BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for the measurement of fluid flow through a conduit, and more particularly, concerns an apparatus and method for the measurement of air flow through a conduit which accounts for measurement errors due to changes in ambient temperature and caused by velocity cooling of the measurement elements within the conduit. 
     2. Description of the Prior Art 
     In many instances where fluids flow through a conduit, tubing or the like, it is desirable to know the velocity of the moving fluid. For purposes of the ensuing discussion, as well as the scope of the present invention, the term &#34;fluid flow&#34; includes liquid flow as well as gas flow, particularly airflow. In conduit units useful for heating or air conditioning purposes, the velocity of airflow through the ductwork is an important parameter in determining the efficiency and proper operability of the system. Accordingly, a reliable technique for measuring airflow through these heating, air conditioning or like airflow systems is desirable to obtain this important characteristic. Moreover, the measurement of the airflow through the conduit should be accurate, as well as reliable. 
     Early techniques for determining airflow through a conduit relied upon an instrument known as an anemometer. The anemometer relied upon a windmill effect with a series of cups extending from a rotatable axle. Air captured by the cups would cause the cups to spin thereby rotating the axle, the speed of which could be measured to correspond to velocity of the air moving through the system. For heating, air conditioning or the like ductwork systems, however, a mechanically operated instrument such as an anemometer would not always be feasible or practicable. 
     Early electronic anemometers deviated from the mechanical spinning technique and relied upon electrical resistance for measuring airflow. For example, an electrically conductive wire would be positioned within the conduit and exposed to the air flowing therethrough. Passing an electrical current through this wire would cause the wire to increase its temperature. Since the airflow through the conduit causes a cooling effect once air passes the wire, a drop in temperature of the wire would result. Concurrent with this drop in temperature of the wire would be a change in electrical resistance of the wire. This change in electrical resistance could be related to the velocity of air passing the wire. Such an early anemometer had a number of deficiencies, including only point measurement within the cross-sectional plane of the conduit, and no compensatory factors which took into account the temperature of ambient air or the like. 
     An improved electronic anemometer was described by Pollack in the February 1957 edition of Popular Electronics. Instead of an electrically heated wire element, the improved electronic anemometer relied upon a modified Wheatstone bridge in which two thermistors formed a part of the bridge and were closely matched in nominal resistance so that the bridge could be balanced. Current flowing through the thermistors caused them to heat; one of the thermistors was exposed to moving air, which caused the thermistor to cool and its resistance to rise sharply. Accordingly, the Wheatstone bridge balance was changed, causing the meter to indicate a reading. The amount of the meter deflection was determined by the magnitude of the resistance change, which, in turn, was a function of airflow velocity. 
     While these electronic anemometers are able to measure airflow, accuracy, nevertheless, remained a problem. Some of these problems were addressed in patent application, U.S. Ser. No. 452,790, filed in December 1982, now issued as U.S. Pat. No. 4,494,406, wherein one of the inventors of that application is the inventor of the present invention. The problems addressed in the afore-mentioned patent application included the use of a plurality of measuring resistors, connected in electrical series, and positioned at various locations in a given plane across the fluid flow conduit. Such a resistor arrangement purportedly improves accuracy since many locations of the flowing air are sampled to obtain the velocity determination. Another area addressed by the aforementioned patent application is in the use of a reference resistor sensitive only to changes in ambient temperature. The resistance value of the reference resistor is subtracted from the resistance of the measuring resistors to produce a resistive value in terms of fluid flow. 
     While these improvements in the measurement of airflow through a conduit are noteworthy, problems still persist. Most significantly, the prior art has failed to recognize that the measurement of airflow is a function of the rate of cooling of the heated resistor element by the moving air. Further, the ambient temperature affects the cooling rate of the resistor element as well as changing its resistance due to its temperature versus resistance characteristics. The afore-mentioned Wheatstone bridge circuit compensates only for the latter phenomenon, i.e., airflow at constant temperature. In this regard, the effects of increased rates of heat transfer related to the combined effects of temperature and airflow (velocity) are not accounted for in the prior art devices. As a result, errors in measurement occur with changes in ambient temperature. 
     Other airflow measurement apparatuses are described in U.S. Pat. Nos. 4,214,478; 4,196,622; and 3,951,113. These patented airflow measuring apparatuses also fail to account or compensate for the combined effects of temperature and airflow cooling on the measuring elements. 
     It is, therefore, the purpose of the present invention to provide an apparatus and method for the measurement of airflow through a conduit which accounts for and compensates for the combined effects of temperature and airflow cooling of the measuring element. 
     SUMMARY OF THE INVENTION 
     The apparatus of the present invention measures fluid flow through a conduit. At least one thermally sensitive element is provided which is capable of producing a measurement value related to temperature located within the conduit. This element is exposed to the flow of fluid through the conduit. This apparatus includes means for producing a reference value related to temperature of fluid ambient to the conduit. The reference value is substantially unaffected by heat generated by the producing means itself. Means determines the measurement and reference values and uses these values to produce an indication which substantially eliminates errors due to changes in the ambient temperature and caused by velocity cooling of the thermally sensitive element. The indication is representative of fluid flow through the conduit. 
     In a preferred embodiment of the aspect of the invention, the apparatus measures airflow through a conduit and includes at least one thermally sensitive electrical resistor serving as a measuring resistor located within the conduit and exposed to the flow of air therethrough. A housing, associated with the conduit, encloses a thermally sensitive electrical resistor serving as a reference resistor. The volume and material construction of the housing are sufficient to permit natural convection to occur during operating temperatures of the reference resistor. Air temperature within the enclosed housing is able to be maintained at a level substantially equal to air temperature ambient to the housing. An electrical circuit means is provided for determining the resistance values of the measuring resistor and the reference resistor. This circuit means produces a temperature- and airflow-compensated voltage output based on the resistance values. The voltage output is representative of airflow through the conduit. 
     Another aspect of the present invention is a method for the measurement of fluid flow through a conduit. This method includes determining a first value related to the temperature of fluid flowing through the conduit and determining a second value related to the temperature of fluid ambient to the conduit. The first and second values are used to produce an indication which substantially eliminates errors due to changes in ambient temperature and caused by velocity cooling within the conduit. The measurement method further involves relating the indication to a value of fluid flow through the conduit. 
     In accordance with the principles of the present invention, measurement of airflow through a conduit is reliably and accurately accomplished. Factors either ignored or not appreciated by the prior art, which have a negative influence particularly on the accuracy of measurement, have been identified and utilized in the present invention. Specifically, the measuring resistance element, preferably a thermistor, exhibits a change in electrical resistance with a change in temperature. It has been found that ambient temperature affects the cooling rate of the thermistor as well as the fact that its resistance changes due to its inherent temperature versus resistance characteristics. 
     By devising an electrical circuit which accounts for these temperature and airflow cooling effects, and to compensate for these effects, the accuracy and reliability of airflow measurement is markedly enhanced. In systems such as heating or air conditioning conduits, such enhanced accuracy and reliability of airflow measurement will improve the efficiency and operating performances of such systems. In addition to the novel electrical circuitry features of the present invention, ambient air conditions are accounted for by the unique approach in housing the reference resistor element. By including the reference resistor in an enclosure of sufficient volume whereby natural convection currents simulate an infinite volume of still air, and due to heat transfer by convection and conduction to the ambient surroundings, compensation is made for the effects of ambient temperature on the cooling rate of the thermistor. The features of the present invention are well-adaptable to a variety of airflow systems, such as mentioned above, and are also suitable for liquid flow systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a typical section of ductwork for heating or air conditioning illustrating one arrangement of the measuring and reference resistors of the present invention; 
     FIG. 2 is an enlarged cross-sectional view of the flow cell of the present invention which encompasses a measuring resistor, taken along line 2--2 of FIG. 1; 
     FIG. 3 is an end view of the flow cell of the present invention, illustrated without the screen to smooth turbulent airflow; 
     FIG. 4 is an enlarged cross-sectional view of the reference cell of the present invention which encompasses the reference resistor, taken along line 4--4 of FIG. 1; 
     FIG. 5 is a graphic representation of the resistance versus airflow characteristics of a thermally sensitive electrical resistor, illustrating the varying characteristics at different temperatures; 
     FIG. 6 is an electrical circuit diagram of one suitable circuit in accordance with the principles of the present invention, depicted with only one measuring resistor; and 
     FIG. 7 is a schematic electrical circuit illustrating an arrangement for a network of measuring resistors to obtain an average of the effects of airflow in the conduit. 
    
    
     DETAILED DESCRIPTION 
     While this invention is satisfied by embodiments in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention, with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and is not intended to limit the invention to the embodiment illustrated. The scope of the invention will be measured by the appended claims and their equivalents. 
     Adverting to the drawings and FIG. 1 in particular, there is illustrated a fluid movement conduit 10. This conduit, for example, may be a section of ductwork commonly known and available for heating or air conditioning purposes. Air is caused to flow through conduit 10 in the direction of the arrows illustrated in FIG. 1. A pump, blower or the like (not shown) generally is provided in a central location to cause air to flow through the conduit. 
     In order to accurately measure the airflow through the conduit, the preferred embodiment of the present invention includes a plurality of flow cells 12, each of which includes a measuring resistor, and a reference cell 14. It can be seen in FIG. 1 that conduit 10 has been divided into four compartments by virtue of a vertical plate 15 and a horizontal plate 16. Two of flow cells 12 are mounted on the vertical plate, in different locations, while the other two flow cells are mounted in different locations along one of the walls of the conduit. Brackets 18 are provided to maintain the flow cells and the reference cell in position within the conduit. By dividing the cross-sectional plane of conduit 10 into four compartments, airflow can be measured at four different places so that the average airflow can be determined. It is understood that the inclusion of four flow cells for measuring purposes is merely suggestive of one embodiment of the present invention, and that there may be fewer or greater numbers of flow cells for airflow measurement. In fact, the principles of the present invention are satisfied by the inclusion of only one flow cell with a measuring resistor along with one reference cell with a reference resistor. 
     While it is preferred to include reference cell 14 inside conduit 10 as illustrated in FIG. 1, it is also feasible to mount reference cell 14 outside of the conduit. Of course, if the reference cell is outside of conduit 10, it will be sensing ambient air conditions which are different from the ambient air conditions inside the conduit when air is flowing therethrough. Therefore, since ambient air conditions affect the measurement of airflow, it is preferred to position the reference cell within the air flowing inside the conduit. 
     Turning now to FIGS. 2 and 3, the details of flow cell 12 are illustrated. Flow cell 12 preferably includes a cylindrically shaped flow tube 20 oriented within conduit 10 so that air may flow into a first open end 21 and out of a second open end 22. A thermally sensitive electrical resistor 24, such as a thermistor, is mounted within flow tube 20 by virtue of two electrically conductive supports 25 and 26, respectively. These electrically conductive supports are held in position by an insulating support 28. Electrical lead wires 29 and 30 are connected to electrically conducted supports 25 and 26, respectively. Two open-mesh, screen members 31 and 32 with a plurality of openings 33 therethrough are included across the cross-sectional plane of flow tube 20. Screen 31 is positioned upstream of resistor 24, whereas screen 32 is positioned downstream of the resistor. The open-mesh nature of the screens disrupt the airflow through flow tube 20 thereby reducing the turbulence of air passing over thermally sensitive element 24. It can be seen that resistor 24 is placed directly in the airflow moving through conduit 10 so that once heated, the cooling effects of air will be perceived on the resistor. Since resistor 24 is preferably a thermistor, a change in electrical resistance will result as the temperature of the thermistor changes. As will be explained hereinafter, this change of resistance can be related to the airflow or velocity of air through the conduit. 
     Referring now to FIG. 4, reference cell 14 is illustrated. A preferably cylindrical tube 35 forms the outer periphery of reference cell 14. It is further preferred that the inside diameter of tube 35 be similar to the inside diameter of tube 20 of flow cell 12 so that the environments of the respective resistors within may be matched. Mounted within tube 35 is a thermally sensitive electrical resistor 36, which may be a thermistor such as previously described with respect to thermistor 24. Two electrically conductive supports 38 and 39 are provided to maintain resistor 36 in position. An electrically nonconductive material 40 is provided to mount electrically conductive supports 38 and 39. Two lead wires 41 and 42 are connected to conductive supports 38 and 39, respectively. 
     Flow cell 14, however, is adapted to prevent air from flowing through tube 35. In this regard, two end plates 42 and 44 cover the ends of tube 35 to provide a housing which encloses resistor 36. Of course, provision is made to bring lead wires 41 and 42 out of the enclosed housing by means of small holes which may be potted or sealed, if appropriate. It is preferred that tube 35 and end plates 42 and 44 be made of thermally conductive material, such as metal, to facilitate a thermal exchange between the inside and the outside of flow cell 14. 
