Abstract:
Apparatus for detecting the magnitude of a property of a fluid including a body mounted on a torsion spring that is oscillated by a feedback of alternately an in-phase and an out-of-phase signal from an amplifier having a gain adequate to sustain the oscillations. If the in-phase signal has a frequency directly proportional to f n  and the out-of-phase signal has a frequency directly proportional to f h , then the magnitude of the property p is determined by the digital computation of ##EQU1## where K and B are constants. The property p may be any one of several properties including but not limited to viscosity.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the detection of a property of a fluid, and more particularly to viscosity or other fluid property magnitude detection. 
     In the past, prior art viscosimeters have had relatively poor accuracy. 
     SUMMARY OF THE INVENTION 
     In accordance with the system of the present invention, the above-described and other disadvantages of the prior art are overcome by oscillating a body in a fluid with an amplified feedback impressed upon a driver with an in-phase and out-of-phase signal at alternate periods of time. It has then been found that if the oscillation frequency is directly proportional to f n  and f h  in the two sets of alternate periods, respectively, then the magnitude of a property of the fluid, including but not limited to viscosity, will be ##EQU2## where K and B are constants. 
     In accordance with the invention, means are provided to compute p in accordance with the above equation. 
     The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which are to be regarded as merely illustrative: 
     FIG. 1 is a vertical sectional view through a drag body assembly, partly in elevation; 
     FIG. 2 is a top plan view of the assembly shown in FIG. 1; 
     FIG. 3 is a transverse sectional view taken on the line 3--3 of the assembly shown in FIG. 1; 
     FIG. 4 is a broken away sectional view, partly in elevation, illustrating the mount of a piezoelectric crystal shown in FIGS. 1 and 2; 
     FIG. 5 is a vertical sectional view, partly in elevation, of an alternative embodiment of the present invention; 
     FIG. 6 is a block diagram of a system of the present invention employing a drag body assembly which may be of the type shown either in FIGS. 1, 2, 3 and 4, or in FIG. 5; 
     FIG. 7 is a block diagram of a frequency multiplier shown in FIG. 6; 
     FIG. 8 is a block diagram of a digital phase shifter shown in FIG. 6; 
     FIG. 9 is a graph of a group of waveforms characteristic of the operation of the digital phase shifter of FIG. 8; 
     FIG. 10 is a block diagram of a reversible counter shown in FIG. 6 with a rate multiplier; 
     FIG. 11 is a block diagram illustrating a portion of two registers and a gating circuit shown in FIG. 10; 
     FIG. 12 is a block diagram of a triangle wave generator shown in FIG. 6; and 
     FIGS. 13 and 14 are graphs of groups of waveforms characteristic of the operation of the system of the invention shown in FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a drag body assembly 20 may be employed which includes a cylindrical container 21 closed at its lower end, and a torsion spring assembly 22 having a disk 23 fixed inside of container 21 at the top thereof, all being shown in FIG. 1. 
     Disk 23 may be threaded into container 21, if desired. If so, a fluid sample may be inserted into container 21 by unscrewing disk 23 and injecting the sample into container 21 through the upper end thereof. 
     All of the structure shown inside container 21 and spaced from the internal cylindrical wall of container 21 may be integrally made from a single isotropic piece of metal or other material. However, there is one exception to this statement. A cylindrical portion or drag body 24 has a hole 25 therethrough in which a permanent magnet 26 is fixed. The location of permanent magnet 26 may be still better understood from the view of FIG. 3. A disk 27 is integral with disk 23. Four torsion strips 28, 29, 30 and 31 connect disk 27 to a disk 28&#39; fixed relative to the torsion strips 28 to 31. Torsion strips 28 to 31 may also be observed in FIG. 2. 
     In FIGS. 1 and 2, a piezoelectric crystal is illustrated at 32. 
     In FIG. 1, the diameter of drag body 24 is illustrated at A&#39;. In the same FIG. 1, the inside diameter of container 21 is illustrated at B&#39;. The inside diameter B&#39; is greater than the outside diameter A&#39; by an amount equal to 60 mils, if desired. 
     As shown in FIG. 1, container 21 is fixed. As shown in FIG. 3, a permanent magnet 33, a ferromagnetic core 34 and a coil 35 around core 34 are fixed relative to container 21. The axis of permanent magnet 33, core 34 and coil 35 is perpendicular to the vertical axis of drag body 24 and makes and angle θ with the axis of permanent magnet 26 equal to, for example, 45°. 
     As shown in FIG. 4, crystal 32 may be mounted in any conventional way having an upper electrode 36, a lower electrode 37 and a bonding layer 38 bonding the lower electrode 37 to disk 23. 
     Upper electrode 36 has an electrical lead 39 connected therefrom. Lower electrode 37 has an electrical lead 40 connected therefrom. Both leads 39 and 40 are also shown in FIG. 1. 
     An alternative embodiment of the drag body assembly 20 is illustrated at 41 in FIG. 5. The drag body assembly 41 may be identical to drag body assembly 20 shown in FIG. 1 except that disk 23&#39; corresponding to disk 23 in FIGS. 5 and 1, respectively, is provided with a hole 42 therethrough into which a tube 43 is sealed. If desired, the inside diameter of tube 43 may be approximately equal to or exactly equal to the spacing between each oppositely disposed pair of torsion strips 28 and 30, and 29 and 31. The hole 42 in disk 23&#39; in FIG. 5 also extends completely through disk 27&#39; corresponding to disk 27 in FIG. 1. 
     Drag body assembly 41 also is provided with a container 21&#39; which may be identical to container 21 shown in FIG. 1, except that it is provided with a hole 44 therethrough into which a tube 45 is sealed. In accordance with the foregoing, as viewed in FIG. 5, fluid may be slowly circulated upwardly or downwardly in tubes 43 and 45. 
     Fluid can pass between the interior cylindrical surface of container 21 and drag body 24 in the space therebetween, the same having larger and smaller inside and outside diameters, respectively, at B&#39; and A&#39; shown in FIG. 1 and as described previously. 
     The system of the present invention is shown in FIG. 6 including a drag body assembly 20&#34; which may be identical to assembly 20 or assembly 41 shown in FIGS. 1 and 5, respectively. 
     Assembly 20&#34; has a piezoelectric crystal 32&#39; corresponding, for example, to crystal 32 shown in FIG. 1, a coil 35&#39; corresponding to coil 35 shown in FIG. 3. Assembly 20&#34; with a differential amplifier 46, a squarer 47, a frequency multiplier 52, a divide-by-250 divider 63, either a divide-by-two divider D1 or a digital phase shifter 51, an electronic switch 48, a triangle wave generator 49, and a power amplifier 50 form a closed loop electromechanical oscillator wherein the gain is adequate to cause drag body 24, for example, in FIG. 3 to rotate back and forth or oscillate about its vertical axis, as viewed in FIG. 1. The peak-to-peak rotation is relatively small in amplitude (perhaps about one degree), and when the upper end of core 34 in FIG. 3 becomes positive, it provides additional attraction for the south pole of permanent magnet 26. Permanent magnet 26 provides permanent bias which may or may not allow the south pole of permanent magnet 26 to rise to or above the position in which it is shown in FIG. 3. When the upper end of core 34 becomes a south pole, both it and torsion spring assembly 22 tend to return permanent magnet 26 toward the position shown in FIG. 3, torsion spring assembly 22 resisting downward movement of the south pole of permanent magnet 26. 
     Coil 35&#39; in FIG. 6 is supplied with an in-phase voltage when switch 48 is in a position not shown. When switch 48 is in the position shown, coil 35&#39; is supplied with a signal which is, for example, 45 electrical degrees lagging the phase of output of amplifier 46. 
     The phase of the input to coil 35&#39; when it lags the output of amplifier 46, in electrical degrees, is independent of frequency because of the use of digital phase shifter 51 shown in FIG. 6. 
     The said electromechanical oscillator is thus operated with a feedback without a phase shift during, for example, one second, and with the said phase shift for another second. This is done alternately so long as the system is in operation. 
     If desired, the output of a frequency multiplier 52 at a junction 53 may be described as f n  when the said electromechanical oscillator is operating without a phase shift. The output of frequency multiplier 52, when a phase shift exists in the oscillatory loop, may be described as f h . 
     Other components of the system of FIG. 6 are a 1-megahertz clock 53&#39;, a divide-by-one thousand divider 54, a divide-by-one thousand divider 55, a divide-by-two divider 56, an electronic switch 57, an inverter 58, a differentiator 59, a flip-flop 60, a counter 61, and AND gate 62, a divide-by-two hundred fifty divider 63, a double-pole, double-throw electronic switch 64, an up-down or reversible counter 65, a rate multiplier 66, a rate multiplier 67, a switch matrix 68, an offset digital computer 69 having a switch matrix 70, and a display unit 71. 
