Abstract:
A smoke concentration measurement system includes: a beam extension chamber having a body defining a smoke channel having a longitudinal axis, the smoke channel having a smoke inlet and a smoke outlet at opposite ends of the longitudinal axis, a first reflective surface located on a first lateral side of the longitudinal axis, an entrance window located on the first lateral side of the longitudinal axis, a second reflective surface located on a second lateral side of the longitudinal axis, and an exit window located on the second lateral side of the longitudinal axis. The system also includes a laser light source adapted to emit laser light onto the entrance window, the laser light reflecting back and forth between the first reflective surface and the second reflective surface toward the exit window, and a first light measurement device adapted to receive laser light exiting the exit window. Other features, as well as a method of measuring the concentration of smoke, are also described.

Description:
TECHNICAL FIELD 
       [0001]    This patent application relates generally to methods and systems for testing properties of smoke. More specifically, this patent application relates to methods and systems for measuring the concentration of smoke. 
       BACKGROUND 
       [0002]    Methods and systems for measuring the concentration of smoke are known. Some known techniques pass a laser through a stream of the smoke, and measure the amount of laser light extinction caused by the smoke. The amount of laser light extinction can then be correlated to the concentration of the smoke. 
         [0003]    Known techniques typically suffer from low sensitivity at low smoke concentrations. In order to compensate for low sensitivity, traditional systems have extended the path length of the laser beam. However, extending the path length results in an area averaged measurement of the smoke concentration, which typically requires an assumption that the smoke concentration is uniform along the path length. 
       SUMMARY 
       [0004]    According to an embodiment, the invention provides a smoke concentration measurement system, comprising: a beam extension chamber comprising: a body defining a smoke channel having a longitudinal axis, the smoke channel having a smoke inlet and a smoke outlet at opposite ends of the longitudinal axis, a first reflective surface located on a first lateral side of the longitudinal axis, an entrance window located on the first lateral side of the longitudinal axis, a second reflective surface located on a second lateral side of the longitudinal axis, and an exit window located on the second lateral side of the longitudinal axis; a laser light source adapted to emit laser light onto the entrance window, the laser light reflecting back and forth between the first reflective surface and the second reflective surface toward the exit window; and a first light measurement device adapted to receive laser light exiting the exit window. According to embodiments, the system can further comprise a smoke collection element, such as a gravimetric filter, in fluid communication with the smoke channel, wherein the smoke collection element is located downstream from the beam extension chamber. 
         [0005]    According to an embodiment, the invention provides a method of measuring the concentration of smoke, comprising: directing a flow of smoke through a smoke channel from a smoke inlet to a smoke outlet, wherein the smoke channel defines a longitudinal axis between the smoke inlet and the smoke outlet; projecting laser light into the smoke channel through an entrance window; deflecting the laser light back-and-forth across the longitudinal axis from the entrance window to an exit window; and measuring the intensity of laser light exiting the exit window. According to embodiments, the method can further include measuring the concentration of the smoke by gravimetric filtering. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing and other features and advantages will be apparent from the following, more particular, description of various exemplary embodiments, as illustrated in the accompanying drawings, wherein like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. 
           [0007]      FIG. 1  is top view of an embodiment of a smoke concentration measurement system. 
           [0008]      FIG. 2  is a side perspective view of an embodiment of a beam extension chamber of  FIG. 1 . 
           [0009]      FIG. 3A  is a side view of an embodiment of a body of the beam extension chamber of  FIG. 2 . 
           [0010]      FIG. 3B  is a cross-sectional view of the body of  FIG. 3A , taken along line  3 B- 3 B. 
           [0011]      FIG. 4A  is a side view of an alternative embodiment of a body. 
           [0012]      FIG. 4B  is a partial cross-sectional view of the body of  FIG. 4A , taken along line  4 B- 4 B. 
           [0013]      FIG. 4C  is an end view of the body of  FIG. 4A . 
           [0014]      FIG. 5A  is a top view of an embodiment of a lateral adjustment bracket for use with the body of  FIG. 4A . 
           [0015]      FIG. 5B  is a side view of the lateral adjustment bracket of  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Various embodiments of the invention are discussed in detail below. While specific embodiments are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations can be used without departing from the spirit and scope of the invention. 
