GB736006A - Improvements in and relating to microbalances and allied instruments - Google Patents

Abstract

736,006. Microbalances. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Feb. 16, 1953 [Feb. 20, 1952], No. 4528/52. Class 40 (1). An instrument for measuring small forces comprises a beam mounted to permit deflection, having a member to or at which said force may be applied, an electrode movable with the beam and at least one fixed electrode co-operating therewith to form a capacitor variable in accordance with movements of the movable electrode, together with a circuit for measuring changes in capacitance and producing a voltage proportional thereto, which is applied between a restoring electrode on the beam and a fixed one to provide an electrostatic restoring force opposing beam deflection. In one embodiment (Figs. 1 and 2) for measuring small pressures, the centre plate 12 of a Knudsen gauge, which is mounted at one end of beam 10 is subject to a thermal molecular force proportional to pressure. An electrode 15 mounted on the opposite end of the centrally pivoted beam forms with fixed plates 16, 17 a pair of displacement sensing capacitors forming adjacent arms of an A.C. bridge. The unbalance voltage corresponding in sense and magnitude to the beam displacement, is fed after amplification at 21 to phase-sensitive detectors 22. The fluctuating direct voltages, instantaneously proportional to beam displacement, from the detectors are amplified and filtered at 23 and applied through a stabilizing network 24 (and Fig. 4, not shown) which advances the phase by an amount proportional to beam velocity, sensitivity being adjustable by a variable potentiometer forming part of this network. The voltages between plates 16, 17 in the balanced condition is used as a measure of the pressure. The fixed plates 16, 17 made of metal foil stretched over insulating rods 34a, 34b, by coil springs 36 attached to both ends; are maintained at different temperatures by having a heater coil 40 on one plate. The insulated rods are mounted in pairs on clamps 32 carried by rods 31 bolted to end plates 30, whereas the wire 11 about which the beam pivots is mounted centrally between the four rods and held taut by leaf spring 39 to which it is attached, the beam zero being set with the aid of a magnetic member 43 on the wire. The whole beam assembly is housed in a pressure chamber, and the axis of rotation of the beam may be horizontal or vertical. A further embodiment employs a counterbalanced beam having only one set of plates, which serve both as Knudsen gauge and as sensing capacitors. The fixed plates form the ends of vessels through which water at a controlled temperature is circulated, these vessels and the electrical connections to the plates being sealed into a gas envelope. In a less sensitive arrangement the moving plate is mounted on a cantilever spring: