GB561444A - Improved electrical apparatus for the detection and indication of gases - Google Patents

Abstract

561,444. Wheatstone bridges. STANDARD OIL DEVELOPMENT CO. Jan. 28, 1943, No. 1450. Convention date, Nov. 19, 1941. [Class 37] Apparatus for detecting the presence of a certain gas or gases in gaseous mixtures includes a potentiometer and a Wheatstone bridge, means for passing a gaseous mixture to be tested over one of the arms of the Wheatstone bridge, and means for removing the potentiometer at intervals and automatically standardizing the Wheatstone bridge during the interval. The Wheatstone bridge 10 comprises a standard resistance 14 and a measuring resistance 16, both preferably of platinum. The resistance 14 is sealed in air, whereas the resistance 16 is arranged so that the gas to be tested is passed over it. The resistances 14, 16 form two arms of the bridge, the other two arms being formed by resistances 18, 22. Connecting the resistances .14, 16 is a compensating resistance 26 with a resistance 24 in parallel. A contact 27 slidably engages the resistance 26. Direct current is applied to one diagonal of the bridge by wires 32, 34, the other diagonal being-connected to resistances 42, 44 from which a portion of the out-of-balance voltage is tapped by a slider 46 and measured by a potentiometer 12. Linked switches 48, 74 when in the fullline position connect a' galvanometer 40 between the slider 46 and a point on the potentiometer. When the switches 48, 74 are in the dotted-line position, the galvanometer 40 is in series with a standard cell 67 across a resistance 64. A resistance 92 is then adjusted until the galvanometer is in balance, the voltage across the resistance 64 then being equal and opposite to that of the standard cell. The switches 48, 74 are then moved to the full-line position and the resistance 113 is adjusted by automatic means controlled by the galvanometer 40 until balance is obtained. A pointer then indicates on a scale the percentage of combustible gas in the air being passed through the measuring cell in the bridge. The Wheatstone bridge is standardized automatically every 45 minutes, a switch 123 being moved to the dotted-line position so as to disconnect the bridge from the potentiometer and connect the line 38 directly to the galvanometer 40. At the same time a two-way valve is electromagnetically operated and causes air instead of gaseous mixture to pass over the resistance 16 and the bridge is automatically balanced by means of the resistance 26. U.S.A. Specification 1,935,732 is referred to.