         [0017]    Embodiments of the smoke concentration system described herein can provide an approximately point source measurement of smoke at low concentrations. For example, the system can be implemented to evaluate the performance of smoke detectors in large data centers, however, other applications are possible. 
         [0018]    Conventional measurement of smoke concentration using laser light extinction tend to be limited by the low-end sensitivity of the system, which may be directly related to the optical path length of the system. Some conventional systems have addressed this problem by extending the laser beam across a long path length in the smoke flow. However, this typically results in an area averaged measurement of smoke concentration, which requires an assumption that the smoke concentration is uniform along the optical path length. Embodiments of the system described herein can remove or reduce the need for a long path length in the smoke flow, thereby allowing for an approximately point source measurement of the smoke concentration; thereby substantially nullifying any assumption of the distribution of smoke concentration along the laser path length. 
         [0019]    Embodiments of the system described herein can also provide a secondary measurement of smoke concentration, e.g., to validate the laser light extinction based measurement. The secondary measurement can be acquired using filters (e.g., gravimetric filters) located in the smoke flow, as will be described in more detail below. The secondary measurement can provide increased accuracy over a system that only utilizes a laser light extinction based measurement, for example, due to assumptions that may be required to convert measurements from the laser system (e.g., photodiode voltages) to smoke concentration. According to embodiments, the gravimetric smoke measurements represent a time-averaged smoke concentration over the entire collection period. Comparison of the total smoke measured by the laser system and the gravimetric filter, both over the same time period, can provide a validation of the assumptions required to calculate smoke concentration from the laser system. Embodiments of the invention can provide a compact and portable configuration of a multi-pass laser extinction measurement system, having increased low-end sensitivity and adjustability compared to conventional systems. 
         [0020]    Referring to  FIG. 1 , embodiments of the smoke concentration measurement system  100  can include a laser-light-based meter  102 , as well as a gravimetric-based meter  104 . The laser-light-based meter  102  can include a laser light source  106 , such as a 1.2 mW stabilized Helium-Neon (HeNe) laser, however, other embodiments are possible. According to embodiments, the laser light source  106  can operate at a wavelength of between about 4×10 7  m and 8×10 7  m, e.g., about 6.328×10 7  m, however, other embodiments are possible. The laser-light-based meter  102  can also include a beam extension chamber  108  through which sample smoke flows, a first light measurement device  110 , and a second light measurement device  112 . According to embodiments, the first and/or second light measurement devices  110 ,  112  can comprise silicon photodiodes, however, other embodiments are possible. Although not shown, control system such as a computer, PLC, or other similar device, can receive and process output signals from the first and second light measurement devices  110 . 
         [0021]    Still referring to  FIG. 1 , the laser-light-based meter  102  can further include a beam splitter  114 , such as a non-polarizing beam splitter, that divides laser light output from the laser light source  106  into a measurement beam A and a reference beam B, as will be described in more detail, below. An adjustable mirror  116 , such as a dielectric mirror, can be provided between the beam splitter  114  and the beam extension chamber  108 . According to embodiments, the mirror  116  can be located on an adjustable mount that provides for adjustment of the mirror&#39;s angle a. By adjusting the angle a, the angle at which the measurement beam A contacts the beam extension chamber  108  can be adjusted. 
         [0022]    Still referring to  FIG. 1 , the beam extension chamber  108  can include a body  118  defining a smoke channel  120  (see  FIG. 3B ) defining an inlet  122  that receives gas flow  124  from a test enclosure, e.g., a test fire. A vacuum pump or other structure can be provided to draw the gas flow  124  through the beam extension chamber  108  and other components. The smoke channel  120  also defines an outlet  126  through which the gas flow  124  exits the smoke channel  120 . Embodiments of the beam extension chamber  108  can define a longitudinal axis  128  between the inlet  122  and outlet  126 , as shown in  FIG. 1 . 
         [0023]    Still referring to  FIG. 1 , the beam extension chamber  108  can include first and second reflective surfaces  130 ,  132  located on opposite sides of the longitudinal axis  128 . According to embodiments, the reflective surfaces  130 ,  132  can comprise dielectric mirrors mounted to the body  118 , however, other configurations are possible. The beam extension chamber  108  can also include an entrance window  134  located upstream of the first reflective surface  130 , and an exit window  136  located downstream of the second reflective surface  132 . According to embodiments, the entrance and exit windows  134 ,  136  can comprise substantially transparent substrates, such as AR-coated broadband precision windows mounted to the body  118 , however, other configurations are possible. 
