Patent GB-920100-A (GB)
Title: Apparatus for measuring specific gravity of fluids
Inventor: WIGHTMAN ERIC JEFFERY
IPC: G01N9/06
Filed: 1960-09-23
Abstract: 920,100. Measuring specific gravity electrically. SPERRY GYROSCOPE CO. Ltd. Dec. 20, 1961 [Sept. 23, 1960], No. 32762/60. Class 40 (1). Intermittent or continuous density measurements of a moving fluid are made by apparatus comprising a vessel through which the fluid flows, and which is capable of vertical movement in accordance with fluid density, a pick-up producing a signal proportional to the displacement of the vessel, and an electromagnetic transducer which exerts a force tending to restore the vessel to its previous position, said signal being also proportional to fluid density. As shown in Fig. 1, fluid is passed through a U-tube pivoted at the end remote from the U and which is counterbalanced so as to be capable of vertical movement. Changes in weight of the tube due to changes in fluid density cause the U-tube to move, these movements being detected by an inductive pick-up 3, the output from which is an A.C. signal whose phase is dependent on the sense of the tube movement, and magnitude proportional to the amount of the deviation. The pick-up output is amplified at 4 and fed to a discriminator 5 whose D.C. output is proportional to the magnitude of its input signal and polarity dependent on the sense of the tube displacement. The discriminator output is smoothed at 6 and passed successively to an amplifier 7 and a power amplifier 8, the output of which is fed to an electromagnetic transducer 2 which produces a force on the U-tube tending to restore it to its original position. The amplifier 8 output signal is thus a function of fluid density and is indicated by a meter 9 and a recorder 10. Structure, Figs. 2 and 3.-A stainless steel U-tube 11 is secured to a cross-member 13 by flanges 12 the member 13 being pivoted to an upright 14 by crossed ligaments 19, 20 mutually normal to one another and at 45 degrees to the vertical. Flexible bellows 16, 18 lined with synthetic rubber couple the open ends of tube 11 to member 14 and to extensions 17 to which conduits carrying the fluid to be evaluated are connected. The U-tube is counter-balanced by a weight 22 able to be moved along a threaded portion 23 of an arm 21 pivoted to a bracket 25 by crossed ligaments. A cross-member 30 carries an adjusting-screw 31 bearing against a push-rod 32 in contact with the movable member of an electromagnetic transducer 2 mounted on base member 15. An inductive displacement pick-up 3 is supported by a bracket 33 and rests on cross-member 30. Adjustable screws 36 mounted on a bracket 35 limit tube movement. The displacement pick-up (Figs. 4 and 5) comprises a metal case 37 containing two similar coils 38 wound on core members 39, the polepieces 40 of which are mutually separated by a gap 41. A movable member comprises a cylindrical slug 40 of magnetic material having two rods 43, 44 screwed into its ends, rod 44 carrying a probe tip 45. Helical springs 46, 47 align the movable member with the case axis. The pick-up's electrical circuit according to Fig. 5 shows the coils 38 bridge connected with external resistors 50, 51. An A.C. voltage is applied to terminals 52, 54 and an output voltage is obtained from terminals 55, 56 when the slug 42 is displaced from its central position, the size and phase of this output being proportional to the magnitude and sense of the slug displacement. The electromagnetic transducer (Fig. 6) comprises a permanent magnet 57 and moving coil member 58 carrying a coil 62 and supported and aligned by means of diaphragms 64, 65. A push-rod 32 located in the top of member 58 contacts the U-tube mounted adjusting screw 31. Tube circuit shown in Fig. 7 comprises items designated 6, 7 and 8 in Fig. 1. The output from the discriminator 5 is applied to terminals 70, 71 and smoothed by capacitors 72 and resistor 75. Amplifier 7 comprises transistors 76, 77 bridge connected with resistors 78, 79 between D.C. supply lines 80, 81, the bridge being balanced by variable resistor 83. An amplified version of the discriminator output is then applied to the base electrodes of bridgeconnected transistors 86, 87 forming the power amplifier 8. Whereas the output of amplifier 7 is able to vary both in its polarity and magnitude, that from amplifier 8 is required to have constant polarity and hence the bridge arms are made asymmetrical. One arm is constituted by transistor 87 and another comprises transistor 86 with a series resistor 88. Resistors 89 and 90 complete the bridge, resistor 90 having a higher value than resistor 89. One bridge diagonal is connected between power lines 80, 81 and the other terminates in conductors 91, 92 which supply the electromagnetic transducer 2. Amplifier 8 output signal as explained above is a function of liquid density and circuits which provide outputs which are linear functions of that amplifier are respectively applied to a meter 93 and a pen recorder 94. It is desirable that the transducer 2 should remain slightly energized at the lower end of the apparatus range and hence provision is made to zero the meter 93 on this residual drive for the transducer 2 by means of slider 100 of a variable resistor 97, similar zero adjustment for the recorder 94 being effected by slider 115 of variable resistor 98. The sensitivity of the meter 93 can be altered by a ratio of 20: 1 by a switch 102 which allows differing fractions of the amplifier output voltage to be developed across the meter drive circuit. The sensitivity of the pen recorder 94 is alterable by a ratio of 20 : 1 by ganged switches 118, 119. The correct relationship between the amplifier 8 output and the recorder scale is set by slider 117 of variable resistor 114. The overall sensitivity of the whole measuring apparatus may be set to the 100 parts in the 1000 degrees scale of specific gravity when the switches 102, 118, 119 are set for low sensitivity, and 5 degrees in the same range when set for high sensitivity. The starting-point of the range selected within a range of 0-1100 degrees is set by adjustment of the position of the balancing weight described above.