GB520993A - Improvements in or relating to apparatus for testing gas for combustible constituents - Google Patents

Abstract

520,993. Testing for combustible gases ; electric tests. MINE SAFETY APPLIANCES CO. Sept. 6, 1938, No. 26037. Convention date, Aug. 9. [Class 1 (i)] [Also in Group XXXVI] Apparatus for testing gases for combustible constituents comprises a Wheatstone bridge circuit having in one branch a testing filament, e.g. of platinum coated with catalytic material, arranged to be heated to a temperature causing combustion of the combustible constituents, and in a parallel branch a compensating element of considerably higher resistance so that the filament may be operated by a dry battery, changes of current due to combustion being indicated by an ammeter &c. To avoid the necessity for separate adjustment of operating voltage and bridge balance, the compensating element may have characteristics similar to but not identical with those of the testing filament, the arrangement being such that adjustment of a single rheostat in the battery circuit zeroizes the instrument at the correct operating voltage for the filament. As shown in Fig. 3, the testing filament 19, in series with a resistance 25, is balanced by a compensating element comprising a resistance 27 and ballast lamp 28, in series with a resistance 26 ; a single rheostat 32 controls both bridge balance and operating voltage. The apparatus, Fig. 2, comprises a case 10 divided by a partition 11 into an upper compartment housing the combustion cell 12 which is connected by pipes 13, 15 to a gas inlet 14 and hand pump 16 respectively, and a lower compartment housing the dry battery 44. In use, the apparatus is zeroized, in air, with the aid of a milliammeter 33, by adjusting resistance 32 by knob 32a. The pump 16 is then operated to draw suspected air through the cell 12 and the highest reading of the ammeter noted.