         [0024]    The gravimetric-based meter  104  can include a smoke collection element  140 , such as a filter housing, located downstream of the beam extension chamber  108  to collect smoke samples. According to embodiments, the smoke collection element can removably house a filter or other collection medium. According to embodiments, the filter can comprise a 2 micron quartz filter, however, other embodiments are possible. A pressure gauge  142 , such as a 0-20 psia pressure transducer, can be provided in the gas flow  124  downstream of the extension chamber  108 . Additionally or alternatively, one or more flow gauges  144 , such as a 0-100 L/min mass flow meter, can be provided in the gas flow  124  downstream of the beam extension chamber  108 . The pressure gauge  142  and/or flow gauge(s)  144  can measure and optionally record the flow rate and pressure of the gas flow  124 . Additionally or alternatively, the pressure gauge  142  and/or flow gauge(s)  144  can provide feedback to the vacuum pump in case adjustments in the gas flow  124  are necessary. 
         [0025]    Referring to  FIG. 2 , a heating element  150  can be coupled to the body  118  of the beam extension chamber  108 . For example, the heating element  150  can comprise heat tape provided around all or a portion of the body  118 . To reduce thermophoretic smoke deposition on optical surfaces of the beam extension chamber  108 , the heating element  150  can maintain the chamber  108  at or above the temperature of the sample gas, e.g., at about 20° C. higher than the sample gas temperature. A controller (not shown) including a temperature gauge  151  can adjust and maintain the temperature of the heating element  150 . 
         [0026]    Referring to  FIGS. 1 and 2 , laser light exiting the laser light source  106  is split using the beam splitter  114  into the measurement beam A and the reference beam B. The reference beam B is directed by the beam splitter  114  onto the second light measurement device  112 . Light intensity measured by the second light measurement device  112  can be used as a reference measurement of the light intensity exiting the laser light source  106 . The beam splitter  114  directs the measurement beam A onto the adjustable mirror  116 , which in turn directs the measurement beam onto the beam extension chamber  108 , where the beam can enter the smoke channel  120  through entrance window  134 . 
         [0027]    As shown in  FIG. 1 , once the measurement beam A enters the smoke channel  120 , the beam is reflected back and forth across the smoke channel  120  by the first and second reflective surfaces  130 ,  132 , thereby extending the optical path length of the beam. Once the beam reaches the exit window  134 , it exits the smoke channel  120  and is received by the first light measurement device  110 , as shown in  FIG. 2 . A controller, such as a computer, PLC, or other device can measure the attenuation of the beam, e.g., by comparing the light intensity measured by the first and second light measurement devices  110 ,  112 . The degree of attenuation can then be correlated to a concentration of smoke following through the smoke channel  120  between the entrance window  134  and the exit window  136 . 
         [0028]    The angle at which the measurement beam A enters the smoke channel  120  can be varied by changing the angle a of the adjustable mirror  116 . This adjustment in turn determines the number of passes (mirror reflections) the beam makes between the entrance window  134  and exit window  136 , and therefore determines the overall path length within the smoke channel  120 . According to an embodiment, the optical path length can be calculated based on the number of reflection points on each reflective surface, the perpendicular distance separating the reflective surfaces, and the angle of the beam between the reflective surfaces. 
         [0029]    According to an embodiment, the smoke channel  120  can define a length (e.g., between smoke inlet  122  and smoke outlet  126 ) of between about 2 inches and about 8 inches, for example, between about 4 inches and about 6 inches, and the measurement beam can make between about 15 and 40 passes between the entrance window and the exit window. According to such embodiments, the optical path length (L) can be between about 2 feet and about 8 feet, for example, between about 4 feet and about 6 feet. According to an embodiment having a separation distance (S) of 1.36 inches and a number of reflection points per mirror (N) of 21, the optical path length L is about 4.85 feet. One of ordinary skill will appreciate from this disclosure, however, that other dimensions and configurations than those described above are possible. 
