Abstract:
A dual sensor fire detector includes a smoke sensor and a gas sensor. A source of radiant energy emits a primary beam that is formed into first and second beams. One beam is directed into a smoke sensing chamber. The other is directed to the gas sensor. Outputs from the smoke sensor and the gas sensor are combined to make a fire determination.

Description:
FIELD 
     The application pertains to fire detectors. More particularly, the application pertains to such detectors that incorporate both a photoelectric smoke sensor and a solid state gas sensor. 
     BACKGROUND 
     There are several types of photoelectric smoke detectors. Most detectors use only forward scattering detectors with a light source in the near infrared. Some detectors use a dual angle sensing chamber, which measures both the forward and backward light scattered from particles in order to gain some insight into particle size. 
     Some detectors use more than one wavelength of light. Others use a combination of angles and wavelengths. Some detectors use a photoelectric sensing chamber combined with heat, gas, or light sensing, i.e., multi-criteria smoke detectors. One example of a photoelectric smoke sensor is disclosed in U.S. Pat. No. 6,521,907, entitled “Miniature Photoelectric Sensing Chamber,” which issued Feb. 18, 2003. One example of a multi-criteria detector is disclosed in U.S. Pat. No. 6,967,582, entitled “Detector With Ambient Photon Sensor and Other Sensors,” which issued Nov. 22, 2005. Both U.S. Pat. No. 6,521,907 and U.S. Pat. No. 6,967,582 are owned by the assignee hereof and incorporated herein by reference. 
     Photoelectric smoke sensors that use near infrared light (850 to 950 nm) are generally known to be better at detecting smoldering fires since those types of fires produce larger particles. Ionization-type smoke sensors tend to detect flaming fires better. Ionization sensing chambers are better at detecting small particles produced by the flaming fires. Ionization-based detectors are falling out of favor due to increased environmental regulations. 
     Smoke detectors are commercially available that use blue light emitting diodes (LED&#39;s). When blue LED&#39;s are used in forward scattering photoelectric smoke sensing chambers, a sensor&#39;s response to small particles improves. This is predicted by the Mie scattering theory, which says that particles will preferentially scatter light in the forward direction when the wavelength of light approaches the particle size. Small particles are typically produced by flaming fires. 
     At least some known photoelectric smoke sensors include an optic block that carries a light source, such as an LED, and a light sensitive element, such as a photodiode. The source and the light sensitive element are arranged at a prescribed angle to one another in order to detect scattered light. A housing surrounds the optic block and serves to exclude ambient light and direct the flow of ambient airborne particulate matter. 
     MOS (metal oxide semiconductor) gas sensors are typically heated to 200 to 400° C. for proper operation. This required heating can be achieved by using a resistance heater, causing high power consumption. Some thick film MOS gas sensors draw up to 500 mW, while thin film or MEMS devices may draw an order of magnitude less. This high power consumption limits the number of applications where they can be used. For example, system connected fire detectors require low power consumption due to battery backup requirements in the National Fire Alarm Code. 
     MOS gas sensors also tend to not be selective to one gas, but sensitive to a whole class of gases, e.g., oxidizing gases. Radiant energy can be directed onto such sensors to increase their sensitivity instead of heating them. Doing so reduces the amount of power required to operate them. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a multi-sensor fire detector in accordance herewith; and 
         FIG. 2  is an enlarged perspective view of a mounting block usable in the detector of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     While disclosed embodiments can take many different forms, specific embodiments hereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof as well as the best mode of practicing the same and is not intended to limit the claims hereof to the specific embodiment illustrated. 
     In one aspect hereof, a smoke sensing chamber includes a blue or UV light source where the light source is used not only for measuring particles of smoke with light scattering, but also enhancing the operation of an MOS gas sensor. Flaming fires can be detected if the gas sensor oxide is chosen to be WO 3  for NO 2  detection since flaming fires produce NO 2 . Alternately, if SnO 2  is chosen for the oxide to sense CO, then both smoldering and flaming fires could be detected. 
     Light or radiant energy from the light source is directed in two directions such that it creates the necessary scattering volume for the smoke sensing chamber, for example, a photoelectric sensing chamber, and it shines on the MOS gas sensor&#39;s gas sensitive oxide in order to enhance operation thereof. In another aspect, the light source can be intermittently activated to reduce power requirements. In an alternate embodiment, two different sources, activated intermittently, could be used. 
     Radiant energy from the source can be divided into beams. One beam can be directed into the scattering volume. The other can be directed at the gas sensor. 
     An optical or mechanical element can be used to form two different beams. One optical element is a beam splitter. Wavelengths for the emitted radiant energy can range from blue (465 nanometers) to ultraviolet (365 nanometers). 
     The MOS gas sensor may be heated, but at a lower level than is ordinarily required or not heated at all. The gas sensor may be occasionally heated in order to clean the sensor and restore it to a baseline condition. Advantageously, various different oxides may be used in the MOS gas sensor, including tin oxide, tungsten oxide, chrome titanium oxide, etc., depending on what gases need to be sensed. 
       FIGS. 1 and 2  illustrate various aspects of an exemplary dual sensor fire detector  10  in accordance herewith. The detector  10  can be carried in a housing  12  that defines an internal scattering volume  14 . The housing  12  defines openings  16 , as would be understood by those of skill in the art, to provide for ingress of ambient airborne particulate matter, for example, smoke from a fire in an adjacent region R being monitored by the detector  10  and gases produced by such fire. 
     The housing  12  also carries a mounting or optical block  20 . The block  20 , in turn, carries a source of radiant energy  22 , such as a blue emitting LED or a laser with a wavelength in a range as discussed above. The source  22  emits radiant energy as a beam B 1  directed to a divider element  24 . The divider element  24 , which could be mechanical or optical, such as a beam splitter, forms two different beams B 2 , B 3 . 
     The beam B 2  is directed into the scattering volume  14 . Light scattered by airborne smoke particulate, indicated generally as B 4 , is incident on a photosensor  26 . 
     The beam B 3  is incident on a metal oxide gas sensor  28  and activates that sensor to respond to gases that enter the housing  12  via a pathway  28   a  and are incident on the sensor  28  as discussed above. 
     Control circuits  30  carried by housing  12  could be implemented, in part, by a programmable processor  30   a  that executes pre-stored control circuitry  30   b  present in a non-transitory computer readable storage medium. The control circuits  30  are coupled to the source  22  to activate the same via a conductor  30   c.    
     The control circuits  30  receive gas indicating signals via a conductor  28   b  and smoke indicating signals via a conductor  26   a . Signals on the lines  28   b  and  26   a  can be processed to make a fire determination. 
     Input/output interface circuits  32  coupled to the control circuits  30  communicate with a displaced alarm system S via a wired or wireless medium  34 . 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. 
     Further, logic flows depicted in the figures do not require the particular order shown or sequential order to achieve desirable results. Other steps may be provided, steps may be eliminated from the described flows, and other components may be add to or removed from the described embodiments.