Patent GB-672693-A (GB)
Title: Improvements in or relating to measuring instruments for physical quantities
IPC: G01L9/00
Filed: 1949-02-03
Abstract: 672,693. Electric indicating systems: testing materials acoustically. DION, G. H. Feb. 3, 1949. [Feb. 13, 1948] No. 3056/49. Addition to 663,946. Classes 40 (i) and 118 (ii). In apparatus, of the kind described in the parent Specification, for measuring the deviation, in response to a physical quantity to be measured, of a solid boundary from its mean position in a constant pressure homogeneous gas, wherein an electroacoustic transducer having a vibrator element parallel to the boundary is connected in a branch of an A.C. energized impedance measuring circuit having an unbalance indicator ; the vibrator element radiates acoustically within a critically dimensioned resonance chamber having a restricted opening which is spaced from the boundary by a distance such that initially the motional impedance of the transducer balances the impedance measuring circuit and subsequent deviation of the boundary from its mean position varies the motional impedance to produce a corresponding unbalance indication, and the transducer coil impedance is initially tuned to the frequency of the supply so that its imaginary component disappears. Fig. 2a shows a device for the accurate measurement of small distances wherein a transducer diaphragm 1 is vibrated by an electromagnet within a pressure chamber 2 (whose critical volume is determined by a given complex formula) having a small opening 3 in a flange 4 placed at a distance d from a rigid plate. The diaphragm movement is subjected to a motional radiation resistance dependent on the distance d, and the electromagnet is energized by an alternating current at the resonant frequency of the diaphragm, so that if the imaginary component of the electrical impedance of the device is reduced to zero by an impedance Z1 of opposite sign at the vibration frequency the input impedance is a pure resistance which is a measure of the distance between the flange 4 and the fixed plate. Two similar devices may be connected in a Wheatstone bridge which is energized at the vibration frequency and normally balanced when the distances d are equal, and the unbalance current in an A.C. measuring instrument indicates variations in one of the distances. Pressure or temperature may be measured by moving the rigid plate relatively to the transducer by a manometer capsule (Fig. 4, not shown), or by bimetallic strips, and the bridge unbalance current may operate a relay controlling temperature or pressure. The unbalance current may be amplified and retransmitted to operate remote repeaters. In an altimeter, two transducers 3, 4 (Fig. 5) are respectively coupled into the anode and cathode circuits of a valve T whose grid is supplied with an alternating voltage E at'the diaphragm resonant frequency, and the rigid plates are movable by manometer capsules 1, 2 one of which is responsive to atmospheric pressure and the other is maintained at ground level pressure. The ratio of the alternating output voltage V of the valve to the input voltage is shown to be equal to the ratio of the manometer pressures. In an airspeed indicator, one manometer capsule responds to the pressure in a Pitot tube and the other is maintained at atmospheric pressure. The Specification, as open to inspection under Sect. 91, describes applications wherein the smoothness of two surfaces are compared by their respective effects on the viscosity of the fluid between each surface and a transducer, and consequently on the transducer motional impedances ; and wherein a transducer is coupled to induce standing waves in a metal body or in a fluid confined in a tube, so that the measured transducer motional impedance gives an indication of flaws in the metal, or of the gas pressure. The transducer may be in the form of a piston which is electromagnetically vibrated similarly to a moving coil loudspeaker. This subject-matter does not appear in the Specification as accepted.