Abstract:
The present invention relates to a flame detector for detection of the presence of a flame or spark in front of the detector comprising a UV sensitive photocathode ( 12; 21 ) and an anode ( 14; 22 ), respectively, wherein the UV sensitive photocathode is oriented such that UV light from a flame present in front of the detector can strike the photocathode; a voltage supply unit ( 18; 23 ) connected to the UV sensitive photocathode and to the anode to force photoelectrons emitted from the UV sensitive photocathode when struck by UV light from a flame present in front of the detector to move towards the anode; and a readout arrangement ( 15-17; 24 ) adapted to detect charges induced by electrons moving towards the anode to thereby detect the presence of a flame in front of the detector. The flame detector can be combined with an alarm unit ( 33 ) to form an automatic alarm ( 31 ).

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to flame and spark detection, and is usable in a variety of fields including for instance fire alarming and flame monitoring of e.g. oil pipe flames and rocket launches, but also to detect electrical coronas, discharges, and to detect lightening at day time as a fast warning, or for triggering fast switch-off of equipment. 
     DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION 
     Many simple automatic fire alarms of today are based on a smoke detector. However, in some situations a smoke detector alone does not give reliable information about fire related accidents. Thus, more sophisticated and reliable devices are under continuous development. 
     A common approach is to simultaneously use two or more detectors of different nature, for example a smoke detector and an infrared detector. However, even such a detector combination may sometimes give false information. In attempts to overcome such limitation there was invented an infrared detection device measuring not only infrared emission from the fire, but also a frequency of flame oscillations. This gives a clear signature of a fire and also improves the signal-to-noise ratio. There have also been attempts to combine IR and UV sensitive detectors. However, the UV detectors used are very expensive and of low sensitivity. 
     Nevertheless, there are a few drawbacks of such approaches. Firstly, such technique is complex and thus costly, the infrared detector needs for instance cooling. Furthermore, the power consumption of such an approach is rather high. 
     Infrared detectors as such have typically not very good signal-to-noise or signal-to-background ratios. Thus, there is a need of another detecting principle, which is simple and reliable. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a flame detector, which provides for high signal-to-noise and signal-to-background ratios to thereby obtain a reliable detection. 
     A further object of the invention is to provide such a flame detector, which provides for high sensitivity. 
     Still a further object of the invention is to provide such a flame detector, which provides for low power consumption. 
     Yet a further object of the present invention is to provide such a flame detector, which is effective, fast, accurate, reliable, and of low cost. 
     These objects among others are, according to the present invention, attained by flame detectors, automatic fire alarms, and methods related thereto as claimed in the appended claims. 
     By the provision of a gaseous-based detector provided with a photocathode as claimed in the appended claims for detection of flames and discharges a simple and cheap detector is obtained, which is also reliable and exhibits excellent detecting characteristics. 
     Further, the detector is insensible to visible light, has high output signals for simple signal processing, and it can be manufactured in large size, i.e. with large sensitive area (square meters) of extremely high sensitivity. 
     Further characteristics of the invention and advantages thereof will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying FIGS. 1-4, which are given by way of illustration only, and thus are not limitative of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  illustrates schematically, in a cross-sectional  7  top view, a flame detector according to a first embodiment of the present invention, and FIGS. 1 b  and  1   c  illustrate other embodiments of the anode. FIG. 1 d  illustrates a lens or mirror according to an embodiment of the invention. 
     FIG. 2 illustrates schematically, in a perspective view, a flame detector according to a second embodiment of the present invention, wherein a top cover of the detector is removed for illustrative purposes. 
     FIG. 3 illustrates schematically, in a cross-sectional top view, a flame detector according to a third embodiment of the present invention. 
     FIG. 4 illustrates schematically, in a block diagram, an automatic fire alarm according to the present invention, where the fire alarm includes any one of the flame detectors of FIGS.  1 - 3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference now to FIG. 1 a first embodiment of a flame detector according to the present invention comprises a detection chamber  11 , which is preferably gas tight and filled with a gas suitable for electron multiplication, e.g. methane, ethane, CO 2  or gas mixtures of e.g. argon-isobutane or argon-CO 2 . 
     At the front side of chamber  11  there is arranged a UV photon sensitive photocathode  12  on a UV-transparent window  13  such that UV light from a flame (not illustrated) present in front of the detector can strike the UV sensitive photocathode and get absorbed. Most flames in air have very strong molecular band emission in the wavelength range of 150-280 nm (CH and OH), which can be used for clear fire identification. Also electrical discharges in air have strong emission lines in the wavelength region 150-280 nm. 
     The photocathode used should preferably have a wavelength dependent quantum efficiency, which makes it only sensitive to UV and VUV light. Typically the photocathode should only be sensitive to wavelengths shorter than 300 nm, or even more preferably to wavelengths shorter than 240 nm. In this way no bandpass filter is needed in front of the detector. Of course, a photocathode material, which is sensitive to longer wavelengths, could be used in combination with a bandpass filter in front of the detector that only transmits UV light. 
     The photocathode  12  is preferably disc-shaped with a diameter D and arranged together with window  13  to constitute an integrated part of the walls of chamber  11 . Suitable photocathode materials are CuI, CsTe and CsI as the quantum efficiency of CuI, CsTe and CsI overlaps well with the flame emission spectra. At the same time such detectors are practically insensitive to visible light. Further, the photocathode shall be thin such that photoelectrons can be emitted from a surface opposite to the surface facing the UV light, i.e. within chamber  11 . Thus the photocathode  12  may be provided as a thin to very thin layer on the entrance window  13 . The photocathode can alternatively be a gaseous or liquid material. 
     A gaseous photocathode is realized by removing the thin photocathode layer  12  and mixing the gas suitable for electron multiplication in chamber  11  with a gas suitable to be used as a photocathode material, e.g. gaseous TMAE, TEA or etylferrocene. Thus electrons may be released anywhere within chamber  11 . 
     A liquid photocathode is realized in a similar manner by removing the thin photocathode layer  12  and filling a bottom portion of chamber  11  with a liquid suitable to be used as a photocathode material, e.g. liquid TMA, TMAE, TEA or etylferrocene. In yet an alternative version the gas suitable for avalanche amplification in chamber  11  is removed and the complete chamber  11  is filled with a liquid or a mixture of liquids, e.g. the above mentioned optionally together with a liquid suitable for avalanche amplification. However, such a detector would not be very sensitive since no or only poor avalanche amplification will occur. 
     An anode in the form of a single wire  14  of a diameter Ø is arranged parallel to, and at a distance d behind, the UV sensitive photocathode  12 . The anode wire diameter Ø shall preferably be larger than 0.01 mm, more preferably larger than 0.1, even more preferably between 0.1 and 3 mm, and most preferably between 0.3 and 1 mm. The inter-electrode distance d shall preferably be less than the diameter D of the photocathode  12 . 
     Further, there is arranged a readout arrangement  15 - 17  close to the anode wire  14  in chamber  11 , the readout arrangement including a readout element  15  possibly supported by a dielectric support structure  16  and a signal connection  17  connecting the readout element  15  to the exterior of chamber  11 . The readout arrangement  15 - 17  could also be a single conducting element. 
     A voltage supply unit  18  is connected to the photocathode  12 , to the anode wire  14 , and to the readout element  15  as schematically indicated in FIG. 1, such that an electric field is created between the photocathode  12  and the anode  14  and a concentrated electric field is created close to and around the anode wire  14 . The voltage supply unit may be powered by a portable battery. 
     When UV photons from the flame hit the UV sensitive photocathode  12 , electrons will be released, which will be accelerated in the electric field and move towards the anode wire and by interaction with the gas in chamber  11  optionally cause avalanche amplification. Three modes of operation of such a detector can be distinguished: proportional, Geiger and streamer modes. The detector operates in proportional mode when a weak electric field between the cathode and the anode (particularly close to the anode) is applied, in Geiger mode when the electric field is increased, and in streamer mode at very high electric fields. The voltages needed are depending on the geometry used and the distance d and may be several hundred volts or more, but typically very low currents are flowing. 
     In the proportional mode all processes in the gaseous detector terminate when the ions from the primary avalanche have been collected at the electrodes. A flame detector according to the present invention, operating in the proportional mode, may have very high gain up to 10 5  or higher and good time resolution, e.g. below a nanosecond. Furthermore, the detector has a wide dynamic range allowing it to measure the UV-light intensity over a wide range of intensities. 
     In the Geiger and streamer modes the primary avalanche may trigger a sequence of secondary avalanches. As a result, in these modes, the output signal from the detector is usually larger than a few volts. The time resolution in this mode is typically 0.1-1 μs. 
     At very low voltages no avalanche multiplication will be obtained, but, nevertheless, there may be applications where the number electrons released from the photocathode will be sufficient for detection. 
     The detector design, i.e. the choices of the diameter Ø of the anode wire and the inter-electrode distance d, are important as they strongly affect the quantum efficiency of the photocathode. 
     The output of the readout arrangement  15 - 17  may be further connected to processing and/or decision circuitry (not illustrated) to process the signal further. 
     It shall be appreciated that the readout element  15  may be dispensed with and that the readout may be performed directly at the anode wire  14 . 
     It shall further be appreciated that other electrodes, e.g. mesh electrodes, may be used to divide chamber  11  into different drift and avalanche amplification regions. 
     It shall still further be appreciated that instead of a single wire anode  14 , a multiwire anode arrangement  114  or planar anode, e.g., including an array of anode elements  214  may be employed. In fact, any type of micropattern or electric field focusing geometry is applicable. 
     In such an instance a lens or mirror  300  may be arranged in front of the detector to image a flame in front of the detector onto the UV sensitive photo cathode  12 , and the readout arrangement may be adapted to detect charges induced by electrons moving towards each anode wire or element separately, and to convert these separately detected charges into a readout signal indicative of the image of the flame in front of the detector on the UV sensitive photocathode. Hereby, two-dimensional imaging of a flame (or at least of the UV light emitted in the flame) may be performed. 
