United States Patent
`
`[19]
`
`[11]
`
`Patent Number:
`
`4,524,610
`
`Date of Patent:
`Jun. 25, 1985
`[45]
`Fitzgerald et al.
`
`[54]
`
`IN-LINE VIBRATORY
`VISCOMETER-DENSITOMETER
`
`FOREIGN PATENT DOCUMENTS
`401908
`2/1984 U.S.S.R.
`........................... .. 73/32A
`
`[75]
`
`Inventors:
`
`J. Vincent Fitzgerald, Metuchen;
`Frank J. Matusik, Piscataway;
`Donald W. Nelson, Voorhees, all of
`N.J.
`.
`
`Primary Examiner—Stewart J. Levy
`Assistant Examiner—Hezron E. Williams
`Attorney, Agent, or Firm—Arthur L. Lessler
`
`ABSTRACT
`[57]
`for
`A rotational vibratory viscometer-densitometer
`in-line process control and similar applications, having
`an elastic hollow metal tube extending between two
`clamps, and a relatively rigid transverse yoke secured to
`the tube at a point midway between the clamps. The
`yoke has magnetically permeable ends and a magneti-
`cally permeable center portion. Electromagnets adja-
`cent one end of the yoke and the center portion thereof
`interact with the yoke to cause the tube to oscillate
`simultaneously in torsion and in flexure at the natural
`frequency of the tube in combination with the fluid
`within it. The amplitude of torsional oscillation is main-
`tained constant by a torsional detector and control cir-
`cuit, and the power required to maintain said amplitude
`is determined, said power being a measure of the viscos-
`ity of the fluid flowing through the tube. The frequency
`of flexural oscillation of the tube is determined by a
`flexural detector and associated circuit, said frequency
`being a measure of the density of the fluid flowing
`through the tube.
`
`24 Claims, 11 Drawing Figures
`
`[73]
`
`Assignee: National Metal and Refining
`Company, Ltd., Edison, N.J.
`
`[21]
`
`Appl. No.: 528,744
`
`[22]
`
`Filed:
`
`Sep. 2, 1983
`
`Int. Cl.3 ........................................... .. G0lN 11/16
`U.S. Cl.
`..................................... .. 73/54; 73/32 A;
`73/59
`Field of Search ................. .. 73/59, 54, 32 A, 179,
`73/60, 702
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`............................ .. 73/59
`6/1958 Roth et al.
`2,839,917
`7/1960 Bernstein .......
`73/32 A
`2,943,476
`4/1965 Banks ......................... .. 73/59
`3,177,705
`5/1968 Fitzgerald et al.
`73/59
`3,382,706
`1/1973 Oppliger ............. ..
`73/59
`3,710,614
`1/1973 Fitzgerald et al.
`..... ..
`.. 73/59
`3,712,117
`3,762,429 10/1973 Fitzgerald et al.
`137/92
`4,217,774
`8/1980 Agar ................................... 73/32 A
`
`
`
`[51]
`[52]
`
`[58]
`
`[56]
`
`MAGNETIC POLE PIECE 25b
`
`MAGNETIC POLE PIECE
`
`TORSIONAL/FLEXURAL
`DETECTOR COIL 2:1
`
`Micro Motion 1031
`
`1
`
`Micro Motion 1031
`
`

`
`.U.S. Patent
`
`Jun. 25,1985‘
`
`Sheet1of6I
`
`4,524,610
`
`/ FLOW OF FLUID
`A
`I
`
`MAGNETIC POLE PIECE 55
`
`-
`
`FLEXURAL DRIVER COIL lb
`
`
`CLAMPED AT EACH END
`
`DETECTOR COIL 2
`
`COMPLIANT TUBE
`
`.-L1
`LIQUID
`
`FIG" 2
`
`2
`
`

`
`.U.S. Patent_ Jun. 25,1985
`
`Sheet2of6
`
`4,524,610
`
`66b
`
`::-_
`V"
`
`2d
`
`FLEXURAL onecron
`
`67b
` FLOWING
`
`LIQUID
`
`CLAMPED AT EACH END
`
`F|G.4b
`
`3
`
`

`
`U.S. Patent
`
`Jun. 25, 1985
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`
`U.S. Patent
`
`Jun. 25, 1985
`
`Sheet5 of6
`
`4,524,610
`
`VISCOSITY ITORSIONAII MODE
`
`
`
`/LOVI VISCOSITY
`
`\\(/NORMALIZED HIGH VISCOSITY
`\
`
`E E
`«Z» 3;
`'5' S
`>< .5
`,_
`L-. 2
`8 E
`?» E
`> 5:
`
`
`
