Abstract:
An airflow measuring assembly and method for an air handling system, such as an HVAC system. The assembly is constructed and arranged to direct a measurable air stream to quantify airflow through an air handling system. The assembly includes a plate structure with at least one aperture and airflow sensors. The assembly is adapted for placement in a slot of an air handler and may simulate the flow resistance of HVAC filters. The airflow sensors may be positioned with respect to the plate structure to provide averaged total and static pressure signals. A manometer may be connected to the assembly to provide pressure differential readings and which may be converted via mathematic formulas to provide the volumetric airflow rate through the air handling system.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/127,117 filed on Mar. 31, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to airflow measuring assemblies and particularly to airflow measuring assemblies and methods to measure airflow in air handling equipment. The assemblies of the invention are constructed and arranged for placement in an air handling system and to condition or direct the airflow of the system into a controlled fluid stream for analysis. The measuring assemblies and methods provide accurate and reproducible airflow measurements for the air handling equipment being monitored or tested. More particularly, the assemblies of this invention are constructed and arranged for use in forced air HVAC distribution systems. The airflow measuring assembly is constructed to be placed in a predetermined location of a forced air distribution system, for example, in a slot proximate the air handler unit. 
     It is important for air handling equipment, such as furnaces, heat pumps and air conditioners to have proper airflow to insure efficient operation of the HVAC systems. Specified airflow or air volume rates are required to be within specified airflow ranges for particular air handling systems. The flow of air is typically measured to insure that a forced air distribution system, for example, was properly designed and installed, and is operating according to the specification. For example, it is important to avoid low airflow in heat pumps, furnaces and air conditioners because reduced airflow results in performance inefficiency and can damage the compressor. Determining an unusually low or high airflow may indicate that a leak in the system or insufficient ventilation exists. 
     A forced air distribution system typically includes an air handler unit, ductwork, registers, dampers, filters, etc. Although the airflow measuring assembly may be utilized in connection with any air handler unit, which contains the blower, the assembly may also be utilized at any location within the air distribution system. Preferably, the assembly is used in the filter position of the air distribution system. Depending upon the similarity of flow resistance between the assembly and the filter typically installed in the air distribution system, a correction factor is calculated and used in the method of the invention. 
     Airflow is preferably measured, however, proximate the air handler unit of the air handling assembly, where proper airflow is important for efficient operation of the equipment. Although the airflow measuring assembly may be positioned, permanently or removably, at a number of locations in an air handling assembly, it has been found that the air filter slot typically provided in air handling equipment provides an efficient, easy and convenient place for such airflow measurement. However, it is within the purview of the invention to utilize other positions or specified slots in an air handling assembly to make such airflow measurements. Thus, the airflow measuring assembly of the invention may be pre-installed in a furnace proximate the blower, may be positioned for use in a specified slot across the airstream, may be positioned in the filter slot of air handling equipment and in the filter slot of a forced air distribution system, whether proximate the blower unit or remote therefrom. 
     In residential furnaces, for example, a specified volume of air is heated and distributed throughout a house and is returned to the furnace by the air return duct system. It is desirable to measure the flow of air through the furnace, in order to insure airflow within the specified operating range of a furnace. The airflow measuring assemblies of this invention measure airflow in a furnace, such as at the filter position, and are constructed to be placed into the furnace filter slot or a similar position in the furnace. This placement has been found to provide a convenient, accessible location for the accurate air volume flow rate measurements. The airflow measuring assemblies are constructed and arranged to condition and direct the airflow therethrough and to measure the airflow via the use of an anemometer, for example, or by measuring a pressure signal generated by the assembly. In the latter assembly, a differential pressure signal generated from single or multiple designed pressure signal locations within the controlled fluid stream can be obtained and converted into volumetric airflow utilizing mathematical relationships. The assemblies of the invention may also be constructed and arranged to simulate the resistance of a typical filter for accuracy and reproducibility of the airflow measurements. Furthermore, a correction factor based on air handler system pressures measured with a typical filter, and again, with the assembly can be derived for improved accuracy. The correction factor accounts for the difference in airflow resistance between the test assembly and filter and is used to correct the measured airflow of the system by the test assembly. 
