GB1580524A - Flow meter - Google Patents

Abstract

A flow meter for the accurate determination of the rate of flow of a flowing medium and for the determination of the speed of sound in this medium has a transformer arrangement (24, 26) through which the flowing medium flows. Two cylindrical transformers (24, 26) are provided, of which alternately one transmits acoustic pressure waves and the other receives these pressure waves. The phase difference between the transmitted and the received signals is stored for two consecutive transmit/receive cycles, the difference between two consecutively stored phase differences specifying the magnitude and the direction of the flow of the flowing medium, and the sum of the stored phase differences being a measure of the speed of sound in the flowing medium. The flow meter has a device for automatically compensating changes in the composition of the flowing medium, with the result that the specified rate of flow is correct at any time. <IMAGE>

Description

(54) FLOW METER (71) We, THE PERKIN-ELMER CORPORATION, a Body Corporate, organized and existing under the laws of the State of New York, United States of America, having a principal place of business at Main Avenue, Norwalk, Connecticut 06856, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to flowmeters and particularly to acoustical flowmeters for measuring the rate of flow of a fluid along a confined path.

A typical acoustical wave flowmeter is disclosed in U.S. Patent No. 3,109,112. This flowmeter has a pair of transducers for generating and receiving compressional waves in either the audible or ultrasonic frequency range and which are located within an enclosure through which the fluid flows. The transducers are alternately caused to transmit and receive so that compressional waves are produced in the gas by the transmitting transducer and received at the other transducer. By measuring the phase difference between the transmitted and the received waves in both directions, the velocity of the gas passing through the transducer is determined. The transducers described in the above U.S. patent may be modified so as substantially to eliminate any problems due to gas composition or temperature changes in the gas.

The above arrangement requires that the transducers be located in the flow path of the gas whose velocity is to be measured, or located in a cavity in the conduit wall. In either case the normal flow of the fluid is substantially altered when passing the transducer thereby causing the accuracy of the meter to suffer. Additionally, sedimentary particles or the like in the fluid can collect around the transducers thereby impairing their transmitting and receiving characteristics.

In an effort to overcome these and other difficulties of known acoustical flowmeters, a flowmeter is described in U.S. Patent No.

4,003,252 which is designed to have no obstructions along the flow path and in which the transducers are generally cylindrical in shape and are disposed within the walls of the fluid conduit thereby eliminating all obstructions in the flow path as well as eliminating cavities in the conduit wall in which debris might collect.

Nevertheless, this flowmeter may become inaccurate or fail to function entirely whenever the composition of the gas flowing therethrough is different from that for which the meter is calibrated. Indeed, this is a common problem with most acoustical flowmeters as the velocity of acoustical compressions within a gas is a function of its chemical composition so that typical acoustical flowmeter accuracy depends on calibrating it for the specific gas whose flow is to be measured.

Requiring re-calibration each time the flowmeter is used with a different gas is, at best, an inconvenience. In applications where the gas composition changes as the flowmeter is used as, for example, in pulmonary function analyzers, known acoustic wave flowmeters are inaccurate unless corrective feedback is provided from a gas analyzer to compensate for gas composition change.

According to the present invention a meter for measuring rate of fluid flow comprising a member formed with a passage defining a path of fluid flow, and having a pair of electro-acoustic transducers spaced along the path, each of which transducers is capable of transmitting and receiving acoustic compression waves and of producing a signal on receipt of such waves has the two transducers connected in a control circuit for producing alternately two cycles of operation, in the first of which one transducer transmits and the other produces a receipt signal and in the other of which the other transducer transmits and the first produces a receipt signal, the frequency of the acoustic compression waves produced by each transducer being automatically adjusted by regulation of the control circuit to a value where a resonant echo occurs at the other transducer, the meter also including means for measuring the phase difference in each cycle between the transmitted acoustic compression wave and the received signal and for producing a difference signal dependent on the difference between the two phase differences, and the magnitude and sign of which are directly related to the instantaneous fluid flow rate and its direction.

The term "resonant echo" is used to mean resonance of the acoustic compression wave within the passage in the locality of each transducer caused by reflections of the wave off the walls of the passage. This effect depends both on the frequency of the wave and the velocity of the wave through the gas and, as just described, the former is automatically adjusted to allow for variations of the latter, so as to maintain the state of resonance. As will be described in more detail later, the automatic adjustment of the frequency leads to increased accuracy of measurement regardless of changes in fluid composition or temperature during use.

A flowmeter in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the flowmeter including a transducer assembly and a block diagram of electronic circuitry associated therewith; Figure 2 is a detailed circuit diagram of a timing pulse generator for controlling the operation of the circuit of Figures 4 and 5; Figure 3 is a timing pulse chart for the circuit of Figure 2; and Figures 4 to 6 are a detailed circuit diagram of a circuit which utilizes the timing pulses generated by the circuit of Figure 2.

Referring first to Figure 1, the flowmeter illustrated includes a transducer assembly 10, shown in longitudinal section, which comprises a substantially cylindrical body having a central opening extending through a support member 14 through which gas flows from the left to right as indicated by the arrows 12. Gas can flow through the assembly 10, however, in the opposite direction if so desired.

The transducer assembly 10 is made generally in accordance with U.S. Patent No. 4,003,252 and accordingly, may additionally include a cylindrical casing made of metal or other suitable material (not shown in Figure 1) which surrounds the support member 14. The support member 14 itself is preferably fabricated from a polyurethane foam, foam rubber or other material having good acoustical damping properties. Two annular notches 16 and 18 are formed in the inner wall of the support member 14 and are disposed in spaced relation to each other along the path of fluid flow as indicated by the arrows 12.

Two additional annular notches 20 and 22 are disposed at opposite ends of the support member 14.

Disposed respectively within and in contact with the surfaces defining each annular notch 16 and 18 is a cylindrical transducer 24 and 26 each having an inner cylindrical bore passing therethrough which is substantially co-extensive with the inner cylindrical bores 28 which extend between the annular notches of the support member 14.

Transducers with other geometry such as arcuate transducers may also be used with appropriate changes to the remaining parts of the transducer assembly 10 so as to maintain a centrally located bore therethrough with no obstructions and no cavities in the bore wall to provide a locus for particulate matter to collect.

Disposed respectively within each annular notch 20 and 22 is a terminal ring 30 and 32 which is preferably made of sound absorbing material such as a polyurethane damping foam such as Y-370 Vibration Damping Tape manufactured by the 3M Corporation. Other materials having similar sound absorbing properties may also be used for the terminal rings 30 and 32. The terminal rings 30 and 32 each have an inner cylindrical bore which is substantially co-extensive with the adjacent cylindrical bores 28 of the support member 14. Accordingly, the fluid flow path through the transducer assembly 10 as indicated by the arrows 12, has a substantially continuous wall so that the fluid flowing therethrough encounters neither protuberances nor cavities.

The transducers 24 and 26 may comprise one of a number of conventional devices capable of operating in radial or hoop mode for generating acoustical compressions within the gas passing through the assembly 10. Examples of suitable materials for the transducers 24 and 26 include hollow cylindrical bodies made of polyvinylfluoridene or other high polymer organic piezoelectric materials, ceramic transducers fabricated from barium titan ate, lead zirconate titanate, or other polarized polycrystalline ferroelectric ceramic materials, quartz, tourmaline or equivalent electromechanical devices.

The inner and outer surfaces of the transducer 24 and 26 have conductive coatings thereon which constitute the electrical drive electrodes. The conductive coatings on the inner surfaces of the transducers 24 and 26 are respectively connected via wires 34 and 36 to externally located crystal driver/receiver circuits 38 and 40. The conductive coatings on the outer surfaces of the transducers 24 and 26 are also respectively connected via wires 42 and 44 to the crystal driver/receiver circuits 38 and 40. The electrical connection wire pairs 34, 42 and 36, 44 for the transducers 24 and 26 respectively pass through openings 46 and 48 which respectively communicate from outside the assembly 10 to the transducers within annular notches 16 and 18.

The operation of the transducer assembly 10 in co-operation with the electronic circuitry is fundamentally the same as described in the above U.S. Patent No.

4,003,252. Basically the transducer assembly is connected by flexible hoses, tubing or the like to an external source of fluid whose flow rate is to be determined.

The fluid itself may comprise either a gas or a liquid although the illustrated embodiment is designed particularly for measuring gas flow as it passes through the transducer assembly 10 in an axial direction indicated by the arrows 12 or in the opposite direction. A control circuit 50 is coupled respectively by lines 52 and 54 to the transducer driver/receiver circuits 38 and 40. The control circuit 50 is operative to cause one of the transducer driver/receiver circuits 38 or 40 to transmit an electrical signal to the respectively coupled cylindrical transducer 24 or 26 while the other driver/receiver circuit 40 or 38 is conditioned to receive a signal from the coupled transducer 26 or 24. As indicated earlier, the transducer 24 or 26 which receives electrical signals from one driver/receiver circuit responds thereto by producing an acoustic compression in the fluid.At the same time the other transducer coupled to the other driver/receiver circuit 38 or 40 responds to the acoustic compressions in the fluid by producing an electrical received signal which is sensed by the coupled driver/receiver circuit.

The acoustic compressions produced at one of the transducers 24 or 26 take a finite time to travel from the transmitting transducer to the receiving transducer with the propagation time dependent on the direction and the velocity of fluid flow through the transducer 10 and the velocity of sound in the fluid flowing within the transducer 10. These relationships are described in greater detail in the above mentioned U.S. Patent.

As an alternative to the above described transducers, each transducer 24 and 26 can comprise a pair of transducer units mounted in the assembly 10 in a similar manner, one of which units is specifically for transmitting, i.e. producing acoustic compressions, and the other is a receiving transducer, i.e. to produce a received signal in response to acoustic compressions in the gas.

The transducer assembly 10 and the coupled circuitry, operate in the following manner. The control circuit 50 first causes one transducer, either transducer 24 or 26, to produce acoustic compressions in the fluid and at the same time the other transducer, either 26 or 24, is conditioned to receive the acoustic compression from the first transducer. This mode of operation comprises a first transmit-receive cycle. A transmit signal is simultaneously sent from the control circuit 50 over either line 52 or 54 to one driver/receiver circuit 38 or 40 and over the line 56 to a phase detector 58.

A received signal from the receiving transducer 24 or 26 is coupled by either transducer driver/receiver circuit 38 or 40 over lines 60 or 62 respectively to the phase detector 58. The phase detector 58 responds to the transmit signal and to the received signal by producing a signal on line 64 which indicates the phase difference between the transmitted and the received signal. This information is coupled over line 64 to a phase adder and subtractor 66 which includes storage means for storing temporarily the phase difference received from the phase detector 58.

The control circuit 50 then causes the transducers 24 and 26 to change roles. That is, the other transducer 24 or 26 is placed in transmit mode to produce acoustic compressions in the fluid and the first transducer 24 or 26 is placed in receive mode. This mode of operation comprises a second transmit-receive cycle. The phase detector 58 is operative during the second transmit-receive cycle to determine the phase difference between the transmitted signal and the received signal and this second phase difference is transmitted over the line 64 to the phase adder and subtractor 66 which also temporarily stores this second phase difference.

The phase adder and subtractor 66, once the two phase differences have been stored therein, calculates both a sum and a difference between the two stored phase differences. The difference between the two stored phase differences with one further correction described in connection with Figure 6 is presented on the output line 68 which, when the flowmeter is calibrated, is indicative of the flow rate through the transducer assembly 10. In addition, the algebraic sign of the difference between the two phase differences calculated by the phase detector 58 is indicative of the direction of flow through the transducer assembly 10 with a positive algebraic sign representing one arbitrary direction of fluid flow through the assembly 10 and a negative algebraic sign representing the opposite direction of fluid flow.

The sum of the two phase differences as calculated by the phase adder and subtractor 66 is presented on the output line 70. The magnitude of the signal on the line 70 is proportional to the velocity of sound in the gas passing through the assembly 10 compared to the velocity of sound in the reference gas used for calibration. The sum is also transmitted over the line 72 to the control circuit 50 which is utilized thereby in a manner described in greater detail below. In addition, it is used to correct the flow value on line 68. The value presented on the output line 70 is a relative one and the flowmeter must be calibrated so that the magnitude of the value on line 70 can be interpreted.

As indicated generally above, a failing of the flowmeter described in the above mentioned U.S. patent is that the meter there described is not operational in environments where the velocity of sound in the gas passing through the transducer changes dynamically during its use. We have found that the cylindrical transducer system functions by building up a strong resonant echo across the diameter of the tube at one or more characteristics (or Eigen) frequencies known as the natural cavity resonant frequencies. For example, the relationship may be approximately 2.3 A=D, where A is the wavelength of sound in the fluid (cm) and D is the inside diameter of the crystal and tube (cm).

The reason for the failure of the above described flowmeter is due to the fact that the velocity of the acoustic compressions within the gas passing through the flow detecting transducers varies as a function of the gas composition. This causes the amplitude of the acoustic compressions, detected by the transducer in the assembly which is in receive mode to vary because the inner diameter of the transducers is no longer equal to the Eigen wave length thereby causing the magnitude of the signal produced by the receiving transducer to fall off dramatically. Accordingly, the received signal becomes more difficult to detect thereby introducing the possibility of detection error or complete failure to detect the acoustic compression.This failure can be overcome by changing the inner diameter dimension of the transducers within the assembly or by changing the frequency of the acoustic compressions produced by the transmitting transducer. As the transducer assembly itself is not suitable for inner diameter adjustment of the transducers, clearly, the better approach to solving this problem is to adjust the frequency of the acoustic compressions produced by the transmitting transducer. Accordingly, the sum of the phase differences as calculated by the phase adder and subtractor 66 is transmitted over the line 72 to the control circuit 50. As indicated above, the signal on the line 72 is related to the velocity of sound in the gas passing through the assembly 10.

This signal is utilized by the control circuit 50 to change the frequency of the acoustic compressions produced by the transmitting transducer during each transmit-receive cycle so as to maintain the wavelength constant and thereby to maximize the signal received by the receiving transducer.

We have also found, after correcting the frequency in order to hold the wavelength constant, that we must multiply the apparent flow rate by the velocity of sound in order to produce an accurate flow rate measurement independent of change of gas composition.

Experience has shown that flowmeters of the type described in the above identified Patent are sometimes subject to error due to reflection of acoustic compressions from other parts of the system into which the transducer is connected. These reflections frequently result from the acoustic compressions bouncing off other fluid transmitting fittings which are coupled to the transducer assembly. Problems have also resulted when the transducer itself is used in breath analyzers where it is physically located in close proximity to the mouth of the individual whose breath is being analyzed.

The present invention overcomes the reflection problem by providing the terminal rings 30 and 32 which are disposed at opposite ends of the assembly 10. These rings 30 and 32 are made of an acoustic damping material such as that mentioned earlier which is operative to substantially reduce the amplitude of acoustic compressions passing therethrough.

Indeed, the magnitude of the damping provided thereby, if the proper material is selected for the rings 30 and 32, is sufficient so that one end of the assembly 10 may be inserted in close proximity to an individuals mouth whose breath is being analyzed and the problems experienced with other sonic flowmeters due to acoustic reflections is substantially eliminated. The damping provided by the rings 30 and 32 also substantially eliminates problems with reflections when the assembly 10 is coupled into the fluid carrying systems as well.

The foregoing discussion in connection with Figure 1 has generally described the circuitry and operation of the invention.

The circuitry of Figures 2, 4-6 comprise one actual implementation of the invention for use in pulmonary testing equipment, however, those of skill in the art will recognize that alternate circuits may be used. and the components may have to be changed to optimize the circuit for use in other applications for the invention.

The circuitry of Figure 2 comprises a pulse generator for producing control pulses to operate the circuits shown in Figures 46. Figure 3, on the other hand, is a pulse diagram showing the pulse train for different outputs from the circuit of Figure 2. Briefly, the circuit of Figure 2 comprises an oscillator circuit, integrated circuit type CD4047, to which is coupled a 121K resistor and a 4700 PF capacitor for controlling the output frequency which, for the resistor and capacitor mentioned, is 400 Hz. Two series coupled J-K flip-flops, each comprising one half of a CD4027 integrated circuit, are utilized to produce timing pulses at a rate slower than that produced by the oscillator integrated circuit CD4047.

The signals produced by the oscillator and the flip-flops are combined by the AND and NAND gates of Figure 2 so as to produce the respective pulse strings as shown in Figure 3. A pulse diagram for the output Q0 is not shown in Figure 3, however, it comprises a square wave having a frequency twice that for Q, and one leading positive going edge of a pulse on the line labeled Q0 occurs at the same time as the leading positive going edge of a pulse on the output of CD4047 labeled Q1.

Referring now to Figure 5, integrated circuit CD4046 has a voltage controlled oscillator (VCO) which produces a square wave signal at the output pin 4 whose frequency is controlled by the resistors connected in series between pin 11 and the ground as well as by the voltage applied at pin 9. For the particular resistors shown in Figure 5 with a voltage at pin 9 of about +7.5 volts, the frequency of the square wave output from the voltage controlled oscillator at pin 4 is nominally 51 KHz.

Changing the voltage appearing at input pin 9 is operative to shift the frequency of the voltage controlled oscillator. The circuit of Figure 5 will cause the frequency of the VCO to change dynamically to respond to changes in the velocity of sound in the gas in a manner described later in greater detail.

The square wave signal from the voltage controlled oscillator at pin 4 connects via a line 100 to pin 3 of circuit CD4046 which internally connects to a phase detector (DET). The square wave signal also connects via a line 102 to a NAND gate whose output connects to an operational amplifier LM318 and operational multiplier XR2208. The function of these circuits is to produce a sine wave signal at point 104 having the same frequency as the square wave signal appearing at line 102. Those of skill in the art will recognize, however, that the circuitry between line 102 and point 104 comprises only one circuit of many known circuits for converting a square wave into a sine wave signal and that other equivalent circuits may be used.

Disposed between point 104 and the output terminal AC are three operational amplifiers comprising circuit types LM 318 and 8043C. The LM318 operational amplifier is for amplifying of the sine wave at point 104. The two 8043C operational amplifiers are for adjusting the phase of the signal appearing at the output terminal AC with adjustment resistors 106 and 108 being operative to adjust the phase of the signal at the output terminal AC by about 360".

These resistors are adjusted during calibration of the flowmeter and are preferably adjusted so that the signal at pin 9 of circuit CD4046 is +7.5 volts with no air flow through the transducer assembly 10 (Figure 1). The description that follows will describe the mechanism by which phase adjustment of the signal at the output terminal AC causes the voltage at pin 9 to change.

The circuitry of Figure 4 comprises the transducer driver/receiver circuits and shows how they are connected to the transducers 24 and 26. The circuitry of Figure 4 is divided into a driver section appearing to the left of the dotted line 110 and a receiver section, excluding the transducers 24 and 26, appearing to the right of the dotted line 110.

The sine wave signal from the circuitry of Figure 5 is connected to the input terminal AC in Figure 4 and thereafter is coupled to two transducer driver circuits, the first driver circuit including Q5 and the second driver circuit including Q6. These transistors Q5 and Q6 gate the sine wave signal from the input terminal AC to the respectively coupled transducer 24 or 26 thereby placing the transducer into transmit mode. The gating signals are developed by the transistor pairs Ql, Q3 and Q2, Q4 and the respectively coupled circuitry which includes that circuitry of Figure 2 for developing the gate signals which appear at terminals X and Y. As the gate signals at terminals X and Y occur at different times and alternate as shown in Figure 3, the transducers 24 and 26 are alternately placed in transmit mode.

The receiver circuits 116 and 118 are also directly coupled respectively to the transducers 24 and 26, however, the receiver circuits 116 and 118 are either operative or inoperative to respond to signals developed by the coupled transducers 24 or 26 depending on whether the respectively coupled input shorting transistor Q7 or Q8 is either off or on.

Transistor Q7, for example, is controlled by a gating pulse signal Q3 and shorts line 112 to ground whenever the signal Q3 is positive. In a similar manner, the transistor Q8 shorts line 114 to ground whenever the gate signal Q3 is positive. From the pulse chart of Figure 3, it can be seen that Q3 is positive whenever X is positive and hence receiver 116 is inoperative whenever the transducer 24 is in transmit mode and receiving signals from the input terminal AC. Similarly, receiver 118 is inoperative due to Q3 being positive whenever the transducer 26 is in transmit mode. As such, whenever a given transducer 24 or 26 is in transmit mode, the respectively coupled receiver circuit 116 or 118 is inoperative.

However, whenever one transducer is in transmit mode, the receiver circuit 116 or 118 coupled to the other transducer 24 or 26 is enabled because the corresponding gate signal Q3 or Q3 is at ground potential.

For example, during a first transmit-receive cycle, the transducer 24 is in transmit mode and receives signals from the input terminal AC, the transistor Q7 is turned on by the signal Q3 to turn receiver 116 off by grounding line 112 and the receiver circuit 118 is operatively coupled to transducer 26 because transistor Q8 is turned off due to gate signal Q3 being at ground potential. At the same time, the gate signal Y is at ground potential thereby preventing the signal appearing at the input terminal AC from being coupled via transistor Q6 to the transducer 26. The second transmit-receive cycle occurs when transducer 26 receives signals from the input terminal AC, transistor Q7 is turned off by Q3, transistor Q8 is turned on by Q3, gate signal Y is positive and the gate signal X is at ground potential.As such, transducer 26 is in transmit mode, receiver 116 is operative and receiver 118 is inoperative.

The receiver circuit 116 coupled to the transducer 24 includes two operational amplifiers 120 and 122 connected in series and operative whenever transistor Q7 is not turned on to amplify any signal appearing at the input line 112. The amplified signal appears at pin 6 of operational amplifier 122 which is connected to the inverting input of a comparator circuit 124. The comparator circuit 124 is enabled whenever the gate signal A is at logic zero which allows the output signal to appear at pin 7.

The output at pin 7 is at a positive voltage (approximately + 15 volts) when the sine wave input to pin 3 of the comparator 124 is of negative potential and at zero volts whenever the input at pin 3 is of positive potential. Comparator 124 thus converts the sine wave input into a square wave output. Comparator circuit 126 which couples thereto is inoperative during the period when comparator circuit 124 is operative and vice versa.

The receiver 118 includes two series connected operational amplifiers 130 and 132 which amplify the signal appearing on the input line 114 and present that amplified signal to the output pin 6 of the operational amplifier 132. This output signal at pin 6 of amplifier 132 is applied to the inverting input of a comparator circuit 126 which is enabled whenever the gate signal B is at logic zero. The output at pin 7 of the comparator circuit 126 is a square wave having a voltage of approximately plus 15 volts whenever the input at pin 3 is of negative potential and at approximately zero volts whenever the input at pin 3 is of positive potential. Since gate signal A and B occur at different times, the operation of the comparator circuit 124 or 126 does not affect the operation of the other comparator circuit 126 or 124.

Accordingly, the comparator circuits 124 and 126 are independently operative to produce square wave signals at their respective outputs from the sinusoidal signals which are developed across the coupled transducer 24 or 26 whenever that transducer is in receive mode and the required gating signals are present to actuate the receiver circuit 116 or 118. As the output pins of the comparator circuits are coupled together at terminal SIG, the signal appearing at terminal SIG represents the logical AND of the signals developed at the output of comparators 124 and 126.

The output from the receiver circuits 116 and 118 is transmitted via the terminal labeled SIG in Figure 4 to the corresponding input terminal labeled SIG in Figure 5 and then to pin 14 of the integrated circuit CD4046 which is internally connected to the phase detector circuit contained therein. The phase detector itself functionally operates the same as an EXCLUSIVE OR circuit whose output is internally connected to pin 2 of integrated circuit CD4046 and has a logic one output level whenever only one input to the phase detector is also at a logic one level.Accordingly, the signal appearing at pin 2 of integrated circuit CD4046 comprises a square wave signal which is pulse width modulated where the width of each pulse is related to the phase difference between the signal transmitted, i.e. the signal on line 100 and the received signal, i.e., the signal appearing at pin 14 of integrated circuit CD4046.

In normal operation of the flowmeter according to the invention, if the composition of the gas passing through the transducer assembly 10 (Figure 1) changes from that when the meter was calibrated, the phase of the signal appearing at the input terminal SIG in Figure 5 is different relative to the signal phase appearing at that input terminal when circuitry was calibrated. Accordingly, the phase detector output will be different thereby causing the voltage across the one microfared capacitor of the low pass filter comprising a 1 meg ohm resistor and a 1 microfared capacitor coupled to the output of operational amplifier 156 to change relative to that when the meter was calibrated in a manner which is described later in greater detail.

This change in voltage across the 1 microfarad capacitor causes the voltage at pin 9 of integrated circuit CD4046 to change thereby causing the frequency of the signal produced by the voltage controlled oscillator to change as well. The system will continue to adjust the voltage controlled oscillator frequency until the phase difference between the transmitted and the received signal no longer causes a change in voltage across the one microfarad capacitor in the low-pass filter.

The ability to change operating frequency in response to gas composition change is critical to the ability of the present invention to operate where prior art systems cannot. It can be demonstrated that the transducers of the invention are able to produce maximum energy transfer from the transmitting to the receiving transducer when operated at an Eigen (characteristic) frequency which depends on the inner diameter of the transducer.

Accordingly, when air is the gas in the transducer assembly when the meter is calibrated, the frequency of the voltage controlled oscillator corresponds to that which produces acoustic compressions having a fractional number of half wave lengths across the transducer diameter in air which corresponds to an Eigen frequency. If the gas density thereafter changes, the velocity of acoustic compressions therein also changes and hence a different phase difference is detected by the phase detector. This causes the frequency of acoustic compressions produced by the voltage controlled oscillator to change in a manner described in greater detail later and it can be demonstrated that the new frequency corresponds to one where the diameter of each transducer is again the Eigen value number of half wavelengths at the new frequency in the gas then in the transducer assembly.As such, maximum energy transfer between a transmitting and a receiving transducer is maintained.

The pulsewidth modulated signal appearing at the output of the phase detector comprises one of two inputs to an AND gate 40 whose output is coupled to an integrator circuit indicated generally at 142.

The second input to the AND gate 140 is an integrate enable signal IE which is a logic one whenever either gate signal A or the gate signal B is logic zero, a condition indicating that the phase detector output corresponds to the phase difference between a transmitted and a received signal. As such, the pulse width modulated signal is applied via the AND gate 140 when it is enabled by the integrate enable signal IE. The output of the integrator circuit 142 appears at pin 6 of the operational amplifier LM318 and comprises an integrated level during the integrate enable period whose final level is related to the phase difference between the transmitted signal at one transducer and the signal received at the other transducer and is coupled via a line 144 to input pin 5 of two different sample and hold circuits 146 and 148.Each sample and hold circuit 146 or 148 samples the voltage appearing at pin 5 when a gate signal is applied to each respective control input on gate pin 6. The sampled voltage appears at pin 11 and has the same level as appeared at pin 5 when the gate signal was present. The voltage at pin 11 of each sample and hold circuit 146 or 148 remains unchanged between gate pulses at pin 6.

The sample and hold circuit 146 is gated to the sample mode whenever the signal appearing at input terminal U is logic one.

In a similar manner, the sample and hold circuit 148 is gated to the sample mode whenever the input terminal D is logic one.

Between the sampling operations at either of the sample and hold circuits 146 and 148, an integrator reset signal appears at the input terminal IR and is operative to turn transistor Q12 on to short circuit a capacitor appearing between pin 6 and pin 2 of the operational amplifier within the integrator circuit 142. This resets the integrator so that its output voltage is zero.

In operation, the sample and hold circuits 146 and 148 are operative to store DC voltages which are representative of the phase difference between the transmitted signal at one transducer and the received signal at the other transducer. In the case of the sample and hold circuit 146 which is gated by the gate signal at input terminal U, a voltage is stored which, according to an arbitrary definition, corresponds to the phase difference between the signal transmitted by the downstream transducer 26 and the signal generated in response thereto at the upstream transducer 24, i.e., a so-called upstream phase difference.On the other hand, the sample and hold circuit 148 is operative in response to the gate signal D to store a voltage representing the phase difference between the signal transmitted by the upstream transducer 24 and the signal generated in response thereto by the downstream transducer 26, i.e., a so-called downstream phase difference.

The output from the sample and hold circuits 146 and 148 are respectively coupled to the inverting and the noninverting input of an operational amplifier 150 which is configured to produce a voltage at its output pin 14 equal to the difference between the voltage applied at the non-inverting input terminal and the voltage appearing at the inverting input terminal (D-u) As indicated earlier, this difference is representative of the uncorrected flow rate of a gas through the transducer assembly 10 (Figure 1).In order to properly calibrate the flowmeter, the non-inverting input terminal of the operational amplifier has an offset adjustment network, indicated generally at 152, connected thereto which is operative to adjust the voltage at the non-inverting input terminal so that the voltage appearing at the output pin 14 is zero whenever the flow rate through the transducer assembly 10 is also zero. This offset adjust circuit 152 compensates for the various circuit offsets especially those for the sample and hold circuits 146 and 148.

The output of the sample and hold circuits 146 and 148 are each coupled via a 20 K resistor to input pin 5 of a further sample and hold circuit 154. Since the outputs of sample and hold circuits 146 and 148 are coupled in the manner shown, the voltage at pin 5 of the sample and hold circuit 154 comprises one half of the sum of the two phase differences which are stored in the two sample and hold circuits 146 and 148. This sum is stored within the sample and hold circuit 154 in response to a summing signal received at the input terminal labelled 5. The output of the sample and hold circuit 154 is coupled by a further operational amplifier 156 which produces a DC voltage at output pin 8 and is related to the sum of the phase differences stored in the sample and hold circuits 146 and 148 at the time when the gate signal occurred.As indicated earlier, this voltage (0,+8,) at the output pin 8 of the amplifier 156 comprises a relative indication of the velocity of sound in the gas in the transducer assembly 10.

The voltage (D+U) appearing at pin 8 of amplifier 156 is fed back to a low pass filter comprising a I Meg ohm resistor and a one microfarad capacitor. The voltage across this one microfarad capacitor is coupled to pin 9 of circuit CD4046 which connects internally to the VCO and thereby adjusts its frequency of operation. Accordingly, the voltage controlled oscillator frequency is changed as the velocity of sound in the gas in transducer 10 changes.

The circuitry of Figure 4 and Figure 5 in co-operation with the circuits of Figure 2 is operative to produce a signal at output pin 14 of amplifier 150 which relates to the uncorrected flow through the transducer assembly 10 and a further output at pin 8 of amplifier 156 which is related to the velocity of sound in the gas in the transducer assembly 10. In accordance with the conventions established in defining transducer 24 as the upstream transducer and transducer 26 as the downstream transducer, whenever the voltage appearing at pin 14 of amplifier 150 is negative, this negative voltage indicates that fluid indeed is flowing through the transducer assembly in a direction from the upstream transducer 24 toward the downstream transducer 26.

Additionally, the magnitude of the voltage appearing at pin 14 of amplifier 150 is related to and can be corrected by the circuitry of Figure 6 to indicate the flow rate for the fluid passing through the assembly 10. On the other hand, if the voltage at pin 14 of amplifier 150 is positive, this indicates that fluid is flowing through the assembly 10 in a direction from the downstream transducer 26 toward the upstream transducer 24. Again, the magnitude of the voltage appearing at pin 14 of amplifier 150 corresponds to the uncorrected flow rate through the transducer assembly 10.

For the circuit shown in Figure 5, however, the output of amplifier 156 is merely an indication related to the velocity of sound in the gas. In order to determine whether the velocity is greater or less than that for which the instrument was calibrated, one must record the amplitude at the time it is calibrated and then compare the current reading with that previously recorded value. The circuit, however, can be readily modified so that the output voltage is equal to zero whenever the calibrating fluid is present in the transducer assembly. Then, if the velocity of sound in the fluid subsequently changes, the output voltage either goes positive or negative and the sign of the voltage corresponds to whether the velocity has increased or decreased compared to the fluid for which the instrument was calibrated.The magnitude of the output voltage then corresponds to the relative difference between the sound velocity in the gas currently passing through the transducers and the sound velocity in the gas in the transducer assembly at the time it was calibrated. In order to use the output voltage to provide an indication of relative sound velocity more circuits are necessary since the output of amplifier 156 is dedicated for use as an error voltage to adjust the frequency of the voltage controlled oscillator VCO and as an input to the circuitry of Figure 6.

A further alternative configuration permits the output voltage to be adjusted to equal one when, for example, air is present in the transducer assembly. Then, whenever the sound velocity of gas flowing through the transducer assembly changes, the magnitude of the output voltage corresponds to the velocity in the gas relative to the velocity in air.

The circuitry of Figure 6 includes a circuitry for accepting the uncorrected flow rate from the output of amplifier 150 in Figure 5 and producing a corrected flow rate output indication. It was found that the output of amplifier 150 had an error which is proportional to fl/f2 where fl is the initial calibration frequency of the VCO in Figure 5 and f2 is the frequency of the VCO when a gas having a different speed of sound is in the transducer. The circuitry of Figure 6 multiplies the uncorrected flow rate output of amplifier 150 by fl/f2 to produce a corrected flow rate signal at pin 7 of amplifier LM324B in Figure 6.

The VCO error voltage which appears at the output of amplifier 156 in Figure 5 is proportional to frequency and is used to establish the correction factor for correcting the flow rate. The operational amplifier LM308 of Figure 6 acts as a signal conditioner and receives the VCO error voltage (D+U) By adjustment of the zero adjust resistor coupled to the amplifier LM308, a zero volt output at pin 6 is established when the error voltage is at its nominal level (+7.5 volt). The signal conditioner provides an output voltage of +1.0 volts per +5 KHz deviation.

The output at pin 6 of LM308 then modulates a duty cycle modulator consisting of amplifiers LM324A and LM311 and circuitry coupled thereto.

When the VCO operates at the calibration frequency, the output of LM308 is zero, the duty cycle adjust resistor coupled to LM324A is adjusted to 50, whereat FET switches Q14 and Q15 are off 50% of the time and are on 50Oo of the time. As a result, the gain of amplifier LM324B is one and no correction is introduced to the uncorrected flow from amplifier 150 in Figure 5 and the output at pin 7 of LM324B represents the flow rate of gas through the transducer.

When the VCO operating frequency changes, a voltage appears at pin 6 of LM308 in Figure 6 which is positive for a decrease in frequency and negative for an increase in frequency. This voltage modulates the duty cycle generator causing the duty cycle to change. The duty cycle change causes the gain of amplifier LM324B to change in accordance with the change of VCO frequency so that the uncorrected flow rate signal from Figure 5 is changed by LM324B so that the output is proportional to flow rate through the transducer.

The foregoing description of an acoustical wave flowmeter has been made with particular emphasis on a preferred electronic circuit which co-operates with a transducer assembly to provide not only gas flow and direction but also a measurement of relative sound velocity in the flowing gas.

The description has made some emphasis on the fact that the invention is suitable as a flowmeter for measuring direction of flow, flow and sound velocity in a gas, however, the apparatus is equally operable for measuring direction of flow, flow and sound velocity in liquids as well although some circuit elements may require change in value to optimize performance of the flowmeter for applications other than gas flow direction, flow and sound velocity measurements. Furthermore, those of skill in the art will recognize other modifications to the flowmeter which might be made without deparing from the spirit and scope of the invention as defined in the claims.

For example, in place of the phase detector and the phase sum and difference calculators, the invention may utilize other equivalent means to calculate a representation proportional to or equal to the speed of acoustic compressions in the transducer traveling from the transmitting to the receiving transducer. Each calculated speed has two components, one being the speed of fluid flow and the other being the speed of acoustic compressions in the fluid without fluid flow. One such equivalent speed calculator may include means to determine the time difference between the start of acoustic compressions at a transmitting transducer and the time when the receiving transducer produces a received signal in response to the acoustic compression. Each time difference calculated is also proportional to the speed of acoustic compressions traveling from the transmitting to the receiving transducer.

Further benefits of the described flowmeter may be obtained by combining or rearranging the signals representing various parameters. For example, changes in gas composition which cause a change in molecular weight of the gas mixture also cause a change in the velocity of sound.

Thus a transition from gas mixture A having an average molecular weight M to A' gas mixture B having an average molecular weight MB, and the fraction of gas mixture A mixed with gas mixture B may be measured. Such a technique may be used with a gas glowmeter, for example to provide a proportional measurement of the fraction of carbon dioxide in the exhaled breath compared to that in the inhaled mixture.

Since the velocity of sound is given by:

I ykT c= meterYsecond NI M where y=ratio of specific heat at constant pressure to that at constant volume, k=Boltzmans constant, 1 .38x 10-23 joules/"C, T=absolute temperature, "Celsius, M=mass of the molecules in the gas, kilograms, it can be seen that molecular weight is proportional to c-2 and for those skilled in the art, an electrical output can be produced which is proportional to changes in molecular weight; other variables remaining constant. It is apparent that this simple case can be extended to combine changes in specific heats and temperature with changes in molecular weight such that gas mixture A is typified by one set of conditions and gas mixture B by another set of conditions.

Additionally, where pressure or density is changing radically, the volumetric flow may be corrected to approximately standard conditions or true mass flow by combining a pressure measurement with parameters available from the described flowmeter.

Mass flow is given by, M=V mp/kT kilograms/second where p=pressure, newtons/meter2.

Thus from the equation for the velocity of sound, c, above M=V p y/C2 To those skilled in the art, it is apparent that a pressure transducer can be connected to measure the absolute pressure in the flowmeter. Further, the output signal of the pressure transducer multiplied by the volumetric flow, V, from the flowmeter and divided by the square of the velocity of sound, c, also a signal from the flowmeter, along with appropriate constants, can provide an approximate value for true mass flow.

While a constant value for y will cause an error in value for some changes in gas mixture, there are many where the changes in y will be insignificant. The use of the above described parameters to produce a signal for volumetric flow reduced to standard temperature and pressure will also be apparent to those skilled in the art.

Having described a flowmeter system capable of accurately measuring flow rate independent of composition it is also apparent that the techniques may be applied with benefit to other sonic flowmeters wherein the transducers are not cylindrical or arcuate and thus may be in the flow path or cause some obstruction or recess along the flow path. That is, any sonic flowmeter may benefit from these techniques when errors are caused by changes in velocity of sound in the fluid.

WHAT WE CLAIM IS: 1. A meter for measuring rate of fluid flow comprising a member formed with a passage defining a path of fluid flow and having a pair of electro-acoustic transducers spaced along the path, each of which transducers is capable of transmitting and receiving acoustic compression waves and of producing a signal on receipt of such waves, the two transducers being connected in a control circuit for producing alternately two cycles of operation, in the first of which one transducer transmits and the other produces a receipt signal and in the other of which the other transducer transmits and the first produces a receipt signal, the frequency of the acoustic compression waves produced by each transducer being automatically adjusted by regulation of the control circuit to a value where a resonant echo occurs at the other transducer, the meter also including means for measuring the phase difference in each cycle between the transmitted acoustic compression wave and the received signal and for producing a difference signal dependent on the difference between the two phase differences, and the magnitude and sign of which are directly related to the instantaneous fluid flow rate and its direction.

2. A meter according to Claim 1, wherein each transducer comprises a transmitting and a separate receiving unit.

3. A meter according to any one of the preceding claims, wherein the passage in the member is of circular cross-section and each transducer comprises a hollow

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. or rearranging the signals representing various parameters. For example, changes in gas composition which cause a change in molecular weight of the gas mixture also cause a change in the velocity of sound. Thus a transition from gas mixture A having an average molecular weight M to A' gas mixture B having an average molecular weight MB, and the fraction of gas mixture A mixed with gas mixture B may be measured. Such a technique may be used with a gas glowmeter, for example to provide a proportional measurement of the fraction of carbon dioxide in the exhaled breath compared to that in the inhaled mixture. Since the velocity of sound is given by: I ykT c= meterYsecond NI M where y=ratio of specific heat at constant pressure to that at constant volume, k=Boltzmans constant, 1 .38x 10-23 joules/"C, T=absolute temperature, "Celsius, M=mass of the molecules in the gas, kilograms, it can be seen that molecular weight is proportional to c-2 and for those skilled in the art, an electrical output can be produced which is proportional to changes in molecular weight; other variables remaining constant. It is apparent that this simple case can be extended to combine changes in specific heats and temperature with changes in molecular weight such that gas mixture A is typified by one set of conditions and gas mixture B by another set of conditions. Additionally, where pressure or density is changing radically, the volumetric flow may be corrected to approximately standard conditions or true mass flow by combining a pressure measurement with parameters available from the described flowmeter. Mass flow is given by, M=V mp/kT kilograms/second where p=pressure, newtons/meter2. Thus from the equation for the velocity of sound, c, above M=V p y/C2 To those skilled in the art, it is apparent that a pressure transducer can be connected to measure the absolute pressure in the flowmeter. Further, the output signal of the pressure transducer multiplied by the volumetric flow, V, from the flowmeter and divided by the square of the velocity of sound, c, also a signal from the flowmeter, along with appropriate constants, can provide an approximate value for true mass flow. While a constant value for y will cause an error in value for some changes in gas mixture, there are many where the changes in y will be insignificant. The use of the above described parameters to produce a signal for volumetric flow reduced to standard temperature and pressure will also be apparent to those skilled in the art. Having described a flowmeter system capable of accurately measuring flow rate independent of composition it is also apparent that the techniques may be applied with benefit to other sonic flowmeters wherein the transducers are not cylindrical or arcuate and thus may be in the flow path or cause some obstruction or recess along the flow path. That is, any sonic flowmeter may benefit from these techniques when errors are caused by changes in velocity of sound in the fluid. WHAT WE CLAIM IS:

1. A meter for measuring rate of fluid flow comprising a member formed with a passage defining a path of fluid flow and having a pair of electro-acoustic transducers spaced along the path, each of which transducers is capable of transmitting and receiving acoustic compression waves and of producing a signal on receipt of such waves, the two transducers being connected in a control circuit for producing alternately two cycles of operation, in the first of which one transducer transmits and the other produces a receipt signal and in the other of which the other transducer transmits and the first produces a receipt signal, the frequency of the acoustic compression waves produced by each transducer being automatically adjusted by regulation of the control circuit to a value where a resonant echo occurs at the other transducer, the meter also including means for measuring the phase difference in each cycle between the transmitted acoustic compression wave and the received signal and for producing a difference signal dependent on the difference between the two phase differences, and the magnitude and sign of which are directly related to the instantaneous fluid flow rate and its direction.

2. A meter according to Claim 1, wherein each transducer comprises a transmitting and a separate receiving unit.

3. A meter according to any one of the preceding claims, wherein the passage in the member is of circular cross-section and each transducer comprises a hollow

cylindrical body with its inner surface substantially co-extensive with the surface of the passage so that the transducers do not obstruct the fluid flow.

4. A meter according to any one of the preceding claims, wherein the frequency is automatically adjusted to a value such that the internal diameter of each transducer is substantially equal to 2.3 A where A is the wavelength of the compression waves produced by the transducers in the fluid.

5. A meter according to any one of the preceding claims, wherein each transducer comprises a crystal capable of electromechanical operation.

6. A meter according to Claim 5, wherein the crystal material is one of the following: polyvinylfluoridene, barium titanate, lead zirconate titanate, quartz, tourmaline, high polymer organic piezoelectric material or polarized polycrystalline ferro-electric ceramic material.

7. A meter according to any one of the preceding claims and additionally including means for producing a signal dependent on the sum of the two phase differences, the magnitude of which is related to the velocity of sound in the fluid.

8. A meter according to any one of the preceding claims, wherein regulation of the control circuit to automatically adjust the frequency of the acoustic compression waves is effected by means which includes an oscillator for producing a signal at a frequency determined at least in part by the sum of the two said phase differences.

9. A meter according to Claim 7 or Claim 8 and including means responsive to the said sum for adjusting the frequency of the oscillator.

10. A meter according to any one of the preceding claims additionally including acoustic damping material disposed along the path of fluid flow at a position or positions such as to substantially reduce acoustic compression waves from either transducer from leaving the path and for substantially reducing reflected acoustic compression waves from entering the path.

11. A flowmeter substantially as described and as illustrated with reference to the accompanying drawings.