`DENSlTY(FLEXURAL) MODE
`
`HIGH
`oeusm
`
`' Low
`oeusm
`
`men vnscosmr
`
`FIG. 7
`
`6
`
`

`
`U.S. Patent
`
`Jun. 25,1985
`
`Sheet6 of6
`
`4,524,610
`
`F|G.9
`
`FIG.IO
`
`70 F|G.8
`
`
`
`FLEXURALDRIVER
`
`7
`
`

`
`1
`
`IN-LINE VIBRATORY
`VISCOMETER-DENSITOMETER
`
`4,524,610
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to an improved vibratory vis-
`cometer transducer and circuit for measuring the vis-
`cosity of fluids, which is particularly suitable for, but
`not limited to in-line process control applications; and
`to an in-line instrument capable of measuring both the
`viscosity and the density of a fluid.
`Rotational vibratory viscometers are well known in
`the art, and generally comprise (i) a transducer having a
`tip immersible in a fluid the viscosity of which is to be
`determined, (ii) an electromagnetic drive coil for caus-
`ing the tip to rotationally oscillate with a very small
`angular amplitude, (iii) a feedback control circuit for
`maintaining the angular amplitude of oscillation of the
`tip at a predetermined constant value irrespective of the
`viscosity of the fluid, and (iv) a circuit for determining
`the power supplied to the drive coil, usually by squaring
`the current supplied to said coil, which power is a mea-
`sure of the viscosity of the fluid.
`A viscometer of this type is described, for example, in
`“Viscometer for Energy Saving”, J. V. Fitzgerald, F. J.
`Matusik, and P. C. Scarna, Jr., Measurements & Control
`April 1980. Similar viscometers are described in the
`references cited in said article, as well as in U.S. Pat.
`Nos.
`3,382,706;
`3,710,614;
`3,712,117;
`3,672,429;
`3,875,791; and 4,299,119; and in copending U.S. patent
`application Ser. No. 483,142, filed Apr. 8, 1983 and
`assigned to the assignee of the present application.
`Such viscometers, however, are not well suited for
`in-line process control applications where the viscosity
`of a fluid flowing in a pipe has to be continuously moni-
`tored, in that the prior art vibratory viscometers require
`immersion of the transducer tip in the fluid stream,
`resulting in undesirable turbulence and restriction of
`flow.
`In cases where the fluid stream contains sus-
`pended particles, these tend to build up on the viscome-
`ter tip, altering its characteristics. Further, since vibra-
`tory viscometers inherently measure the viscosity-den-
`sity product of the fluid, in order to determine actual
`viscosity it is necessary to determine the density of the
`fluid; and prior art vibratory viscometers require either
`a manually set density input or a signal from a separate
`densitometer.
`
`Accordingly, an object of the present invention is to
`provide an improved vibratory viscometer transducer
`and circuit suitable for in-line process control applica-
`tions.
`
`SUMMARY OF THE INVENTION
`
`As herein described, there is provided a transducer
`assembly for an in-line vibratory viscometer, compris-
`ing: a tube exhibiting elasticity; means for securing first
`and second longitudinally spaced sections of said tube
`to provide oscillation nodes at said sections; a transverse
`yoke secured to a third section of said tube intermediate
`said first and second sections thereof; drive means com-
`prising means operatively associated with said yoke for
`exciting and maintaining oscillation of said tube be-
`tween said nodes; and detector means for detecting said
`oscillation.
