JP2008175668A - Fluid flow measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fluid flow measuring device with high measurement resolution maintained even if a received waveform changes concurrently with environmental changes. <P>SOLUTION: An ultrasonic signal transmitted from a first vibrator 2 based on an AC signal output from a transmission means 6 is received by a second vibrator 3, and then zero-cross points of its received wave are detected by a prescribed number by a waveform detection means 8. A timing means 9 measures elapsed time from the start of transmission to the respective zero-cross points based on a reference clock. Since a frequency setting means 13 sets a reference clock frequency so that phase relations to the reference clock differ at the respective zero-cross points, the phase relations can be kept appropriate even if the received waveform changes due to environmental changes, etc. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a flow rate measuring device that detects a flow rate by measuring a propagation time of an ultrasonic signal and measures a flow rate of a fluid.

  Conventionally, as this kind of fluid flow measuring device, for example, a device as shown in Patent Document 1 has been proposed.

  In FIG. 7, an ultrasonic transducer 32 and a transducer 33 are provided in a part of the fluid conduit 31 so as to be opposite to each other in the direction of flow. A burst signal is transmitted, ultrasonic waves are generated in the flow direction from the vibrator 32, the ultrasonic waves are received by the vibrator 33, and when detected by the amplifier circuit 37 and the comparison means 38, the timekeeping means 35 stops timing, The calculation means 39 performs calculation processing on the time measurement result of the time measurement means 35 to obtain the propagation time. When the predetermined number of measurements is completed, the transducer is switched by the switching means 40, ultrasonic waves are generated against the flow from the transducer 33, and propagated in the same procedure as when ultrasonic waves are generated in the flow direction. Seeking time.

In FIG. 7, the sound velocity is C, the flow velocity is v, the distance between the two vibrators is L, the angle formed by the ultrasonic wave propagation direction and the flow direction is θ, and the vibrator is arranged on the upstream side of the pipe. transmitting ultrasonic waves from the 32, t ₁ the propagation time in the case of receiving a two vibrators 33 arranged on the downstream _side, when the propagation time of the reverse direction is t ₂ t ₁ and t ₂ is obtained by the following formula be able to.

t ₁ = L / (C + v cos θ) (Formula 1)
t ₂ = L / (C−v cos θ) (Formula 2)
(Formula 1) and (Formula 2) are modified, and the flow velocity v is obtained by (Formula 3).

v = L · (1 / t ₁ −1 / t ₂ ) / ₂ cos θ (Formula 3)
The flow rate of the fluid can be obtained by multiplying the value obtained by (Equation 3) by the cross-sectional area of the fluid conduit.

  FIG. 8 is a signal characteristic diagram of the flow measuring device. FIG. 8 shows the waveform of an ultrasonic wave received after amplification by the amplifier circuit 37, the output waveform of the comparison means 38, and the waveform of a reference clock (not shown) provided in the time measuring means 35. The comparison means 38 is configured to convert the comparison result between the reference voltage (zero point) of the AC signal and the received signal into a binary value of “1” and “0” and output the result. The point at which the output of the comparison means 38 changes from “1” to “0” or from “0” to “1” is the point (zero cross point) at which the received signal passes the reference voltage (zero point). The time measuring means 35 measures the elapsed times ta and tb from the transmission start of the transmission circuit 36 to the zero cross points A and B based on the reference clock, and the time calculating means 39 obtains the average value of the two values as the propagation time. . That is, the average value of the detection times of the zero cross points detected by the comparison means 38 is obtained to obtain the propagation times indicated by (Equation 1) and (Equation 2).

The time resolution in the fluid flow measuring apparatus configured as described above will be described. For example, consider a case where two zero-cross points A and B are detected as shown in FIG. When the propagation time of the ultrasonic wave changes due to some factor (temperature change, flow rate change, etc.), point A and point B move relatively back and forth on the time axis while maintaining the same time interval. It becomes. Since the phase difference between the reference clocks at the zero-cross points A and B shown in FIG. 8 is 180 degrees, the measurement results at the zero-cross points A and B are alternately 1 every time the propagation time changes by 1/2 clock. It will change by the amount of the clock. Therefore, a time resolution that is half that of the reference clock can be obtained.

Similarly, when the number of zero cross points is increased, a time resolution higher than the cycle of the reference clock can be obtained by setting the phase relationship of the reference clocks at each zero cross point to be different. Therefore, by adjusting the relationship between the reference clock frequency and the reception signal frequency in advance, it is possible to obtain a measurement result with high resolution while keeping the frequency of the reference clock low. Can be provided.
Japanese Patent Laid-Open No. 10-30947

  However, the conventional configuration does not disclose details of a method for controlling the phase relationship between the reference clock and the received waveform. Even if the phase relationship can be set optimally, when the frequency of the ultrasonic waveform changes due to environmental conditions, the disclosed configuration can appropriately set the phase relationship between the two in accordance with the environmental conditions. It was difficult.

  The present invention solves the above-described conventional problems, and provides a fluid flow measurement device that always has high resolution measurement performance regardless of environmental changes.