     Dividers 54 and 55 are connected in succession from the output of clock 53&#39;. Divider 55 has an output lead 72 which is high for one second and low for the following second, the same being a slow square wave. Output lead 72 of divider 55 is connected to the input of inverter 58, controls the position of switch 64, controls the updating of counter 65, and controls the alternate modes of oscillation of the said oscillatory loop by controlling the position of switch 48. 
     It is a nuance of the operation of the system of FIG. 6 that f h  is larger than f n . The system is therefore cycled by control of switch 64 to cause counter 65 to count f h  for one second in the up direction, and to count f n  down when switch 64 is in the position not shown, in this position, f n  being impressed upon the down input of counter 65. In this way, the updated minimum count stored in counter 65 represents the difference between f h  and f n . The stored value is then multiplied by the pulses appearing at the output lead 73 of switch 57. However, switch 57 is not closed for a full second. The closure of switch 57 is directly proportional to the period of f n  (the reciprocal of f n ). The closure of switch 57 is controlled by the &#34;0&#34; output of flip-flop 60. 
     Inverter 58 and differentiator 59 reset flip-flop 60 on the falling edge of the square wave output on lead 72 of divider 55. This is the end of the time period for f h . Note that a junction is provided at 74 which is connected to junction 53 at the output of frequency multiplier 52. Moreover, a lead 75 is connected from junction 74 to one input of AND gate 62. AND gate 62 also receives another input from the &#34;0&#34; output of flip-flop 60. Counter 61 then counts f n  pulses and automatically resets by a reset output 76 thereof. At the same time, counter 61 resets flip-flop 60 to the &#34;1&#34; state. 
     Counter 61 is set to a count so that the reset pulse at output lead 76 thereof will occur prior to one second from the output pulse of differentiator 59. The range of the instrument will be determined by how short counter 61 should be. However, the frequency shift is always, or nearly always, quite small. Squarer 47 is connected to a junction 77 which is also connected to the input of frequency multiplier 52. The frequency of the signal appearing at junction 77 when the output of frequency multiplier 52 is f n  may be 200.000 hertz. The frequency of the signal appearing at junction 77 when f h  exists may be 202.500 hertz. 
     Counter 61 may reset on the count of 100,000; 150,000; or more or less, but may be close to, but must be less than, a count which would cause switch 57 to be closed for more than one second. 
     The number stored in counter 65 is then multiplied by the number of pulses appearing on output lead 73 of switch 57 during the time that switch 57 is closed. 
     Switch matrix 68 in combination with rate multiplier 67 merely reduces the number of pulses at an output lead 79 of rate multiplier 66 so that the output of rate multiplier 67 is a number of pulses equal to ##EQU3## The factor K is set by adjustment of switch matrix 68. The factor B in the equation set forth hereinbefore is subtracted by computer 69 by setting matrix 70. The output of computer 69 is impressed upon a display unit 71. 
     Frequency multiplier 52 may, for example, multiply the frequency of the square wave appearing at junction 77 by a factor of 1,000. 
     Triangle wave generator 49 may be replaced with a sine wave generator. It also may be replaced with a number of other components. The purpose of triangle wave generator 49 is to get a reasonably smooth fundamental current through coil 35&#39;. The current which triangle wave generator 49 produces in coil 35&#39; is illustrated at i1 in FIG. 13. 
     As can be determined by inspection, dividers 54 and 55 merely are employed to produce a two-second period square wave on output lead 72 of divider 55 responsive to the output of clock 53&#39;. Divider 56 reduces the unnecessarily large pulse rate at the output of clock 53&#39; from which divider 56 is connected so that the input to rate multiplier 66 will not be unnecessarily large. The clock rate may be reduced, divider 56 eliminated, and the size of one or both of the dividers 54 and 55 reduced, if desired. 
     In some cases, the size of counter 61 may be increased or decreased, as desired. 
     The combination of the structures illustrated in FIG. 6 is new. Only a few of the individual components of the system of FIG. 6 are per se new. 
     Divider 63 reduces the unnecessarily large frequency appearing at the output of frequency multiplier 52 to provide an input for digital phase shifter 51. 