     Although the utilization of the airflow measuring assembly of the invention at or near the air handler may be preferred, the assembly may also be used at other locations of an air handler assembly. For example, filter holders may be built into ductwork or positioned at the return grille of the system. If the assembly is installed remotely from the furnace or air handler, duct leakage between the assembly and the air handler may exist and needs to be taken into consideration. Further, if the pressure drop across the filter is significantly different from that of the airflow measuring assembly, this difference is taken into consideration by calculating a correction factor. 
     Various airflow measuring assemblies and methods to estimate volumetric airflow have been proposed and used in the past, however, these prior art assemblies and methods have drawbacks and difficulties. Prior art methods and devices have often been found to be burdensome, difficult to use and yield inaccurate and unrepeatable results for purposes of measuring airflow in air handling equipment. For example, prior art devices typically require more care in proper use to obtain accurate results than installers or others have time to provide. Prior art assemblies and techniques include the utilization of heating elements in an air stream and a calibrated fan assembly used to deliver air to the return side of the air handler. However, accuracy in result and the time consumption required, to utilize these prior art techniques are undesirable for general testing usage. Other prior art assemblies use pitot tubes or like devices to measure air velocity pressures at various locations in a system and convert the measurement to volumetric airflow. Other prior art assemblies measure airflow at air outlets by trapping air in large capture hood devices. Capture hood devices are often difficult to seal over registers and return grilles due to obstructions and their respective locations, i.e., due to furniture, under cabinet locations, etc. And, because capture hood devices measure airflow at the registers and return grilles, any duct leakage is not accounted for when airflow at the air handler is desired. 
     Still other prior art devices attempt to have as little effect as possible on air flow by minimizing the pressure loss through the measuring device. The latter devices tend to be very sensitive to upstream conditions and require long lengths of straight duct to be accurate. Such lengths of straight duct often do not exist in actual installed systems. On the other hand, the assembly and method of this invention have been found through experimentation to be insensitive to upstream conditions. In summary, known prior art assemblies and methods are difficult and cumbersome to use and do not necessarily provide accurate airflow measurement at the air handler, where such measurement is most useful and yield more accurate and reproducible results. 
     Prior art assemblies, as far as is known, have not been developed to measure airflow at the air filter location of an air handler, for example. The airflow measuring assembly of the present invention overcomes the difficulties in use and inefficiencies of the prior art. The measuring assemblies of the present invention provide fast, easy and accurate means to measure the airflow in a forced air HVAC system or the like by placement of the assembly into the filter section or a similar position within the system. 
     Another benefit of the measuring assembly of the present invention is that it permits a more accurate derivation of duct efficiency. Duct efficiency equations are generally dependent upon airflow through the air handler. The accurate measurements provided by the present invention thereby yield more accurate calculations of duct efficiency. 
     It is a further object of this invention to provide airflow measuring assemblies which are adjustable in size to accommodate various air handler filter slot dimensions. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an airflow measuring assembly and method which conditions and directs a fluid stream for analysis. The invention further relates to an airflow measuring assembly having airflow sensors for providing a differential pressure signal to determine volumetric airflow in air handling equipment. The assemblies are constructed and arranged to be positioned into a filter slot, for example, or other position of an air handler system. The assembly in a filter slot preferably approximates the filter resistance in conditioning the airflow to provide easy and accurate airflow measurements. Airflow correction factors are calculated in the method of the invention. Correction factors either based on filter parameters or measured at the time of testing may be utilized. For example, charts or tables with empirical data may be developed with correction factors for various models and sizes of filters. Alternatively, to correct for the change in airflow at the time of testing, a representative static pressure in the air handling system is measured both when the filter is in place and again when the measuring assembly is in place. For example, for two static pressure readings P 1  and P 2 , respectively, the corrected flow is the flow through the test assembly multiplied by the square root of P 1 /P 2 . 
     The airflow measuring assemblies of the present invention are comprised of a plate structure having at least one aperture therethrough to precondition and direct an airflow stream for measurement. The plate structure is preferably adjustable in area to accommodate various air handler slots, such as filter slots, or duct sizes. The plate structure is constructed so that inserts or extensions may be added to the assemblies to insure a proper fit and minimal leakage. The plate structure may have a plurality of apertures with airflow sensors and may utilize various sensor and aperture patterns to precondition and direct airflow and to approximate filter resistance to thereby provide accurate and measurable airflow streams. 
