Abstract:
A transit time ultrasonic fluid flow sensor is configured as a self contained modular unit that does not rely on acoustic transmissions through or reflected by a pipe for operation. A highly efficient transducer assembly transmits and receives signals which are well isolated from each other and which enables simplification of the supporting electronics. Continuous wave transmission and reception of the acoustic signals, and provision for acoustic path and electronic drifts enable the sensors to be physically small and yet to exhibit high sensitivity and measurement precision. Some versions of the apparatus sense flow along two orthogonal axes. In addition, electrolytic cleaning is provided for some versions of the sensor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/298,558 filed Apr. 22, 1999, and Ser. No. 09/356,960 filed Jul. 19, 1999, both now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     TECHNICAL FIELD 
     The present invention relates to a simplified method for measuring the flow rate of a fluid whereby the propagation times of ultrasonic signals transmitted through the fluid can be detected to determine fluid flow rate. The invention further relates to improvement of a probe flow sensor configuration and to its installation, and similar sensing devices. 
     DISCUSSION 
     Transit-time ultrasonic flow sensors, also known as “time-of-flight” ultrasonic flow sensors, detect the acoustic propagation time difference between the upstream and downstream ultrasonic transmissions resulting from the movement of the flowing fluid and process this information to derive a fluid flow rate. Several different sensor configurations have been used including: 1) direct measurement of the propagation time of a pulse emitted by a first transducer and received by a second transducer where the change in time is a function of fluid flow rate; 2) dual “sing-around” sound velocimeters, where the difference in “sing-around” frequency between the velocimeters is a function of the fluid flow rate; 3) sensors producing continuous waves using two widely different high frequency carriers but commonly modulated with another much lower frequency signal where the phase difference of the modulated signal on the received carriers is a function of the fluid flow rate; and 4) sensors producing bursts of continuous waves using a single frequency on a pair of transducers, the burst duration being less than the acoustic propagation time between the transducers, where the time between the received transmissions is a function of flow rate. 
     The transducers of transit-time ultrasonic flow sensors are most often field mounted, whereby they are individually attached to the outside of a pipe, thereby offering the advantage of not having to break into the pipe as with other meters, in order to make the flow measurement. However, the uncertainty of the pipe wall integrity and the effects of its surface condition, and the uncertainties of locating, attaching and acoustically coupling the transducers to the pipe, and the uncertainties of the reflection from the interior of the pipe when it is used to complete the acoustic path between the transducers, can often lead to substantial measurement error. Even when the transducers are in contact with the fluid being measured (wetted), their mechanical location, which is again most often accomplished in the field, may result in misalignment or being disposed at the wrong distance or angle, again resulting in measurement error. As a result, these sensors are usually equipped with supporting electronics containing sophisticated diagnostic means for confirming proper installation and operation. Overall, these sensors are relatively expensive and have a reputation for sometimes producing erroneous measurements. 
     It is therefore an object of the present invention to provide means for reducing the complexity and cost of transit-time ultrasonic flow sensors, to improve their measurement reliability, and to make their installation easier. 
     It is a further object of the present invention to provide a cost effective means for enabling transit-time flow sensors to measure a fluid flow rate along two axes. 
     SUMMARY OF THE INVENTION 
     The above and other objects are satisfied with a transit-time flow sensor configured as a single modular unit as exemplified in accordance with preferred embodiments of the present invention. One of the preferred embodiments includes an insertion probe with two permanently mounted transducers which enters a relatively small opening in a pipe carrying a flowing fluid. A probe-mounted acoustically reflective surface is also provided to enable the acoustic path to be completed within the entity of the probe so that it does not depend upon any other reflective surface for its operation. This sensor is thus more simple in construction, easier to install correctly, and provides more reliable operation. The probe may of course be supplied factory mounted and calibrated in a short section of pipe, and thereby be considered a “full bore” sensor for replacement between two similar pipe sections. 
     The transducers are mounted one upstream and one downstream, in line with and at an angle to the fluid flow, and are directly wetted by the fluid. In this configuration the sensor is isolated from the attenuation and multipath problems which occur when the transducers are pipe mounted. The supporting electronics may be simplified in concept and incorporate cost effective components while still offering good measurement precision. 
     The method of flow rate sensing used with the present invention is different from the four methods listed previously in the “Discussion” hereof, in that it uses a variable frequency acoustic signal which is continuously transmitted by either one or the other transducers as they alternate between the transmitting and receiving functions. The relatively low alternation frequency is the exclusive modulation source and the primary detection of time difference occurs at the acoustic frequency without using an intermediate frequency. Since this method in its basic form is unstable because of both acoustic path and electronic related drifts and uncertainties, special provisions have been employed to correct these deficiencies so that high flow sensing sensitivity and zero stability, along with low noise level, is obtained. 
     In another preferred embodiment, two pairs of transducers are used within a single sensor housing and are located to determine fluid flow rate components along two orthogonal axes. 
     In a further preferred embodiment, an electric current is passed between surfaces of the sensor in the proximity of the acoustic path so that, by the process of electrolysis, those surfaces will be maintained clean. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referencing the following drawings in which: 
     FIG. 1 illustrates a simplified side cross-sectional view of a preferred embodiment of the ultrasonic transit-time flow sensor of the present invention; 
     FIG. 1A illustrates a simplified end cross-sectional view of the sensor in FIG. 1; 
     FIG. 2 illustrates a block diagram indicating the functional blocks of electronic circuitry of the sensor in accordance with a preferred embodiment of the present invention; and 
     FIG. 3 illustrates an arrangement of two pairs of transducer elements in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG.  1  and to FIG. 1A, a flow sensor  10  in accordance with a preferred embodiment of the present invention as it would be typically mounted in a pipe, is shown. Fluid, the flow direction of which is represented by arrow  12  enters pipe  14  and passes between piezoelectric transducers  16  and  18 , and a reflector  20 . Transducers  16  and  18  are each mounted to reduce internal acoustic reflections and to direct transfer of acoustic energy between each other so that at least a component of the acoustic energy is along an axis parallel to the direction  12  of flow. A material  34  such as cork can be used as an acoustic energy absorbing material surrounding each transducer and as a barrier between the transducers, as may be required to provide the acoustic isolation needed for proper operation. The dominant acoustic energy transferred between the transducers  16  and  18 , is beamed along the lines  22 . A housing  24  mounting the transducers  16  and  18 , joins with a stem  26  upon which is mounted an electronics enclosure  28 . Reflector  20  is supported by posts  36 . The stem  26  is mounted through hot tap fitting  30  to a pipe mounted fitting  32 . 
     During operation one of the transducers  16  or  18  transmits while the other receives. Then they alternate functional states so that the transducer formerly transmitting is now receiving, and the transducer formerly receiving is now transmitting. Acoustic energy is being transmitted by one or the other transducer  16  or  18 , all of the time and received by one or the other transducer  16  or  18 , all of the time. The difference in phase shift between the acoustic energy signals of the two functional states is representative of fluid flow rate. With these and other refinements, the acoustically self contained module which sensor  10  exemplifies thereby exhibits low noise and high stability and measurement precision. The sensor  10  further enables the electronics to be simplified and the mechanical assembly to be standardized to cover a wide range of pipe sizes. Furthermore, the risk of improper operation due to installation or pipe related factors is greatly reduced. 
     FIG. 2 illustrates the block diagram of a representative electronic circuit for the present invention. Oscillator  246  generates a high frequency carrier signal which is amplified by amplifier  248  and routed through relay  260  to transducer  16 . The acoustic energy received by transducer  18  is converted to the corresponding electrical signal which is routed through relay  260  to amplifier  238 , and then to limiter  240 . The output from limiter  240  enters Schmitt trigger  268  which then provides an inverted signal to phase detector  244  and Schmitt trigger  270 . The output signal from Schmitt trigger  270  enters phase detector  242 . Both phase detectors  244  and  242  also receive a common input from oscillator  246 , and each provides an output signal to the corresponding low pass filters  250  and  251 , whose outputs then enter differential amplifier  252 . The alternating signal component of the signal from amplifier  252  passes through differentiating circuit  253  to buffer amplifier  255  and then to inverting buffer amplifier  257 . The output of amplifier  257  is then routed by relay  258  to the low pass filters  254  and  280  and then to differential amplifier  256 . The output from amplifier  256  enters the switch  249  and is routed through buffer amplifier  272 , low pass filter  261 , and amplifier  259  for providing the rate output signal. 
     Oscillator  278  activates relays  260  and  258 . Oscillator  266  activates switches  276  and  249 . Amplifier  274  detects the voltage difference between the output from low pass filter  250  and a reference voltage “A”, and produces an output signal which passes through low pass filter  263  to the frequency control  264  to vary the frequency of oscillator  246  as in a phase locked loop. The output from low pass filter  263  is also switched to a reference voltage “B” when switch  276  is activated by oscillator  266 . Balanced circuits are indicated to reduce drift and increase dynamic operating range. 
     Oscillator  246  produces a continuous carrier signal at a high frequency, for example 4 MHz. This signal is increased in magnitude by amplifier  248  and routed through relay  260  to transducer  16  which emits a corresponding acoustic signal. A portion of that acoustic signal is received by transducer  18  which converts it into a corresponding electrical signal that is routed by relay  260  to amplifier  238  to increase its magnitude and then to limiter  240  to produce a relatively large output signal for the smallest received signal of interest to insure that the Schmitt trigger  268  is reliably switched between its trigger points. The Schmitt trigger  268  drives the second Schmitt trigger  270  so that phase detectors  244  and  242 , provide  180  degree phase opposed received signals while their other inputs from oscillator  246  are of the same phase. The outputs from the phase detectors  244  and  242 , are therefore phase opposed and enter low pass filters  250  and  251  to remove the high frequency components prior to entering differential amplifier  252 . The differential amplifier  252  extracts their difference voltage, the magnitude of that voltage being a function of the phase difference between the oscillator  246  signal and the signal provided by transducer  18 . 
     Relay  258  and  260  are activated by oscillator  278  which produces a square wave at a relatively low frequency, typically 100 Hz. Relay  260  comprises means or alternating the functional states of receiving and transmitting for the two transducers  16  and  18 , so that the difference in the transit times between the upstream and downstream acoustic paths resulting from relative movement between the transducers and the fluid flow being sensed produces an alternating signal at the output of amplifier  252 . The DC component of the output signal of amplifier  252  is removed by differentiating circuit  253  so that only the magnitude of the alternating component of that signal enters buffer amplifier  255  and then its inverting amplifier  257 . DC drifts present at the output of amplifier  252  are thereby eliminated so that they will not affect the output and, in particular, the zero stability of the rate output signal. 
     The phase opposed signals from amplifier  255  and inverter  257 , are synchronously switched in relay  258  at the 100 Hz frequency provided by oscillator  278 . This provides rectification of the signals therefrom, which are then smoothed by low pass filters  254  and  280 , prior to entering differential amplifier  256 . 
     The output signal from amplifier  256  is then a DC voltage responsive to the magnitude of the difference in transit time between the mode of operation where transducer  16  transmits while transducer  18  receives, and the mode of operation where transducer  18  transmits while transducer  16  receives. The output from amplifier  256  enters the normally closed switch  249 , which has in parallel with its output a capacitor for retaining the most recent amplifier  256  output voltage in the event switch  249  is opened. Amplifier  272  is a high impedance buffer amplifier to minimally discharge the capacitor when switch  249  opens and provides the signal through lowpass filter  261  to output amplifier  259 . 
     Transducers exhibit the equivalent of acoustic phase drift in characteristics due to aging, temperature, mechanical stress and other factors, which affect the precision of the flow rate measurement. The mechanical mounting of the transducers and their relationship to the acoustic reflecting surfaces that they interact with, particularly if those surfaces are of plastic, may be unstable over a period of time considering moisture absorption, accumulation of surface contamination and stress relief. Mechanical stress and temperature can also cause acoustic phase drift. Because of the desire to produce a small insertion probe, the spacing between the transducers is relatively small and relatively small mechanical changes increase the drift problem. The higher frequencies desired to increase phase detection sensitivity because of the small transducer-to-transducer spacing, result in further instabilities. Add the above drift promoting factors to the phase drifts produced by the supporting electronics, particularly when they are of relatively low cost commercial quality and designed with a clear intent to reduce costs, and the total impact of these drifts can be catastrophic for a transit-time flow sensing product. It can now be appreciated that the prior art does not overcome these difficulties to produce a commercial product competitive with probes relying on other flow sensing technologies, while the present invention is successful at doing so. 