     All of the differentiators disclosed herein may be of the type illustrated at 80 shown in FIG. 8, the same being entirely conventional. Clock 53&#39; may be entirely conventional. The same is true of dividers 54, 55, 56 and 63. 
     The electronic switches 48, 57 and 64 may be entirely conventional. The same is true of flip-flop 60 and counter 61. The same is also true of AND gate 62 shown in FIG. 6. The same is true of differential amplifier 46, squarer 47 and power amplifier 50 shown in FIG. 6. The same is true of frequency multiplier 52, reversible counter 65, rate multiplier 66, rate multiplier 67, switch matrix 68, computer 69, switch matrix 70 and display unit 71. The same is also true of inverter 58. 
     Rate multipliers 66 and 67 may be identical to those disclosed in U.S. Pat. No. 3,878,374, or they may be otherwise conventional, as desired. The same is true of switch matrix 68, computer 69, switch matrix 70 and display unit 71. That is, all of the components 66, 67, 68, 69, 70 and 71 may be identical to the corresponding components disclosed in U.S. Pat. No. 3,878,374. 
     Counter 65 may be entirely conventional with or without updating. For example, see the numerous reversible counters in Handbook of Logic Circuits by John D. Lenk, (Reston Publishing Company, Inc., Reston, Virginia, copyright 1972). 
     OPERATION 
     In the operation of the system of FIG. 6, when the signal on output lead 72 of divider 55 goes high, the switches 48, 57 and 64 assume the positions shown. Counter 65 then counts up. This is true because switch 48, in the position shown, connects the output of digital phase shifter 51 to triangle wave generator 49, and f h  appears at the output of frequency multiplier 52, f h  being larger than f n . 
     When the signal on output lead 72 of divider 55 goes low, the output of inverter 58 causes differentiator 59 to reset flip-flop 60, and AND gate 62 then passes the f n  pulses from lead 75 to the input of counter 61. Due to the said reset of flip-flop 60 by differentiator 59, the &#34;0&#34; output of flip-flop 60 is high, and the connection 81 therefrom to switch 57 closes the switch 57. Thus, at the same time counter 65 is counting down, two registers therein, to be described, are employed to count down and to store at the same time. The stored difference between frequencies f h  and f n  is then multiplied by the pulses then appearing on the output lead 73 of switch 57 which is connected to the input of rate multiplier 66. The number of pulses impressed upon the input of rate multiplier 66 is then directly proportional to the period which is the reciprocal of f n . In the equation set forth hereinbefore, the division by f n  is then accomplished by multiplying by the period of f n , such period being represented by the number of the input pulses to rate multiplier 66 in each successive group thereof. 
     For the operation of components 67, 68, 69, 70 and 71, see the said patent. 
     If the system of FIG. 6 is employed to indicate viscosity, display unit 71 will do so. 
     The system of FIG. 6 is calibrated against a standard empirically. Calibration may be employed by using the method of least squares, or more easily, and perhaps less accurately, by reading display unit 71 for fluids of two different viscosities, measuring f h  and f n  at both of the viscosities, forming two simultaneous equations for the two different viscosities, and solving for the two unknowns K and B. 
     For the use of simultaneous equations and calibrating equipments empirically, see U.S. Pat. No. 3,677,067. 
     Frequency multiplier 52 shown in FIG. 6 may be of the type shown in FIG. 7 or may be otherwise entirely conventional. Frequency multiplier 52 is provided with a voltage controlled oscillator (VCO) 82, a squarer 83, a divide-by-one thousand divider 84, a phase detector 85 and a low pass filter 86 connected in succession in that order. The output of low pass filter 86 is connected to the input of VCO 82 over a lead 87. The input to the frequency multiplier 52 is impressed upon phase detector 85 over an input lead 88. 
     Digital phase shifter 51 is shown in FIG. 8 including an inverter 90, differentiator 80 and a divide-by-two divider 91 connected in succession in that order. 
     The waveforms of FIG. 9 relate to FIG. 8. The waveforms of FIG. 9 are labeled in FIG. 8. The waveform e10 is the output of divide-by-two divider D2 in FIG. 12. The product of the amount divider 91 (FIG. 7) divides by and the amount divider 63 (FIG. 6) divides by must equal 500 because the frequency multiplier 52 in FIG. 6 multiplies by 1,000, and the frequency at the output of divider D2 (FIG. 12) must be the same frequency as the frequency of the waveform appearing at junction 77 regardless of the position of switch 48 shown in FIG. 6. The same is true of the divisor products of dividers D1 and 63 in FIG. 6. Note will be taken that the output e10 has been phase shifted 45 degrees in FIG. 9 because the trailing edge of input waveform e6 has been employed to trigger the leading edge of the waveform of e10. 