  In order to solve the above-described conventional problems, a fluid flow measurement device according to the present invention includes a transmission unit that outputs an AC signal to a first transducer and a plurality of ultrasonic signals received by the second transducer in succession. Waveform detecting means for detecting a predetermined number of zero-cross points generated in advance, and timing means for measuring an elapsed time from transmission of the first vibrator to the zero-cross point detected by the waveform detecting means based on a reference clock; Frequency setting means for setting the reference clock frequency of the oscillator, and the frequency setting means sets the reference clock frequency so that the phase relationship of the reference clocks at each zero cross point is different.

  According to the above invention, even when the received waveform changes, the phase relationship of the reference clock at each zero cross point can be maintained appropriately.

  The fluid flow measurement device of the present invention can maintain a high measurement resolution even when the received waveform changes as the environment changes.

According to a first aspect of the present invention, there is provided a first vibrator for transmitting an ultrasonic signal provided in a fluid conduit, a second vibrator for receiving the ultrasonic signal, and a transmitting means for outputting an AC signal to the first vibrator. A waveform detecting means for detecting a predetermined number of zero-cross points that are continuously generated a plurality of times by the ultrasonic signal received by the second vibrator, an oscillator for generating a reference clock having a constant frequency, Time measuring means for measuring an elapsed time from transmission of the first vibrator to a zero cross point detected by the waveform detecting means based on a reference clock; and a propagation time of the ultrasonic signal based on a measurement result of the time measuring means. A time calculating means for obtaining, a time difference calculating means for obtaining an occurrence time interval of each zero cross point, and a frequency setting means for setting a reference clock frequency of the oscillator,
The frequency setting means sets the reference clock frequency so that the output of the time difference calculation means satisfies a predetermined condition.

  The second invention is characterized in that the frequency setting means sets the reference clock frequency so that the phase relationship of the reference clock at each zero cross point is different.

  According to the first and second inventions, even when the received waveform changes, the phase relationship of the reference clocks at each zero cross point can be appropriately maintained.

  According to a third aspect of the invention, there is provided phase difference detection means for obtaining the phase difference of the reference clocks at two different zero-cross points detected by the waveform detection means instead of the time difference calculation means, and the frequency setting means includes the phase difference detection means The reference clock frequency of the oscillator is set so that the detected phase difference matches the target value.

  In addition, it is possible to finely adjust the phase relationship of the reference clock at each zero cross point.

  According to a fourth aspect of the present invention, the frequency setting means sets the reference clock frequency of the oscillator by setting the target value of the phase difference detected by the phase difference detection means to a value obtained by dividing 360 degrees by the number of zero cross detections. To do.

  In the fifth invention, the frequency setting means adjusts the reference clock frequency by repeating the setting operation until the deviation between the phase difference detected by the phase difference detecting means and the phase difference target value becomes less than a predetermined value. It is characterized by.

  According to the fourth and fifth inventions, it is possible to always maintain a high measurement resolution and to finely adjust the phase relationship of the reference clock at each zero cross point.

  According to a sixth aspect of the invention, the frequency setting means invalidates the output of the time calculating means when the deviation between the phase difference detected by the phase difference detecting means and the phase difference target value is larger than a predetermined value. It is a feature.

  And the reliability of a measured value can be improved.

  According to a seventh aspect of the present invention, there is provided a second transmitter that starts operation at a zero-cross point and stops operation at a rising point of a reference clock, and second timing means that counts the count number of the second transmitter, and detects a phase difference The means is characterized in that the phase difference between the reference clocks at two different zero-cross points detected by the waveform detecting means is obtained based on the count number counted by the second time measuring means.

  In addition, it is possible to always maintain a high measurement resolution and to finely adjust the phase relationship of the reference clock at each zero cross point.

  According to an eighth aspect of the present invention, there is provided temperature detection means for detecting a fluid temperature, storage means for storing the relationship between the fluid temperature, the reference clock frequency of the oscillator, and the phase difference detected by the phase difference detection means, It comprises learning means for selecting a reference clock frequency for the phase difference target value based on the stored contents, and the frequency setting means sets the value selected by the learning means as the reference clock frequency. is there.

  And even if the received waveform changes as the fluid temperature changes, the phase relationship between the received waveform and the reference clock can always be kept appropriate.

  The ninth invention is characterized in that the temperature detecting means is constituted by an arithmetic circuit for calculating the fluid temperature using the output result of the time calculating means.

  And it is possible to cope with temperature changes with an inexpensive configuration without adding a special device, and even when the received waveform changes with changes in fluid temperature, the received waveform and the reference clock are always changed. The phase relationship can be kept appropriate.

  A tenth aspect of the invention stores a relationship between a flow rate calculation unit that calculates a fluid flow rate using a measurement result of a time calculation unit, a fluid flow rate, a reference clock frequency of an oscillator, and a phase difference detected by a phase difference detection unit. And a learning means for selecting a reference clock frequency for the phase difference target value based on the storage contents of the storage means, and the frequency setting means sets the value selected by the learning means as the reference clock frequency. It is characterized by that.

  And even if the received waveform changes with the change in flow rate, the phase relationship between the received waveform and the reference clock can always be kept appropriate.

