Measuring and testing – Specific gravity or density of liquid or solid
Patent
1992-02-18
1995-01-24
Chapman, John E.
Measuring and testing
Specific gravity or density of liquid or solid
73433, 73580, G01N 900
Patent
active
053833490
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to fluid densitometers.
Acoustic resonance densitometers are now well known for use in measuring fluid density. Typically, a sample chamber is formed as part of a mechanical resonator system which is driven into self resonance by incorporating appropriate transducers and an electronic amplifier in a closed loop positive feedback circuit. The sample chamber is often a straight or U shaped tube through which the sample flows.
The commercially available systems are expensive even in the case of low resolution instruments. This is to a large extent because the mechanical resonators are difficult and expensive to make, it being necessary to control the acoustic properties within very close limits. There are typically large numbers of machining and heat treatment steps with high reject rates in quality control.
It is an object of this invention to provide a liquid densitometer which is of low cost yet which offers a reasonable resolution and reproducibility in measurements.
SUMMARY OF THE INVENTION
Accordingly the present invention consists in one aspect in a liquid densitometer comprising a base; a diaphragm suspended in the base for vibrational movement relative thereto; an electromagnetic coupling means having cooperating elements mounted respectively on the base and the diaphragm and having an electrical such that vibrational movement of the diaphragm is excited on the application of a periodic electrical signal thereto; a sample chamber for receiving a fixed volume of a liquid sample and associated with the diaphragm to load massively the diaphragm in accordance with the density of the sample. The frequency of vibration of the diaphragm is measured and compared with a predetermined frequency of vibration at a known massive loading of the diaphragm to obtain therefrom a measure of the density of the fluid sample.
Preferably, there is provided a positive feedback loop connected with the electrical terminal and serving in use to drive the diaphragm into vibrational resonance.
Advantageously, the positive feedback loop includes automatic gain control to ensure vibration of the diaphragm at constant amplitude.
In another aspect, the present invention consists in a liquid densitometer, comprising a base; a concave diaphragm; a suspension connected between the outer periphery of the diaphragm and the base to provide for generally axial vibrational movement of the diaphragm relative to the base; and electrical coil means provided on the diaphragm; a magnet supported on the base and positioned to generate a magnetic field at the coil; a driver for applying an electrical signal to the coil to excite vibrational movement; and a sample chamber for receiving a fixed volume of a liquid sample and associated with the diaphragm to load massively the diaphragm in accordance with the density of the sample. The frequency of vibration of the diaphragm is measured and compared with a predetermined frequency of vibration at a known massive loading of the diaphragm to obtain a measure of the density of the sample.
In a further aspect, the present invention consists in a fluid densitometer comprising a base; a diaphragm having a drive axis; an annular suspension disposed circumferentially of said drive axis and acting between the diaphragm and the base to provide for axial vibration of the diaphragm; a pair of coils provided on the diaphragm at respective opposite sides of the annular suspension; A magnet supported on the base and positioned to generate a magnetic field at each of said coils; an amplifier connected to receive a feedback signal from one of the coils and connected with another of the coils to provide a drive signal thereto, the arrangement being such as to excite a resonant frequency of axial vibration of the diaphragm; and a sample chamber for receiving a fixed volume of a liquid sample and associated with the diaphragm to load massively the diaphragm means in accordance with the density of the sample. A frequency meter is connected with at least one of the coi
REFERENCES:
patent: 3295360 (1967-03-01), Dimeff
patent: 3745811 (1973-07-01), Dure et al.
patent: 4429564 (1984-02-01), Ikeda et al.
patent: 4628739 (1986-12-01), Bruggen et al.
patent: 4872335 (1989-10-01), Tsuruoka et al.
Chapman John E.
Public Health Laboratory Service Board
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Inventor
Made construction of very low cost, relatively high resolution, liquid densitometer possible employing small design changes in plastic coned loudspeakers. Compared to high resolution tubular glass or silicon acoustic resonance densitometrs costing ten to twelve thousand dollars this invention could do the same thing for less than two hundred! Main problem is that applications for densitometers are limited, thus little commercial value.
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