Abstract:
A scheme for attenuating reflected microwave radiation ( 11 ) reflected from distant objects ( 10 ) in a flow measuring device. A microwave transducer ( 3 ) is mounted on a feedpipe ( 5 ) or adjacent to a region in which matrial ( 2 ) is permitted to freely fall. The feedpipe ( 5 ) permits the introduction of electromagnetic radiation ( 6 ) into a larger mass transporting conduit ( 1 ). As particulate material ( 2 ) passes through the electromagnetic wavefront ( 7 ) the reflected signal ( 9 ) is sensed by the transducer ( 3 ) and the velocity of the material ( 2 ) can be calculated. A radar absorbent material ( 22 ) is used to line the conduit ( 1 ) or surround the region in which material is freely falling, thereby reducing the magnitude of any electromagnetic energy ( 6 ) that passes through the absorbent material ( 22 ).

Description:
1. FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of devices used for measuring the mass flow rate of particulate matter through a conduit, guide or region of free fall and more particularly to those devices which radiate electromagnetic energy toward the particulates flowing within the pipe, conduit or region and subsequently perform data processing of the reflected energy to determine the flow rate.  
         2. BACKGROUND OF THE INVENTION  
         [0002]    Ultrasonic methods for determining the presence and rate of a gas/solid two phase flow within a conduit are well known. A typical gas/solid two phase flow, such as coal particles entrained in an air flow, generally comprises a rope like structure of coal particles travelling in the pipe. There are some techniques which attempt to measure the amount of coal in the pipe, but there are drawbacks that make them unacceptable for continuous long term measurements. There are optical methods, but the optical sensors are easily fouled and require frequent maintenance. Other methods require the physical insertion of a probe into the flow path, but the probe(s) either become fouled or are abraded and heated to the point of failure in the harsh environment.  
           [0003]    Trial and error methods commonly used in coal power plant operation can result in poor efficiency and increased air pollution. In order to optimize combustion the amount of coal and the amount of air delivered to the burner must be known. Many other examples of powder and granule flow exist in other fields such as the food processing and material manufacturing industries.  
           [0004]    One class of flowrate measuring devices transmits microwave energy through the flowing material and a portion of the radiated energy is reflected from the material. At higher flow concentrations, the microwave energy does not penetrate uniformly through the material flow. Much of the energy is reflected by the material or absorbed by the material closest to the to the transducer or transmitter. The material flowing at the farthest or opposite side of the pipe will be exposed to less energy and therefore have less contribution to the total reflected energy received by the transducer or receiver.  
           [0005]    Another type of flow measurement device utilizing microwave energy relies on the attenuation of the transmitted energy caused by the material flowing through the conduit. In this method, there is a separate transmitter and a separate receiver. When no material is flowing, the received signal is at a maximum. As the quantity of material flowing within the pipe increases, the received signal is diminished. The amount of flow is generally assumed to be proportional to the decrease in signal strength.  
           [0006]    The flow characteristics in multiphase flow, such as the density and location of the flowing material, are not linear functions and instead present a turbulent and chaotic pattern which does not lend itself to straightforward mathematical analysis. The most accurate method of measurement would be to expose all of the flowing material within a pipe to the same levels of electromagnetic energy. This is not usually possible because the transmitter radiates its energy via a directional antenna, typically a horn assembly which is tuned for a specific frequency, direction and beamwidth. The resulting radiated beam may thus illuminate either all or only part of the material flowing through the pipe depending on the beam angle and the distance between the material and the horn antenna. Even if all of the material flow is within the radiation pattern formed by the transmitting horn, the microwave intensity or electromagnetic flux is not uniform throughout the irradiated volume. Some of these signal intensity problems may be corrected by linearization algorithms or by using multiple receivers strategically placed on opposite sides of the conduit to measure the loss of energy caused by either absorption or reflection of the electromagnetic radiation due to the presence of the flowing material. An example of a flow meter using multiple receivers is disclosed in U.S. Pat. No. 5,600,073, issued to Hill.  
           [0007]    Microwave flow technology has been shown to work well when the concentrations of material being measured are quite small, that is, the volumetric ratio of coal particles to the conveying airstream, for example, is on the order of 0.001.  
           [0008]    Under such circumstances, very little of the radiated microwave signal is reflected or attenuated by the flowing material, resulting in a relatively uniform electromagnetic flux density throughout the volume being measured. The more uniform flux concentration results in a reflected signal that is relatively linear over the range of flowing material concentrations being measured. The low concentrations of material result in a reflected signal of relatively low magnitude, thereby requiring the use of amplification to produce a signal usable for further processing.  
           [0009]    The pipe or conduit in which the material is conveyed is frequently of a type that is microwave transparent or which does not block or reflect a major portion of the transmitted microwave signal. The radiated microwave signal may pass from the transmitter through the first wall of the pipe, completely through the flowing material, and then through the second wall of the pipe.  
           [0010]    One problem with microwave transparent conduits is that the radiated electromagnetic energy may pass completely through both walls of the pipe and continue into regions beyond the pipe where no flowing material is being transported and hence where no flow measurement is desired. If the radiated energy encounters reflective material beyond the boundaries of the pipe walls, energy may be reflected back through the pipe and the flowing material, eventually being detected by the receiver. As mentioned already, the received signal from the flowing material may be very low due to the low concentrations of particulate matter within the pipe.  
           [0011]    A reflection from some large object outside of the pipe, even if relatively distant from the transducer, may produce a received signal that is roughly equivalent in strength to the signal reflected from the flowing material. Such a signal will obviously produce a false indication of the magnitude of material flowing within the pipe.  
           [0012]    Simple techniques such as high or low pass filtering may remove some of the unwanted signal depending on the relative frequency attributable to material flowing within the pipe and the frequency of the interfering signal. Usually, the frequency separation is not sufficient to achieve success by this method. Further, there are cases where the unwanted signal is not due to the background movement of people or conveying equipment, but is instead inherent in the flow measuring site. For example, the pipe and transducer, although firmly clamped together, may vibrate or move during normal operation. This creates relative movement between the transducer and any wall or other nearby stationary objects falling within the beamwidth of the horn antenna. This relative movement again results in a false measurement signal. The transducer could be mounted on the wall or floor so as to eliminate this source of relative movement, but then the vibration of the pipe in which the material flows would itself create relative movement with the transducer. While the majority of the radiated signal will pass through the pipe wall there will still be some reflection from the pipe wall itself unless the pipe material has the same electromagnetic properties as the surrounding atmosphere, which is highly unlikely.  
           [0013]    In most practical situations the relative movement of the pipe with respect to the transducer will result in an incorrect flow measurement.  
           [0014]    In some situations the flowing material which is to be measured is transported within a metallic pipe which is inherently opaque to microwave radiation. In this case the transducer is mounted within another metallic pipe which perforates or penetrates the wall of the material transport pipe such that the radiated energy is permitted to pass into the interior volume of the material transport pipe. A microwave transparent plug may be placed in the end of the feed pipe to prevent material from travelling into the feed pipe and toward the transducer. Since the radiated and reflected microwave signals are confined within the metallic transport and feed pipes, measurement errors cannot be introduced by the relative motion of objects outside of the pipes. However, this method of flow measurement does introduce other problems.  
           [0015]    First, the metallic conductor may behave as a waveguide or other resonant or tuned chamber. The conducting material is typically chosen to one of many reasons such as abrasion resistance, static electricity control, explosion proofing, pressure characteristics or structural properties. Substitution of another material in order to create microwave transparency or preserve microwave opacity may not be practical. Under these circumstances the radiated microwave signal may be propagated within the material transport pipe for long distances with minimal attenuation.  
           [0016]    Since the particulate matter is usually being conveyed pneumatically, there is typically a fan with its associated moving blades spanning the cross sectional area of the conduit. Reflection of the transmitted electromagnetic energy from the moving blades can create interference and hence degradation of the signal reflected from the material being transported and measured.  
           [0017]    A second problem can be caused by vibration of the metallic pipe which can be induced by its attachment to moving machinery such as the aforementioned fan. Since microwave radiation is inherently of very short wavelengths, the physical motion induced by vibration of the pipe may be an appreciable fraction of those wavelengths. Because the metallic pipe is acting as a tuned circuit its vibration or periodic translation through some appreciable fraction of a wavelength can alter the characteristics of the reflected signal in unpredictable and hence uncompensatable ways.  
