Abstract:
A flowmeter has a variable area, vertically-oriented duct through which flows fluid whose flow rate is to be determined. A float in the duct assumes a position in the duct that depends on the flow rate of fluid in the duct. A window is located at an end of the duct in alignment with a longitudinal axis of the duct. A transducer unit projects a signal beam comprising either light in the ultraviolet, visible, or infrared spectrum, or ultrasound energy. The signal beam propagates in a beam through the window and along the axis toward the float. The transducer includes a sensor that detects the signal energy returned by the float by reflection or some other mechanism to the transducer unit. The intensity or delay time in the returned signal energy indicates the position of the float in the duct, from which the fluid flow rate may be determined.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation in part application filed under 35 U.S.C. § 111(a) claiming priority, under 35 U.S.C. § 119, of application Ser. No. 11/125,500, previously filed May 10, 2005 under 35 U.S.C. § 111(b). 
     
    
     BACKGROUND  
       [0002]     A variable area volumetric flow rate meter, hereafter simply VA flowmeter, uses a vertical sensing duct through which flows a fluid whose volumetric flow rate is to be measured. The sensing duct has a cross section area that smoothly increases along the length of the sensing bore, typically increasing upwards but that may also increase downwards. Typically, the fluid flows into an end of the duct having the smaller cross section area, and out of the duct at the larger cross section area although variations on this design are possible.  
         [0003]     In any duct of changing cross section area, velocity at each point along the duct, of a fluid flowing in the duct varies inversely to the area at that point. While the situation is somewhat different for flow of a compressible gas in such a duct, for low flow velocities, in general the velocity will also decrease as duct area increases.  
         [0004]     The sensing duct for such a flowmeter contains a float occupying a fraction of the duct area. Normally, the specific gravity of the float is somewhat higher than the fluid flowing in the duct, so the term “float” is a bit misleading. When dealing with liquids however, it is actually possible to use a float that does float. In such a case, buoyancy of the float is counteracted by downward liquid flow. The following discussion assumes a float slightly heavier than the fluid flowing through the sensing duct.  
         [0005]     Fluid flowing through the duct and past the float creates a lift force on the float that shifts the float from the bottom of the duct. The lift force has viscous drag and momentum components. The lift force depends on the fluid velocity around the float, increasing with increasing flow velocity.  
         [0006]     The float will rise or fall to a point where the drag created by the velocity of the adjacent fluid flowing past the float exactly equals the gravitational force provided by the float less the buoyant force on the float applied by the fluid in the duct. Regardless of the volumetric flow rate in the duct, the float will always reach the point in the duct where the velocity of the adjacent fluid exactly balances the net of float weight less buoyant force on the float. The product of the area of the duct where the float finds equilibrium and the velocity of the fluid adjacent to the float equals the volumetric flow rate of fluid through the duct.  
         [0007]     If the fluid is a gas, buoyancy force is very small, indeed may even be ignored, and the gravitational force predominates. If the fluid is a liquid, the buoyancy force may be significant. In an upwardly diverging duct through which a liquid flows, relative specific gravities of the liquid and the float are important in determining the float position for a given flow rate. A downwardly diverging duct with a float whose specific gravity is less than the liquid in the duct is most useful for measuring small liquid flow rates because buoyancy and gravity forces oppose creating a relatively small net force. Normally, the float specific gravity will be larger than that of the fluid, perhaps substantially larger in the case where the fluid is a gas.  
         [0008]     Where fluid flow is measured in an upwardly diverging duct, the float specific gravity is normally greater than that of the fluid. The fluid itself; the specific gravity, shape, size, and total weight of the float; and the duct geometry should all be chosen so that the available range of the float&#39;s vertical position in the duct allows the expected range of flow rates to be measured.  
         [0009]     The flow rate can be calibrated against the float position to accurately indicate the flow rate. In the simplest situation, an operator provides several different known fluid flow rates to the flowmeter and records the position of the float for each flow rate. This provides a table in which the operator can interpolate to determine the flow rate with good accuracy.  
         [0010]     Some applications for these flowmeters require them to handle corrosive fluids without contaminating the fluid flowing through the flowmeters. For most of these types of fluids, materials exist that are inert with respect to the fluid. All of the flowmeter surfaces in contact with the fluid must comprise such inert material to avoid contaminating the fluid.  