     Specifically, resistor 36 is included in the present invention in order to serve as a reference resistor. As a reference resistor, its purpose is to detect ambient air conditions of the air flowing through the conduit and to provide a mechanism for subtracting the effects of ambient air during the measurement of air flowing through the conduit. However, since reference resistor 36 generates heat, the housing which encloses it will therefore be affected by such heat. In fact, if the housing surrounding the reference resistor is sufficiently small, the air inside the reference cell will act as an insulating blanket. The net result of such a structure will be an inaccurate measurement of ambient air. According to the present invention, it has been determined that reference resistor 36 should be enclosed in a volume sufficiently large to permit natural convection to occur during operating temperatures of reference resistor 36. For example, and without in any way suggesting a limitation, reference resistor 36 for purposes of the present invention may operate in still air between 8° and 50° C. When heated to its operating temperature, reference resistor 36 reaches an equilibrium with the ambient temperatures surrounding thermally conductive reference cell 14 due to convection currents within the enclosure and thermal conduction to the outside atmosphere. In order to assure adequate heat transfer by convection, it has been determined that the volume within the flow cell 14 should be at least fifty times the volume of reference resistor 36, and that such a volume ratio of 100:1 is preferable for the present invention. Such a volume ratio and the structure of flow cell 14 permit an accurate measurement of ambient temperature even when reference resistor is heated to its operating temperature. Reference resistor 36 thus acts as a point of reference simulating the resistance of measuring resistor 24 as if no airflow existed. Accordingly, the effects of ambient temperature on the cooling rate of measuring thermistor 24 are taken into account by the construction of flow cell 14 as described herein. 
     As alluded to above, the cooling rate of measuring thermistor 24 is also affected by the fact that its resistance is changed due to its temperature-related resistance characteristics. In FIG. 5, a graphic illustration of resistance versus airflow characteristics of a thermally sensitive electrical resistor is depicted. Resistance is equated to a voltage at a constant current. It can be seen that the resistance of a thermally sensitive resistor, such as measuring resistor 24, increases, in linear fashion, with increased rates of heat transfer due to airflow or velocity. In addition, as the three parameters of temperature at 35° C., 25° C. and 15° C. illustrate, as the temperatures decreases, the resistance change is greater over the same range. As a result of such disparity, the voltage difference from zero velocity to the velocity represented at V N  is not the same at all temperatures. This phenomenon is referred to as the velocity temperature effect or wind chill factor. As pointed out above, the ambient temperature effects should be eliminated from the determination of airflow related to the cooling of the thermistor exposed to the airflow. However, as the resistance and airflow characteristics of FIG. 5 illustrate, the resistance at zero velocity, represented by reference resistor 36, cannot be used as a reference in a bridge circuit if true accuracy of measurement is the desired goal. This change of slope of resistance between zero velocity and velocity at V N , at different temperatures, is taken into account in accordance with the principles of the present invention. 
     In particular, it has been determined that the ratio of the change of slope with respect to change of temperature is equal to the ratio of the change of resistance of reference resistor 36 with respect to temperature. Once having determined this, the resistance of measuring resistor 24 may be modified by this aforementioned ratio. This modification of resistance of measuring resistor 24 acts as if there were no change in slope due to temperature changes thereby eliminating the velocity temperature effect, as described above, on measuring resistor 24. The resistance value at zero velocity represented by the resistance of reference resistor 36 may now be subtracted to obtain an error-free airflow measurement. 
     FIG. 6 illustrates one electrical circuit diagram which compensates for the temperature effects on velocity measurement due to ambient temperature effects as well as the resistance changes due to temperature characteristics of the thermally sensitive resistor. Inaccuracies result if a fixed voltage, representing the zero velocity reading, is subtracted from the voltage corresponding to the resistance of the measuring resistor at some velocity, V N . The present invention overcomes this temperature phenomenon by calibrating the velocity of measuring resistor 24 at a certain, fixed temperature, such as, for example, 25° C. Any voltage calibrated velocity at a known temperature can then be related to the same calibration curve at a different temperature. A temperature- and airflow-compensated voltage output representative of airflow through the conduit can be determined according to the following relationship: ##EQU1## where, V AF  is the voltage corresponding to airflow. 
     V M  is the voltage related to measuring resistor 24. 
     V R  is the voltage related to reference resistor 36. 
     V C  is the voltage related to a calibrated resistance of the measuring resistor at zero velocity. 
     V M  V C  /V R  is the voltage related to the compensated resistance value. 
     These voltage parameters, as well as their mathematical relationship, are produced in accordance with the present invention by the circuit illustrated in FIG. 6. 
     As seen in FIG. 6, the circuit includes reference resistor 36 and measuring resistor 24, both of which are preferably thermistors. In the embodiment being described, there is only one measuring resistor included in the circuit, although more than one measuring resistor may be included as will hereinafter be described. Resistors 51 and 52 are arranged with thermistors 24 and 36 in a bridge circuit, referred to generally by numeral 54. The bridge circuit output terminals are connected to a logarithmic amplifier multiplier-divider circuit 55 designated by the dashed lines in FIG. 6. Circuit 55 multiplies the voltage developed across measuring resistor 24 by the ratio of the voltage related to the calibrated resistance of measuring resistor 24 at zero velocity to the voltage of measuring resistor 36. This calibrated resistance is developed at the adjustable resistor 56. Operational amplifiers 58, 59 and 60 with their associated resistors 61,62,63,64,65 and 66 and transistors 68,69 and 70 form logarithmic amplifiers. An operational amplifier with transistor 74 and resistor 75 forms an antilog amplifier. Capacitors 76,78 and 79 provide phase compensation to compensate for the effects of the gains added by the amplifiers. 
     At amplifier 72, its voltage output bears an antilog relationship to the sum and difference of the output voltages of amplifiers 58,59 and 60 as expressed by the following relationship: 
     
         V.sub.72 =antilog (V.sub.58 -V.sub.59 +V.sub.60). 
    
     As a result, V OUT  may be calculated as follows: 
     
         V.sub.OUT =V.sub.24 V.sub.56 /V.sub.36 
    
     which is equivalent to the following relationship: 
     
         V.sub.OUT =V.sub.M V.sub.C /V.sub.R 
    
     where these voltage parameters are as described above. The output of multiplier-divider circuit 55 is connected to a summing amplifier 80 through resistor 81. From variable resistor 56, the reference voltage is connected to an inverting amplifier 82 through input resistor 84 and feedback resistor 85 to establish unity gain. The output of amplifier 82 is connected to summing amplifier 80 through resistor 86. Output voltage of amplifier 80 is equivalent to V AF , the voltage corresponding to airflow. This output voltage of amplifier 80 is represented by the equation V AF  =V OUT  -V C . Accordingly, mass airflow through the conduit is measured which takes into account and compensates for the combined effects of temperature and velocity on the measuring and reference resistors. The voltage corresponding to airflow through the conduit, as calculated above, substantially eliminates errors due to changes in ambient temperature and caused by velocity cooling of the thermally sensitive resistor which measures the airflow. 
     A number of measuring resistors may be placed in different locations within the conduit to provide an average measurement of airflow. Such an arrangement is illustrated in FIG. 1 where a measuring resistor is included within each flow cell 12. In such an arrangement, the electrical resistors may be included in an electrical network 90 such as illustrated in FIG. 7. Individual thermally sensitive electrical resistors, such as thermistors 91,92,93,94 and 95, each with a constant current sufficient to heat the thermistors to a temperature, for example, above 100° C. to insure good sensitivity with changes in velocity, are connected to a voltage follower 96 in such a way that all the voltages may be averaged. If the resistance values of thermistors 91-95 are all equal, then the following relationship applies: ##EQU2## 
     It is therefore possible to average the effects of velocity on a network of thermistors in a flowing airstream which in turn yields a voltage representing the average velocity in a particular plane across the conduit. V OUT   (V AVG ) may be substituted for measuring resistor 24 in bridge circuit 54 of the electrical circuit set forth in FIG. 6. 
     Thus, the present invention provides an apparatus and method for the measurement of airflow through a conduit which accounts for the increased rates of heat transfer related to the combined effects of temperature and velocity (wind chill factor). Errors in airflow measurement are eliminated by virtue of the principles of the present invention.
An apparatus for the measurement of fluid flow through a conduit comprises at least one thermally sensitive element capable of producing a measurement value related to temperature located within the conduit. This element is exposed to the flow of fluid through the conduit. A second thermally sensitive element is capable of producing a reference value related to temperature of fluid ambient to the conduit. The reference value is substantially unaffected by heat generated by the second thermally sensitive element. An electrical circuit is provided for determining the measurement and reference values and for using the same to produce an indication which substantially eliminates errors due to changes in the ambient temperature and caused by velocity cooling of the first thermally sensitive element. This indication is representative of fluid flow through the conduit.