Method and system for measuring fluid parameters by ultrasonic m

Details Method and system for measuring fluid parameters by ultrasonic m Method and system for measuring fluid parameters by ultrasonic m

1996-11-13

1998-09-08

Oda, Christine K.

Measuring and testing

Instrument proving or calibrating

Apparatus for measuring by use of vibration or apparatus for...

73 186, 73645, 73579, 73 32A, G01H 500

Patent

active

058046984

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention:
This invention relates to methods and systems for measuring fluid parameters by ultrasonic methods. More particularly, the system of the present invention uses ultrasonic methods for measuring thermodynamic properties of a liquid sample.
2. Description of the Related Art:
The function of the volume of a liquid vs. pressure and/or temperature ("P-V-T data") is of importance in many fields of science and technology. P-V-T data are the basis for a complete thermodynamic characterization of condensed matter, including liquids, by the respective equation of state.
Various methods for measuring P-V-T data are known in the art. Particularly promising are ultrasonic methods because of the inherent potential to obtain higher accuracies than most other methods, because the velocity of sound in condensed matter can be determined with an accuracy in the order of 10.sup.6.
Ultrasonic methods for determining parameters of a material generally involve the measuring of the velocity and optionally the attenuation of ultrasonic waves in the material to be investigated at various pressures and temperatures. The adiabatic compressibility of a material is directly correlated to the velocity of sound, thus, the P-V-T data of the material can be derived from measurements of the velocity of sound waves, particularly ultrasonic waves, at various pressures and temperatures, if the absolute value of the density of the material under investigation at the temperature of interest at normal pressure is known.
The acoustic impedance Z of a liquid is a function of its density .rho. and the velocity of sound c in the liquid:
The acoustic resonances f.sub.n of a layer of a sample liquid in an acoustic resonator cell (in short "liquid resonances") and the resonances of the emitter and receiver transducers of the cell (which are assumed as equal) at the fundamental frequencies f.sub.0, and at the odd multiples thereof are coupling with each other, causing a mutual interaction of these resonances. Therefore the reflection of sound waves in an ultrasonic resonator at the interfaces between the transducers and the sample liquid is a function of the acoustic impedance Z of the sample liquid and the acoustic impedance Z.sub.0 of the transducer material.
The coupling effects are dependent on the differences of the frequency of the transducer resonance and the frequency of the respective resonance of the-liquid on the one hand, and on the acoustic impedances of the sample liquid and transducer material on the other hand.
More specific theoretical considerations of the behaviour of an ultrasonic resonator have shown that when the transducers are air-backed the reflection conditions at liquid resonance frequencies f.sub.n .apprxeq.nf.sub..pi. (f.sub..pi. =f.sub.0 /2) are almost ideal, i.e. the amplitudes and phases of the liquid resonances can be taken as independent of the resonances of the transducers. It is therefore easy to evaluate the values of the velocity and absorption of sound in the sample liquid at these frequencies.
However, liquid resonances near the resonance frequency of the transducers are adversely affected by the non-ideal conditions of sound wave reflection. This has been discussed in various theoretical treatises. In all these theoretical discussions the effects of the non-ideal reflection conditions on the liquid resonances are described as a function of the acoustic impedances of liquid and transducer material on the one hand, and as a function of the distance of the resonance frequency of the respective liquid resonance from the resonance frequency of the transducer on the other hand, see e.g. A. P. Sarvazyan and T. V. Chalikian, Ultrasonics 29 (1991) 119-124. If the acoustic impedance of the transducer material is known, the impedance of the sample liquid and hence its density can be evaluated from the changes of the liquid resonances caused by the non-ideal reflection conditions.
Sarvazyan and Chalikian (l. c.) also disclose an apparatus for ultrasonic measurements of li

REFERENCES:
patent: 3828607 (1974-08-01), Janzen et al.
A.P. Sarvazyan et al., Theoretical Analysis of an Ultrasonic Interferometer for Precise Measurements at High Pressures, Ultrasonics 1991, vol. 29, Mar. 30, 1990, pp. 119-124.

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