         [0011]     Determining the float position in the sensing duct is sometimes difficult. Magnetic position sensing requires magnetic material in the float. Corrosive fluids may attack such magnetic materials. Even if the magnetic material is completely embedded in the float, users are concerned that the corrosive fluid may penetrate the float and cause contamination.  
         [0012]     Alternatively, an optical or other sensor may be located along the length of the sensing duct, but such a sensor must be quite large and complex. Such sensing requires the sensing duct to be made of transparent material, or at least have a transparent window along the sensing duct length, which complicates the sensing duct structure.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0013]     A flowmeter for providing a signal indicating fluid flow rate through the flowmeter has a vertical, tapered duct defined by a duct wall. An axis extends the length of the duct along the fluid flow. A first opening in the duct receives flow of fluid whose flow rate is to be measured. Fluid exits the duct through a second opening. A float within the duct shifts position along the duct axis. The float&#39;s vertical position in the duct indicates the fluid flow rate.  
         [0014]     The flowmeter further comprises a window forming a portion of the duct wall in alignment with the duct axis. The window is transparent to a signal beam and positioned adjacent to one opening of the duct. The signal beam may comprise light in the visible, infrared, ultraviolet, or visible spectrum, or may comprise ultrasound energy. A signal beam source external to the duct and adjacent to the window transmits the signal beam through the window toward the float.  
         [0015]     The float has a feature that returns a portion of the signal beam shining on the float toward the window. The feature may be a beam-reflecting surface on the float. Where the beam is light, the feature may be a property of the float material that causes the beam to diffuse or diffract and return to the window. The float may contain an embedded element such as a metal plate that reflects light.  
         [0016]     A signal beam sensor is located external to the duct and receives through the window, signal beam energy returned from the float, and that the signal beam source had previously transmitted toward the float. The signal beam sensor provides a sensor signal indicating the level of signal beam energy received by the signal beam sensor. For an ultrasound signal beam, the same transducer may comprise both the source and the sensor.  
         [0017]     A controller receives the sensor signal and computes the position of the float therefrom. By calibrating the fluid flow in the duct with the float position, the controller can provide a signal indicating the rate of fluid flow in the duct. At the present time, the intensity of the returned light signal beam seems to most effectively indicate the float position. The relatively low speed of sound allows the controller to measure the time for individual pulses of ultrasound energy to return to the sensor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows a simplified version of the invention.  
         [0019]      FIGS. 2 and 2   a  are block diagrams of transducers for producing a signal beam useful in measuring position of a float in a VA flowmeter.  
         [0020]      FIG. 3  is a two-transducer version of the invention.  
         [0021]      FIG. 4  is a detail of one design of a float used by the invention.  
         [0022]      FIG. 5  is a block diagram of a controller suitable for operating the flowmeter.  
         [0023]      FIG. 6  is a flow chart of firmware for calculating the current value of the flow rate.  
         [0024]      FIG. 7  is a flow chart of firmware for assisting in determining parameters for calculating the current value of the flow rate.  
         [0025]      FIG. 8  is a block diagram of microprocessor-based hardware for linearizing sensor output. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]      FIG. 1  shows in section a simplified variable area flowmeter  10  incorporating one version of the invention. Flowmeter  10  has a duct  35  defined by a wall  22 . Duct  35  has an inlet port  39  where fluid flow enters duct  35  and an outlet port  55  where fluid exits duct  35 . Duct  35  diverges upwards, so that the cross section area of duct adjacent to outlet port  55  is substantially larger than the cross section area adjacent to inlet port  39 . Duct  35  has a longitudinal axis extending along the duct in the direction of fluid flow.  
         [0027]     A float  30  is present within duct  35 . The term “float” in this context is a bit misleading, since float  30  will normally, for reasons already explained, have a specific gravity substantially greater than the fluid flowing in duct  35 , and thus does not float at the top of duct  35 . The frustro-conical shape shown for float  30  in  FIG. 1  is suitable for a variety of flowmeter designs.  
         [0028]     The dimensions of float  30  should be sufficiently large relative to the duct  35  diameters to maintain the float  30  axis substantially in alignment with the axis of duct  35 . Alternatively, a guide rod  36  attached at either one end or both ends (as shown) of duct  35  is useful for maintaining the axis of float  30  in approximate parallel alignment with the axis of duct  35   
         [0029]     The VA flowmeter  10  shown in  FIG. 1  uses a transducer unit  50  to detect the position of float  30  within duct  35 . Unit  50  is shown in more detail in  FIG. 2  as including a signal beam source  61  and a signal beam detector  60 . In one version of this invention, the signal beam is light. In this case, signal beam source  61  is preferably a laser diode or LED that may provide either a visible, an IR, or a UV beam  43  directed more or less parallel to the axis of duct  35  and toward float  30 .  