     The airflow measuring assembly of the invention is constructed and arranged to provide a pressure differential measurement comprised of an upstream pressure signal and a downstream pressure signal which are generated from designed pressure signal locations in the controlled airflow stream. The upstream pressure signal may be equated to the total pressure in the air stream before going through the conditioning aperture of the assembly. The downstream pressure signal may be equated to the static pressure after conditioning, which depends on the volumetric flow rate, plate aperture geometry, fluid density and viscous losses. The differential between the total pressure and the static pressure is defined as the dynamic pressure. In this case, the readily measurable dynamic pressure correlates to the airflow of the fluid distribution system. 
     Airflow measuring means or air pressure sensors are positioned within each plate aperture to measure airflow. For example, an anemometer may be used to measure airflow and alternatively, total and static pressure sensors, which may be interconnected to provide an average value of the respective pressure signals, may also be used to provide the dynamic air pressure and thus provides the airflow measurement. In the latter method, apertures in the air pressure sensors to measure total and static pressure are disposed behind the plane of the plate structure. The assemblies of the latter invention are provided with connectors for attachment to a differential pressure manometer. The manometer provides the difference between the total and static pressure signals and from which airflow may be calculated. 
     To provide an accurate and repeatable volumetric airflow rate for air handling equipment is the primary object of the present invention. The flow plate provides a controlled fluid flow stream which may be measured by various sensors which can be converted to volumetric airflow. For example, using a mathematical relationship, the differential pressure signals may be converted to volumetric airflow. In summary, the volumetric airflow rate provided by the present invention aids in verifying that the air handling equipment was properly designed and installed and is operating within the airflow range specified for efficient use. The volumetric airflow rate can be used as inputs to calculate the duct and/or furnace efficiency. 
     These and other benefits of this invention will become clear from the following description by reference to the drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional representation of air handling equipment and showing a filter slot therein; 
     FIG. 2 is an elevational view of the inlet side of an embodiment of the airflow measuring assembly of this invention; 
     FIG. 3 is a rear elevational view of the airflow measuring assembly of FIG. 2; 
     FIG. 4 is a front perspective view of the airflow measuring assembly of FIG. 2; 
     FIG. 5 is an enlarged view of the pressure fitting assembly of the airflow measuring assembly of FIG. 2; 
     FIG. 6 is a rear perspective view of the airflow measuring assembly of FIG. 2; 
     FIG. 7 is a front view of an airflow measuring aperture of the airflow measuring assembly of FIG. 2; 
     FIG. 8 is a lateral perspective view of another alternate embodiment of the airflow measuring assembly of this. invention; 
     FIG. 9 is a sectional view of the airflow measuring assembly of FIG. 8; 
     FIG. 9A is an alternate embodiment of the airflow measuring assembly of FIG. 9; 
     FIG. 10 is a front plan view of the airflow measuring sensor of FIG. 8; 
     FIG. 11 is a sectional view of the airflow measuring sensor of FIG. 10; 
     FIG. 12 is a sectional view showing an alternate embodiment of an airflow measuring sensor; 
     FIG. 13 is a sectional view showing an airflow measuring assembly utilizing hot wire anemometers to measure airflow through the plate apertures; and 
     FIG. 14 is a front elevational view showing an adjustable airflow measuring assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to an airflow measuring assembly which directs a controlled fluid stream for measuring airflow at a predetermined position of air handling equipment, i.e., in forced air HVAC distribution systems. The measuring assembly of this invention is particularly efficient and accurate in permitting the volumetric airflow and the efficiency of a duct system and/or furnace to be calculated. 
     The airflow measuring assemblies of this invention are designed to include use in forced air distribution systems. A typical forced air furnace will be used herein to describe the construction and use of the airflow measuring assemblies of the invention. A furnace, such as a forced air heating system, pulls air at room temperature by means of a blower into the heat exchange area of the furnace and then sends conditioned air through a duct and register distributing system of a house or building. Air return ducts channel air back to the furnace using the pulling force of the blower, which drives the air through the air filter typically positioned in the slot of the furnace. 