     Counter 65 and rate multiplier 66 are again shown in FIG. 10. Counter 65 may or may not be conventional, as desired. Counter 65 employs a conventional logic circuit 92, a conventional counter-register 93 connected therefrom, a conventional gating circuit 94 connected from register 93, a conventional storage register 95 connected from gating circuit 94, storage register 95 being connected to rate multiplier 66. 
     Counter 65 is reset on the leading edge of the output pulse of divider 55 in FIG. 6 by a connection therefrom through a differentiator 96. The output of differentiator 96 is connected to logic circuit 92 and gating circuit 94. As is conventional, gating circuit 94 is employed to transfer the contents of counter-register 93 to storage register 95. The pulses appearing on the input lead to rate multiplier 66 from switch 57 are multiplied by the number stored in register 95, as is conventional. 
     The transfer of the contents of register 93 to register 95 through gating circuit 94 is demonstrated by a stage in each of the registers 93 and 95 corresponding to each other, and NAND gates in gating circuit 94 which accomplish the transfer function. 
     In FIG. 11, the output of differentiator 96 is impressed upon an input lead 97 which is connected to one input of each of two NAND gates 98 and 99 connected respectively to the set and reset inputs of a flip-flop 100. NAND gate 99 also receives an input from the &#34;0&#34; output of a flip-flop 101. NAND gate 98 also receives an input from the &#34;1&#34; output of flip-flop 101. Flip-flop 101 represents a stage in register 93 which corresponds with a stage in register 95 including flip-flop 100. 
     Triangle wave generator 49 is shown in FIG. 12 including an electronic switch 102 operated by the output (pole) of switch 48 shown in FIG. 6. 
     In FIG. 12, a square wave having a regulated positive and negative value, the absolute value of each being regulated to an identical value, if possible, is impressed upon an input junction 103 of a conventional integrator 104 having a feedback capacitor 105, an amplifier 106 and an electronic reset switch 107 operated by the output of a differentiator 116. Triangle wave generator 49 has an input lead 109 connected from the pole of switch 48 shown in FIG. 6. In FIG. 12, input lead 109, as stated previously, is connected to control the position of switch 102, but also forms a junction 110 with a lead 111. 
     Divider D2, inverter 115 and differentiator 116 are employed to zero integrator 104 and thereby to prevent drift. As shown, integrator 104 is zeroed each time the triangular wave output 113 thereof passes through zero volts at 114. Zeroing may be performed less often, if desired. Zeroing may be employed once per cycle of waveform 113, or less often. Inverter 115 and a differentiator 116 are connected from lead 111 to reset switch 107. 
     In accordance with the foregoing, the leading and trailing edges of each pulse received over input lead 109 is detected by differentiator 116 and pulses at those points in time are employed to close switch 107 at the trailing edges of the pulses appearing on input lead 109. 
     In FIG. 13, when switch 48 is in the position not shown in FIG. 6, e1 represents the square wave appearing at the output of switch 48 and on the input lead 109 of triangle wave generator 49 shown in FIGS. 6 and 12, respectively. The same waveform when switch 48 is in the positions shown in FIG. 6 appears at e2. Although, as shown, the waveforms (e1 and e2) may have no time correspondence, they are shown in time correspondence to illustrate that the waveform which always appears at junction 77 in FIG. 6 is 45 degrees out of phase with the output of divider D2 in FIG. 12 when switch 48 is in the position shown, whereas the square wave appearing at junction 77 (FIG. 6) is in phase with the output of divider D2 (FIG. 12) when switch 48 is in the position not shown in FIG. 6. Note will be taken that the waveform e2 lags waveform e1 by 45 degrees in FIG. 13. 
     As stated previously, the output of switch 48 in its two different positions have no time correspondence because the switch 48 in FIG. 6 is in only one position at a time, obviously. The phases of the two different output signals of switch 48 on the pole thereof as shown in FIG. 6 therefore are unrelated. Although the frequency of the square wave appearing at junction 77 in FIG. 6 is always nearly the same, it is in fact the two different frequencies thereat by which viscosity is calculated. The properties of a fluid other than viscosity may also be calculated in the same way as disclosed in FIG. 6. 
