Patent GB-594594-A (GB)
Title: Improvements in or relating to instruments for indicating the ratio or product of two variables
IPC: G01P5/175
Filed: 1945-03-15
Abstract: 594,594. Pressure gauges; indicating- apparatus. K.D.G. INSTRUMENTS, Ltd., and EVERINGTON. G. D. March 15, 1945, No. 6567. [Classes 106 (ii) and 106 (iv)] An instrument for giving indications dependent on a product or ratio of two variables comprises two resilient diaphragms responsive to fluid pressures varying in accordance with said variables and each having associated therewith means whereby the deflection is proportional to the logarithm of the force due to fluid pressure, a transmission for adding or subtracting the logarithmic deflections of said diaphragms and arranged to move an operating member in sliding engagement with an .edge or track on a pivoted indicating member, which operating member is arranged to move in a plane at an angle to said edge or track and parallel with the pivot axis of the indicating member. An instrument for use in aircraft for indicating the Mach number is shown, the Mach member being a function of the ratio P-S - where P is the pressure developed due to S air velocity and S is the static pressure. Capsules 10 and 11 are evacuated and connected to the pitot head of the aircraft respectively and are surrounded by air at the static pressure, the instrument casing being in communication with the static head. Thus, the inward deflection of the capsule 10 is dependent on S and the outward deflection of the capsule 11 is dependent on P-S. The free diaphragm of the capsule 11 has attached to it a stirrup, having pivotally mounted therein a lever 20 and carrying a screw, the pointed end of which is opposite to a pallet fixed to a spring blade 26 anchored at its other end between two blocks clamped against a frame member. To the frame member is secured a block 30 in which is located a row of adjusting screws which, when suitably adjusted, vary the effective length of the spring 26 in such a manner that the deflection of the diaphragm bears a logarithmic relation to the varying pressure on the diaphragm. The other capsule 10 carries a screw-threaded spindle on which is adjustably mounted a small disc 33, the rim of which engages one end of the lever 20. The end of the spindle is provided with an adjusting nut 37 which engages a second spring blade 36, clamped at its other end and provided with a similar block 41 of adjusting screws in order to obtain a logarithmic response. The lever 20 is arranged to subtract the deflection of the capsule 10 from that of the capsule 11 so that the movement of its upper end is proportional to P-S log -. The upper end of the lever engages S a radial edge of a toothed arcuate member (Fig. 5) which engages with a pinion on the pointer spindle. By suitably dimensioning the toothed arcuate member and the relative position of the upper end of the lever, the pointer may be arranged to indicate Mach numbers from 0.5 to 1.0 over a substantially evenly divided scale. It is preferable to form the diaphragms so that their deflections would be approximately logarithmic without the use of the spring blades and for this purpose, the diaphragms are formed from beryllium copper and have circumferential corrugations. Each capsule is formed from two diaphragms joined together by means of peripheral flanges.