Article for the 23rd Sensing Forum

Theme:

Physical Quantity Measured by aVibration Viscometer(Re: JCSS Standardization of Viscosity)

Presented by:

Naoto Izumo
R&D Division, A&D Company, Limited

October 2 ~ 3, 2006
Tsukuba Center Inc.
Tsukuba, JAPANPhysical Quantity Measured by a Vibration Viscometer
Subtitle: The JCSS Standardization of Viscosity
Naoto Izumo
R&D Division, A&D Co., Ltd.
Higashi-Ikebukuro, Toshima-ku, Tokyo 170-0013 Japan
Abstract
The objective of this article is to introduce a viscometer that utilizes a new viscosimetry
measuring method. In addition, the article will recommend a new unit system, which isutilized in the vibration viscometer. Using examples, the article explains JCSS viscosity
standardization and recent requirements for viscosity measurements.
Keywords:

Vibration Viscometer, Static Viscosity (Viscosity × Density), Viscosity
JCSS, Cloud Point

Introduction
The following is an introduction of the vibration viscometer, a new method formeasuring viscosity. In addition to providing a description of the physical quantity thatis measured using the vibration viscometer, a new unit system for viscosity will beproposed. Furthermore, there is an explanation regarding the Japan Calibration
Service System (JCSS) standardization of viscosity and viscosity measurements using
actual examples. There is also discussion of recent requirements for measuringviscosity.
History and Development of Viscosity Measurement
The history of viscosity measurements is extensive and is believed to date back to whenpeople began measuring the viscosities of engine oils with the advent of the automobile
industry in the United States. In the U.S., it had become necessary to control theviscosities of engine oils as a method of maintaining the performance of engines. Even
today, viscosities of engine oils are standardized for both high and low temperatures,such as 5W-30. It is believed that if the viscosity of oil reaches below 2.6 cp (the viscosityof purified water at 20˚C is approximately 1 cp = 1 mPa·s : 1 milliPascal second) theengine will burn out. This has recently become an important issue when developing
energy-saving engine oils for the purpose of improving fuel efficiency. Furthermore, thedemand for viscosity measurements aimed at maintaining quality in the field of
cutting-edge technologies has been increasing. This is due in part to the expansion of
new markets for viscosity-related applications, which now include resist inks for liquidcrystals, abrasives for semi-conductors, glass coating materials, powder particle size
distribution, polymeric emulsion and cloud point measurement of surface-active agents.
Moreover, recently there have been discussions of performing viscosity measurements ofhuman blood. Studies have shown that high blood viscosity increases the possibility ofsudden death due to diseases affecting the circulatory system. The viscosity of humanblood, although dependent on the measurement method, is generally believed to fall
somewhere between approximately 3 and 10 mPa·s.
Actual examples of the measurements of the viscosity and temperature of engine oil andcloud point measurements of nonionic surface-active agents have been presented inFigures 1 and 2 for reference.

Viscosity change of gasoline engine oil

Figure 1.

Cloud point measurement

Figure 2.

Definition of Viscosity and the JCSS Standardization
Viscosity is defined using the relative motion between two boards that have been placed
opposite to each other in a sample liquid. Viscosity is the proportional constant when
the interactive force (shear stress) per unit area generated in the planar direction
between the opposing two boards, and the velocity gradient (shear rate), calculated by
dividing the relative displacement speed by the distance between the two boards, areproportional. Based on this definition, there is a proportional relationship between theshear stress and the shear rate. The fluid is called a Newtonian fluid in the event thatthe shear stress and the shear rate are proportional, and viscosity is indicated as aconstant stable value. On the other hand, if the proportional relationship with the shear

