Patent GB-505802-A (GB)
Title: Improvements in apparatus for indicating the presence of an explosive or combustible gas
IPC: G01N25/36
Filed: 1938-04-23
Abstract: 505,802. Gas detectors. GES. FUR NAUTISCHE UND TIEFBOHRTECHNISCHE INSTRUMENTE, and MARTIENSSEN, O. April 23, 1938, No. 12247. Convention date, June 8, 1937. [Class 1 (i)] Apparatus for indicating the presence of combustible gases, e.g. firedamp, is used in association with an electric safety lamp and comprises a switch device responsive, to heat generated by a heated element consisting of a platinum or platinum alloy wire activated with rare metal salts in the presence of methane and which operates to influence of interrupt intermittently the energizing cut-rent for the lamp. Preferably the switch also cuts off the current to the detector wire as well as to the lamp, so that a more rapid response is obtained, and to overcome effects due to varying external air temperature and variation in the voltage of the battery supplying current, the switch is made responsive to the difference in temperature between adjacent wires, one activated and one non-activated. In a practical Embodiment, the gas detector is combined with an electric safety lamp, carrying its own battery, and is housed in a combustion chamber formed of cylindrical concentric screens, preferably three in number. One end of the chamber carries an observation glass and at the other end is mounted a baseplate 12 of insulating material carrying the gas detector and switch and through which the necessary electrical connections are made. Figs. 2 and 3 show a switch device in the form of U -shaped capillary tube 13 with widened ends, which contain hydrogen or a rare gas. The middle capillary contains a mercury thread 14, and a contact pin 15 is fused into the middle of this tube. Connection is made between this pin and one pole of the battery. Into one shank of the tube 13 is fixed a platinum wire 16 which normally makes contact with the mercury thread, the other end being connected to a pin 17 passing through the base plate and connected to one terminal of the lamp by spring 25. The other lamp terminal is connected to the metallic casing and hence ! to the other pole of the battery. The wires 18 and 19 of the detector are connected in series and arranged in close proximity to the ends of the U-tube, the activated wire 18 being uppermost. Connection to the battery is made via pin 17 and pin 26 connected to the casing. The wires are approximately 0À08 mm. diameter and are so arranged as to first glow visibly on passage of current. In the presence of methane, coil 18 becomes hotter owing to combustion, coil 19 becomes cooler owing to the higher heat conductivity of the surrounding air, and the mercury thread moves and breaks contact with wire 16, thus cutting off the current to the lamp and the wires. The ends of the capillary rapidly assume normal temperature, and the original conditions are restored. If the methane content is still present, the cycle is renewed. Figs. 4 and 5 show an arrangement in which the detector wires 18a and 19a are in close proximity to two bi-metallic strips 21 and 22 which normally make contact at their free. ends 23, the electrical connections being as before. In the presence of methane, the greater heat generated by coil 18a will cause unequal bending of the strips and interrupt the contact, with the same result as described before. Fig. 6 shows a device in which the capillary tube is replaced by a copper tube 15b having a very elastic conductive diaphragm 20 mounted at the centre and against which the end of a conducting pin 16b normally rests. The activated wire 18b is adjacent the right-hand chamber of tube 15b and owing to the greater pressure generated in this end of the chamber in the presence of methane, the diaphragm 20 moves and breaks the contact with the pin 16b. The contact pressure may be adjusted by spring 38 and screw 39. Fig. 7 shows diagrammatically the electrical connections of the device shown in Figs. 2 and 3. Adjustment may be made so that the device is responsive to a methane content varying from 1 ¢-5 per cent. Specification 237,970, [Class 1 (i)], is referred to: