Abstract:
A method and apparatus of determining gas transport time from an input port to first and second ultrasonic detectors in a flow conduit for an aspirated smoke detector are provided. The method includes first measuring a transit time of ambient air between the detectors. Then, a different gas is injected into the conduit, and the time of injection is stored. The transit time is measured at least intermittently between the two ultrasonic detectors until a change therein is detected. A time of detected change is subtracted from the time of injection to establish a transport time for the aspirated smoke detector.

Description:
FIELD 
     The application pertains to systems and methods of using ultrasonic transducers in aspirated smoke detectors to measure transport times. More particularly, the application pertains to such systems and methods that use ultrasonic transducers to measure transit times through two different gases in an aspirating smoke detector to determine transport times. 
     BACKGROUND 
     Various configurations of aspirated smoke detectors are known to be useful in harsh environments, where maintenance can be difficult or where aesthetics matter. Several embodiments thereof are disclosed in Griffith et al., U.S. Pat. No. 7,669,457, entitled “Apparatus and Method of Smoke Detection”. The &#39;457 patent issued Mar. 2, 2010, is assigned to the assignee hereof, and is incorporated herein by reference. 
     When the installation of an aspirating smoke detector and associated conduit or pipe network is performed, it is necessary and required that the installer measure the transport time of the installed apparatus. This measurement is performed to verify that one of the core parts of the installation, the pipe network, has been correctly installed. 
     Transport time is the time for air/smoke to flow from a sampling point to the smoke sensing element in the aspirating device. The transport time will preferably not include any processing time and is specifically limited to the time it takes to transport air/smoke from the sampling point to the sensing element. 
     The theoretical transport time can be estimated by software tools. The installer compares software results with the measured transport time to verify that, in the real system, there are no pipe leakages, errors in setup of the device, issues with a fan, incorrect pipe assembly, etc. 
     The typical way that installers measure transport time in a real system is to use smoke pellets or cotton wicks to produce smoke to flow into a selected opening or hole of the pipe network connected to the aspirated detector. The transport time is measured as the time between the start of smoke inflow and the signaling of the presence of smoke at the aspirated device. The device is set to the highest level of sensibility. 
     The measured time is not the transport time as defined. It is the transport time plus the processing time to respond to inflowing smoke, which depends on the characteristics of the sensing element and associated alarm/filtering algorithms. The problem is that the processing time is difficult to calculate because it may vary from one test to another, due, for example, to unknown polling intervals, and, with time, due to changes in the alarm threshold. This additional time causes an inaccurate and non-reproducible determination of the transport time, affecting the verification of the system. It would be desirable to eliminate these inaccuracies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a block diagram of a system in accordance herewith; 
         FIG. 1B  illustrates additional details of the system of  FIG. 1A ; 
         FIG. 2  is a graph which illustrates operation of aspects of the system of  FIG. 1A ; and 
         FIG. 3  is a flow diagram illustrating processing of the system of  FIG. 1A . 
     
    
    
     DETAILED DESCRIPTION 
     While disclosed embodiments can take many different forms, specific embodiments thereof 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 thereof as well as the best mode of practicing the same and is not intended to limit the application or claims to the specific embodiment illustrated. 
     In one aspect, embodiments hereof can measure transport time without the additional processing time. The starting point is that some commercial aspirating smoke devices are equipped with ultrasonic transducers to measure the air flow of the pipe network. An internal controller measures the transit times of an ultrasonic pulse propagating along an internal path, for example, with and against the direction of the flow. A pair of ultrasonic transducers can be used for this purpose. The sampled transit times are processed by the programmable controller to obtain the transport time. 
     We have recognized that the transit times, which could be measured with and against the flow, are affected by the density of the flowing medium. For example, assuming the same flow speed, the transit time sampled, if the medium is air, is different from the transit time sampled if the medium is tetrafluoroethane. Using our technique, as discussed below, the transit times can be measured excluding additional processing times. 
     In this regard, the speed of sound c in gases is given by the Newton-Laplace equation:
 
 c =SQRT( K/r )
 
where:
         K is a coefficient of stiffness (the modulus of bulk elasticity)   r is the density of the gas   (SQRT=square root)
 
Thus, the speed of sound decreases with the increasing density of the gas.
       