     Referring to FIG. 1, a representation of air handling equipment, such as a furnace  12  is shown. The furnace  12  is shown comprised of return plenum  13 , filter slot  11 , blower  14 , supply plenum  15 , duct  16  and return air duct  18 . The blower  14  is positioned to pull cool air from return plenum  13  through filter slot  11 . The output air  19  is conditioned (heated, cooled or filtered) by a heat exchanger or the like and pushed by blower  14  through supply plenum  15  and into duct  16  to registers (not shown). The return air  17  is pulled by blower  14 , enters the furnace  12  through return air duct  18 , and flows into return plenum  13  and through the filter slot  11 . The airflow assemblies of the present invention are constructed and arranged for positioning and use in filter slot  11 . Filter locations in air handler equipment may be located in various system locations, i.e., the return plenum or a return grille, for example. 
     FIG. 2 shows a front view of airflow measuring assembly  10 . Assembly  10  is shown comprising a plate structure  20 , a total pressure measuring assembly  51 , an airflow measurement apertures  54 , and having plate apertures  55 . The apertures  55  are of a predetermined size and, although shown to be circular, may be of other configurations. Measurement structure  51  having apertures  54  can be seen through apertures  55  of the front  21  of plate structure  20 . FIG. 3 is a rear view of airflow measuring assembly  10  and the plate structure  20  is shown having a rear side  22  and apertures  55 . Measurement structures  51  and  52  are shown attached to the rear side  22  of plate structure  20 . 
     Referring to FIGS. 4-7, airflow measuring assembly embodiment  10  is further shown. Airflow measuring assembly  10  is shown comprised of plate structure  20 , a total pressure measuring structure  51 , static pressure measuring structure  52  and a connection assembly  56 . Measuring structures  51  and  52  are shown generally as E-shaped tubing structures and are constructed and arranged to provide averaged total pressure and the static pressure signals, respectively. Measuring structures  51  and  52  are positioned and attached behind plate structure  20 . Measuring structure  51  contains airflow measurement apertures  54  which are preferably front-facing and located in the center of apertures  55  of the plate structure  20 . Apertures  54  permit air pressure to act on the total pressure measuring assembly  51  at several predetermined locations of plate  20 . Through the apertures  54  air pressure acts on structure  51  where the individual pressure signals are combined with those of the other apertures  54 . This arrangement provides an average total pressure reading from structure  51 . Apertures  53 , similar to apertures  54 , are provided with respect to static pressure measurement structure  52 . The apertures may be positioned on the top, bottom or rear of the tubing structure  52 . Thus, apertures  53  may be positioned within a 180 degree range in the tube structures  52 . Through these individual apertures  53  air pressure in the structure  52  is combined or averaged to provide an average static pressure signal. 
     As particularly shown in FIGS. 4-6, connection assembly  56  provides averaged total and static pressure signals or measurements from structures  51  and  52 , respectively. Tubes  59  and  60  having terminal fittings  61  and  62  extend from connection assembly  56 . Connection fittings  61  and  62  are attachable to a manometer, or the like, to provide the difference between the total and static pressure signals. FIG. 5 shows the connection assembly  56  of embodiment  10  comprised of total pressure tubing  59  and static pressure tubing  60  extending through apertures  57  and  58  in plate structure  20 . Tubing  59  extends from coupling  64  of structure  51  and provides the averaged total pressure. Tubing  60  extends from a similar coupling of structure  52  and provides the averaged static pressure. As shown in FIG. 4, averaged total and static pressure signal tubing  59  and  60  extend from connection assembly  56  and terminate at manometer connection fittings  61  and  62  which connect to a manometer to provide the total and static pressure signal difference. 
     Referring to FIG. 6, a rear view of airflow measuring assembly  10  is shown comprising plate structure  20 , averaged total and static measurement structures  51  and  52 , connection assembly  56  and apertures  55  in plate  20 . Measurement structures  51  and  52  are shown attached to the rear of plate structure  20  by fastening brackets or fastening clips  46 , however, the structures may be attached by any other known means. 
     Particularly, FIG. 7 shows an airflow measurement aperture  54 . Air flows through aperture  54  and into total pressure measurement structure  51 . In measurement structure  51  air pressure from all apertures  54  combine to form an averaged total pressure which is measured. As shown in the drawings, apertures  54  are positioned in generally the center of a concave or hemispherical indentation  63  of rigid tubing structure  51 . The concave indentation  63  creates a total or stagnated pressure zone which is relatively insensitive to oncoming fluid streamline directions or angles. 
