Patent GB-830463-A (GB)
IPC: G01N11/16
Filed: 1954-11-13
Abstract: 830,463. Magnetostrictive viscometers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Feb. 13, 1956 [Nov. 13, 1954], No.32954/54. Class 40 (1). [Also in Groups XXXV and XXXIX] A viscometer comprises a magnetostrictive or piezo-electric transducer for producing torsional oscillations which are imparted to the test material by a stem mounted on a diaphragm which keeps the test material from the transducer. For very high viscosities the stem may be in two parts separated by the test material. The transducer may be intermittently excited at the natural frequency or a harmonic so that oscillations die away between pulses of excitation. Viscosity may be measured with the aid of a bridge comprising adjustable elements which simulate the characteristics of the vibratory system and of the test material and in effect measures the change of impedance of the vibratory system due to contact with the test material. Alternatively a circuit may be used which supplies a current varying periodically each side of resonance to an extent which is determined automatically by the effective Q of the vibrator system. The extent of the sweep, which can be indicated then shows the viscosity of the test material. Magnetostrictive vibrators. As shown in Fig. 1, a ferrite tube 12 is supported at a velocity node by a mica disc 13 and is magnetized axially by D.C. excited coils 23 and has a toroidal winding 12 supplied with bursts of oscillations at the natural frequency for torsional vibration of the tube or a harmonic thereof. Secured to the tube is a stainless steel stem 14 having the same natural frequency as the tube and supported at a velocity node by a diaphragm 17. The mechanical impedances at the junction are matched. Tube 14 is closed at the end by a hemispherical formation or may be enlarged into a sphere or other solid of revolution. Diaphragm 17 is designed to give a 90-degree phase shift to torsional vibrations travelling outward to the pressure node provided by the securing ring 18. The magnetic circuit is completed by steel discs 22 and the connections to the toroidal winding are taken out on the mica diaphragm B. This construction may be simplified by making the lengths of the two tubes (2n - 1)#/4 and providing a single mounting at the junction. In the modification, Fig. 6, the D.C. flux is provided by the temporary passage of a heavy current through a copper rod 33 held axially in the tube by steel pins 35, and capable of slight downward movement into contact with a copper disc 32. The oscillating assembly is supported on a diaphragm (not shown) at a velocity node of the steel tube. The A.C. field is provided by a coil 37. Piezo electric vibrators. As shown in Fig. 7, a piezo electric element built in two or more sections and preferably of barium titanate; barium lead titanate, zirconate or niobate, producing torsional vibration when suitably excited is cemented to a steel tube 43 supported at velocity nodes by diaphragms formed as low-pass filters cutting off well below the operating frequency. In a modification, Fig. 9 (not shown), a shorter steel tube is used and is supported by a single diaphragm at the central. velocity node of the oscillating system. Modified vibrator construction. The construction described in the preceding paragraphs may be modified by the provision of a hollow diaphragm formed as a low-pass filter and supplied with cooling fluid as shown in Fig. 10. The closed steel tube may be replaced by an open-ended tube flared at the driving end and having a short cylindrical portion for engaging the test material, Fig. 11 (not shown). In another arrangement, Fig. 12, a #/2 driver is supported by a central diaphragm (not shown) and has a tapering stem 62 also #/2 in length followed by a #/2 cylindrical port (not shown) closed hemispherically. This construction is adapted for very, viscous materials by separating the tapering and cylindrical parts of the stem at 64 and inserting the test material in the gap. The part 63 is then 3 #/4 in length and is supported at #/2 from the gap 64. In general the features of the transducers described can be interchanged. Circuit arrangement. Fig. 4, for direct reading or null method. The bridge formed by the windings of a differential transformer T, the transducer V and a network representing separately the impedances of the transducer and of the liquid is excited by bursts of oscillations of the natural frequency of the transducer or a harmonic. The elements Re, Le, Cm, Rmi are first adjusted to balance the bridge with Zm short-circuited and the transducer clear of the test material. The transducer is then inserted in the test material and the unbalance may be measured on a meter calibrated in viscosity with Zm still short-circuited or the shortcircuit may be removed from Zm and the viscosity calculated from the values of Zm that restore balance. Circuit arrangement, Fig. 13, with frequency sweep of energizing oscillator. Oscillator O is tuned by a current-variable impedance, e.g. a capacitor driven by a moving-coil arrangement or a saturable choke, and its output as well as feeding a bridge arrangement comprising the transducer UV and balancing elements C B , R B  provides after rectification and smoothing at R 3 , Css an adjustable datum potential at P 1  which acts in opposition to the rectified bridge output at point P. The resultant potential is switched at regular intervals between the input circuits of valves V 1 , V 2  the outputs of which are switched synchronously to a valve V 3  controlling the value of the adjustable impedance. The bridge is balanced at resonance and the rest of the circuit is so dimensioned and adjusted that the frequency sweep resulting from the switching operation is such as to bring the maximum bridge output to a value equal to 1/#2 : the current in the transducer at resonance. At this point the potential developed at P tends to stop the sweep. The amplitude of the sweep thus depends on the effective Q of the transducer and so furnishes a measure of the viscosity of the associated material.