`Also herein described is a transducer assembly for an
`in-line vibratory viscometer, comprising: a tube exhibit-
`ing elasticity; means for securing flrst and second longi-
`tudinally spaced sections of said tube to provide oscilla-
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`tion nodes at said sections; a transverse yoke secured to
`a third section of said tube intermediate said first and
`second sections thereof; drive means comprising means
`operatively associated with said yoke for exciting and
`maintaining oscillation of said tube between said nodes,
`said drive means comprising: flexural drive means for
`causing said tube to flexurally oscillate in deflection
`about the longitudinal axis of the tube, torsional drive
`means for causing said tube to rotationally oscillate in
`torsion about said longitudinal axis, flexural detector
`means responsive to the flexural oscillation of said tube,
`and torsional detector means responsive to the torsional
`oscillation of said tube.
`According to another aspect of the invention there is
`provided an in-line vibratory viscometer, comprising: a
`transducer having: a tube exhibiting elasticity, means
`for securing first and second longitudinally spaced sec-
`tions of said tube to provide oscillation nodes at said
`sections, a transverse yoke secured to a third section of
`said tube intermediate said first and second sections
`thereof, drive means comprising means operatively
`associated with said yoke for exciting and maintaining
`torsional oscillation of said tube between said nodes,
`and detector means for detecting said torsional oscilla-
`tion; first circuit means connected between said drive
`means and said detector means for providing a torsional
`oscillation mode control signal to said drive means to
`maintain said tube in torsional oscillation at substan-
`tially the natural frequency thereof and at a predeter-
`mined constant amplitude; and second circuit means
`coupled to said drive means and said detector means for
`providing a viscosity-density product output signal
`indicative of the viscosity-density product of any fluid
`within said tube, said viscosity-density product output
`signal corresponding to the power required to sustain a
`predetermined angular amplitude of torsional mode
`oscillation of said tube with said fluid therein.
`
`IN THE DRAWING
`
`FIG. 1 is a perspective view of an in-line viscometer
`transducer according to a first embodiment of the pres-
`ent invention, for measurement of fluid viscosity;
`FIG. 2 is a partially cut-away perspective view of a
`modified form of said transducer;
`FIG. 3 is a perspective view of an in-line viscometer
`transducer according to a second embodiment of the
`invention, for alternate measurement of fluid viscosity
`and fluid density;
`FIG. 4a is a cross-sectional view of an in-line viscom-
`eter transducer according to a third embodiment of the
`present invention, capable of simultaneously determin-
`ing both fluid viscosity and fluid density;
`FIG. 4b is a partially cut-away perspective view of an
`in-line viscometer transducer according to a modified
`third embodiment of the present invention, capable of
`simultaneously determining both fluid viscosity and
`fluid density;
`FIG. 5 is a functional block diagram of a viscometer-
`densitometer utilizing the transducer of FIG. 3 for alter-
`nately determining fluid viscosity and fluid density;
`FIG. 6 is a functional block diagram of a viscometer-
`densitometer utilizing the transducer of FIG. 4a or
`FIG. 4b for simultaneously determining viscosity and
`density;
`FIG. 7 is a graph useful in explaining the operation of
`the transducer and circuit of the invention;
`
`8
`
`

`
`4,524,610
`
`3
`FIG. 8 is a perspective view of an in-line viscometer
`transducer according to a fourth embodiment of the
`present invention and suitable for laboratory usage;
`FIG. 9 is a side elevation view of an in-line viscome-
`ter transducer according to a fifth embodiment of the
`present invention; and
`FIG. 10 is a side elevation view of an in-line viscome-
`ter according to a sixth embodiment of the present in-
`vention.
`In the drawing, those numerals which correspond to
`numerals in the drawing of copending U.S. patent appli-
`cation Ser. No. 483,142, filed Apr. 8, 1983,
`identify
`elements which are of similar construction and which
`perform similar functions.
`DETAILED DESCRIPTION
`
`10
`
`15
`
`4
`tance of the fluid is coupled to the internal wall of the
`tube by viscous friction forces and dampens the tor-
`sional vibration of the tube, so that additional power
`must be supplied to the driver coil 1a to maintain the
`tube at the predetermined amplitude of torsional oscilla-
`tion, said power being a measure of the viscosity-den-
`sity product of the fluid.