     FIG. 8 shows an alternate embodiment  23  of an airflow measuring assembly comprised of frame structure  24 , front plate  26  and airflow sensors  25 . Airflow sensor assemblies  25 ,  49  and  41  are further shown in FIGS. 9-12, and are further described below. Embodiments  10  and  23  show a variety of airflow sensor patterns that may be utilized in the assemblies of the present invention. The airflow sensors and the pattern utilized in the assembly may simulate the resistance of the normal air filter used in the furnace. Sensors  25  are shown arranged in airflow measuring assembly  23 . Front plate  26 , center plate  27  and rear plate  28  are shown arranged and held in frame structure  24 . Plenum  35  is shown formed by front plate  26  and center plate  27 . Plenum  36  is shown formed by center plate  27  and rear plate  28 . Connector or pressure fitting  37  is shown extending from frame structure  24  and also extends into plenum  35  of assembly  23 . Connector or pressure fitting  38  is also shown extending from frame structure  24  and extends into plenum  36 . The connector or pressure fittings  37  and  38  are preferably constructed and arranged to receive a differential pressure manometer to read the pressure difference signal generated from assembly  23 . 
     FIG. 9 shows a sectional view of sensors  25  of airflow measuring assembly  23 . Plenums  35  and  36  are shown formed by front plate  26  and center plate  27 , and by center plate  27  and rear plate  28 , respectively. A tubular member  31  is shown spanning across the sensor  25  to interconnect plenum  35  and the member  31  in FIG. 9 is shown located near the front  29  of sensor  25 . An aperture  32  is shown positioned in the front of each tubular member  31 . Side apertures  33  and  34  are shown in communication with plenum  36  and are shown in FIG. 9 to be positioned in the sides  30  of airflow sensor  25  behind tubular member  31  and near the rear  40  of sensor  25 . Airflow depicted by arrow  39  is shown directed through the airflow sensor  25 . Aperture(s)  32 , preferably having an indented area thereabout, provides an averaged total pressure to be obtained from plenum  35 . The side apertures  33  and  34  in airflow sensor  25 , in communication with plenum  36 , permits and averaged static pressure reading to be obtained. The difference between the averaged total and static pressure signals is the dynamic pressure or the pressure difference of the assembly  23  and may be obtained at connectors  37  and  38  as shown in FIG.  8 . 
     In FIG. 9A, the airflow measuring assembly embodiment  50  is shown to have an airflow sensor  49  which does not have side apertures, unlike the structure of sensor  25 . Instead, rear plate  48  is shown to have an aperture  47  which causes plenum  36  to provide an averaged static pressure. 
     Referring to FIG. 8, the two connectors, such as pressure fittings  37  and  38 , are adapted to be attached to a manometer and are in communication with plenums  35  and  36 . As shown in FIG. 9, plenums  35  and  36  receive the total pressure and the static pressure, respectively, of the air flowing through assembly  23  of the present invention. Therefore, the manometer can read the dynamic pressure across the assembly. The dynamic pressure is important because when known, the volumetric airflow can be determined. Dynamic pressure, ΔP, and volumetric flow rate, Q, are related by the following equation: 
     
       
         Q=KΔP n   
       
     
     where K is a constant and n typically has a value of 0.5. K depends on the density of the fluid, the area of the apertures in the plate structure, the relative spacing of these apertures, the viscosity of the fluid, etc., and is determined upon the calibration of the measuring assembly. For example, with a pressure difference of 20 Pascals and a K value of 137 ft 3 /min/Pa 0.5  yields an airflow of approximately 613 ft 3 /min. 
     FIG. 10 shows a front view of pressure sensor  25 . Tubular member  31  extends along a diameter of sensor  25 . Aperture  32  is preferably located on the front and in the center of tubular member  31 . The front  29  and annular side wall  30  of pressure sensor  25  can be seen. As shown in FIG. 11, tubular member  31  runs across sensor  25  within annular side wall  30 , and between front plate  26  and center plate  27 . Aperture  32  is shown contained on tubular member  31 . Side aperture  33  is shown on sensor side  30  between center plate  27  and rear plate  28 . 
     FIG. 12 shows an alternate embodiment  41  of an airflow sensor. Like embodiment  25 , embodiment  41  is constructed and arranged to be contained in and to communicate with plates  26 - 28 . Embodiment  41  comprises annular side wall  44  and tubular member  31  with front-facing aperture  32 . Side aperture  45  is contained between front plate  26  and center plate  27  and leads to plenum  42  which contains the static pressure. Tubular member  31  runs within annular side wall  44 , between center plate  27  and rear plate  28 , and is in communication with plenum  43  which permits a total pressure measurement. In alternate embodiment  41 , the side apertures are contained in the annular side wall nearer the front of the sensor than the tubular member. 
