Measuring and testing – Specific gravity or density of liquid or solid
Patent
1996-08-29
1998-11-17
Williams, Hezron E.
Measuring and testing
Specific gravity or density of liquid or solid
73 5426, 73 5427, 73 5441, 73 6179, 73 6442, 73 6453, 73861355, 73DIG1, 310316, 310338, G01N 932, G01N 2902, G01N 2924
Patent
active
058378855
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
From "The Marconi Review", Vol. 43, Nr. 218, page 156 ff, a method and a device for measuring the characteristics of an oscillating system is known. Resonating sensors are described in R. M. Langdon, J. Phys., E: Sci. Instrum., Vol.18, 1985, p. 103-115 and in W. Gopel et al., Sensors, Verlag VCH, Weinheim, 1994. These three references are designated as integral parts of the present application. A mechanical oscillator is excited near its eigenfrequency with a first transducer at a stabilised frequency. Usually the response of the oscillator is measured with a second transducer. For example the damping of the oscillator can be obtained from the frequency difference generated by shifting the phase between two values in a phase-locked loop. One of the most important drawbacks of this procedure is the fact that cross-talk between the two transducers can hardly be avoided, especially when the oscillating system has small dimensions or if the measured signal is too weak.
EP-A-297 032 discloses a viscosimeter. It comprises a resonating system with a cylindrical torsional resonator. A piezoelectric excitation transducer is coupled to the resonator and is excited in the vicinity of a resonance frequency of the resonator with a sinusoidal signal. A measuring transducer measures the resulting vibration of the resonator. In a phase locked feedback loop between the two transducers the phase angle is shifted between two given values. From an evaluation of the resulting frequency shift the viscosity of a fluid surrounding the resonator is calculated.
In Patent Abstracts of Japan, vol. 10, no 48 (P-431) a vessel is torsionally elastically suspended. A coil is fixed to the vessel, the coil co-operating with permanent magnets. The coil is connected to two amplifiers via a change over switch. In a first stage the coil receives a pulse signal from the first amplifier to rotate the vessel. Then the switch is charged over and the resulting voltage output of the coil is amplified by the second amplifier. From the attenuation of this signal and from its wave form the blood clotting process is deducted.
The present invention proposes a method and a device for measuring the characteristics of an oscillating system. Such characteristics include the resonance frequencies and the corresponding damping coefficients of the oscillating system. One of the main features of the invention is that it solves the problem of cross-talk.
Thanks to the use of a gated alternate sensing and excitation scheme, cross-talk between two transducers can be completely avoided. Thus resonators (for example mechanical oscillators) can be built in very small sizes. Another advantage of the method is the fact that it allows the use of transducers that are prone to high cross-talk like electromagnetic transducers. Because the method proposed in this invention allows the construction of very small resonators, it can be used for example to build blood viscometers which are capable of measuring on-line (directly as it comes from the vein) or in-vivo (in the vein or artery) the viscosity of blood, or of any other body fluid since the volume necessary for the measurement is small (<1 cm.sup.3). The direct measurement of the viscosity of blood without addition of anticoagulants would have for example a great significance as a diagnostic instrument capable of evaluating the action of certain drugs or of the health state of high risk patients. Another important application is the monitoring of blood viscosity in surgical operations (for example in heart-lung machines). The concept of a blood viscometer presented here can also be used for any other biological fluid. Such applications were not possible hitherto, because, amongst other reasons, till now, viscometers could not be built in sufficiently small sizes.
Another advantage of the method proposed in the present invention is the fact that the same single transducer can be used for both excitation and sensing. This simplifies the fabrication of the transducer and contributes also for example to the fa
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The Marconi Review, vol. 43, No. 218, 1980 Great Baddow, UK, pp. 156-175. R.M. Langdon, `Vibratory process control transducer`.
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Dual Jurg
Goodbread Joseph
Hausler Klaus
Sayir Mahir
Miller Rose M.
Williams Hezron E.
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