Patent GB-664684-A (GB)
Title: Improvements in or relating to a pressure indicating or measuring device
IPC: G01L9/00
Filed: 1948-11-23
Abstract: 664,684. Electric indicating systems. STEVENS, A. H. (Sheffield Corporation). Nov. 23, 1948. No. 30393/48. Class 40 (i). A pressure indicating or measuring device comprises an annular cup-shaped housing of magnetic material with a central wound polepiece, the magnetic circuit being closed by a diaphragm of magnetic material, having a low acceleration response which is peripherally secured to the housing so that flexure of the middle of the diaphragm under pressure varies the reluctance of the magnetic circuit and the impedance of the core windings. In Fig. 2, and Figs. 1, 3 (not shown), a resilient magnetic diaphragm is gripped at the edges between hollow pressure tight magnetic housings 20, 21 having central polepieces 24, 25, symmetrically spaced from the diaphragm. Spools 12, 15 on the polepieces are respectively wound with interleaved transformer primaries 10, 13 and secondaries 11, 14. The windings are brought out through pressure seals and differential pressures are applicable to the two compartments separated by the diaphragm through fittings 31, 32. The primary windings are energised in series or parallel with AC of e.g. 1,000 c/s from an oscillator 50, Fig. 5, and the secondaries are connected through coupling transformers to bridge rectifiers 67, 68 whose outputs are connected in balanced opposition to a direct current meter 69. Zero balance is adjustable by the tapped resistor 63, and the differential pressures distort the diaphragm to inversely vary the reluctances of the magnetic circuits through the housings 20, 21 so that the meter 69 indicates the magnitude and sense of the pressure difference. The secondaries may be single or multiple, wound to give a stepup ratio, connected reinforcingly to an A.C. meter, or connected in series opposition through a coupling transformer to a .single bridge rectifier and D.C. meter Fig. 4, (not shown). One compartment may be connected to a reference pressure (e.g. atmospheric) or sealed off. Fluid flow may be indicated in response to the differential pressures across a constriction, and the measuring instrument may be a recording oscillograph. In Fig. 7 and Fig. 6 (not shown), a flexible diaphragm 26a is flush mounted in a pressurised surface 71 (e.g. an aerofoil) and the surfaces may be ground smooth for measurements at supersonic speeds. Magnetic housings 20a, 21a have hollow polepieces 24a, 25a each wound as above with primary and secondary windings, separated by an apertured resilient magnetic diaphragm 75 having a central non- magnetic ring 78a which is threaded to a sleeve member 80, connected by a differing thread to a central stud on the inside of the diaphragm 26a, so that rotation of the sleeve adjusts the static flexure of the diaphragms. The windings are brought out through pressure seals for connection to indicating devices as above, and a reference pressure is applicable to the interior of the diaphragm 26a fitting through 81, which may alternatively be sealed off. The device is applicable to gauging the size of objects displacing the member 80. In a single-ended arrangement Figs. 8, 9, 10 (not shown), the reluctance of a single magnetic circuit is varied by an external pressure diaphragm to control the output of a single transformer, which is rectified and balanced in opposition against a rectified voltage from the A.C. source. The device is applicable to pressure measurements in the cylinder of an I.C. engine, and instead of reference pressure fluid, a coolant may be supplied to the back of the diaphragm, and escapes through ports. The sensitivity may be increased by connecting condensers and resistors in the primary and secondary circuits to make them resonant with the supply frequency. In modifications, Figs. 12, 13, 14, 15 (not shown), differing in detail, the primary and secondary windings of each transformer are identical and may be connected as two or four arms of an A.C. fed Wheatstone bridge whose unbalance voltage is a measure of the differential pressure. In a further modification for measuring the pressures in an I.C. engine cylinder, Figs. 17, 18 (not shown), a plug member screwing into the cylinder carries the pressure diaphragm, which is adjustably coupled to an apertured magnetic diaphragm varying the reluctances of the magnetic circuits of the transformers, which may be bridge connected. The interior reference pressure is applicable by a circulating coolant. The pressure diaphragms may be of steel or phosphor bronze, while to ensure temperature stability the material of the magnetic housing members and diaphragms may be of low expansion coefficient, and that of the windings may be of low resistance coefficient. The pressure seals may be of glass and a metal of equal expansion coefficient.