Patent GB-681921-A (GB)
Title: Improvements in or relating to diaphragm type pressure responsive devices
IPC: G01L7/08
Filed: 1950-03-07
Abstract: 681,921. Pressure gauges. K.D.G. INSTRUMENTS, Ltd., and MACK, F. March 7, 1951 [March 7, 1950], No. 5795/50. Class 106 (ii). A diaphragm type pressure responsive device having an operating diaphragm connected to an indicator or to a control mechanism has, in addition, a transmitting diaphragm which is gripped around its periphery and is otherwise free to be deflected to a limited extent over the whole of its area by the pressure to be gauged, the movement of the transmitting diaphragm being conveyed to the operating diaphragm by fluid contained between the two diaphragms, and the movement of the transmitting diaphragm being stopped after a predetermined deflection to restrict the pressure on the operating diaphragm. As shown, the construction is arranged to measure the difference of two pressures. The device comprises cover parts 10 and 11 having recesses 12 and 13 respectively on their inner faces. A sealing ring 14 is at the bottom of each recess. The cover parts have inlet passages 15 and 16 respectively for connection with two fluid mediums whose difference of pressure is to be indicated. In the two recesses are dividing walls 21 and 22, their outer faces having shallow spherical recesses 23, 24 respectively. The two dividing walls abut on opposite sides of the periphery of an operating diaphragm 26. The operating diaphragm may be pressed from beryllium copper sheet so as to provide a number of concentric corrugations. The periphery of a transmitting diaphragm 27 is gripped between the dividing wall 21 and its sealing washer 14, whilst a transmitting diaphragm 28 is similarly gripped between the dividing wall 22 and its sealing washer 14. The transmitting diaphragms are of thin, non- resilient or flabby material such as silver foil of a thickness of about three thousandths of an inch. The spaces 29, 30 on the inner sides of the diaphragms 27 and 28 communicate through small passages 31, 32 with the space 33 between the inner faces of the dividing walls 21, 22, this space 33 being divided into two chambers by the operating diaphragm 26. In the space 33 is a movement 35 by which the deflection of the diaphragm 26 is transmitted to a spindle 36 by a magnetic transmission 38, which spindle either operates an indicator or a control mechanism. The space 33 between the two partitions 21 and 22 and the spaces 29 and 30 between the diaphragms 27 and 28 and the partitions are filled with an incompressible liquid such as water. The characteristic of the diaphragm 26 is selected so that it will deflect a predetermined maximum amount for a predetermined maximum difference in pressure in the fluids which communicate with the outer sides of the diaphragms 27 and 28. Thus, should one of the pressures on the transmitting diaphragms 27, 28 fail and the other pressure exceed that for which the diaphragm 26 is rated, the particular transmitting diaphragm submitted to pressure will contact with one of the spherical faces 23, 24 and thus, although the pressure on the outer side of a transmitting diaphragm may be greater than the operating diaphragm would stand, the actual pressure on the operating diaphragm will be maintained within safe limits. The space 33 is arranged to be small so as to reduce as much as possible the effect of expansion and contraction of the operating fluid due to changes in temperature. The volumes of the spherical recesses 29 and 30 are made rather greater than that corresponding to the volume swept out by the operating diaphragm for the maximum pressure change required to be indicated in order to compensate for temperature changes in the liquid medium in the indicator.