     The time dimension C&#39; shown in waveform e1 may be substantially equal to the time width of each pulse in waveform e2 in FIG. 13, although, as stated previously, there is a finite difference which is important, and which is employed to compute viscosity, et cetera. The pulse width C&#39; may be, for example, 5 milliseconds or so. 
     When switch 48 in FIG. 6 is in the position shown, e1 may represent the waveform appearing at junction 77, and e2 may represent the waveform at the output of divider D2 in FIG. 12. The dimension F&#39; in comparing waveforms e1 and e2 in FIG. 13 may be equal to C&#39; divided by 4. 
     The current in coil 35&#39; may be similar regardless of the position of switch 48. It may have a shape as indicated at i1 shown in FIG. 13, for example, when switch 48 is in a position not shown. 
     Over the period D&#39; shown in FIG. 14, e3 is the train of pulses at the output of frequency multiplier 52 (f h ). During the period E&#39;, e 3  is the train of pulses at the output of frequency multiplier 52 (f n ). 
     The pulses shown in e4 in FIG. 14 represent the output pulses of divider 56 shown in FIG. 6. After counter 65 has counted up during a period corresponding to period D&#39; but immediately prior thereto, and counter 65 has counted down during a period corresponding to E&#39; during a period immediately prior thereto, the difference contained in register 93 (FIG. 10) is then transferred to register 95 at the beginning of period D&#39; shown in FIG. 14. Counter 65 then counts up again at frequency f h , and the multiplication provided by rate multiplier 66 shown in FIG. 6 is not performed until the beginning of period E&#39; shown in FIG. 14. At the same time, counter-register 93 will be counting down during period E&#39; . 
     The pulses at e5 in FIG. 14 represent the output pulses of switch 57 shown in FIG. 6 which are passed to the input of rate multiplier 66 in FIG. 6. The input pulses to rate multiplier 66 shown in FIG. 6 come in groups because switch 57 is not closed except over a period which may be slightly less than the period E&#39; shown in FIG. 14 or may be substantially less than the period E&#39;. The period during which one pulse group of the pulses e5 in FIG. 14 are produced depends upon the size of counter 61 shown in FIG. 6. 
     The word &#34;switch&#34; is hereby defined for use herein and for use in the claims to mean any kind of switch, but preferably an electronic or electrical switch. Further, the word &#34;switch&#34; is hereby defined herein for use herein and for use in the claims to mean preferably an electronic switch. 
     The flow through container 21&#39; shown in FIG. 5, and as described hereinbefore, is preferably moderate, slow, or very slow. 
     Because the energization of coil 35 shown in FIG. 3 causes the oscillatory rotation of drag body 24, it will be appreciated that container 21 and perhaps all of the structures therein are preferably non-magnetic except for permanent magnet 26 shown in FIGS. 1 and 3. 
     The equation set forth hereinbefore may be employed to measure density if viscosity is maintained substantially or exactly constant. Similarly, in order to display viscosity at display unit 71 shown in FIG. 6, it can be important to maintain the fluid in container 21, for example, shown in FIG. 1 at a constant temperature so as to maintain the density of the fluid therein constant. 
     Torsion spring assembly 22 shown in FIG. 1 may be provided with any conventional torsion spring, torque tube or torque rod, if desired. 
     Also, as stated previously, one or more magnitudes of one or more respective fluid properties may appear at the output of computer 69 in FIG. 6 and be used in combination with any one or more components for the computation of other variables in a larger system. In addition, the system of FIG. 6 may be employed to indicate the magnitude of a fluid property at display unit 71, or display unit 71 may be replaced by a process controller or otherwise. This is true regardless of what magnitude of what fluid property is represented by the output of computer 69 in FIG. 6. 
     The word &#34;fluid&#34; as used herein and for use in the claims is hereby defined to be a liquid unless the invention is useful in the determination of a fluid property of one or more or all gases. 
     It is important to note that digital phase shifter 51 shown in FIG. 6 is a phase shifter which shifts the phase of the square wave appearing at junction 77 shown in FIG. 6 independent of the frequency of the square wave appearing at the said junction 77. 
     The maximum rotary movement of drag body 24 in FIGS. 1 and 3 may or may not be 1.0 degree.