Patent GB-1088940-A (GB)
Title: Improvements in or relating to pressure responsive vibrating tubes
IPC: G01L9/00
Filed: 1964-11-25
Abstract: 1,088,940. Electrostatic motors. ROSEMOUNT ENGINEERING CO. Nov. 25, 1964 [Nov. 26, 1963], No. 47994/64. Heading H2A. [Also in Division G1] In a transducer, a tube of non-circular crosssection is supported at one or more positions which constitute nodal points when the tube is vibrated at its natural frequency, the tube being adapted to be subjected to a differential pressure. The transducer may be used for the measurement of pressure or temperature, or as a frequency standard. As shown, Fig. 1, a flattened tube 10 with closed ends is vibrated in a transverse mode at its natural frequency by an electrical drive circuit comprising two plates 41, 44, equidistantly disposed between the flat surfaces of the tube near one end thereof, and a feedback amplifier 55. A test pressure is applied to the interior of the tube via a capillary tube at 29; this pressure determines the natural frequency of vibration, and a signal from terminals 36, 58 is fed to a frequencymeasuring device (not shown) to give an indication of the pressure. Tube 10 is supported by the capillary tube and by tensioned wires at nodal points on lines 12, 13 for vibration at the fundamental frequency; if a harmonic of the natural frequency is utilized, corresponding nodal support points are used. Conductive weights may be fixed to each end of tube 10 to add mass thereto. If the tube is mounted vertical, support at a single nodal point will suffice; in Fig. 5 (not shown) an elliptical tube (62) is held vertical by the capillary tube (64) used to admit the test pressure, the driving plates (68, 69) being located near the end of the tube adjacent the support. In a modification (Figs. 6, 7, not shown), a flattened tube (70) is supported vertical at a node by a capillary tube (71) and a tensioned wire (74), the tube being steadied at a lower node by pointed screws (84, 85) in supports (79, 80), which latter also provide mountings for driving plates (41, 44). If the driving-plates are displaced to one side of the longitudinal axis of the tube, the latter will vibrate in a torsional mode, either horizontally (Figs. 13, 14, not shown) or vertically (Figs. 15, 16, not shown); the support for the tube may be located at a point on the transverse axis or at points where the longitudinal axis bisects the ends of the tube (Fig. 28, not shown). By mounting " dumb-bell " weights to each end of the tube (Figs. 18-20, not shown), the weights being of a different metal from that of the tube, some temperature compensation is achieved. The vibrating tube may be filled with oil or other liquid (Fig. 21, not shown), the test pressure being applied to a diaphragm to compress the oil. An electromagnetic drive may replace the electro-static arrangement. In Figs. 9, 10 and Fig. 11 (not shown), respectively, the transversely vibrated tube comprises a tuning- fork and an inverted U-tube of magnetic material having in each case a driving and pick-up coil between the ends of its tines; in Fig. 22 (not shown) a U-shaped coil-bearing core has polepieces shaped to conform to one side of an elliptical tube of magnetic material to vibrate it torsionally. In a piezo-electric embodiment (Figs. 24-27, not shown) the vibrating tube is made of quartz with four longitudinal metallized strip electrodes, and is connected in a Pierce oscillator to determine the frequency of oscillation thereof. Instead of the test-pressure being admitted to the interior of the tube (in any embodiment), the tube may be sealed and supported within a housing to which the test pressure is admitted (Figs. 8A, 8B, 17, not shown); in Figs. 29-31 (not shown) an openended tube is supported at its nodes (for transverse vibration) by diaphragms, the portion of the tube between the diaphragms being enclosed in a gas-tight container for subjection to exterior pressure therein. Mono-crystalline semi-conductor material, e.g. Ge, Si, InSb, GaSb, SiC, may be used for the vibrating tube in some embodiments.