Patent GB-993252-A (GB)
Title: Transducers responsive to fluid pressure
Inventor: ECCLES EDWARD STUART
IPC: G01L13/00
Filed: 1962-01-15
Abstract: 993,252. Measuring electrically. BRISTOL SIDDELEY ENGINES Ltd. April 11, 1963 [Jan. 15, 1962], No. 1399/62. Heading G1N. [Also in Division H2] In a fluid pressure transducer deviations from a datum position of a boundary between the fluid and a liquid of higher density which is being rotated are sensed, the speed of rotation is varied so as to oppose any deviation, and an output signal representative of the speed of rotation is derived. A body containing the liquid 26 consists of inner and outer perforated sleeves 18, 20 between which is a diaphragm 6 which defines the boundary between the liquid and the fluid whose pressure is being measured. A second boundary 4 exists between the liquid and an evacuated space. The body forms the rotor of a permanent magnet or hysteresis type electric motor. Sleeve 20 forms one electrode of a capacitor, the other electrode being liquid 26, if it is conductive, or diaphragm 6. The capacitor is connected into a transformer ratio arm type circuit, constituting a Blumlein bridge, by means of which the motor speed is controlled. The speed is measured by a tachometer, a pulse pickup on the transducer, an optical pickup, or, if sleeve 18 is a permanent magnet, by an auxiliary stator winding. Temperature effects are nullified by axially extending space 2 at a point between boundaries 4 and 6, and providing an adjustable plug 34. In a modification (Fig. 3) the body consists of an insulating block 35 containing a passage 8 in a plane containing the axis of rotation. Contacts 46, 48 are provided, contact 46 co-operating with a platinum ball 54 floating on mercury 56. Instead of a second probe 48, ball 54 may bridge a pair of contacts at 46. In either case the contacts are connected in series with the driving motor, which will hunt about the'required speed. If a non-conducting liquid is used two pairs of contacts are provided at 46, contact 54 bridging one pair on inward movement and the other on outward movement, for respective acceleration and deceleration of the motor. One limb of passage 8 may be offset circumferentially, and the resulting negative feedback effect may increase stability. Alternatively the passage may have three limbs meeting at a common junction, the limbs lying in a plane perpendicular to the axis of rotation. The limb running rearwards relative to the direction of rotation is open to the input pressure. Surging of the mercury in either case is reduced by filling part of the evacuated inner space with a liquid of low density. Temperature effects are reduced by proportioning passage 8, and second order effects are counteracted by making one end or the other of passage 8 divergent. The pressure may be transmitted through a diaphragm to a further fluid acting on the transducer. A Machmeter for measuring Mach 2. 5 to 4 consists of two such transducers, one exposed to ram air pressure P, and the other to static air pressure P 0 . The resulting output frequencies, representing the motor speeds and proportional to #P, are divided to give a signal representing the Mach number. Frequency f, may trigger "on" a bi-stable circuit, which is triggered "off" by f 0 , the average output voltage representing the quotient. The transducer may be adapted to measure the difference between two pressures, the second pressure being applied to the previously evacuated space at the inner boundary of the liquid.