           [0018]    A third problem may be caused by the creation of localized amplitude and phase variations in the reflected signal within the metallic pipe, these variations not being aligned in a predictable way with the radiated microwave beam. These variations or standing waves are caused by the addition and subtraction of the travelling electromagnetic waves within the pipe. The pipe sizes used in conveying the particulate material may be on the order of a few to many wavelengths of the microwave wavelength.  
           [0019]    As the microwaves travel through the pipe, they add and subtract in complex ways that would appear almost random but which could in fact be predicted if all of the characteristics of the conducting medium were precisely known. This effect occurs not only along the longitudinal axis of the pipe but also throughout its cross sectional area. The final result of these interactions is a complex three dimensional field. These effects are greatest a short distance from the transducer and decrease as the distance from the transducer becomes greater.  
           [0020]    If the material flow through the pipe was constant and homogeneous, the reflected signal attributable to the material flow would be a summation or integration of the signals reflected from the material along the length of the pipe or conductor. Unfortunately, the material often flows in what is termed a “rope”, meaning one or more generally longitudinal strands in which the material concentration is very much higher than in adjacent regions of the pipe. The position of the ropes themselves may vary in a chaotic fashion. This is not to say the mass flowrate is varying in either concentration or velocity. Rather, the instantaneous cross sectional concentration at any given point in the pipe may vary widely and unpredictably. The radiated beam may not intersect a representative region of the pipe cross section at any predictable time, and hence traditional integration techniques will not yield accurate mass flow rate measurements.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention addresses some of the problems of the prior art and in particular the problem of false measurements caused by the relative movement of the microwave transducer with respect to other objects. This is accomplished by restricting or confining the radiated and reflected microwave signal to the volume inside the pipe or other conveying guide and by not allowing passage of the microwave signal into a region where other relative movement may be detected. In the present invention a microwave absorbing material is used to prevent the transmitted microwave is signal from leaving the volume where particulate material is actually flowing. The microwave signal is substantially absorbed by the material. Any signal that does pass through the material and is reflected must again pass through the absorbent material before being detected by the transducer. This reduces the presence of false signals to a level where they are either undetectable or not a significant factor with respect to the signal reflected from the flowing material.  
           [0022]    In the present invention, the microwave transducer is rigidly affixed to the conveying pipe or conduit. The microwave absorbing material is wrapped around the pipe or conduit in the region adjacent to the transducer. Preferably the transducer assembly as well as the microwave horn, feed or antenna is also wrapped with the absorbent material. An open path must be maintained to permit the radiated transducer signal to illuminate the flowing material within the pipe without obstruction.  
           [0023]    The microwave absorbing material is firmly attached to all components so that no relative movement between the material and the transducer can occur. Typically, the radar absorbing material is wrapped within another more durable material in order to provide environmental protection.  
           [0024]    With the microwave absorbing material in place, the Doppler shifted microwave signal is reflected from the material moving within the pipe. Any microwave signal that passes through the moving particulate matter and the wall of the pipe encounters the microwave absorbing material. This absorbent material has characteristics that tend to either directly absorb or at least scatter any intercepted microwave energy. Relatively little energy escapes, the exact quantity depending on the specific characteristics of the radar absorbing material being used. Any residual energy that is subsequently reflected back through the pipe wall from moving objects is further attenuated and dissipated by the absorbent material.  
           [0025]    The present invention also uses microwave absorbing material to address interference caused by objects moving in the pipe, pipe vibration and by standing wave interference of the microwave field. Since microwave absorbing material cannot typically withstand the environmental conditions within the pipe while particulate material is being transported, such material cannot be used to directly line the interior pipe wall.  
           [0026]    The present invention addresses this problem by using a suitably robust microwave transparent material to line the inside of the pipe while maintaining the desired material transport characteristics of the pipe.  
           [0027]    A suitable liner material may be plastic or a hard abrasion resistant material such as ceramic or basalt. The microwave absorbing material is wrapped around the pipe liner. The microwave absorbing material is formed to include an opening to permit passage of the microwave signal into the pipe cavity. This microwave absorbent liner assembly is inserted into a larger pipe which has been formed to include the feed pipe which houses the microwave transducer. The entire liner/feed pipe assembly can then be inserted as a substitute section of the particulate matter conveying conduit.  