     Further, the readout arrangement may be adapted to detect charges induced by the moving electrons temporally resolved to thereby detect temporal properties of the flame in front of the detector, particularly a frequency of flame oscillations of the flame in front of the detector, to increase the reliability of the detector. 
     A position-sensitive detector of the present invention can further be used in various technical fields, such as, e.g., for monitoring of flames, e.g., oil pipe flames and rocket launches, but also to detect electrical coronas or discharges on high voltage units, and to detect lightning during daytime as an early warning or for triggering fast switch-off of equipment. 
     With reference next to FIG. 2 a second embodiment of the flame detector of the present invention comprises a cylindrical UV sensitive photocathode  21  and an anode wire  22  arranged symmetrically within the photocathode  21 . A voltage supply unit  23  is connected to the photocathode  21  and the anode wire  22  to obtain a suitable electric field. Further, a readout arrangement  24  is connected to the anode wire  22  to detect charges induced therein. The cylindrical photocathode defines the sidewalls of a chamber  25 . A top cover (not illustrated) and a bottom cover  26  encloses the chamber  25 , preferably in a gas-tight manner, and chamber  25  is filled with a gas or gas mixture suitable for avalanche amplification of electrons. 
     By such detector design a 360° view angle is achieved and thus a flame in virtually any direction may be detected. 
     It shall be appreciated that the cylindrical photocathode  21  may be covered by a bandpass filter (not illustrated) with a narrow pass band as was discussed in connection with the FIG. 1 embodiment. 
     It shall further be appreciated that if a cylindrical shape is difficult or costly to manufacture a similar operation will be achieved if three or more planar photocathodes (not illustrated) are arranged around the anode wire to cover all or most directions in the horizontal plane. In fact, practically any geometry where the UV-light impinges on the photocathode and the emitted electrons move towards the anode is possible. 
     It shall still further be appreciated that a multi-element anode (not illustrated) may be used with this large view angle embodiment. For instance, a plurality of anode wires, strips or other pattern geometry may be arranged on a dielectric support with a photocathode arrangement, and if individual readout of the wires are performed not only presence of a flame may be detected, but also in which direction from the detector the flame exists. This effect can be made more efficient by dividing the volume 25 into optically segmented volumes using optical blinds between the anode wires. 
     With reference next to FIG. 3 a third embodiment of the flame detector of the present invention is identical with the first embodiment except for the location of the photocathode  12 . Here the thin photocathode layer  12  is provided on the readout element  15  of the readout arrangement  15 - 17 , and only window  13  is arranged at the front of the detector, where window  13  is made of e.g. quartz or MgF 2  and is possibly provided with a UV band pass filter (not illustrated). Window  13  and readout element  15 /photocathode  12  are adapted to operate as cathodes in the detector and are thus, together with the anode wire  14 , connected to the voltage supply unit  18 . 
     In an alternative version, the anode wire  14  can be dispensed with, and thus the voltages are applied such that window  13  operates as anode in the detector. 
     With reference finally to FIG. 4 an automatic fire alarm  31  of the present invention comprises a flame detector  32  and an alarm unit  33  connected to the detector  32 . The detector  32  is the flame detector of any of the first, second, or third embodiments of the present invention and is thus adapted to detect the presence of a flame in front of the detector. The alarm unit is adapted to generate a fire alarm signal in response to a detected presence of a flame in front of the detector  32 . 
     Optionally the UV detector can be used in combination with one or several other detector types, e.g. IR and smoke detectors where the logic in the alarm unit  33  decides whether there is an alarm or not based on the different signals from these detectors. 
     Further, the alarm unit includes a sound generator  34  and a transmitter  35 , each connected to the alarm unit  31 . The sound generator  34  is adapted to generate a sound in response to the fire alarm signal to alert the environment of the presence of a nearby flame. The transmitter  35  adapted to transmit (through a wire or wireless) the fire alarm signal to a remotely located site, e.g. an emergency service center. 
     Several UV detectors can also be placed at different positions, all facing the same point. In this way a 3-D reconstruction of the possible flame is possible which can be used by the logic in the alarm unit to further discriminate false alarms. 
     Advantages of the flame detector and the automatic fire alarm of the present invention include: 
     Low cost. 
     Low power consumption, and thus batteries can be used as voltage supply. 
     High sensitivity for flame radiation. 
     The detector is practically insensitive to visible light. 
     No UV transmitting filter is needed. 
     High output signals. When operated in Geiger mode, for example, the amplitude of the output signal is up to few volts, and thus no additional amplifier is needed to process the signal. 
     The inventive detector has practically no noise pulses. 
     High signal-to-noise and signal-to-background ratios. 
     No cooling is needed. 
     It will be obvious that the invention may be varied in a plurality of ways. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.