HEAT DETECTOR

Abstract: This idea relates to a heat detector using an optical fiber as heat sensitive line, in which a modulation light is emitted from a light emitting element through the optical fiber, and output of the photoelectric element is checked for attenua­ tion. This detector is capable of readily discriminating a fire by attentuation of the light from the light source caused by damage in the optical fiber in case of fire.

Description: Heat Detector This idea relates to a heat detector using an optical fiber as heat sensing wire. There have been proposed various heat detectors which use optical fibers being vulnerable in a low temperature range of some 60 ¯ 3000C as heat sensing wires and which detect fires by decrease in light reaching the photoelectric elements as caused when the optical fibers are damaged by heat or flame in case of fire, and the light from the light emitting elements leaks out through the damaged portions. For their heat sensing wires these heat detectors use an optical fiber which is composed of only a plastic core fusing at a low temperature (some 60 s 300"C), an optical fiber with its core and clad layer made of plastic fusing at a low temperature, or an optical fiber with its core made of quartz glass which does not fuse at a low temperature and its clad layer made of plastic fusing at a low temperature. Of these optical fibers, the ones composed of the plastic core alone or the core and clad layer, both being made of plastic, fuse and open with heat from or flame of fire while in the case of the one composed of the quartz glass core and the plastic clad layer, only the clad layer fuses and the core does not open. If these optical fibers have an open or partial fusion caused by heat or flame in case of fire, all or part of the light from the light source leaks out through the damaged portion, and consequently the photoelectric element receives no light or onl-y a part of the light from the light source. In practice, however, there exist the sunlight, illumination light etc. around the optical fiber. If the optical fiber opens or partially fuses due to heat or flame in case of fire, the enviromental light such as the sunlight, illumination light or light from flame enters the optical fiber from the damaged portion. Therefore, even if the optical fiber is damaged by fire, the photoelectric element receives the environmental lights and the light from flame entering through the damaged portion in lieu of the light from the light source, and accordingly there is scarecely any change in light intensity reaching the photoelectric element before and after the optical fiber is damaged, and it wae quite difficult to discriminate the fire. In view of the above point, the present idea aims at offering a heat detector which uses an optical fiber as heat sensing wire and is capable of readily discriminating a fire by detecting, without fail, such decrease of light from the light source as caused by damage in the optical fiber in case of fire. An embodiment according to this idea is described below with reference to a drawing, wherein 1 is an optical fiber with its core and clad layer made of plastics having different refraction rates and which fuses at a low temperature (some 60 ¯ 3000C), 2 is a light emitting element such as a light emitting diode provided at one end of optical fiber 1, 3 is a light emitting current feed circuit to feed light emitting current to light emitting element 2, 4 is a modulation circuit to modulate the light emitting current from light emitting current feed circuit 3 by signal with specific frequency f from oscillator 5 and to feed the modulated light to light emitting element 2, 6 is a photoelectric element such as solar cell provided at the other end of optical fiber 1, 7 is an amplifier to amplify the output of photoelectric element 6, 8 is a bandpass filter through which only the output modulated by specific frequency f out of the amplified output from amplifier 7 can pass, and 9 is a fire discriminating circuit comprising a comparator which gives out a fire signal when the output through the bandpass filter has fallen below the predetermined level. In normal condition, the heat detector of this composition operates as follows. The light emitting current from light emitting current feed circuit 3 is normally modulated by modulation circuit 4 and fed to light emitting element 2. With this modulated light emitting current, light emitting element 2 emits modulated light to optical fiber 1. On the other hand photoelectric element 6 receives the modulated light from light emitting element 2 through optical fiber 1. The output of photoelectric element 6 is amplified by amplifier 7, and then fed to fire discriminating circuit 9 through bandpass filter 8. Fire discriminating circuit 9 does not give out a fire signal as long as the modulated light output is above the predetermined level. If a fire breaks out and optical fiber 1 fuses to open with heat from or flame of the fire, propagation of the modulated light from light emitting element 2 is interrupted at the opened portion, and external light such as sunlight, illumination light, and light of the flame enter optical fiber 1 from the opened portion. Consequently, photoelectric element 6 no longer receives the modulated light from light emitting element 2 but the external light and light of the flame, thus generating output which does not contain modulated light portion with specific frequency f, and which is amplified by amplifier 7 and sent to bandpass filter 8. Nevertheless, since this output sent to bandpass filter 8 contains no modulated light portion with frequency f, no output develops at bandpass filter 8. As a result of this, the modulated light output in fire discriminating circuit 9 falls below the predetermined value, i.e. 0, fire discriminating circuit 9 determines it as fire and sends a fire signal to fire control panel which is not shown in the drawing. It is also feasible to make the light emitting current from light emitting current feed circuit 3 as pulse output, to modulate the light emitting current pulse by output of oscillator 5 in modulation circuit 4 and to feed the modulated light emitting current pulse to light emitting element 2. In this case, it will do if operation of amplifier 7 or fire discriminating circuit 9 is synchronized with light emitting current output of light emitting current feed circuit 3. It is also possible to use an optical fiber which is composed of only plastic core fusing at a low temperature or an optical fiber with its core made of quartz glass which does not fuse at a low temperature and its clad layer made of plastic fusing at a low temperature. According to the present idea, occurence of a fire is discriminated by emitting modulated light to the optical fiber from the light emitting element and checking if there is attenuation in the modulated light intensity reaching the photoelectric element. Therefore, it offers a heat detector using an optical fiber as heat sensing wire which is capable of readily discriminating fire by detecting, without fail, such attenuation of light from the light source as caused by damage in the optical fiber in case of fire. Brief Description of Drawing The drawing is a schematic diagram of an embodiment of the heat detector according to this idea. 1 ............ Optical fiber which is damaged at a low temperature 2 ............ Light emitting element 3 ............ Light emitting current feed circuit 4 ............ Modulation circuit 5 ............ Oscillator 6 ............ Photoelectric element 7 ............ Amplifier 8 ............ Band-pass filter 9 ............ Fire discriminating circuit

Claims: Claims 1. A heat detector having a light emitting element, a circuit which feeds modulated light emitting current to the light emitting element, an optical fiber through which the modulated light from the light emitting element is fed and which is vulnerable such as break, fusing etc. at a low temperature, a photo electric element which receives light from the optical fiber, and a circuit which detects output of the modulated light portion of the output of the photo electric element to discriminate whether a fire has broken out or not. 2. A heat detector as set forth in Claim 1, wherein the circuit which feeds the modulated light emitting current is formed in such a way that the modulated light emitting current is intermittently fed to the light emitting element, and the circuit which discri minates whether a fire has broken out or not is formed in such a way as detects the output in synchronous with the modulated light emitting current output.

Inventors: OKAZAKI, TOSHIAKI NOHMI BOSAI KOGYO CO., LTD.
Applicants: NOHMI BOSAI KOGYO CO., LTD.
Application year: 1983
Main classification: G08B17
IPC codes: G01K11, G08B17, G02B6