         [0030]     The signal beam may also comprise ultrasound energy, in which signal beam source  61  may comprise a sound transducer such a piezoelectric transducer capable of generating an ultrasound signal. Such transducer devices are now widely used in echocardiogram devices.  
         [0031]     Regardless of the type of beam produced, signal beam source  61  receives operating power from a power source and signal processor  20  on a path  45 . A signal beam detector  60  provides a signal on path  46  indicating the intensity of the energy in the signal beam  42  returned from float  30 . Where the signal beam is light, detector  60  is preferably a photodiode.  
         [0032]     If the signal beam comprises ultrasound energy, then detector  60  comprises a piezoelectric transducer. In a preferred version of an ultrasound source  61 , the same component comprises both source  61  and sensor  60 , as suggested in  FIG. 2   a.    
         [0033]     A system  10  having a source  61  producing ultrasound may operate in a pulsed mode. The delay time for the reflected signal beam is on the order of hundreds of μsec. for an ultrasound signal beam where the spacing from transducer  61  to float  30  is several inches. This time interval is easy to measure with modern electronic components.  
         [0034]     A window  52  that is transparent to the signal beam provided by source  61  is placed at one end of duct  35  in alignment with the duct  35  axis. In  FIG. 1 , window  52  is placed near outlet port  55 , but may be also advantageously placed near inlet port  39 . Transducer  50  is mounted on the outside surface of window  52  so that energy from source  61  passes through window  52  and the fluid within duct  35 , and impinges on float  30 . For ultrasound beam, a window made of hard plastic with a thickness of 0.01-0.10 in. is suitable.  
         [0035]     In some arrangements for light signal beams, sensor  60  may be at one end of duct  35  and source  61  may be at the other end. Such an arrangement has a window transparent to the signal beam for each of the signal beam source  61  and the sensor  60 .  
         [0036]     Arrow  42  symbolizes the signal beam from source  61  reflecting from or otherwise returning from float  30  through the fluid within duct  35  to detector  60 . For a light signal beam, the intensity of the returning signal beam increases with decreasing distance between the top of float  30  and transducer unit  50  in a predictable and repeatable way.  
         [0037]     A system  10  using a single ultrasound signal beam source/sensor  61 / 60  preferably operates in a pulsed range-finding mode, where the time for a pulse to return to sensor  60  is a precise measure of the distance of float  30  from source/sensor  61 / 60 . In such a system  10 , processor  20  issues a drive pulse perhaps 200-400 μsec. long each 1 to 25 ms. Of course, the float  30  position measurement is critically dependent on the sound velocity within the fluid flowing in duct  35 . Processor  20  includes a timer element  21  or other means such as signal phase shift for measuring the time between release of each pulse of ultrasound energy and the return or echo of that pulse received at sensor  60 .  
         [0038]     Controller  20  periodically provides power on path  45  to activate signal beam source  61 . In the case of a light signal beam, the energy returning to detector  60  from float  30  causes detector  60  to provide a signal on path  46 . The signal on path  46  indicates the signal beam energy intensity at detector  60 , and is usually quite constant for a given float position and fluid type. That is, the signal on path  46  for a given light signal beam energy level and float position typically does not change significantly over time for a particular fluid.  
         [0039]     Therefore, the signal on path  46  can be empirically correlated with the flow rate. Typically, the signal will be processed and digitized before use. The position of float  30  in the sensing tube provides information in the returned light or ultrasound from which the float  30  position can be derived. The float position and the intensity of the either the returned light beam or the delay in return of an ultrasound pulse may be correlated during a calibration process. The intensity of or delay in the returned energy may then be used to indicate float position, which indicates the flow rate. Controller  20  provides the flow rate in a flow rate signal carried on path  47 . The flow rate signal can be used to provide a visible indication of the flow rate, or in a closed loop system for process control, for example.  
         [0040]     Accuracy of float  30  position improves by using the temperature of the fluid in duct  35  in the calculations. For example, sound velocity in a fluid depends to some extent on the temperature of the fluid. A temperature sensor  54  mounted within duct  35  provides a fluid temperature signal on path  55  to processor  20 . Processor  20  can  
         [0041]     Flowmeter  10  is particularly well-suited for use with corrosive fluids. No metallic materials need contact the fluid flowing in duct  35 . Every component in the system may be non-reactive for the particular fluid involved. No complicated float position sensing along the length of duct  35  is necessary. The walls  22  need not even be transparent.  
         [0042]     For light-based float  30  sensing, signal beam source  61  may comprise a laser diode light source projecting a collimated, coherent beam through the transparent window axially along the sensing duct. Using a high-intensity multi-wavelength incandescent light source is another option but may have less good results due to intensity drift over time. A reflective top  25  on the float  30  may reflect a portion of the beam from the light source  61  back through the window  52  to detector  60 .  
         [0043]     A number of alternatives exist for the structure of float  30  that returns the light signal beam to sensor  60 . Float  30  may have a reflective element  31  ( FIG. 4 ) embedded in float  30 ′. Float  30  may have a diffraction grating on the surface  25  facing source  61  that diffracts the light beam impinging on float  30 .  
         [0044]     Another option that may be useful is to form float  30  from a material that is partially transparent or translucent to the signal beam. Such a type of material allows an amount of signal beam energy to return to the sensor  60  through diffusion and backscatter rather than through reflection.  
         [0045]     For a signal beam comprising light energy, Teflon (Reg. trademark of Dupont Corp.), PFA, PTFE, FEP, and TFM, sapphire (possibly too expensive for most applications), various ceramics, and non-Teflon CTFE are some of the materials that seem to have the proper amounts of translucency and that are also inert and non-contaminating with respect to a wide range of corrosive materials. The sensing duct may have a reflective outer surface in this case to increase the intensity of the returned light.  
         [0046]     In one version of the invention the float top surface  25  is slightly convex, producing a diverging reflected light beam  42 , although other float top shapes may be suitable as well. A diverging beam allows the beam reflection to reliably impinge on detector  60  regardless of the angular orientation of the float. A radius of curvature of the reflective float top on the order of 2.5 cm. may be suitable. In any case, the float top surface  25  preferably has a uniform finish so that rotation of the float will not in itself affect the intensity of the returned light. Convex, concave, flat, and grooved float surfaces may also provide returned light that accurately indicates float position.  
         [0047]     Suitable values for light beam wavelength, intensity, and size depend on the characteristics of the fluid. For example, when measuring the flow of fluids such as corrosive aqueous liquids and organic solvents, a light beam with a wavelength of 850 nm., an intensity of 1.25 mw., and a cross section diameter of 4.6 mm. is suitable.  
         [0048]     The distance the beam travels through the fluid also reduces beam intensity. In actual practice, both float distance-based attenuation and fluid attenuation may cooperate to attenuate the beam in a way that allows accurate float position sensing.  
         [0049]     Float position sensing can also use a piezoelectric element  61 / 60  that generates ultrasound pulses that reflected back to element  61 / 60  from float  30 . The delay time for the reflected sound depends on the float position in the sensing tube as well as the characteristics of the fluid. Since sound travels much more slowly than does light, time delays is much easier to measure and result in more accurate float position sensing. The window through which the sound beam passes must then be of the type that is transparent to that sound, or at least does not attenuate the sound substantially.  
         [0050]     One possible transducer device suitable to function as source/sensor  61 / 60  is available from International Transducer Corp., Santa Barbara, Calif. and has Model No. ITC-9072. The ITC-9072 device can function as both a source and sensor for ultrasound. It has a suggested operating frequency of 150 kHz. Other devices have different operating frequencies. Best judgment at this time suggests that a frequency in the range of 40-5000 kHz will provide good results. The signal beam must be able to pass easily through window  52 , and yet reflect strongly from float  30 . At the same time, the fluid flowing through duct  30  should not significantly attenuate the signal. The use of a element  31  that strongly reflects ultrasound may improve performance.  
         [0051]      FIG. 3  shows a further version of a flowmeter  10 ′ that is in many ways similar to flowmeter  10  shown in  FIG. 1 , and that uses a signal beam comprising light. Similar elements have similar or identical reference numbers.  
         [0052]     Inlet port  69  provides fluid to duct  35 . A second window  72  is positioned at the small end of duct  35 . A transducer unit  80  whose design is essentially identical to that of transducer unit  50  has a light source positioned to project light toward the bottom end of float  30  as indicated by beam  63 . Light returns from float  30  as beam  62  indicates and is detected by a detector in transducer unit  80 .  