     It may at first seem obvious that the phase locked loop would alone be sufficient to provide the frequency control of oscillator  246  so that the average output from phase detector  242  is maintained in the center of its operating range. However, this is not the case. Consider, for example, that while the frequency range of the oscillator needs to be great enough to compensate for the few acoustic wavelengths of maximum possible phase drift, it should be no greater in order to minimize oscillator  246  phase noise and drift. This would be, for example, the equivalent of one or two acoustic wavelengths on either side of its center frequency. However, should phase lock occur with oscillator  246  operating close to a limit of its frequency range, and subsequent phase drift requires oscillator  246  to change its frequency in the direction that it is incapable of accommodating, the resultant will be sensor performance degradation, if not actual failure. Oscillator  246  might become phase locked close to a frequency limit when operating power is applied, for example, or because the acoustic path has been broken, for example, because of insufficient flowing fluid or absorption of the acoustic energy. This could also occur because of reflection or scattering of the acoustic beams due to composition of the flowing fluid and then being reestablished, or because of phase drift inherent in the sensor. In any case, a means is required to both initialize and maintain the frequency of oscillator  246  and phase detector  242  in the nominal center of their operating ranges. 
     The above is accomplished by providing a switch  249  which is activated by oscillator  266 , and which produces a pulse at a very low frequency, for example once every minute, to enable the frequency control circuits to reset to the center of their operating range. During the time of this pulse and for a short time afterwards to enable the circuits to stabilize, a total duration of, for example, two seconds, the output from amplifier  259  is maintained at the value existing just before the pulse occurred to avoid upsetting the rate output signal. 
     Oscillator  266  also activates relay  276  which connects during the oscillator  266  pulse, the reference voltage “B” to the capacitor of long time constant filter  263  to force it to the voltage which acts on frequency control  264  to adjust the frequency of oscillator  246  to the nominal center of its operating range. At all other times, the signal representative of the phase difference between the oscillator  246  and Schmitt trigger  270 , as it appears at the output of low pass filter  250 , is used in conjunction with amplifier  274  and low pass filter  263  as in a conventional phase locked loop. Reference voltage A is of the magnitude corresponding to the detector  242  center of its operating range so that amplifier  274  will vary its output voltage to maintain the voltage on its positive input essentially equal to that on its negative input, thereby locking the loop. The low pass filter  263  has a relatively long time constant so that its output does not contain a significant 100 Hz component, enabling the phase locked loop to respond only to the average of the phase detector  242  output, while the full alternating component of its signal enters amplifier  252 . 
     An alternative method of control is to have a phase shift block with a similar number of acoustic cycles of phase shift capacity located between oscillator  246  and the phase detectors  242  and  244 , which is similarly controlled by phase detector  242 , low pass filter  250 , amplifier  274  and low pass filter  263  to maintain the phase detector  250  output centered in its operating range. In this case, the phase shift block is periodically connected by switch  276  to the reference “B” voltage to force its operation to the nominal center of its operating range. In this method, oscillator  246  is not frequency controlled and the frequency control block  264  is omitted. 
     Reset of the phase locked loop occurs periodically as a function of the oscillator  266  frequency. Reset can also occur by alternate means, for example, by detecting when power is first applied, when the average output from phase detector  242  is detected to be sufficiently removed from its center value, when the acoustic signal is reestablished after loss or when other criteria are met. 
     The use of balanced circuits, as in the case of phase detectors  242  and  244 , for example, reduces the upsetting effect of the reset pulse. This is because the oscillator  246  is never phase locked exactly to the center of its frequency range and must make an abrupt frequency change before resuming stable phase lock. In some applications this disruption may be small enough to be insignificant so that relay  249  and amplifier  272  may be omitted. Furthermore, relay  249  and amplifier  272  may be omitted if the reset pulse occurs infrequently enough for the intended sensor application, although it produces a significant change in the rate output signal. 
     The reset means and overall high sensitivity signal-to-noise ratio employed in the present invention, as exemplified in particular in FIG. 2, make the sensor very tolerant of operating conditions. Flow probes can now be made economically with small dimensions which make them easy to install. These probes can have their transducers set at small angles, such as 75 degrees with respect to the flow axis, and thereby be relatively small so that the hole in the pipe through which the probe passes during installation is likewise, relatively small. For example, a probe may be made with a 0.8″ transducer  16  to transducer  18  spacing and fitting through a 1.0″ hole in the wall of a pipe. Furthermore the probe will be relatively tolerant of the distance between the reflector and the transducers  16  and  18 , so that it can even be supplied without the reflector and rely instead on the opposite inside wall of the pipe that they would be mounted on to act as the reflector and complete the acoustic path between transducers  16  and  18 . 
     A relatively small amount of low pass filtering enables the amplifier  256  output to respond to rapidly changing flow rates so that characteristics of the fluid system in which the probe is used, such as pulsations due to pump impellers, can be detected and provided as an output signal indicative of impeller condition. The sensor  10  may then additionally serve as a flow system maintenance monitor. 