`If the internal surface of the tube 50 is smooth, the
`viscous shear between the tube inner surface and the
`fluid within the tube is minimal, resulting in high preci-
`sion of measurement but relatively low transducer sensi-
`tivity.
`The sensitivity of the transducer 100 can be increased
`by roughening the internal surface of the tube 50 to
`increase the viscous shear applied to the fluid within the
`tube. This can be accomplished by abrasion of the tube
`surface or by stamping the tube to generate internal
`surface irregularities. Alternatively, a material having
`surface irregularities providing a large surface area,
`such as surgical cloth, can be secured to the internal
`surface of the tube. It is been found that such modifica-
`
`tions can be made without adversely affecting the per-
`formance of the transducer.
`
`It has also been found that the measured viscosity-
`density product is substantially independent of the flow
`rate of the fluid, for variations in flow rate of up to
`about 30%. The permissible variation in flow rate is
`greater for an internally smooth tube than for one
`which has been roughened, since there are turbulence
`effects in the latter case.
`
`For greater precision of measurement, especially
`where there is a chance of trapping bubbles, the tube 50
`should be maintained in a vertical position. Otherwise,
`the position of the tube 50 can be varied, with only a
`relatively minor effect on the measurement.
`While the tube 50 is preferably straight and of circu-
`lar cross-section, it may be curved and may have other
`(i.e. non-circular) cross-sections; or even a cross-section
`which varies along the length of the tube. For example,
`FIG. 9 shows a transducer 109 having a U-shaped tube
`50a with a yoke 53a situated at the bend of the tube and
`in the plane thereof; and FIG. 10 shows a transducer 10f
`comprising a tube 53b having a central enlarged portion
`53c.
`As shown in FIG. 2, torsional oscillation of the tube
`50 via the yoke 53d (which here consists of two separate
`pieces welded to opposite sides ofthe tube 50 so that the
`yoke extends along a diameter of the tube) generates a
`shear wave which propagates through the fluid within
`the tube toward the tube axis, with a shear rate equal to
`
`27f
`
`where
`
`(1)
`
`fr: frequency of torsional oscillation
`The torsional resonance frequency of a straight tube
`clamped or otherwise secured at both ends and twisted
`into torsional vibration by applying alternating torque
`at the midpoint of the tube between the clamped por-
`tions thereof, is equal to
`
`fT=27r(2KT/D5
`
`where
`
`(2)
`
`KT=spring constant of the tube, i.e. the torque re-
`quired to twist the tube through one radian at the
`midpoint between the clamped portions thereof
`I=moment of inertia of the tube and yoke
`
`Transducer Structure
`
`The in-line viscometer
`
`transducer 10:: shown in
`
`20
`
`25
`
`30
`
`FIGS. 1 and 2 consists of a non-magnetic tube 50, pref-
`erably made of stainless steel and dimensioned to have
`sufficient torsional compliance and elasticity so that it
`can be maintained in torsional oscillation by the tor-
`sional driver coil la at a very small angular amplitude,
`typically on the order of about 0.001 radian, with very
`small internal energy loss in the tube itself when driven
`at the natural frequency of torsional oscillation of the
`tube.
`The tube 50 is secured at each end by clamping or
`heliarc welding to the support blocks 51a and 5117,
`which are attached to a base plate 52. At the midpoint
`of the tube 50 between the support blocks 51:2 and 51b
`a non-magnetic yoke 53, preferably of stainless steel,
`surrounds the tube 50 and is secured to its periphery,
`preferably by heliarc welding.
`The yoke 53 should be positioned at an antinode of 35
`oscillation of the tube 50. If the tube 50 is caused to
`oscillate in its fundamental mode, the antinode will be at
`the midpoint between the support blocks 51a and 51b.
`If, however,
`the tube 50 is caused to oscillate in its
`second harmonic mode, there will be two antinodes,
`each halfway between the midpoint of the tube 50 (be-
`tween said support blocks) and the adjacent support
`block.
