Patent GB-679011-A (GB)
Title: Improvements in or relating to devices sensitive to the ratio between fluid pressures
Inventor: JOHNSON CHRISTOPHER LINLEY
IPC: G05D11/00
Filed: 1948-03-17
Abstract: 679,011. Pressure gauges. ROLLS ROYCE, Ltd. March 8, 1949 [March 17, 1948], No. 8037/48. Class 106 (ii). [Also in Group XXIX] A device for measuring the difference between two pressures and comparing with a standard difference to give an indication of pressure above or below the standard difference comprises a tapping 22 led from inlet 20 of a choked Venturi 21 to a common chamber between a diaphragm 38 dividing a cylinder 23 so that it forms part of two chambers and a second diaphragm 35 dividing a cylindrical extension 36 of the cylinder 23. The chamber on the other side of the diaphragm 38 is connected by the tapping 27 to the throat 28 of the choked Venturi-tube 21. The effective area of the diaphragm 35 is (1-k) times the effective area of the diaphragm 38, where k is the sonic ratio for the gas flowing, with the result that the load applied by the absolute pressure, of the inlet of the Venturi 20 to the diaphragm 35 balances the load applied to the diaphragm 38 by the rod 34. To the left, the cylindrical extension 36 opens into a third cylinder 30 containing a third diaphragm 31. Pressure tappings 32, 33, are provided from two fluid pressure sources, that from the lower pressure source being tapping 32 which leads to the chamber on the left of the diaphragm 31 and that from the higher pressure source being tapping 33 which leads to the chamber having a part of its wall both the diaphragm 31 and the diaphragm 35. The three diaphragms are interconnected by the rod 34 and the diaphragm structure is therefore urged to move to the left by a load equal to Q, the higher pressure, multiplied by the difference in the effective areas of the diaphragms 31 and 35, the diaphragm structure is urged to move to the right by a load equal to R, the lower fluid pressure, multiplied by the effective area of the diaphragm 31, since there is no effective load due to the pressures p1 and p11 derived from the choked Venturi-tube 21, where p1 is the absolute pressure at the inlet of the Venturi-tube and p11 is the absolute pressure at the throat of the Venturi-tube. The diaphragm 31 is so dimensioned that it is larger than the diaphragm 35 and its effective area is m times the difference between its effective area and the effective area of the diaphragm 35. When the selected ratio exists between the two fluid pressures, there is no effective load urging the diaphragm structure to move, but when the ratio of the lower fluid pressure 6 to the higher fluid pressure is below the preselected value, the diaphragm structure will tend to move to the left, and vice versa. If the selected ratio is very large the diaphragm 31 has to be very little larger than the diaphragm 35 and very small changes in the physical dimensions will interfere with accuracy. In a modification to overcome this (Fig. 2) the lower of the two fluid pressures R acts on the diaphragm 41 while the inlet pressure p1 of Venturi-tube 40 acts in the chamber between the diaphragm 41 and the Venturi-diaphragm 42. The throat pressure p11 derived from the choked Venturi-tube 40 acts in the space between the Venturi-diaphragm 42 and the diaphragm 43 and the higher of the two fluid pressures Q acts on the other side of the diaphragm 43. The dimensions of the three diaphragms 41, 42, 43, are inter-related so that the effective area of the diaphragm 41 on which R acts is m-times the affective area of the diaphragm 43 on which Q acts. Thus the diaphragm structure is urged to move to the left when the ratio of Q to R is greater than the preselected value m, and to the right when the ratio is less than the preselected value.