's instructions (in the dark, in closed containers), since they are not long-term stable (half a year). In addition to the check by means of calibration oils, during a traceable calibration, the check of the rotational speed / torque and the mechanical condition (essentially the concentricity of the axis for the rotary viscometers) of the device and various attachments is mandatory. If during calibration a deviation from the values specified by the manufacturer occurs, a new adjustment of the viscometer must be carried out.
Viscosity describes the fluidity (the ability of fluids to flow), i.e. of liquid and gaseous substances. It can be determined with the help of a viscometer. The higher the viscosity of a substance, the more sluggishly it reacts. If substances with a high viscosity are to be moved or mixed with the other substances, more energy must therefore be expended. In order to be able to compare the viscosity of samples, various viscometers have been developed that measure quantities related to fluidity. When using these viscometers, it must be taken into account that not every measuring method is suitable for all measuring ranges of viscosity. It must also be taken into account that the viscosity of a substance is always dependent on the temperature and can also change under the influence of forces.
Viscosity can also be described as the resistance of the flowable substance to the change of the position of its volume particles. The particles can interfere with each other more or less when changing their position. Spherical particles hinder each other less than disc-shaped or irregular ones. Linearly arranged molecules slide past each other more easily than the branched ones, and the small molecules – more easily than the large ones. The increasing number of cross-links also makes it more difficult for the particles to slide past each other.
Applications for viscometers
With a viscometer, either dynamic or kinematic viscosity can be determined. The dynamic viscosity η [Eta] is defined by the shear stress and the velocity gradient perpendicular to the direction of flow. It is measured in pascal seconds (Pa·s) or, according to the CGS system, in poise (P). One poise corresponds to 0.1 pascal seconds. Instead of Pascal seconds (Pa·s), also Newton seconds per square metre (N·/m²) or kilograms per metre and second (kg/(m·s)) can be used in the International System of Units. The kinematic viscosity ν (Ny) results from the quotient of dynamic viscosity and density of a substance. It is given in m² per second (m²/s) or according to the CGS system in stokes. One stoke corresponds to 0.0001 m² per second. For example, the kinematic viscosity is determined with the standardized flow cups. The dynamic viscosity results when the kinematic viscosity is multiplied by the density of the substance.
To simplify the planning of transport and processing, fluids can be classified as low, medium and high viscosity substances. This classification facilitates the assignment to suitable piping and dosing systems. The limits are at about 300 millipascal seconds for the transition between low and medium viscosity substances and at about 8,000 millipascal seconds as the lower limit for the high viscosity substances. Standardized flow cups are only suitable as viscometers for low-viscosity liquid substances. For viscous materials, the viscometers with rotating measuring spindles and adjustable speed can be used. For some substances, the viscometers are also suitable in which the measurement is carried out via the rising of the substance in thin tubes. Special forms of these viscometers with capillaries were named after Ubbelohde and after Cannon and Fenske. Besides, there are also viscometers in which the spread of the sample is recorded in a horizontally positioned measuring pan. This method measures how far the sample flows on the scaled pan bottom in a defined period of time. These viscometers, named after Bostwick, are mainly used in the food industry to determine the consistency, viscosity or flow rate of sauces, soups, ketchup or jams.
The decision about a certain type of viscometer should mainly depend on the purpose of the viscosity measurement. For many applications, liquids, pastes and gels should have a high viscosity at low shear stress to prevent the deposition of ingredients. When there is a requirement for a high shear, however, they should be thinner so that they could mix better and could be applied to the surfaces as a uniformly thin film. For these substances, the viscometer should be able to determine viscosity at variable shear stresses and temperatures. Digital viscometers with a set of rotating spindles, adjustable speed and temperature measurement allow the creation of detailed curves of the viscosity curve.
Flow cups as viscometers
The flow cups for viscosity measurement are hollow bodies with standardized shapes and dimensions. If they have the same shape, there are usually different standard sizes for the lower opening. This type of viscometer is often used to determine the run-out time and viscosity of paints, varnishes and other products that flow well. Paints for application in the spray guns, for example, can only be processed up to a certain viscosity. The measurements can be carried out on the end product as well as on the samples from the production process. The lower the viscosity is, the faster the substance flows out of the flow cup. This time – the run-out time – is directly related to the viscosity. This type of viscometer allows the measurements at different temperatures, but not reproducible tests under the influence of additionally introduced shear forces. The kinematic viscosity is calculated from the run-out times via the fixed factors and from this, with the material-specific density, the dynamic viscosity.
Viscometer as a flow cup according to DIN 53211 (withdrawn in October 1996)
These cups have a volume of 100 ml, a conical taper and a fixed outlet with a diameter of 4 mm. They are usually made of anodized aluminum and have an internally polished stainless steel nozzle. The cup is filled to the brim and then the time until the flowing jet breaks off is stopped. The measurements outside the range of 25 to 150 seconds should not be counted. For the users who switch from measurements according to DIN 53211, which was already withdrawn in 1996, to the valid DIN EN ISO 2431, there are conversion tables for the run-out times.
Run-out cups according to DIN EN ISO 2431, ASTM D 5125
The international standard ASTM D 5125 (2010) - Viscosity test methods for paints and related materials using ISO flow (run-out) cups – also describes a method for viscosity measurement using a run-out cup. The measuring range is extended by means of other internal dimensions, a larger inlet cone and a longer nozzle for which four different diameters are defined. The run-out times for these cups should be between 25 and 100 seconds. Otherwise, the cup with the next larger or smaller opening can be changed. However, the run-out times of different cups may only be compared after conversion.