           [0028]    The microwave signal emitted from the transducer will pass through the wall of the liner and into the flowing particulate matter. Some of the radiated signal will be reflected from the flowing material and pass back through the liner to the transducer receiver. Some of the remaining microwave signal will be absorbed by the flowing material but most of it will pass through the opposite wall of the liner and be absorbed, with relatively little of the signal reaching the metallic pipe wall and hence being reflected. Any reflected signal must then pass back through the liner and will be further attenuated to a level that is insignificant to the flow measurement data processing task. A further advantage of the aforementioned scheme is that any reflected signal will have to bounce through the absorbent material numerous times as it propagates along the length of the pipe. Depending on the transducer feed angle and the length of the lined substitute pipe section, the reflected signal will tend to be highly attenuated before reaching an unlined portion of the conveying pipe since each reflected signal bounce requires two passages through the absorbent liner material.  
           [0029]    Moving objects within the pipe, such as fan blades, and vibration of the pipe will thus not be at a high enough signal level to affect measurement accuracy. Signal concentrations caused by standing waves are also significantly attenuated since their existence requires reflection from the conductive layer which is blocked by the radar absorbing material. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a side elevation depicting a first configuration for measuring particulate mass flow;  
         [0031]    [0031]FIG. 2 is a side elevation depicting a second configuration for measuring particulate mass flow;  
         [0032]    [0032]FIG. 3 is a side elevation depicting a third configuration for measuring particulate mass flow;  
         [0033]    [0033]FIG. 4 is a plan view showing a first mass flow measuring device constructed according to the principles of the present invention;  
         [0034]    [0034]FIG. 5 is a side elevation showing the apparatus depicted in FIG. 4;  
         [0035]    [0035]FIG. 6 is a side elevation depicting a fourth configuration for measuring mass particulate flow;  
         [0036]    [0036]FIG. 7 is a plan view depicting a second mass flow measuring device constructed according to the principles of the present invention;  
         [0037]    [0037]FIG. 8 side elevation of the apparatus depicted in FIG. 7; and  
         [0038]    [0038]FIG. 9 is a side elevation depicting roping of the particulate flow in a metallic pipe. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0039]    Referring generally to FIG. 1, a particulate matter mass flow measuring scheme is depicted. A nonmetallic pipe  1  is shown through which a granular or powdery material  2  flows in the direction of arrow  13 . In order to measure the mass flow rate of the material  2  through the pipe  1  a microwave transducer and its associated electronics  3  is oriented so as to transmit microwave signals  7  across and through the interior volume of pipe  1 . In practice, the transducer  3  is affixed to a flange  4  which mechanically couples the transducer  3  to a feed pipe  5 . The feed pipe  5  is formed so as to present an un obstructed path to emitted microwave radiation  6  leaving the transducer  3  and aimed at the particulate material  2  within pipe  1 . The angle  14  at which the feed pipe enters the conveying pipe  1  cannot be ninety degrees, but must orient the feedpipe either upstream or downstream. A ninety degree angle will not work because of the need to obtain a Doppler shifted signal. A microwave signal pointed across the direction of flow will provide only an indication of flow moving laterally across the pipe. Theoretically the best position for the emitted signal source would be aimed longitudinally directly down the pipe, but this cannot be practically achieved in this case. In practice, the beam source is at as great a departure from ninety degrees as possible, keeping in mind that very oblique angles are not practical either. If the angle chosen is very slight, the emitted microwave signals have a much longer distance to travel to the material  2  and back to the transducer  3 , thereby compromising sensitivity. The specific angle chosen is dependent on several factors such as the diameter  15  of pipe  1 , the wavelength of the emitted microwave signal and the expected velocity of the material  2  as it flows through pipe  1 .  
         [0040]    Some of the emitted signal  6  passes through the interior of pipe  1  as radiation  7 , and that electromagnetic energy which is not reflected or absorbed continues through the wall  16  of pipe  1  as electromagnetic wave  8 . Often there is a vibrating or moving object  10  in a region that is adjacent to conveying pipe  1 , the direction of movement or vibration being depicted by arrow  17 . A portion of the electromagnetic waves  8  will encounter the vibrating object  10  and be reflected as reflected signal  11 . A small amount of the reflected signal  11  will reenter feed pipe  5  where it will be sensed by transducer  3  as reflection  12 . The reflection  12  will generally be indistinguishable from desired signal  9  which is produced by the reflection of transmitted signal  6  as it encounters the flowing material  2  within pipe  1 .  
         [0041]    Referring also to FIG. 2, an alternate flow measuring scheme is shown in which the transducer is mounted adjacent to the nonmetallic pipe  1  without being rigidly affixed to the pipe  1 .  
         [0042]    The flowing material  2  travels in the direction of arrow  13 . The signal  18  emitted by the transducer  3  is free to travel through the pipe  1  and has a radiation pattern that is determined primarily by the characteristics of transducer antenna  19 . Part of the transmitted signal  18  in the pipe  1  is reflected as signal  9  to transducer  3  and received as detected signal  45 .  10  While this arrangement presents a more uniform flux density to the measured material  2 , relative movement  42  between the transducer  3  and pipe  1  is now possible. Further, the radiated signal  18  may be reflected from both the inside as well as the outside of pipe  1 . Relative motion between the transducer  3  and pipe  1  (or any external vibrating object) results in a modulation of the detected signal  45  which is received by transducer  3 . The relative motion may be of a nature such that the resulting detected signal  45  is indistinguishable from the Doppler shifted signal  9  produced by interaction with the flowing product  2 .  
         [0043]    One should emphasize that the signal which is reflected back into transducer  3  by the pipe wall or any other object outside of the pipe  1  is not always a problem. If there is no relative motion between transducer  3  and pipe  1  (or other external object) then the reflected signal produced by such interaction is not Doppler shifted. As an unshifted signal, the transducer  3  and its associated software and signal processing electronics recognize this reflected signal as a stationary component and hence is a component that does not contribute to the flow of product  2 . However, if there is relative motion between the transducer  3  and any object, the relative movement will result in a Doppler shifted reflected signal which may be indistinguishable from the Doppler shifted signal  9  produced by the moving particle flow  2 .  
         [0044]    Referring also to FIG. 3, another flow measuring arrangement is depicted. The microwave transducer  3  is mounted adjacent to but not affixed to pipe  1 , and a separate receiver  20  is mounted opposite to the transducer  3  such that signals passing between transducer  3  and receiver  20  must pass through the pipe  1  and particulate matter  2 . In this arrangement, the transmitted wave  18  is sensed directly by the receiver  20 . Further, while the transducer  3  may be only a transmitter, the transducer  3  may also be a transceiver capable of receiving the reflected waves  21 . In this case a comparison of the signals received by receiver  20  and those received by transducer  3  may be compared to produce more accurate flow measurement data. However, this arrangement still permits vibration and relative movement  17  between the pipe  1 , transducer  3  and receiver  20 , so much of the accuracy gains could be lost by the presence of false or undesired motion signals.  
         [0045]    The use of a microwave absorbing material  22  can be seen in the arrangement of FIGS. 4 and 5, in which the nometallic pipe  1  depicted in FIG. 1 is surrounded or encased by the absorbent material  22 . The microwave transceiver  3  emits a signal  6  through feed pipe  5  which enters pipe  1 . The emitted signal  6  encounters the flowing material  2 . Some of the emitted signal  6  is reflected from the particulate material  2 , thereby producing the Doppler shifted reflected signal  9 . Some of the reflected signal  9  enters feed pipe  5  where it is received by transceiver  3 .  
         [0046]    The angle  14  is selected so that the transmitted signal  6  encounters or senses a relatively high material flow velocity, which thereby tends to maximize the magnitude of the frequency shift of reflected signal  9 . A portion of the originally transmitted signal  6  is also mixed with received signal  9  within the transceiver  3 . The result of this mixing is to create a difference or image frequency in the output of the receiver portion of transceiver  3  according to the formula:  
           dF= 1  Fr−Ft  1=(2* v*Ft )/ c    
         [0047]    where  
         [0048]    dF=the low frequency doppler signal in the receiver output  
         [0049]    Ft=the transmitting or emitted frequency  6   
         [0050]    Fr=the frequency of the doppler shifted reflected signal  9   
         [0051]    v=the speed of the target particulate material  2   
         [0052]    c=the speed of light (300,000,000 meters/second)  
         [0053]    In practice, the flowing material  2  includes portions that are flowing at different velocities, which results in a distribution of received signals  9  of differing amplitudes and differing frequencies. Within the transceiver  3  or connected to it is an amplifier and filter which amplifies the low frequency Doppler signal spectrum dF and which also removes extraneous noise signals.  
         [0054]    The amplified Doppler signal is digitized by circuitry (not shown) associated with the transceiver  3  using a high speed analog to digital converter. The sampling rate used by the converter is chosen to satisfy the Nyquist criteria for accurately determining the maximum frequency of interest within the Doppler signal dF. The sample period (sample rate * number of samples) must allow for determination of the lowest frequency of interest in the Doppler signal dF.  
         [0055]    The next step performed by the processing circuitry of transceiver  3  is to process the array of digitized samples by an appropriate spectral analysis program, such as the Fast Fourier Transform (FFT), which generates an array of signal amplitude versus frequency from the original sample array. Each value of the FFT array corresponds to the amplitude of the received microwave signal that falls within a fixed range of frequencies.  
         [0056]    The amplitude value for each frequency in the spectral analysis is then squared to convert the array to a power spectrum instead of an amplitude spectrum. At this point the power level of the received microwave signal within each frequency range is proportional to the mass density of only the material  2  flowing at the range of velocities which corresponds to that range of frequencies.  
         [0057]    A numeric integration is then performed on the power spectrum as follows: The power level P at each frequency step n is multiplied by the value of n and then each of these product terms is summed. The basic mass flow rate is defined as:  
         Mass Flow Rate=mass density*flow velocity*flow cross section  
         [0058]    Since the flow cross sectional area is a constant, the total mass flow rate can be defined as the sum of the mass density of the material flowing at a given velocity multiplied by that corresponding velocity. Each of the numeric integration terms describes the mass flow rate of only the material flowing at the range of velocities which corresponds to that frequency and thus the velocity step n. The sum of all of these terms is proportional to the total mass flow rate.  
         [0059]    That portion of the emitted signal  6  which is not reflected from or absorbed by the flowing material  2  travels through the wall of pipe  1  and into the radar absorbent material  22 . A relatively large amount of the emitted signal  6  which enters material  22  is absorbed, leaving very little of the signal  6  to enter the region  23  which lies beyond pipe  1 . If the highly attenuated remains of signal  6  encounter any object such as object  10  shown in FIG. 1, the reflection produced will be extremely weak and will have to reenter the absorbent material  22  in order to be sensed by transceiver  3 . By reentering the absorbent material  22  the already weak signal will be further attenuated to the point where its signal strength is negligible. In this manner the effect of any vibrating object in region  23  on the accuracy of the flow measurement of particulate material  2  will be substantially reduced or eliminated by the presence of the radar absorbing material  22  on the exterior of the nonmetallic pipe  1 .  
         [0060]    Referring to FIG. 6, the problem of flow measurement when using a metallic pipe  24  is presented. The microwave transceiver  3  is connected to a metallic feed pipe  25  which is rigidly affixed to metallic material conveying pipe  24 . The transceiver emits within the feedpipe an initially high intensity signal  32 . The pipe  24  acts as a waveguide in this configuration, allowing much of the gradually weakening radiated signal  31  to be reflected back to the transceiver  3 .  
         [0061]    The signal  31  travels along pipe  24  with little attenuation, becoming the propagated signal  27  which eventually encounters some moving object such as the blower fan  29 . The moving object may or may not be directly in the conduit pipe  24  insofar as the signal is readily propagated throughout the interior of, for example, metallic boxes and chutes which may be part of the associated material flow hardware in an actual real world installation. In any event, the object such as fan  29  reflects some downstream radiation  28  as a Doppler shifted signal  30 , some of which successfully makes the return trip through pipe  24  and which is received by transceiver  3 . The Doppler shifted signal  30  may be indistinguishable from the material flow induced signal  26 . In some cases the Doppler shifte signal  30  may be of a magnitude which is much greater than the signal  26  produced by reflections of signal  31  from the material flow.  
         [0062]    With the foregoing in mind, FIG. 9 shows particulate flow in a metallic pipe where roping occurs. As mentioned previously, the particulate flow may form “ropes” or “bands”. These ropes generally are in the center portion of the pipe but frequently move throughout the pipe in a chaotic fashion.  
         [0063]    When microwave signals  31  are emitted from transceiver  3  and the propagated signals  27  encounter reflected signals  30 , variations in microwave field intensity within the pipe  24  occur, resulting in microwave intensities which are much greater at one point within pipe  24  than at another location which is quite nearby. The patterns of varying field intensity are caused by the constructive and destructive interference of the primary transmitted signal  31 , the multiple reflected signals  26 ,  27  and propagated signals  30 .  
         [0064]    The magnitude of the returned signal  26  is upon the mass and physical characteristics of the flowing material  2 , the quantity of the material  2  and the intensity of the transmitted signal  31 . The returned signal  26  will also be dependent upon the absolute position of the mass flow  2  within the pipe. If region  46  represents a region of relatively low microwave flux and region  47  represents a region of relatively high microwave flux, the transit of roped material  2  from region  46  to region  47  will result in a dramatically different return signal  26  to transceiver  3 , even though the mass flow rate through pipe  24  has remained relatively constant.  
         [0065]    As seen in FIGS. 7 and 8, the present invention may be used advantageously by substituting an entire section of metallic pipe  1  with an entire section  33  which has been formed to include a liner of microwave absorbent material  34 . The substitute section is affixed to the existing pipe at flange  42 . The microwave transceiver  3  is attached to feed pipe  35  and aligned along the axis of the feed pipe  35  to emit microwaves  36  into the interior  37  of the substitute pipe section  33 .  
         [0066]    The substitute section  33  is preferably constructed so as to have a metallic exterior. The inner wall  38  of section  33  is lined with radar absorbing material  34 , which is protected by a microwave transparent liner  39 . The emitted waves  36  pass through the liner  39  and into the area of the flowing material  2 . Some of the microwave signal  36  is reflected by the material  2 , the reflected signal  43  passing back through the feedpipe  35  and into the transducer  3 . This signal is reflected amplified, filtered and analyzed to determine the mass flow rate. The microwave signal  36  that is not reflected or attenuated by material  2  continues traveling until recontacting the liner  39  and passing into the microwave absorbing material  34 , where the emitted signal  36  is attenuated. This attenuation retards the reflection of the microwave signal back into the pipe  33 , thus substantially eliminating the problem of further reflections downstream in the pipe where moving objects may be encountered. Further, the attenuation of signal  36  inhibits the relatively high intensity localized microwave flux caused by standing waves created by interaction with the otherwise present reflection of signal  36 . Also note that the outer diameter  40  of section  33  is greater than the inner diameter  41  of pipe  24 . This is necessary so that there will be no aerodynamic discontinuity to the flow of material  2  within pipe  24  and through section  33 . Changes in the flow properties result in chaotic turbulent flow which makes flow measurement more difficult. In general this arrangement results in the vast majority of reflected energy  43  which reaches transceiver  3  being the result of Doppler shifted interaction with the flowing particulate matter  2  as opposed to reflections from object  29  which lie beyond the boundaries of section  33 .  
                                         Flow Rate Measuring Apparatus Parts List                                1   Pipe       2   Flowing Material       3   Transducer       4   Flange       5   Feed Pipe       6   Emitted Signal       7   Microwave Signal       8   Electromagnetic Wave       9   Desired Signal       10   Vibrating Object       11   Reflected Signal       12   Reflection       13   Direction of Arrow       14   Angle       15   Diameter       16   Wall       17   Arrow       18   Transmitted Signal       19   Transducer Antenna       20   Separate Receiver       21   Reflected Waves       22   Absorbent material       23   Region       24   Material Conveying Pipe       25   Metallic Feed Pipe       26   Material Flow Induced Signal       27   Propagated Signals       28   Downstream Radiation       29   Fan       30   Doppler Shifted Signal       31   Gradually Weakening Radiated Signal       32   High Intensity Emitted Signal       33   Substitute Section       34   Microwave Absorbing Material       35   Feed Pipe       36   Microwave Signal       37   Interior       38   Inner Wall       39   Microwave Transparent Liner       40   Outer Diameter       41   Inner Diameter       42   Flange       43   Reflected Energy       44   Relative Movement       45   Detected Signal       46   Region       47   Region