`The yoke 53 extends transversely of the tube 50, and
`has a magnetic pole piece 25a secured to the bottom of 45
`one end and a magnetic pole piece 25b secured to the
`bottom of the other end. The electromagnetic driver
`coil la is mounted on the base plate 52 below the pole
`piece 25b, while an electromagnetic detector coil 2a is
`mounted on the base plate 52 below the pole piece 25a.
`This arrangement allows the driver coil 1a to exert
`force on the pole piece 25b to cause the tube 50 to oscil-
`late in torsion about its longitudinal axis 54; and causes
`the detector coil 2a to interact with the pole piece 25a
`to generate a signal having an amplitude and frequency
`corresponding to the amplitude and frequency of said
`torsional oscillation.
`
`40
`
`50
`
`55
`
`The circuitry which will be hereafter described pro-
`vides a positive feedback loop between the detector and
`driver coils to maintain the tube 50 in torsional oscilla-
`tion at a predetermined constant angular amplitude; and
`provides an output signal corresponding to the power
`provided to the driver coil 50,
`i.e. to the energy re-
`quired to maintain the tube in oscillation at said prede-
`termined angular amplitude.
`When this is done, and a fluid is caused to flow
`through the tube 50 (with the fluid preferably filling the
`entire interior volume of the tube), the viscous resis-
`
`60
`
`65
`
`9
`
`

`
`4,524,610
`
`6
`
`and
`
`KT= G7T(r24-714)/1
`
`(3)
`
`where
`G=modulus of rigidity of the tube material
`r2=outer tube radius
`r1=inner tube radius
`l=length of tube between the clamped portions
`thereof
`
`and
`
`I=7rtp(r24—r14)/2
`
`(4)
`
`where
`t=thickness of tube wall
`
`5
`
`10
`
`1 5
`
`rugated metal. It is also desirable to isolate the base
`plate 52 from its supporting structure by means of a
`resilient vibration absorbing material having high inter-
`nal friction loss, such as foam rubber, acoustic absorp-
`tion material, or the like.
`If so desired, the drive coil la and magnetic pole
`piece 24a must be replaced by an electrostrictive or
`magnetostrictive element rigidly connected between
`the base plate 52 and yoke 53. Such structures would
`exhibit improved resistance to interference effects due
`to external vibrations.
`
`Bimodal Transducer for Measurement of Fluid Density
`as well as Viscosity-Density Product
`
`It is generally known that the density of a fluid can be
`measured by causing flexural resonance of a tube (i.e. so
`that the tube axis bends) containing the fluid whose
`density is to be measured. Commercial densitometers
`based on this principle include the “Liquid Density
`Transmitter” made by Bell & Howell, Pasadena, Calif.;
`the “DPR 2000 On-line Density Measuring System”
`made by Anton Paar K. G., Graz, Austria; the “Vibra-
`tion Type Liquid Density Measuring System” made by
`Yokogawa Electric Works, Tokyo, Japan; the “Dyna-
`trol” made by Automation Products Co., U.S.A. The
`“E-Z Cal Density Gauge” sold by Texas Nuclear,
`Houston, Tex., measures density of the media flowing
`through a pipe by attenuation of radiation.
`Such density gauges can be used to provide measure-
`ments for conversion of the viscosity-density product to
`provide true viscosity data. However, the arrangement
`hereafter described is capable of providing density data
`as well as viscosity data utilizing the same vibratory
`tube as has been described in the preceding portion of
`this specification.
`The tube material should exhibit a relatively low
`coefficient of variation of elastic modulus with tempera-
`ture. By providing a thermocouple or thermostat which
`generates an electrical signal corresponding to tempera-
`ture, the density reading can be adjusted to compensate
`for the temperature variation in tube modulus of elastic-
`ity. The stainless steel tube hereafter described exhib-
`ited a flexural mode frequency shift of 0.06 Hz. per °C.
`While isolation of the tube from external vibrations is
`not as critical for the flexural mode of vibration as for
`the torsional mode, the use of vibration isolation means
`similar to those employed in the torsional mode as de-
`scribed above, is preferred.
