PNNL-13501 - Pacific Northwest National Laboratory

Therefore, in order to extract the viscosity, one also needs
to measure the density.
Acoustic Impedance Measurement (Density-Speed of
Sound Product)
The acoustic impedance of a material is defined as the
product of the density and speed of sound in the material.
For a perpendicular reflection, the reflection coefficient is
dependent upon the acoustic impedance of the wall
material and the acoustic impedance of the liquid.
Speed of Sound in the Liquid
Figure 1 shows 0.2 MHz transducers on opposite sides of
a tank. It also might be possible to use a single transducer
operating as both the sender and receiver of the ultrasonic
wave, i.e., the pulse-echo mode of operation. The time of
flight of the pulse can be used to determine the speed of
sound.
The combination of these three types of measurements
can yield the density, viscosity, and speed of sound in the
liquid.
Approach
To evaluate the feasibility of through-wall measurements,
we conducted a series of experiments varying the
transducer frequency and bandwidth and type of pulse for
both shear and longitudinal waves to investigate the types
of responses that met the criteria that the succeeding
echoes do not overlap. The properties of the transducers
were determined. Design parameters include the
frequency of the transducer and its bandwidth, because
the frequency determines the period of the pulse while the
bandwidth determines the shape of the pulse. The
bonding of the transducer to the wall is another important
aspect, which may be accomplished using epoxy. The
time length of the pulse is determined by the pulse that is
sent to the transducer and the bandwidth of the transducer.
For these investigations, a square-wave pulser provided a
high voltage pulse of short duration to the transducer.
Data were obtained using a digital oscilloscope that has
the capability to obtain fast Fourier transforms and save
the information to disk. Also investigated were the use of
toneburst signals of approximately 6 cycles. Both
methods were equivalent. An experimental apparatus was
designed and constructed, and experiments were
conducted to measure the acoustic impedance of a liquid.
Results and Accomplishments
Shear wave transducers produced vibrations that were
perpendicular to the direction of the wave and that
traveled easily in solids. While longitudinal waves travel
easily in liquids, shear waves penetrate the liquid only
very slightly. This penetration depends upon the viscosity
and increases with an increasing viscosity. The acoustic
impedance of stainless steel was large compared to most
liquids; therefore, the change in viscosity was not
significant enough to be readily observed in our
experiments. However, experiments with a machinable
glass allowed us to differentiate between a range of water
and sugar water samples with differing viscosities. The
results showed that differences in voltages can be seen for
30% sugar water and 50% sugar water, compared to
water.
Summary and Conclusions
A new approach was developed to characterize fluid
density and viscosity by transmitting an ultrasonic signal
through the wall of the pipe or vessel. Experiments were
conducted to evaluate configurations for successful
measurement. These experiments showed the potential to
measure the density through the wall of pipes or vessels.
Experiments showed that the measurement of viscosity
using machinable glass is feasible.
References
Bamberger JA, and MS Greenwood. 2000. “Measuring
slurry density and viscosity in-situ in real time during
pipeline transport using an ultrasonic sensor.”
FEDSM00-11121. Proceedings of the ASME 2000 Fluids
Engineering Division Summer Meeting, June 11-15, 2000,
Boston, Massachusetts. American Society of Mechanical
Engineers, New York.
Bamberger JA, LJ Bond, and MS Greenwood. 1999.
“Ultrasonic measurements for on-line real-time food
process monitoring.” In Proceedings of the Sixth
Conference on Food Engineering, 1999 AIChE Annual
Meeting, Dallas, Texas. PNNL-SA-32024, Pacific
Northwest National Laboratory, Richland, Washington.
Greenwood MS, JR Skorpik, and JA Bamberger. 1999.
“On-line sensor for density and viscosity measurement of
a liquid or slurry for process control in the food industry.”
In Proceeding of the Sixth Conference on Food
Engineering, 1999 AIChE Annual Meeting, Dallas,
Texas. PNNL-SA-32025, Pacific Northwest National
Laboratory, Richland, Washington.
Sensors and Electronics 405