GB1432531A - Ionization fire alarms - Google Patents

Abstract

1432531 Ionization chambers CERBERUS AG 3 April 1974 [3 April 1973] 14840/74 Heading H1D A fire-detector device includes an ionization chamber 6 accessible to the air, at least one radio-active source 9, 10 for ion generation, and two spaced electrodes 7, 8, the ion current therebetween being reduced in the presence of smoke or combustion aerosols, the radio-active source(s) producing ionizing regions that include a portion R only of the space between the electrodes, ion current normally flowing between a portion only of the electrode surfaces and the electrodes extending from the ionization region in the direction of possible air flow through a distance equal to at least five times (e.g 10-20) their separation. The extended electrodes ensure that ions may still reach the electrodes despite air flow. Fig. 2A includes air duct 4, fan 5, short range α or # radiators, e.g. tritium-containing foils, or Ni<SP>63</SP>, Kr<SP>85</SP>, or Am<SP>241</SP>, the range being preferably reduced by an absorbing covering, electrodes 7, 8 preferably being impermeable to air ions and 2 cms. apart, projections 11, limiting ionization extension, and sharp bent edges 12 producing increased field strength to enhance electron capture. Part-cylindrical electrodes are preferred for circular cross-section ducts (Fig. 2B, not shown), or alternatively a wire or pin (14) within a tubular sheath (13), (Fig. 2C, not shown), with single radio-active source (15). Residual ions may be collected by grids (16), (17) (Fig. 2D, not shown). The detector may be used in air ducts with forced air flow or where air flows by convection, the detector then having disc-shaped electrodes (28, 29) (e.g. #1 cm. separation and preferably >10 cm. diameter) with centrally located radio-active source (31) as an annular foil (Figs. 3A, 3B, not shown). To base (20) with an alarm indicator device (e.g. LED 21), the unit is releasably secured by electrical contacts (23) and is built on insulating plate (24) with housing (25) and grid (26) as side walls for air passage, central insulator electrode support (27) and support pegs (32), and a 2 cm. diameter ionized zone R. Bent edges, added pins, or annular projections (33) or grid (26), electrically charged, increase edge efficiency. Leakage-path ribs (34) may be provided (Figs. 3C, 3D) and one electrode may be secured to pegs (32) (Fig. 3D) to improve insulation. The radio-active source may be a ring sunk in a groove in an electrode (Fig. 3D), post (27) may have a central bore with a connection between electrode (29) and fieldeffect transistor (35) or other voltage-sensitive device on plate (24) and preferably encapsulated. Voltages and air speeds are exemplified. Spacings and potentials may be such that operation is at the beginning of, or just below, the saturation range. Operation may be within the saturation region for air ions but not for charged heavy particles. Reduction of ion current from blowing out of the chamber may be compensated by reduced recombination due to wind effects.