`For a tube clamped at both ends the frequency of
`flexural resonance is given by
`
`f1-‘= s(3E1/m13)%
`
`where
`E=the deflection modulus of the tube material
`I=the area moment of inertia of the tube
`m=mass of the tube per unit length
`l=length of tube
`and
`
`1=(a'24—d14)7r/64
`
`(5)
`
`(6)
`
`where
`d2=outer diameter of tube
`d1=inner diameter of tube
`Using equations 5 and 6, the flexural resonance fre-
`quency for a one inch diameter empty type 316 stainless
`
`20
`
`30
`
`35
`
`p=density of tube material
`The above equations assume the effect of the yoke to
`be negligible. Using equations 2 to 4, the torsional oscil-
`lation frequency of a one inch diameter empty type 316
`stainless steel tube having a wall thickness of 0.85 mm.
`and a distance of 22.7 cm. between clamping points was
`calculated to be about 700 Hz.
`In an actual test the resonant frequency of torsional
`oscillation of such a tube was found be be 635 Hz.
`It has also been found that as the viscosity of the fluid 25
`within the tube increases,
`the resonant frequency of
`torsional oscillation decreases up to about 3%. How-
`ever, this frequency change does not adversely affect
`the accuracy of the measurement of viscosity-density
`product.
`The upper and lower left-hand curves in FIG. 7 show
`in solid lines the angular amplitude vs. frequency distri-
`bution for torsional oscillation of the tube 50 when filled
`with low viscosity and high viscosity fluids respec-
`tively, under conditions where the feedback circuit does
`not maintain the oscillation amplitude constant. It is
`evident that the frequency of torsional oscillation is
`slightly reduced as the fluid viscosity increases. The
`dashed line shows the angular amplitude vs. frequency
`distribution with the high viscosity fluid, using the con-
`trol circuit 11 of FIG. 5, which maintains a constant
`amplitude of oscillation.
`In an actual test of the transducer shown in FIG. 1
`with the circuit shown in FIG. 5, the transducer having
`a stainless steel tube with a smooth internal wall surface, 45
`the sensitivity of the transducer was sufficient to enable
`the measurement of the viscosity-density product of
`water (nominally unity) with a resolution of :0.0S
`centipoise >< g/cm3.
`A high frequency of torsional oscillation is desirable 50
`for measuring the viscosity-density product of low vis-
`cosity liquids in order to obtain measurements compara-
`ble with measurements by capillary viscometers or ro-
`tary viscometers. The latter two types of viscometers
`generally operate at relatively low shear rates. Because 55
`low viscosity fluids usually are relatively shear indepen-
`dent, the higher frequency may be used. A higher fre-
`quency is desirable to simplify the electronic circuitry
`and mechanical construction of the apparatus. On the
`other hand,
`low frequency of torsional oscillation is 60
`desirable for shear thinning liquids in order to obtain
`measurements comparable with measurements of such
`liquids by capillary and rotary viscometers.
`In order to minimize interference effects due to exter-
`nal vibration, the tube 50 should, if possible, be coupled 65
`into the fluid pipline at its ends by means of high com-
`pliance flexible sections of tubing. These flexible sec-
`tions may comprise natural or synthetic rubber, or cor-
`
`40
`
`10‘
`
`10
`
`

`
`7
`steel tube having a wall thickness of 0.85 mm. and a
`distance of 22.7 cm. between clamping points was cal-
`culated to be about 957 Hz; and a change of 7.0 Hz.
`corresponded to a fluid density change of about 0.33
`g/cm3. One experimental design of this type exhibited a
`flexural resonance frequency of 844 Hz., and a change
`of 10.1 Hz. corresponded to a fluid density change of
`about 0.33 g/cm3. Since the resonant frequency can be
`accurately measured by available digital
`frequency
`measuring equipment,
`it
`is evident
`that
`the flexural
`resonance method described above is capable of very
`high resolution density measurements.
`The transducer 10b shown in FIG. 3 is a modified
`form of the transducer shown in FIG. 1, and is capable
`of sustained oscillation in either a flexural mode or a
`torsional mode.
