GB1329475A - Ionization fire alarm - Google Patents

Abstract

1329475 Ionization fire detectors PREUSSAG AG FELIERSCHUTZ 17 May 1971 [ 16 May 1970 30 April 1971] 15285/71 Heading GIN In an ionization fire detector including a radioactive source the source has an activity less than 0À1 microcuries so that both free ions from smokeless fires which cause an increase in ionization current and smoke colloidal particles which cause decrease in ionization current can be detected. In the arrangement of Fig. 2 the junction of measuring chamber 1 and reference chamber 2 including radioactive sources 30, 31 is connected to output terminals 11, 12 via F.E.T. amplifiers 44, 43, dividers 5, 6 and 7, 8 being adjusted so that there is a voltage at 12 but not at 11. In the case of a smokeless fire the ionization current in chamber 1 is increased and the voltage at 12 becomes zero to operate an alarm. If smoke colloidal particles enter chamber 1 the ionization current decreases and the voltage at 11 increases to operate an alarm. Terminal 11 may be omitted and dividers 5, 6 adjusted so that both an increase or decrease in ionization current can be indicated at 12, Fig. 1 (not shown). The junction of the chambers 1, 2 may be connected to one input of an F.E.T. differential amplifier Fig. 4 (not shown). To produce a fire alarm signal only when the rate of charge of ionization current in chamber 1 exceeds a threshold level i.e. prevent the alarm being triggered due to DUST ACCUMULATION in the chamber the output of a F.E.T. connected to the chamber 1 is connected to a threshold amplifer via a differentiating amplifier. Alternatively or in addition an operational amplifier having a predetermined threshold may be used to actuate an alarm when the DUST ACCUMULATION exceeds a predetermined level, Figs. 5, 6, 7, 8 (not shown). DUST ACCUMULATION may also be detected by monitoring the activity of the radio-active source by means of a radiation detector and using the output of the detector to activate an alarm when the activity falls below a predetermined level, Fig. 9 (not shown) or to vary the conductivity of a transistor in series with the measuring chamber in place of the reference chamber 2 to nullify the effects of dust accumulation Instead of using a radiation detector DUST ACCUMULATION may be detected by insulating the radio-active source from the outer electrode so that a leakage current flows across the insulation when dust accumulates thereon which current may be used to operate an alarm or mitigate the effect of the dust Figs. 11, 12 (not shown). In Fig.13 (not shown) an auxiliary annular inner electrode is provided so that in the quiescent state these two electrodes are at equal potential and the ionization currents are equal. A fire alarm signal is only produced if there is a difference between the two currents.