     The amplifier  256  output may also be used to detect flow variations due to vortex shedding or fluidic oscillators for example. If only flow detection of flow variations is required, the circuit may be simplified by eliminating relay  260 , amplifiers  257 ,  272  and  259 , low pass filters  261  and  280 , and relay  249 . Flow rate pulsations due to Karman vortices as may be produced by a bluff body  38  in the flow stream, whereby the street of vortices pass through one or both acoustic beams of the transducers  16  and  18 , of sensor  10  will be detected whereby the frequency of the pulsations is proportional to flow rate. The bluff body  38  may be located to one side of or between the acoustic beams  22 . However, if sensor  10  is rotated in its fitting  30  by 90 degrees, one of the posts  36  may be used as the bluff body  38  to produce the vortices. 
     In the process of detecting the phase difference between the received and transmitted signals, DC offsets appear at the output signal of amplifier  252  which may drift over a period of time and seriously affect the zero flow stability of the sensor. By removing the DC component of that signal with differentiating circuit  253 , that source of error is eliminated. In the sensor referred to earlier, zero stability, equivalent to a sensed flow rate of 0.05 inches/second, has been observed. This is significant particularly when it is noted that no high stability or matched filters or other sophisticated or inherently expensive components were used, the transducers were not impedance matched for both the receiving and transmitting functions, and transmitted signals were seriously distorted from the desired sinusoidal wave shapes. Furthermore, conventional and relatively low cost commercial components were used throughout to reflect the commercial intent of future products based upon the present invention. Additionally, only three adjustments, these being output zero, output span and center frequency, are required for the entire circuit. 
     Exclusive-OR gates operating at the carrier frequency are preferably used as the phase detectors  242  and  244  to detect the phase shift between the locally generated signal from oscillator  246  and the received signal from transducer  216 . There is no translation of frequency so that phase noise and drift from this source is minimized. The circuit would also function if the phase detectors were supplied with receive signals having the same phase while the signals from oscillator  646  were phase opposed. The carrier frequency used is relatively high, typically 4-10 MHz, so that a substantial flow related phase shift occurs for a physically small probe. 
     It is noteworthy that the transmitting signals may be seriously distorted without seriously affecting the overall operation of the sensor. The output of amplifier  248  may be a square wave, whereby only a small inductive impedance in series with relay  260  is used to provide a semblance of series resonance with the transducer capacitance. Furthermore, The circuit is very tolerant of impedance mismatch of the transducers  16  and  18 , between the functions of transmitting and receiving, so that in an existing practical sensor design there is no provision for such matching. 
     Since the transmitted and received acoustic signals are always present in the general application of the present invention with each transducer operating half of the time to transmit and the other half to receive, phase detection occurs over many carrier cycles. In this manner, signal detection averaging is extensive and the signal-to-noise ratio is high. Therefore, at the relatively low alternating frequency of 100 Hz (compared to a 4 MHz carrier frequency) and the relatively short acoustic transit time (because of the short length of the acoustic path between the transducers  16  and  18 ), any error introduced because of resultant signal delays in the acoustic path between the transducers  16  and  18  will, on the average, be a very small factor of the rate output signal and will therefore be negligible. However, should the transit time become significant, one or more functional blocks for receiving or detecting the rate related signals, such as a portion of relay  260  and amplifiers  238  and  268 , limiter  240 , inverter  270 , and phase detectors  642  and  244 , would be disabled for the required interval during the transducers  16  and  18 , transmitting/receiving alternation. 
     When the acoustic signals are reflected between the transducers with a flat reflector, the received signal is a summation of the transmitted signal having taken more than one path because of beam spreading. As the transmitted frequency changes to maintain phase lock and center phase detectors,  242  and  244 , operating points, the change in the relative magnitudes and phase relationships of the components of the received signal may cause increased noise and drift in the rate output signal. This problem may be reduced by curving the surface of the reflector facing the transducers in the direction along the axis between the transducers with a radius equal to about two times the distance between the midpoint between the transducers and reflector. Because this degree of curvature is very small, it should not affect fluid flow sensing in most applications. The inside surface of a pipe would not provide the same benefit because its curvature is orthogonal to that desired. 
