Viscometers: The Science of Measuring Fluid Flow

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 Viscometers: The Science of Measuring Fluid Flow

Please see our Rheometers / Viscometers section to find manufacturers that sell these products

Viscometers measure the viscosity and flow properties of fluids. Viscosity arises from the internal friction of a fluid and is defined as a liquid’s resistance to flow or shear stress.

Applications of viscometers

Viscometers have wide and varied applications in the materials science and chemical industries for materials such as oil, asphalt, plastics, wax, paints, coatings, and adhesives. They are also used for food and beverages and personal-care products such as cosmetics, shampoo, and toothpaste.

Measuring viscosity is important when considering process conditions for materials that need to be pumped or piped. Viscosity also affects dipping and coating performance, which is particularly relevant to paints and inks.

Viscosity is a very useful indirect measure of material properties including molecular weight and density, both of which affect flow behavior. Viscometers can therefore be used to monitor batch consistency and quality control.

Example applications include:

  • Measuring the flow of engine oil under different temperature conditions
  • Analyzing the flow properties of milk to design suitable piping systems
  • Checking the viscosity of jams and syrups to ensure batch consistency.

Since changes in viscosity reflect changes to molecular weight, viscometers are used to characterize plastics. They are particularly useful in polymer synthesis since the flow rate can be used to determine the change of polymer length through different processes.

In many industrially useful cases, viscosity decreases as enzymatic activity increases. Viscometers are used to determine the activity of enzymes such as cellulase, protease, amylase, and pectinase.

Types of viscometers

There are many ways to measure viscosity, but they break down into two basic methods. Either an object, such as a sphere or a rotor blade, moves through a stationary material, or the material flows through or past a stationary object. In either case, the resistance to flow is measured.

Capillary viscometers

Also known as u-tube or glass viscometers, capillary viscometers are often used in laboratory settings. Viscosity is measured by timing how long it takes for a transparent or translucent fluid to flow between two points of a capillary tube.

For opaque liquids, it is hard to determine when the sample has passed a point in the tube, so it is necessary to use reverse-flow viscometers that only wet the timing section of the viscometer capillary during the actual measurement. The Cannon-Fenske Opaque, Zeitfuchs Cross-Arm, and BS/IP/RF Viscometers from CANNON Instrument Co. (State College, PA; www.cannoninstrument.com) are all reverse-flow types. Reverse-flow viscometers must be cleaned, dried, and refilled before a repeat measurement can be made.

Capillary viscometers can be manual or automatic. Automatic instruments use infrared optical sensors for transparent samples, or thermal sensors for opaque samples.

Rotational viscometers

Simple rotational viscometers, also known as Brookfield type viscometers, use a torsion spring to measure the torque required to rotate a spindle in the material. Changing rotor speed and size allows for the measurement of different ranges of viscosity.

Figure 1 – The SVM 3000 Stabinger Viscometer from Anton Paar measures the dynamic viscosity and density of oils and fuels.

The SVM 3000 (Figure 1) from Anton Paar (Ashland, VA; www.anton-paar.com) uses the Stabinger measuring principle. A lightweight magnetic rotor floats in a liquid-filled tube, which rotates at constant speed. The sample fluid’s viscous forces drive the rotor, leading to an equilibrium rotor speed, which is recorded as a measure of the fluid’s viscosity.

Different spindle geometries such as cylindrical, coaxial cylinder, parallel plate, and cone-plate are used in rheometers to create different shear forces. Rheometers are more complex instruments than viscometers and are used to measure flow rate-dependent viscosity in non-Newtonian fluids.

Rolling ball, falling ball, sphere, and piston viscometers

The Lovis 2000 M/ME from Anton Paar is a microviscometer based on the rolling ball principle. A ball rolls through a closed, sample-filled capillary that is inclined at a defined angle. Inductive sensors determine the ball’s rolling time between two marks.

In a similar way, falling ball or sphere viscometers measure the time it takes for a ball or sphere to fall under gravity through a sample-filled tube inclined at an angle. Falling piston viscometers operate on a similar principle, but they measure the resistance to the piston falling through the material.

Considerations for purchasing viscometers

Choosing a viscometer is a complex process and will depend on the sample type, viscosity, opacity, available volume of sample, required throughput, and level of automation needed. At a minimum, the following factors should be considered:

  • Viscosity range
  • Accuracy
  • Temperature control
  • Sample volume and microvolume capability
  • Automation and computer interfacing
  • Regulatory requirements and industry standards.

Viscosity range

Viscosity is measured in Pascal seconds (Pa·s), in other words, the amount of pressure needed to achieve a given rate of flow. Viscosity is commonly stated in centipoise (cP), which is the unit used by the standards agency ASTM International; 1 cP equals 1 mPa·s. Water at 20 °C has a dynamic viscosity of 1.002 cP. Kinematic viscosity is the ratio of dynamic viscosity to density; the unit of measurement is the Stoke (St).

