GB831812A - Improvements in or relating to fire detectors - Google Patents

Abstract

831,812. Electric tests. MINERVA DETECTOR CO. Ltd. May 1, 1958 [Feb. 1, 1957], No. 3556/57. Class 37. A fire detector, of the kind in which the presence of smoke in an ionization chamber open to the ambient atmosphere renders a discharge tube conductive, comprises a domeshaped gong with a central opening in or through which the ionization chamber lies or projects, a vibrator device for causing a striker intermittently to strike the gong, and a relay in the discharge tube circuit for controlling the operation of the vibrator device. As shown, Fig. 4, to the grid of a discharge tube 8 are connected electrodes 2, 5, in ionization chambers 1, 4, which contain a radio-active substance and which are, respectively, open to the atmosphere and protected from it. Second electrodes 3, 6, of ionization chambers 1, 4, are connected to the cathode and the anode, respectively, of the tube 8, and also to the output of a mains transformer 17 which is rectified at 18 and smoothed by capacitors 25, 26, and resistor 20. When the tube 8 becomes conductive, a relay 21a is energized and closes contact 21b in the circuit of a vibrator device 28, which operates until the discharge tube circuit is broken at reset switch 22; a second, remote vibrator device 47 may be connected in parallel with the first. To test the device, a switch 13 is operated to apply a potential to the grid of tube 8 via a potential divider 30, 32 and ionization chamber 1, such that the tube conducts. Figs. 1 and 2 show the fire detector assembled as a unit structure in which the detector head, comprising chambers 1, 4, and tube 8, is located in a cylindrical casing 33, the chamber 1 having an outer end wall and a circumferential wall of fine wire mesh, and projecting through the central hole in a gong 35. The gong is mounted on a base-plate 34 by means of a mounting-ring 38, a threaded clamping-ring 41, a tubular support 39, and a mounting block 43; alternatively, it may be held on a number of bolts protruding from the base-plate 34, these bolts also supporting a mounting platform for the casing 33. The other components of the device are mounted under the gong on the base-plate in the positions shown, levers 36, 37 projecting beyond the periphery of the gong for manual operation of the test-switch 13 and the reset switch 22 respectively. The apparatus may be modified to be operated by dry cells. Specification 536,594 is referred to.