     In some applications it will be desired to operate the sensor with low power. Considerable power reduction without serious loss of flow measurement precision may obtained by enabling all the sensor&#39;s functional blocks operating at the acoustic path frequency to operate for only a few alternating cycles within a larger period of operation, whereby low pass filter  263  becomes equipped with a sample-and-hold circuit to retain its frequency control voltage during that period. Relay  249  acts in effect as a sample and hold circuit and would be switched to amplifier  256  only during the latter portion of the alternating frequency cycles when the amplifier  256  output signal has stabilized. Since the functional blocks operating at the acoustic path frequency consume almost all of the operating power of the sensor, a large power reduction is possible. For example, with a larger period of one second and the acoustic path functional blocks being active for  100  milliseconds, the power reduction is nearly 90%. 
     The flow sensor configuration of probe  10  is also suitable for use in open channels and in large bodies of water, for example, as it provides for the complete reflective path within itself and can operate with low power. Furthermore, a second set of transducers  370 ,  372  located in an enlarged from of housing  24 , and mounted orthogonally to transducers  16  and  18 , which similarly beam to and receive from reflector  20 , will provide a measurement of flow rate in a direction orthogonal to the first set whereby their rate and directional components enable a resultant flow rate and angle to be determined by electronic computation. Such a transducer arrangement is illustrated in FIG. 3 whereby transducers  16  and  18  of probe  10  establish an acoustic energy beam line  22 . The beam line  22  is reflected by reflector  20  to sense the component of fluid flow which moves horizontally across the page, in the direction of arrow  12 . Transducers  370  and  372 , mounted in the same housing  24 , also establish an acoustic energy beam, represented by lines  374 , to sense the component of flow moving orthogonally to energy beam  22 . It is possible for both sets of transducers  316 , 318  and  370 , 322  to operate at the same time with their own supporting electronics when the acoustic beam angles are narrow if there is otherwise good acoustic and electrical isolation between them. However, they may for example also time share the same electronics or operate at different frequencies. 
     The installation of probe  10  in a pipe for example, typically requires that it be angled to the flow precisely, if the best flow sensing accuracy is to be achieved. Observing an indication of a flow rate measurement while adjusting the probe angle for the maximum indication is useful, but often unsatisfactory, considering that flows are often not constant enough. A pipe mounted flow probe configured as in FIG. 3 however, would do very nicely whereby the second set of transducers  370  and  372 , sensing fluid flow orthogonal to primary flow direction  12  through the pipe, would be aligned to produce an output signal null when the probe angle is exactly correct. As a null indication, the magnitude of the fluid flow  12  rate is not important as long as some flow  12  is occurring so that a steady flow rate is no longer necessary. A null type of indication is also more sensitive and can be made to respond faster. 
     In some fluid handling systems, the fluid flow is not reasonably uniform and straight with a variability depending on flow rate and other factors such as nearby valve states. The affects of these nonuniformaties may also be sensed by the second set of transducers  370  and  372 , with their flow responsive signals being used for correcting for flow measurement being made by transducers  316  and  318 . 
     The second set of transducers  370  and  372 , if not used to provide a precision rate measurement, need not be mechanically located as shown and do not even have to make use of reflector  20 . Transducers  370  and  372  merely need to lie parallel to and facing each other with the flow and acoustic energy passing between them to respond to the component of flow which is not parallel. Since transducers  370  and  372  have the axis between them perpendicular to the fluid flow  312 , they may additionally be used to directly measure the speed of sound of the fluid whereby the output signal obtained therefrom is used, for example, to vary the gain of amplifier  259  to compensate for flow rate errors due to fluid temperature or composition changes. Any suitable supporting electronics configurations including those listed in the earlier “Discussion” hereof may be used to obtain the related output signals. Compensation means may also be had by measuring the temperature of the flowing fluid and using the output signal obtained therefrom to similarly vary the gain of amplifier  259 , when ere is close correlation between the fluid temperature and the speed of sound. 
     When used in flow environments which encourage the accumulation of surface coatings, debris or biogrowths, electrolytic means may be used to clean or maintain clean the acoustically active surfaces. This would consist, in a sea water environment for example, of a positive potential being applied to the flow sensing or nearby surfaces which had been platinum plated so as to cause a corresponding electric current to flow through the water to and generate chlorine gas at those active surfaces. Nearby insulated electrodes or conductive surfaces with a corresponding negative potential complete the current path. Low currents of several milliamperes and less have been found effective in maintaining the surfaces of a small flow sensor of a few square inches in surface area clean in such environments. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms where a transit-time ultrasonic flow sensor is configured as a self contained modular system. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specifications and claims.