Thermo Scientific Gilmont Falling-ball Viscometers (Thermo Fisher Scientific, Waltham, MA; www.thermoscientific.com) offer a simple, accurate, economical, absolute method for measuring viscosity of clear fluids up to 200 cP.

The Viscotek DSV from Malvern Instruments (Malvern, U.K.; www.malvern.com) directly measures the relative viscosity of dilute polymer solutions using pressure sensing technology and has a viscosity range of <1 mPa·s to 20 mPa·s.

In most glass capillary viscometers, the samples flow under gravity, but vacuum viscometers can be used to measure highly viscous materials.

Vacuum viscometers require a very accurately controlled vacuum to pull the liquid through the capillary. CANNON Digital Vacuum Regulators (DVRs) can be used in conjunction with vacuum viscometers for the measurement of asphalt cement at 60 °C (140 °F) according to ASTM D 2171.

The torque range for rotational viscometers dictates the range of viscosity that the instrument can measure. The High Shear CAP-2000+ from Brookfield Engineering Laboratories (Middleboro, MA; www.brookfieldengineering.com) offers a choice of torque range with a standard torque of 181,000 dyne·cm or a low torque of 7970 dyne·cm. The speed of rotation varies between 5 and 1000 rpm, resulting in a viscosity measurement range of 0.2–15,000 cP.

Accuracy

The accuracy of a viscometer is dependent not only on the engineering of the instrument itself, but also on how precisely temperature can be controlled. It is common to make repeated viscosity measurements that are averaged to achieve a result with an associated accuracy.

Viscosity standards are used to check the accuracy of a viscometer and can be important for verifying compliance with industry quality systems. Check that the manufacturer provides standards suitable for your application. Brookfield Silicone and Mineral Oil Standards provide a fluid viscosity value that is constant at 25 °C. The company recommends replacing viscosity standards on an annual basis.

Temperature control

The viscosity of most materials reduces as temperature increases. Materials such as engine oils are subject to routine temperature variations in use, and the measurement of viscosity is required at different temperatures. Therefore, it is very important for viscometers to have precise temperature controls.

The Black Pearl Viscometer from ATS RheoSystems (Bordentown, NJ; www.atsrheosystems.com) comes with built-in Peltier temperature control for all measuring systems. Some systems have the option of adding cooling equipment, leading to temperature ranges below 0 °C.

Sample volume and microvolume capability

Sample volume is important for biological samples, especially in the clinical laboratory. The SV-A Series from A&D Weighing (San Jose, CA; www.andonline.com/weighing) allows for the accurate measurement of sample sizes as small as 2 mL with 1% accuracy. The Cannon-Manning Semi-Micro Extra Low Change U-tube Viscometer from CANNON Instrument Co. will measure kinematic viscosity with as little as 0.5 mL of sample.

Figure 2 – The DV2T Viscometer from Brookfield Engineering features a 5-in. color display to guide users through test creation and data gathering.

Automation and computer interfacing

The simplest viscometers allow one measurement at a time, but for continuous measurements, or measurements at different temperatures, it is useful to have an automated system. Semi-Automatic Kinematic Viscosity Systems from UIC, Inc. (Joliet, IL; www.uicinc.com) can determine the viscosities of up to 12 samples simultaneously and independently.

Some automated instruments can be interfaced to a PC. Check whether the manufacturer’s software can be interfaced to your in-house laboratory system if required.

WinCT-Viscometer software from A&D Weighing uses an RS232C interface to display measurement progress from the SV series Sine-wave Vibro Viscometer in real time on a PC. Measured results can be saved or analyzed.

The DV2T Viscometer from Brookfield Engineering Laboratories (Figure 2) features a 5-in. color display to guide users through test creation and data gathering. Multistep test protocols can be created using the Program Generator software and uploaded to the DV2T through a USB flash drive. Test data can be printed or sent to a PC.

Regulatory requirements and industry standards

Be sure that your chosen viscometer is certified for your required ASTM or ISO industry standards.

The Thermo Scientific HAAKE Falling Ball Viscometer type C from Thermo Fisher Scientific measures the viscosity of transparent Newtonian liquids and gases. It meets the ISO 12058 standard and is accepted as an official reference instrument.

The PolyVISC® Automatic Glass Capillary Viscometer from CANNON Instrument Co. is a benchtop unit for dilute solution polymer viscosity analysis. Both transparent and opaque samples can be measured with the accuracy required by ASTM D 2857.

Viscometer manufacturers

A list of viscometer manufacturers is given in Table 1.

Table 1 – Manufacturers of viscometers

Katriona Scoffin, B.Sc., is a freelance science writer; e-mail: articles@scoffin.co.uk.

Please see our Rheometers / Viscometers section to find manufacturers that sell these products

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