         [0053]     A finger or arm  70  supports float  30  during times of little or no fluid flow through duct  35 . Finger  70  may be very narrow to thereby avoid interfering either with fluid flow or with light beams  62  and  63 .  
         [0054]     Controller  20 ′ provides power for operating the light source within transducer unit  80  on path  65 . A signal carried on path  66  indicates the light level detected by the detector internal to transducer unit  80 .  
         [0055]     Controller  20 ′ operates in a mode somewhat different from that of controller  20  because of extra information provided by the second transducer  80 .  FIG. 5  shows the configuration of a simple controller  20 ′. A microprocessor  80  is designed to execute software or firmware that implements the functions described in  FIGS. 6 and 7 , flow charts that describe firmware for operating flowmeter  10 ′.  
         [0056]     A keypad  84  allows operator input to microprocessor  82 . Keypad  84  has keys  0 - 9  for number entry and has a calibrate key and an enter key.  
         [0057]     A display  87  provides numeric information for an operator. Computations by microprocessor  82  may generate the numbers displayed on display  87 . Microprocessor  82  may also cause display  87  to show keypad  84  entries.  
         [0058]     Microprocessor  82  provides a signal on path  88  causing a light source driver  90  to provide power on path  45  for illuminating light source  60 . A light signal amplifier  94  amplifies the light sensor signal on path  46  and sends the amplified signal to microprocessor  82  on path  92 .  
         [0059]     The firmware defined by  FIGS. 6 and 7  discloses one way that a microprocessor can control operation of flowmeter  10 ′. The implementation of  FIGS. 6 and 7  is for use with a duct  35  whose cross section area increases linearly upwards with distance from finger  70 . The calculations in  FIG. 6  assume that the intensity of beams  42  and  62  varies linearly with the distance of float  50  from finger  70 , which is reasonable. The fluid flowing in duct  35  should be a liquid. One should realize that many other computer-controlled operating modes may be used as well.  
         [0060]      FIG. 6  is a continuously running operating loop for controlling flowmeter  10 ′.  FIG. 7  shows firmware that controls and simplifies calibration of flowmeter  10 ′.  
         [0061]     In  FIG. 6 , connector symbol A  100  indicates the beginning and end of the loop. If the calibrate key is operated, decision element transfers instruction execution to connector element B  140  in  FIG. 7 .  
         [0062]     If calibration is not commanded, then activity element  104  causes light source  60  to illuminate float  30  for 100 ms. Since float  30  may not be totally stable in its position in duct  35 , this provides an opportunity for microprocessor  82  to average the position of float  30 . Activity element  106  samples the light intensity from float  30 , typically several times. Activity element  110  processes the signals from light sensor  60 . This may include multiple sensing instances to measure the average float  30  position accurately. The intensity value for light returned from float  30  is recorded as  150 .  
         [0063]     Activity element  113  delays further processing for 100 ms., in case the light source driver provides the timer function for operating light source  61 . Activity elements  117 ,  120 , and  123  perform a similar function for transducer unit  80 . The intensity of the light returned to the light sensor in transducer  80  is recorded as I 80 . Activity element  124  then calculates the ratio of the two intensities as I I =I 50 /I 80 .  
         [0064]     To determine flow rate, computational element  126  calculates the formula shown. The formula in computational element  126  is an interpolation based on parameters determined by a calibration shown in  FIG. 7 . The interpolation depends for accuracy on the requirement that duct area increases linearly with distance upwards from finger  70 , and on the accuracy of the assumption that the intensity of light in beams  42  and  62  varies linearly with distance of float  30  from transducer units  50  and  80 .  
         [0065]     The flow rate value is provided either in a signal on path  47  or on display  87 . After calculating flow rate and providing the flow rate value for use, microprocessor  82  delays further execution for 100 ms. as shown by activity element  130 , and then reexecutes the loop of  FIG. 6 .  
         [0066]      FIG. 7  discloses how the firmware executed by microprocessor  82  cooperates with an operator to calibrate the system. When calibration is requested by the operator pressing the calibrate key on keypad  84 , activity element  150  causes the display unit  87  to prompt the operator to adjust flow rate through flowmeter  10 ′ at a rate that just barely lifts float  30  from finger  70 . Further instructions of activity element  150  indicate on display  87  the start of a 1 min. interval during which the operator holds a graduated container to receive liquid flowing from the outlet port  55 . Where flow rates are large, a smaller time period, say 10 sec. may be suitable  
         [0067]     Conveniently, microprocessor  82  can time that interval with the instructions of activity element  156 . When the 1 min. interval has expired, the instructions cause display unit  87  to indicate that the operator should stop the flow or remove the container and then use keypad  84  to enter into microprocessor  82  the quantity of fluid held in the container. The instructions of activity element  160  cause microprocessor  82  to convert the amount of liquid in the container to the measured minimum flow rate and record same as FR M . In one version the container may be graduated in flow rates based on a one minute interval rather than a volume, in which case microprocessor  82  can enter FR M  directly.  
         [0068]     The instructions of activity element  163  then sequentially activate the light sources in transducer units  50  and  80 . Units  50  and  80  should be sequentially activated to avoid interference of one light beam  42  or  62  by the other. The light intensities sensed by transducer units  50  and  80  are recorded respectively as I 50  and I 80 . Activity element  163  then calculates and records as I M  the ratio (I 50 /I 80 ) MIN  for these values of I 50  and I 80 .  
         [0069]     Activity element  167  then prompts the operator to increase the fluid flow rate to somewhere near the level that shifts the position of float  30  to close to the top of duct  35 , and for the operator to again run liquid into the container for 1 min.  
         [0070]     Activity element  170  then prompts the operator to use keypad  84  to enter the amount of liquid in the container. The instructions of activity element  173  cause microprocessor  82  to convert the volume of liquid in the container to a near-maximum flow rate and record same as FR L .  
         [0071]     The instructions of activity element  176  then sequentially activate the light sources in transducer units  50  and  80 . The light intensities sensed by transducer units  50  and  80  are recorded respectively as I 50  and I 80 . Activity element  163  then calculates and records as I L  the ratio (I 50 /I 80 ) LRG  for these values of I 50  and I 80 .  
         [0072]     Instruction execution then returns to connector A  100  to calculate flow rates. The formula in calculation element  126  uses the parameters calculated by the firmware of  FIG. 7  and the activities of the operator described in  FIG. 7  to perform an interpolation between the two flow rates used by the firmware of  FIG. 7 . The accuracy of the calculation relies on linearities in the system. These are first, the linearly increasing area of duct  35 , and second, the linear change in the intensity of light beams  42  and  62  sensed at transducer units  50  and  80  with changes in the distance between float  30  and the transducer units  50  and  80 .  
         [0073]     Since this process for interpreting the signal data relies on linear response of the sensor signal, it may be possible to linearize the sensor  60  signal digitally.  FIG. 8  shows a possible system for accomplishing this function. The functionality of  FIG. 8  may be incorporated in signal processor  20 .  
         [0074]      FIG. 8  shows an A/D converter  93  receiving the raw sensor  60  data on path  46 . This data may be either time delay data for an ultrasound system, or light intensity data for a light-based system. If A/D converter  93  has an 8 bit output for example, this corresponds to 256 different output values. 256 values can provide accuracy of approximately 0.02 in. in specifying the position of a float  30  which has a range of movement of 5 in. within duct  35 .  
         [0075]     The digital output of A/D converter  93  is provided to a table lookup function within microprocessor  82 . An EPROM or other non-volatile memory device  95  holds a linearization table with 256 entries. The entries have addresses ranging from 1 to 256. The values in the entries in memory device  95  have been preset to collectively specify linear values for the flow in duct  35 .  
         [0076]     Microprocessor  82  reads the value in memory device  95  having the address specified by the output of A/D converter  93 . The value in the memory device  95  entry specified by the converter  93  output is supplied to D/A converter  97 . The voltage output of converter  97  is linear with respect to flow in duct  35 .  
         [0077]     The values in memory device  95  are typically different for each combination of fluid and flowmeter  10 . The memory device  95  linearization table entries are usually determined empirically.  
         [0078]     In one version of the invention, memory device  95  may be a module that plugs into microprocessor  82 . This module may contain a number of linearization tables. While commissioning a flowmeter system  10 , the installer selects a particular one of these tables previously identified as suitable for the expected fluid. The selection process may be done using a personal computer having a USB port into which memory device  95  is plugged during commissioning. The selection software may be resident on memory device  95 . Such software prompts the installer to identify the type of fluid carried by duct  35 , and the answer to this prompt selects the correct linearization table for use by microprocessor  82 .