`The transducer 10b is similar in construction to the
`transducer 100 except forithe yoke and the associated
`electromagnetic coils. In the transducer 10b the driver
`coil la and the detector coil 2a are the same as those of
`the transducer 10a, and the detector coil 2a serves to
`detect both torsional and flexural vibrations of the yoke
`53e.
`'
`
`For optimum sensitivity, the detector coil 20 should
`be a linear voltage characteristic differential
`trans-
`former, since such devices exhibit relatively high sensi-
`tivity to small magnetic circuit element motions along
`the axis of their coils.
`
`Instead of the detector coil 2a, other displacement or
`motion detectors, such as optical or capacitance detec-
`tors, may be employed.
`A second driver coil 1b is mounted on the base plate
`52 below the yoke 529 and tube 50, and vertically
`aligned with the axis of the tube 50, so that the magnetic
`pole piece 55 on the bottom of the central portion of the
`yoke 53e interacts with the driver coil lb to cause the
`tube 50 to oscillate in a flexural mode, i.e. in a vertical
`plane. Alternatively, the driver coil lb and magnetic
`pole piece 55 may be replaced by an electrostrictive or
`magnetostrictive element connected between the base
`plate 52 and yoke 53e, so long as it exhibits sufficient
`lateral compliance to avoid inhibiting torsional vibra-
`tion of the tube 50.
`
`For measuring the viscosity-density product of the
`fluid within the tube 50, a positive feedback circuit is
`interconnected between the detector coil 2a and the
`torsional driver coil la, and the power supplied to the
`driver coil la in order to maintain a predetermined
`amplitude of torsional oscillation of the tube 50 is mea-
`sured, as previously described with respect to the trans-
`ducer 10a; said power being proportional to the fluid
`viscosity-density product.
`For measuring the density of the fluid within the tube
`50, substantially the same positive feedback circuit is
`interconnected between the detector coil 2a and the
`flexural driver coil 1b, and the resonant frequency of
`flexural oscillation of the tube 50 is measured, said fre-
`quency being proportional
`to the fluid density. For
`maximum accuracy and stability of measurement, the
`amplitude of flexural oscillation should be kept constan-
`t—although amplitude variations have only a minor
`effect on the density measurement.
`As shown in the two right-hand curves of FIG. 7, the
`difference in resonant frequency of flexural oscillation
`of the tube 50 for low density and high density fluids
`within the tube 50 is proportional to the difference in
`density of the fluids, the resonant frequency decreasing
`with increasing fluid density.
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`ll
`
`4,524,610
`
`8
`By using a sampling signal to switch back and forth
`between the torsional and flexural modes of oscillation
`of the tube 50, the transducer 10b can be used to alter-
`nately measure both viscosity-density product and den-
`sity. Preferably the switching rate should be no faster
`than about 5 seconds per cycle for fluids having a vis-
`cosity similar to that of water, to allow sufficient set-
`tling time for accurate measurements. A higher switch-
`ing rate can be used for relatively low viscosity fluids,
`and a lower switching rate should be used for relatively
`high viscosity fluids.
`In this alternate mode sample-and-hold circuits are
`employed to store the viscosity-density product and
`density
`information between successive
`readings
`thereof; and the viscosity-density product value is di-
`vided by the density value to provide a measurement of
`true viscosity of the fluid within the tube 50. The over-
`all circuit for providing these functions is shown in
`FIG. 5.
`
`Oscillation Control; Alternate Viscosity and Density
`Measurement Circuitry
`
`As shown in FIG. 5, oscillation amplitude control
`circuit 11 causes the yoke 53e and tube 50 to oscillate in
`torsion at a constant angular amplitude when the mode
`switch 56 (consisting of ganged sections 560 and 56b) is
`in one position; and causes the yoke 53e and tube 50 to
`oscillate in flexure at a constant linear amplitude when
`the mode switch 56 is in the other position.