     Considering that the density (r) of tetrafluoroethane is greater than air, it means that the speed of sound in this medium is lower than the speed of sound in air. As a consequence, when the tetrafluoroethane reaches the internal path of the ultrasound detectors, the transit time increases. In embodiments hereof, the transport time is established by sampling, recording, and evaluating the ultrasound transit times and detecting flow medium changes as described in more detail below. 
     In practice, the installer inserts a gas different from air in a sampling hole in the conduit or pipe network. The present system indicates when the gas arrives in the smoke sensing element. The interval between the arrival time and the gas insertion time is the transport time. 
       FIGS. 1A and 1B  illustrate an aspirated detector  10  in accordance herewith. Detector  10  is connected to a gas sample delivery pipe network P. Detector  10  is carried, at least in part in a housing  10 - 1 . 
     The embodiment of  FIGS. 1A and 1B  has an ambient air inflow port  12 , a particle separator  14 , as would be understood by those of skill in the art, and an outflow port  16 . The outflow from port  16  is in fluid flow communication with an aspirator  18 . As a result of the pressure differential developed at separator  14 , smaller, lighter particles of airborne particulate matter will be diverted from the flow from port  12  as discussed below. 
     Aspirator  18  can be implemented as a fan, or other element, which produces a reduced pressure at port  16 , thereby drawing ambient air and associated particulate matter into port  12 . 
     Chamber  22 , a smoke detection chamber, receives, via port  22   a , a partial flow of inflowing ambient air with larger particles excluded. Chamber  22  can be implemented as a photoelectric, an ionization, or both, sensing chamber without limitation. Neither the exact details of the separator  14 , nor the smoke detection chamber  22 , are limitations hereof. 
     Control circuits  24  are coupled to aspirator  18  and chamber  22 . Circuits  24 , which could be implemented, at least in part, with a programmed processor  24   a , and associated executable control software  24   b , can activate a photoelectric implementation of chamber  22  via a conductor  26   a . Smoke indicating signals can be received via conductor  26   b  at the control circuits  24 . 
     Circuits  24  can process signals on line  26   b  to establish the presence of a potential or actual fire condition and couple that determination, via a wired or wireless communications medium  28 , to an alarm system control unit  30 . Circuits  24  can also include a local communications interface  24   c , which might be implemented as a USB-type port, for use by an installer. 
     In the detector  10 , larger airborne particles flow from port  12  to port  16  without being diverted into chamber  22 . Hence, pollutants, such as dust particles and the like, will be excluded from chamber  22 . 
     Transport times, measured in a direction along the direction of ambient gas flow into the detector  10 , or perhaps against the direction of such ambient gas flow, can be obtained using ultrasonic transducers transmitters/receiver  12 - 1 ,  12 - 2 , coupled to control circuits  24 . With respect to  FIG. 2 , with ambient air in the inflow pipe  12 , measured directional ultrasonic transit times can be expected to be reasonably constant, within the measurement system accuracy, for constant environmental conditions. 
     When a sample gas G is injected into the pipe network P and inflow conduit  12 , at insertion time A, the transit time will remain substantially constant until time B when the gas G enters the ultrasonic path between transducers  12 - 1 ,  12 - 2 . When the transit time increases, at time B, due to increasing gas density, the difference B-A corresponds to the transport time. It is exclusive of smoke detector processing time. 
     Two different embodiments can be implemented. In a first, the aspirating smoke device  10  directly determines the ultrasound transit times and records when the flow medium inside the device is changed so as to establish the transport time. Alternately, an external device, for example a PC, such as  30 , with dedicated software  30   a , can be connected to the aspirating smoke detector  10  via interfaces  24   c  and  30   b . The computer  30  receives, records, and processes the transit times, deduces when the flow medium changes, and can then establish the respective transport time. 
     As those of skill in the art will understand, embodiments hereof can be implemented using any of the aspiration detectors of the &#39;457 patent, incorporated herein, or alternately, in publicly available aspiration detectors marketed by the assignee hereof as the SYSTEM SENSOR FAAST LT aspirating smoke detector. 
     Application software  30   a  can be provided to evaluate the ultrasound transit times. In this embodiment, the suggested procedure is:
         Installer connects the inlets  12  of the device  10  to the pipe network P.   Installer connects the device  10  to the laptop  30 .   Installer runs the software  30   a  that communicates with the device  10  and retrieves ultrasound transit times.   Installer inserts a gas G different from air in the sampling port of the pipe network being tested. Synchronization between the test software  30   a  and the gas insertion time can be implemented by another installer or with the help of time scheduling functionality of the test software  30   a.      The test software  30   a  senses the transit times and responds to the time when the gas is arrived at the device  10 . The interval, B-A, between the arrival time and the gas insertion time is the transport time.       

       FIG. 3  illustrates a flow diagram of a process  100 . The detector is coupled to a conduit or pipe network as at  102 . Transit time at the detector  10  is measured as at  104 . A different gas can be injected into the conduit or pipe network as at  106 . The time from injection of the gas G to a transit time change at the detector  10  is measured as at  108 . The transport time can then be output as at  110 . 
     While the above exemplary embodiments have been discussed in terms of using electro-acoustic processing to determine when the gas G has arrived at the detector  10 , those of skill will recognize that other types of processing can be used. For example, electro-magnetic processing can be used. For example, if the opacity of air is different than the opacity of the gas G, optical readings can be made to determine when the gas G has arrived at the detector  10  to determine transport time. All such variations come within the spirit and scope hereof, without limitation. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope hereof. 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, or steps may be eliminated, from the described flows, and other components may be added to or removed from the described embodiments.