ISO cup with 3 mm outlet for viscosity 10 to 40 mm²/s
ISO cup with 4 mm outlet for viscosity 25 to 130 mm²/s
ISO cup with 5 mm outlet for viscosity 70 to 360 mm²/s
ISO cup with 6 mm outlet for viscosity 130 to 700 mm²/s
Ford flow cups according to ASTM D 1200
In the standard ASTM D 1200 (2010) – Determination of the viscosity of paints and varnishes; method with the Ford cup – another type of flow cup is described. The run-out time here should be between 20 and 100 seconds. The measurable kinematic viscosities are limited as follows:
Ford cup no. 2 with 2.53 mm run-out for viscosity 25 to 120 mm²/s.
Ford cup no. 3 with 3.40 mm for viscosity 40 to 220 mm²/s
Ford cup no. 4 with 4.12 mm run-out for viscosity 70 to 370 mm²/s
Viscometer with rotating spindle – Rotating viscometer
In a rotating viscometer, the rotating rod or spindle is immersed in the sample and set in rotation by a motor. The more viscous the substance is, the more force must be applied to maintain a constant rotational speed. Since the shape of the spindle also has a great influence on the amount of force required for the rotational movement, different spindle shapes have been developed for the viscosity of different substances. Due to their characteristics, these viscometers with rotating spindle can be used for different tasks to determine the dynamic viscosity.
At constant speed and with spindles that introduce constant shear rates into the sample, it is possible to check whether the viscosity of the substance increases or decreases the longer the spindle is rotated. Thixotropic substances liquefy with longer stirring, rheopexic substances become thicker. If such a time-dependent increase or decrease of the dynamic viscosity is observed, it is essential to take this into account when determining the test procedure and evaluating the measurements. Otherwise, no useful results can be obtained.
Many rotating viscometers offer the possibility of setting different rotational speeds for the spindle. If you measure once at a lower speed and then at a higher speed, you can quickly determine whether the viscosity of a substance changes when higher shear forces are applied.
Modern rotational viscometers are available with a wide variety of equipment optimized for the measurements on samples in the laboratory or on site at the production facilities or storage or transport containers. For the measurements in the laboratory, the viscometers with a stand are often used, where the sample is filled into a beaker. For on-site measurements, the rotating spindle equipped with a flexible cable can be inserted into the available containers. Often these viscometers have an integrated temperature measurement or the possibility to connect a temperature sensor. Viscometers for explosive areas, e.g. in the chemical and mineral oil industries, are specially protected and have ATEX approval.
Viscometer is a simple-in-operation and relatively inexpensive device which is perfect for monitoring of the production process and quality control. Modern devices have been designed in such a way that they are able to measure a wide range of viscosity, and thus, may be applied for measuring viscosity of many fluid materials, for example, paints, chemical and pharmaceutical products and asphalt. Special measuring method allows detecting the viscosity of the components (binders or bitumen) the asphalt is made of, at different temperatures (though, the standard temperature for measurements is 60 °C) and levels of mass consolidation. Bitumen in combination with the mineral materials and other components should make a homogeneous mass, and this process is under permanent control, including temperature regime and time of mixing. Asphalt quality plays an incredibly important role and thus, this process is monitored from the very beginning to the end. The color, homogeneity, bitumen presence in the mixture, water amount and water resistance coefficient – all these parameters should meet the requirements. Viscometer is just one of the device applied for the full profound check of the final product, but it helps to detect the flows in the production and eliminate them immediately, by adjusting the amount of the components and conditions of the production process.
Vacuum capillary viscometer is the most widely applicable type of viscometers for asphalt measurements. To provide reliable and accurate measurement the device must comply with the standards (ASTM D 2171) and be calibrated. To determine the asphalt characteristics a few samples are usually takes from the mixture and carefully studied and checked in the laboratory, for this purpose laboratory model of viscometer is recommended.
Slurries find application in many branches of industries, that is why the knowledge about their flow behavior is so important. it is quite a challenging task, since they often have inhomogeneous structure. Rheological characteristics of the slurry suspensions give a better understanding of how the process should be organized, so that to make the desired final product or process flowless. Slurries have got a number of solid parts (sometimes of different size), various additives which, if not considered, may affect the viscosity and flow tremendously. Besides, temperature and other factors also have their own influence on the fluid, and therefore it is even more important to study the character of the liquid in use. For example, it may be important for choosing the correct pipeline, if it goes about mining or industry.
Depending on the type of slurry (which may contain sand particles, cement, clay and many others), different methods of viscosity measurements may be selected.
In fact, both, rheometer and viscometer (rotational and tube) can be applied for slurry measurements, but the use of the latter makes more sense, since it is easier in operation and provides sufficient information about viscosity of the slurry, without determination of other extra characteristics, which rheometer is used for. A lot depends on the slurry itself and the process, in which it is used. As a rule, its behavioral characteristics are like of the non-Newtonian fluids, and to give a correct description of the slurry flow abilities, several measurements are to be taken. Viscosity is directly connected with the shear rate the fluid demonstrates. It is not obligatory that this rate remains one stable value, that is why viscosity may also differ in the parts with high shear rate form the parts with low shear rate, even though it may be the same slurry. That is why it is recommended to apply a few instruments, depending on the characteristics of a particular part under control (speed, shear rate, temperature) to get accurate results about slurry behavior in a process flow.