`i
`In the torsional oscillation mode the current through
`the torsional driver coil la and resistor 34 is propor-
`tional to the amplitude of torsional oscillation, and the
`square of said current,
`i.e. the square of the voltage
`developed across the resistor 34, corresponds to the
`power supplied to the driver coil 1a to maintain the
`torsional oscillation amplitude, i.e. to the viscosity-den-
`sity product of fluid within the tube 50. This product is
`determined by the operational rectifier and filter 36 and
`variable exponent amplifier squaring circuit 39, and
`stored in the sample-and-hold amplifier 57 between
`successive viscosity-density product readings. The den-
`sity correction divider 40 uses a density signal from the
`density sample-and-hold amplifier 58 to divide the vis-
`cosity-density product signal from the amplifier 57 by
`density, so as to provide a true viscosity signal output
`on line 43, which viscosity signal may be coupled to the
`active filter 44 and display 45 or recorder and/or data
`bus 46 via readout selector switch 59.
`In the flexural oscillation mode the current through
`flexural driver coil lb passes through resistor 34, and
`the corresponding voltage signal (having a frequency
`corresponding to the density of the fluid within the tube
`50) is converted by the frequency to voltage converter
`60 (which may be a suitably calibrated frequency
`counter or a frequency discrimnator circuit) to a signal
`corresponding to fluid density. This density signal
`is
`amplified by the amplifier 61 and stored in the sample-
`and-hold amplifier 58 between successive density read-
`ings; from which amplifier the density signal is used to
`convert the viscosity-density signal from the sample-
`and-hold amplifier 57 to true viscosity and/or to pro-
`vide a density readout via the readout selector switch
`59, as previously described.
`The sampling control oscillator 62, comprising an
`astable multivibrator or clock, alternately switches the
`circuit between the torsional (viscosity-density product
`determining) and flexural (density determining) oscilla-
`tion modes, by switching the ganged switch sections
`
`11
`
`

`
`9
`56a and 56b and simultaneously alternately enabling the
`sample-and-hold amplifiers 57 and S8. The sampling
`rate can be varied by varying the frequency of the oscil-
`lator 62.
`
`In the oscillation amplitude control circuit 11, gain
`controlled amplifier 3 has its input connected to the
`output of detector coil 2a and its output connected
`through a phase compensation circuit 4 to the input of
`driver coil la or 1b, to form a positive feedback loop
`which maintains the tube 50 in torsional or flexural 10
`oscillation at the natural frequency thereof. The ampli-
`tude of the oscillation is maintained constant by control
`of the gain of amplifier 3 via a gain control signal ap-
`plied thereto on line 5.
`An amplitude monitoring circuit 6 has its input con-
`nected to the output of detector coil 2a, and provides on
`its output line 7 an AC signal at the frequency of oscilla-
`tion of the tube 50, said AC signal having an amplitude
`corresponding to the amplitude of oscillation of the tube
`50.
`
`20
`
`15
`
`25
`
`40
`
`The AC signal output of amplitude monitoring circuit
`6 is applied to (i) sample-and-hold circuit 8 and (ii)
`threshold comparator 9. Threshold comparator 9 gener-
`ates an output signal once during each cycle of the AC
`signal on line 7, at a time when said AC signal crosses a
`preset amplitude threshold, i.e. at a time when yoke 53e
`(i.e. tube 50) is in a particular position of its cycle of
`(torsional or flexural) oscillation, preferably at or near
`the peak of the oscillation cycle. That is, the output 30
`signal from threshold comparator 9 always appears in
`successive cycles at the same physical position of the
`yoke 53.2.
`Threshold comparator 9 includes a 90° phase shift
`circuit to shift the AC signal on line 7 so that the zero 35
`crossovers of the phase shifted signal occur at the same
`times as the peaks of said AC signal on line 7. The com-
`parator 9 then compares the phase shifted signal with
`ground, to generate an output signal at each positive-
`going (or negative-going) zero crossover.
`The output of threshold comparator 9 is differenti-
`ated by differentiator circuit 30, with the output of
`circuit 30 being coupled to monostable multivibrator
`31, which provides sampling pulses (one sampling pulse
`per cycle of oscillation of the tube 50) to the sample- 45
`and-hold circuit 8 on line 32.
`The AC signal on line 7, corresponding to amplitude
`of oscillation of the tube 50, is compared by sample-and-
`hold circuit 8 with a DC reference signal (indicative of
`the desired ampli