stress deteriorates due to changes in the shear rate or if the proportional relationship islost due to temporal changes (a fixed viscosity value cannot be determined for the liquid
due to the measurement conditions), all such fluids are collectively called
non-Newtonian fluids. While as demonstrated above, it is easy to define viscosity, thestructures of the devices to conduct actual measurements are not as simple and there
are many structural problems. For example, it is important to stabilize the measuring
environments, such as keeping the measuring temperature constant, for methods likethe cup type, which measures the time taken by the sample liquid to flow from theopening of a given sample cup, the falling-sphere type, which measures the viscosity by
the time needed for a rigid body to fall within the sample liquid, and the capillary type,which measures the time taken by the sample liquid to flow inside a capillary. For therotation type, it is necessary to regulate the rotation of a rotor at a constant speed andsteadily measure the torque required for the rotation. On the other hand, for thevibration type, which calculates the viscosity from the power to drive an oscillator
placed in a sample liquid, technology to steadily vibrate the oscillator at the natural
frequency is essential. Among viscosity measurements based on the measurement
principles above, the underlying theory of the measurement principle that has become
the modeling formula (modeling equation) and the “uncertainties” inherent in themeasurements have been demonstrated with the capillary, rotation and vibration types.
As a result, along with the standard liquids of viscosity, these types of viscometers wereaccredited as the JCSS standard devices and have been uploaded to the official websiteof the National Institute of Technology and Evaluation (NITE) as of April 2006.
Physical Quantity Measured by Each Measurement Method
Next is a brief explanation of the measurement principles for viscosity standardized by
the JCSS:
Capillary type: A liquid filling a given vessel is made to flow to a lower position by
gravity and the viscous behavior of the liquid is measured based on the flow time. Thetime taken by the liquid for the movement is measured and is converted to a viscosity
value using the flow time of internationally standardized water as a reference. Whenusing this measurement principle, the physical quantity to be measured (i.e. time) is
proportional to the viscosity, but inversely proportional to density. Therefore, thisphysical quantity can be expressed as “viscosity/density,” and is called the “kineticviscosity.”
Rotation type: A rotor is placed in a liquid and is constantly rotated. During rotation,the torque necessary for the rotation is proportional to the viscosity. The “viscosity” isthe physical quantity that is measured.
Vibration type: An oscillator placed in a liquid is vibrated at a constant displacement
magnitude. By detecting the power necessary for the vibration, the viscous behavior of
the liquid is measured. The physical quantity to be measured is expressed as “viscosity× density.”
Advantages and Measurement Principles of the Vibration Viscometers
There are two kinds of vibration viscometers, the rotational vibration type and thetuning-fork vibration type – both types rely on the same measurement principle. The
present section is devoted to a detailed explanation of the tuning fork vibration method.
A viscometer using the tuning-fork method has a pair of opposing oscillators of the same
natural frequency. Each of these oscillators is individually synchronized and driven byelectromagnetic power. As the two oscillators move in opposite phases, no outward
reactive force is generated: this is true with a tuning fork. Driving at a natural
frequency with very small damping is also possible. During the viscosity measurement,
the amplitude that is generated is constantly measured and controlled in order tomaintain a fixed amplitude. In addition, the electromagnetic power required to drive
the oscillators is also measured. Viscosity is determined based on variations in driving
power in accordance with viscosity multiplied by the density of the liquid in which theoscillators are immersed. The energy applied to the sample liquid is small because thevibration method causes only minute displacement in the sample liquid. Moreover, as
the thermal capacity of the oscillator is small, interference to the sample substance dueto the measurement can be minimized. Since there is no flowing or churning of thesample liquid, little change is caused mechanically to the physical properties of thesample even after the measurement starts, making a speedy and stable measurement
possible. The viscosity of a liquid is temperature dependent and varies by as much as –2~ –10%/˚C. Hence, small interference by the measuring system can provide benefits
such as decreasing the possibility of temperature variation that can cause changes tothe physical properties of the sample. In addition, by utilizing a tuning-fork vibration,
the viscometer has a high measurement sensitivity and is capable of performing
continuous measurements, ranging from as low as 0.3 mPa·s (1/3 the viscosity ofpurified water) to 10,000 mPa·s. This enables the measurement of the cure processes ofmaterials such as adhesives, gelatin, and egg albumen. For example, the cure processesof albumen proteins with different constituents can be monitored at different
temperatures. The physical quantity measured by the vibration viscometer is, from thetheoretical formula, “viscosity × density” in principle.
Next, is an explanation of the measurement model for the tuning-fork vibration
viscometers. As illustrated in the model of the free vibration system shown in Figure 3,
inertia terms based on the mass of the measuring system, viscous terms based on theviscosity of the liquid, and the spring terms based on the spring constant of themeasuring system can be examined. When the measuring system is driven byelectromagnetic power at the natural frequency determined by the mass and spring
constant of the measuring system, the inertial force and the restorative force of thespring will balance each other, and the energy consumed by the measuring system will
only be the viscous term of the liquid. This information is presented in Formula (1)expressed as a motion equation, where F: Excitation force, m: Mass, C: Viscosity
coefficient, K: Spring constant, x: Amplitude, ωn: Natural frequency of the vibration
systemF=md 2 x
dt 2+cdx
dt+Kx

Spring plate(1)

Displacement sensor
Electromagnetic drive

When Expression (1) is integrated,

Temperature sensor
Samplex=Fc w(2)OscillatornFigure 3. Mechanismofthedetection system

If the amplitude (displacement magnitude) x and the natural frequency ωn are constantvalues, there should be a proportional relationship between the excitation force F andthe viscosity coefficient C.

