Article for the 23 rd Sensing Forum

Kx
dt
dx
c
dt
x
d
m
F
+
+
=
2
2
(1) 
When Expression (1) is integrated, 
n
c
F
x
w
=
(2) 
If the amplitude (displacement magnitude) 
x
and the natural frequency 
ω
n
are constant 
values, there should be a proportional relationship between the excitation force 
F
and 
the 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 the 
rotation viscometers are comparatively simple and 
have a long history of being used for measurements. 
Hence, the “kinetic viscosity” and the “viscosity” have 
long been acknowledged as the unit systems suitable 
for these types of viscometers. Theoretically, the 
measurement principle for the vibration viscometer 
Spring plate 
Displacement sensor 
Electromagnetic drive 
Temperature sensor
Oscillator 
Sample 
Figure 3. Mechanism of the 
detection system
Figure 4. Vibration viscometer 

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was established in Japan approximately half a century ago. It was featured in 
publications in Japan as early as 1958 and was expected to become a new method to 
conduct viscosity measurements. However, because technology to drive oscillators at 
the natural frequency was difficult, the vibration method did not see its way onto the 
market for many years. 
Today, despite the fact that the production technology has become feasible and the 
product has already been introduced to the market, the “viscosity × density” unit 
system has not yet been officially adopted. Therefore, I would like to propose the 
adoption 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 by 
measuring 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 is 
accompanied by a barycenter shift of the liquid as the liquid being measured actually 
flows 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 no 
shifting 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 in 
reciprocating motion within a liquid. The liquid around the oscillators obtains a “shear 
rate,” and the accompanying “shear stress” is in turn loaded to the oscillators. By this 
method, there is neither a barycenter shift nor widespread rotational movement of the 
liquid, which makes it possible to measure viscosity in the resting state. In addition, the 
energy possessed by the measuring system of the vibration type is minimum. This 
means 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 the 
liquid is generated. Thus, I would argue that it is reasonable to term the physical 
quantity “viscosity × density,” the “static viscosity.” By using the terms “kinetic 

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viscosity,” “viscosity,” and “static viscosity,” it becomes possible to accurately express the 
motion 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 “static 
viscosity” 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 the