     The airflow through the apertures may be determined by any means known in the art. For example, hot wire anemometers may be utilized, as shown in alternate embodiment  65  of the airflow measuring device of this invention. FIG. 13 shows embodiment  65  having anemometers  69  and  70  which are positioned across each aperture  67  of the plate structure  66 . Each anemometer  69  is preferably a hot wire anemometer comprising an electrically heated fine wire of platinum for example. The wire of the anemometer is exposed to the air traveling through each aperture. An increase in airflow cools the wire and changes its electrical resistance. In a constant-current anemometer, air velocity is determined by measuring the wire resistance whereas in a constant-resistance anemometer, air velocity is determined by measuring the current required to maintain the wire temperature, and thus the resistance constant. Either type of anemometer, which differ primarily in electric circuitry and instruments utilized, may be used in accordance with the teachings of the present invention. Other anemometer structures, such as rotating vane, swinging vane, vortex shedding and the like, may also be utilized in the teachings of this invention. 
     As shown in FIG. 14, the airflow measuring assemblies of the present invention may have adjustable features to enable the assemblies to be placed into various size furnace slots. The adjustable assembly  71  is shown to have a plate structure  72  having a plurality of plate apertures  73  with airflow sensors  74  positioned with respect to each aperture. The assembly  71  is shown to have a top insert or extension  75 , a bottom insert or extension  77  and side inserts or extensions  76  and  78 . The insert or extension members  75 - 78  may be attached to the plate structure  72  in any manner known in the art. For example the extension members may be hinged, slid or attached to the plate structure  72 , the important feature being that air is directed only for flow through the apertures  73  in plate structure  72 . Further shown are comer inserts or extensions  79 ,  80 ,  81  and  82 . The latter inserts may be attached to inserts  75 - 78 , respectively, to thereby completely fill the furnace slot area and to thereby direct airflow through the plate apertures  73  for measurement. For example, insert or extension  79  may be hinged to or slid from either insert  75  or  76 . 
     The adjustable assembly  71  may therefore be utilized in a range of filter slot sizes. As previously discussed, the airflow measuring assemblies of the present invention may be utilized in sizes small, medium and large. The inserts or extensions  75 - 78  and the comer inserts or extensions  79 - 82  may be incorporated into each of the three sizes to thereby permit all filter slots having dimensions between these sizes to be effectively tested for airflow measuring purposes according to the teachings of this invention. 
     In the method of this invention for measuring airflow in an air handling system, first the filter is checked to ensure it is clean. Next a static pressure probe is inserted at a specified location in the air handling system, for example in the downstream corner of the supply plenum. The air handler is then turned on and the static pressure P 1  is measured. Next, the filter is replaced by the test assembly, the air handler is turned on and the static pressure P 2  is measured at the same specified location. The airflow is now measured through the test assembly. The measured airflow is corrected by multiplying the measured airflow by the calculated correction factor, for example, as described above, and the airflow moving through the air handling system with the filter in place is thereby determined. 
     In summary, the airflow measuring assemblies of the present invention are constructed and arranged to be positioned in a forced air distribution system, such as incorporated into the system, in the filter slot or a similar slot. Airflow sensors or apertures can be arranged in a variety of configurations, sizes and patterns in an airflow measuring assembly in order to approximate the resistance of a filter in a furnace. This simulated flow resistance permits the airflow measuring assembly to provide accurate readings of the airflow through the filter slot of an air handler and which can be used to estimate duct efficiency. The airflow sensors of the present invention are shown comprised of a tubular body which may be molded of a plastic composition or other suitable materials. The assembly frames and plates may by constructed of a plastic, a metallic or other composition which is impervious to airflow. The assembly frame and plate structures are constructed and arranged in small, medium and large size ranges in order to fit into various filter slots. Also, inserts may be added to the sides or tops of the assemblies to provide a proper fit with minimal air leakage. 
     As many changes are possible to the embodiments of this invention, utilizing the teachings thereof, the description above and the accompanying drawings should be interpreted in the illustrative and not the limited sense.