By applying this principle, the tuning-fork vibration

viscometer resonates the two oscillators at the natural frequency with electromagnetic
power and thus realizes highly sensitive viscosity measurements.
Examination of the Unit System for Viscosity
The measurement principles of the capillary and therotation viscometers are comparatively simple andhave a long history of being used for measurements.
Hence, the “kinetic viscosity” and the “viscosity” havelong been acknowledged as the unit systems suitable
for these types of viscometers. Theoretically, themeasurement principle for the vibration viscometer
Figure 4. Vibration viscometerwas established in Japan approximately half a century ago. It was featured inpublications in Japan as early as 1958 and was expected to become a new method toconduct viscosity measurements. However, because technology to drive oscillators at
the natural frequency was difficult, the vibration method did not see its way onto themarket for many years.
Today, despite the fact that the production technology has become feasible and theproduct has already been introduced to the market, the “viscosity × density” unit
system has not yet been officially adopted. Therefore, I would like to propose theadoption of “static viscosity” as the physical quantity for the vibration type, similar to“kinetic viscosity” for the capillary type and “viscosity” for the rotation type. The
reasons for my proposal are as follows:- The “kinetic viscosity” (viscosity/density) of the capillary type can be obtained bymeasuring the time taken by a liquid in a vessel of a given volume to go through a flow
channel of a given diameter. Consequently, the physical quantity to be obtained is in
direct proportion to the viscosity while inversely proportional to the density, which
generates the pressure to the fluid. Meanwhile, this measurement method isaccompanied by a barycenter shift of the liquid as the liquid being measured actuallyflows through the inside of the tube. The kinetic viscosity is an indication of this state of
the liquid and can be considered to be accurate.
- In the rotation viscometer, although the liquid is in rotational movement, there is noshifting of the liquid’s barycenter. Particularly, in the plate-type rotation viscometer, the“shear rate is constant” due to its measurement principle and only the viscosity value
can be obtained, expressed in terms of rotational torque.
- Unlike the two methods above, in the vibration viscometer, the oscillators are inreciprocating motion within a liquid. The liquid around the oscillators obtains a “shearrate,” and the accompanying “shear stress” is in turn loaded to the oscillators. By thismethod, there is neither a barycenter shift nor widespread rotational movement of theliquid, which makes it possible to measure viscosity in the resting state. In addition, theenergy possessed by the measuring system of the vibration type is minimum.

Thismeans that the energy transferred from the measuring system to a liquid is also
minimized. The vibration type is the only method in which no macro movement of theliquid is generated. Thus, I would argue that it is reasonable to term the physicalquantity “viscosity × density,” the “static viscosity.” By using the terms “kinetic

viscosity,” “viscosity,” and “static viscosity,” it becomes possible to accurately express themotion or the state of the liquid measured by each viscosity measuring method.
Examples and Future Prospects of Static Viscosity Measurement
The following are actual measurement examples. In some cases, phenomena that once
seemed impossible in conventional viscosity measurements, such as cloud point
measurements, have been rendered possible, (See Figures 1, 2, 5, & 6).

-Engine oil: Measurement of temperature characteristics of oil from the “staticviscosity” of when temperature is changed-

Cloud point measurement: Detection of the cloud point of a nonionic surface-active
agent from its “static viscosity” change-

Change from a liquid to a solid: Monitoring the cure process of a protein material(egg albumen) from its “static viscosity” change-

Analysis of constituents of a liquid: Inference of constituent elements by measuring
the concentration change of alcohol from the “static viscosity.”

In future measurements of “static viscosity,” applications in the following fields may
also become possible:-Evaluating the viscosity of the base material necessary for the measurement of theparticle size distribution (evaluation of Brownian motion)-

Inferring the molecular weight of a turbid solution by measuring the viscosity-

Measuring the cure processes and temperature characteristics of functional liquids
such as coating materials and inks-

Quantifying the “swallowability” of soft drinks or physical properties of biologicalobjects such as blood viscosity

SV10 value

Theoretical value
Sampel temp. 25 ?C2.5]s 2aPm
[1.5ytisocs 1iV
0.500(Water)

Coagulation and temperature dependencyof chicken egg albumen

Figure 5.

204060Concentration [%]80* Theoretical value is corrected for density
Concentration and viscosity of ethanol solution

Figure 6.

100(Ethanol)

Summary
This article provides a brief explanation of the features of a new viscosity measurement
method, the tuning-fork vibration viscometer, accompanied by actual measurement
examples. The vibration viscometer measures “viscosity × density,” and using water as
an example (the temperature coefficient of the density value is as low as 1/100 of theviscosity value) expresses the “viscosity × density” as the “static viscosity.” As a result, it
has become possible to easily perform continuous measurements of various
viscosity-related physical property changes. Various measuring methods for viscosity
and uncertainties of these methods have already been verified and made publicly
available as the JCSS standardized methods for viscosity measurement. Based on theseexperiences, it is expected that there will be more opportunities for precise viscosity
measurements in such areas as research & development, production, and quality
control in a variety of industries, thereby further contributing to their development.