         [0030]    Referring to  FIGS. 3A and 3B , an embodiment of body  118  is shown. In  FIG. 3A , the first reflective surface  130 , in the form of a mirror, is also shown. The body  118  can be constructed of a metal tube, such as steel, having an outer diameter of between one and three inches, e.g., about 1.75 inches, and an inner diameter of between 0.5 and 2 inches, e.g., about 1 inch. One of ordinary skill in the art will understand, however, that other dimensions are possible. The smoke channel  120  can extend longitudinally through the body  118 . Coupling devices, such as threads  152 ,  154  or other structures can be provided to facilitate attachment to upstream and downstream components. 
         [0031]    Opposite sides of the body  118  can include mounting surfaces  160 ,  162  for the reflective surfaces  130 ,  132  and/or windows  134 ,  136 . For example, with reference to  FIG. 2 , one or more mounting brackets  164  (only one shown) can secure the reflective surface  130  and entrance window  134  in place on the body  118 , e.g., using machine bolts  166  or other fasteners. Although not shown, a similar arrangement can be used for the reflective surface  132  and exit window  136 . Referring to  FIG. 3B , longitudinal slots  168  can be provided in body  118  to provide an optical pathway for the measurement beam between the reflective surfaces  130 ,  132 . The slots  168  can be in registry with at least a portion of the reflective surfaces  130 ,  132  and windows  134 ,  136 . According to embodiments, each slot can have a length of between about 3 inches and 6 inches (e.g., about 5 inches) and width of between about 0.24 inches and 0.75 inches (e.g., about 0.4 inches), however, other sizes are possible. Adjustment slots in the mounting holes for the brackets  164  (see  FIG. 2 ), or other similar structures, can be provided to allow adjustment of the reflective surfaces  130 ,  132  and windows  134 ,  136  along the longitudinal axis  124 . This allows the position of reflective surfaces  130 ,  132  and windows  134 ,  136  to be adjusted in response to changes in the angle a of the adjustable mirror  116 , and resulting changes in the number of reflections per mirror (N) and location of the measurement beam&#39;s entrance point and exit point from the smoke channel  120 . 
         [0032]    Referring to  FIGS. 4A-4C , another embodiment of the body  118  is shown. Body  118  is substantially similar to the version described in connection with  FIGS. 3A ,  3 B, with differences described herein below. Referring to  FIGS. 4A and 4B , the mounting surfaces  160 ,  162  can each be divided into multi-planar portions to allow for independent mounting and adjustment of the reflective surfaces  130 ,  132  and windows  134 ,  136 , respectively. For example, as shown in  FIG. 4B , the mounting surface  160  can be divided into a primary surface  160 A for mounting the first reflective surface  130 , and a secondary surface  160 B, located on a different plane, for mounting the entrance window  134 . The secondary surface  160 B can have a larger area than the primary surface  160 A, as shown, however, other variations are possible. The mounting surface  162  can have a similar configuration to surface  160 , as shown in  FIG. 4B . The windows  134 ,  136  and reflective surfaces  130 ,  132  can include slotted mounting holes to allow for longitudinal adjustment, as described previously in connection with  FIGS. 3A and 3B . 
         [0033]    As shown in  FIG. 4C , the body  118  can include a longitudinal slot  168  extending along all or a part of its lower surface. The longitudinal slot  168  can be dimensioned to receive a lateral adjustment bracket  169 , shown in  FIGS. 5A and 5B . The lateral adjustment bracket  169  can include mounting holes  170  for securing the bracket  169  to a work surface. The bracket  169  can also include slotted holes  172  for securing the bracket  169  to the body  118 , e.g., using screws extending into holes (not shown) in longitudinal slot  168 . The slotted holes  172  can allow the position of the body  118  to be adjusted along the length of adjustment bracket  169 , e.g., for accurate positioning of the body  118  with respect to the measurement beam. As shown in  FIGS. 4A and 4B , the ends of body  118  can be round to receive heating elements on either end of the beam extension chamber  108 . The heating elements can be wired in series to a controller with a thermocouple mechanically fastened to the top of the chamber, to control the temperature of the beam extension chamber  108 . 
         [0034]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. For example, the invention can be applied to the measurement of many other particulates in an air stream and is not limited to the measurement of smoke. Thus, the breadth and scope of the present invention should not be limited by any of the above-described embodiments, but should instead be defined only in accordance with the following claims and their equivalents.