(Embodiment 1)
FIG. 1 is a block diagram of a fluid flow detection device according to Embodiment 1 of the present invention.

  In FIG. 1, a first vibrator 2 that transmits ultrasonic waves is arranged in the middle of the fluid conduit 1 on the upstream side of the flow, and a second vibrator 3 that receives ultrasonic waves transmitted from the first vibrator 2 is provided. Located downstream of the flow. The first vibrator 2 and the second vibrator 3 are connected to a subsequent processing block through switching means 4 that reverses the role of transmission and reception. That is, by the action of the switching means 4, it is possible to make the first vibrator 2 the transmitting side and the second vibrator 3 the receiving side.

  The trigger means 5 outputs a trigger signal instructing the start of measurement, and an AC signal is output from the transmission means 6 in synchronization with this signal. The output signal of the transmission unit 6 is output to the first transducer 2 via the switching unit 4, and an ultrasonic signal is output from the first transducer 2. The ultrasonic signal transmitted from the first transducer 2 and received by the second transducer 3 is amplified by the amplification circuit 7 via the switching unit 4 and then output to the waveform detection unit 8.

  The waveform detection means 8 compares the size of the ultrasonic signal, which is an AC signal, with its zero point, determines that the point where the magnitude relationship is reversed is the zero cross point, and detects the zero cross point by a predetermined number. After that, the operation is stopped. Simultaneously with the output of the trigger means 5, the time measuring means 9 measures the elapsed time from the start of measurement to the zero cross point based on the reference clock supplied from the oscillator 10.

  The measurement result of the time measuring means 9 is output to the time calculating means 11 and the time difference calculating means 12, where the time calculating means 11 obtains the propagation time of the ultrasonic signal, and the time difference calculating means 12 obtains the occurrence time interval of each zero cross point. . Then, the frequency setting unit 13 sets the reference clock frequency output from the oscillator 10 so that the output of the time difference calculation unit 12 satisfies a predetermined condition.

  FIG. 2 is a signal characteristic diagram of the fluid flow detection device according to the present embodiment. For example, consider a case where the received waveform as shown in FIG. 2 is measured by the time measuring means 9 and the time difference calculating means 12. The time measuring means 9 is a timer counter that operates based on a reference clock output from the oscillator 10. If the measured value at the zero cross point A is Ca, the measured values Cb, Cc, Cd at the zero cross points B, C, D are as follows.

Cb = Ca + 4 (Formula 4)
Cc = Ca + 8 (Formula 5)
Cd = Ca + 12 (Formula 6)
These values are output to the time difference calculation means 12, and the interval between the zero cross points is obtained. Assuming that the intervals A to B, B to C, and C to D are respectively Cab, Cbc, and Ccd, each value is obtained as follows.

Cab = Cbc = Ccd = 4 (Formula 7)
(Expression 7) indicates that the zero cross time interval is an integral multiple (4 times) of the reference clock, and indicates that the phase relationship of the reference clock at each zero cross point is equal. Therefore, the frequency setting means 13 sets the reference clock frequency so that this relationship is broken, that is, all three values are not equal.

  The frequency setting means 13 is constituted by, for example, a CR oscillation circuit and can easily adjust the signal frequency by changing the resistance value. The frequency setting means 13 selects the optimum frequency by gradually changing the signal frequency while referring to the measurement result of the time difference calculation means 12. After the frequency is optimized, the reference clock frequency is fixed.

  As described above, since the frequency setting means 13 sets the reference clock frequency so that the phase relationship of the reference clock at each zero cross point is different, even if the received waveform changes, the reference at each zero cross point is set. The clock phase relationship can be maintained appropriately.

(Embodiment 2)
FIG. 3 is a block diagram of the flow measuring apparatus according to the second embodiment of the present invention. In FIG. 3, since the main part is the same as FIG. 1, detailed description is abbreviate | omitted and only a different part is demonstrated.

  In FIG. 3, the second time measuring means 15 obtains the time difference between the reference clock and the zero cross point based on the second clock generated from the second oscillator 16. The measurement results of the time measuring means 9 and the second time measuring means 15 are output to the time calculating means 11 and the phase difference detecting means 14, where the time calculating means 11 is the propagation time of the ultrasonic signal and the phase difference detecting means 14 is the adjacent zero cross point. The phase difference of the reference clock between them is obtained. The frequency setting unit 13 sets the reference clock frequency output from the oscillator 10 according to the output of the phase difference detection unit 14.

  FIG. 4 is a signal characteristic diagram of the fluid flow detection device according to the second exemplary embodiment. At the zero cross point A, the second oscillator 16 and the second time means 15 start to operate, and measure the time difference Taa 'to the next reference clock rising point A'. Even at the next zero cross point B, the second oscillator 16 and the time measuring means 15 start to operate, and measure the time difference Tbb 'up to the next reference clock rising point B'. It is clear that Taa 'and Tbb' are values indicating the phase of the reference clock at the zero cross points A and B. Here, the count number from the zero cross point A to the next reference clock rising point A ′ measured by the second time measuring means 15 is Cx, the count number from the zero cross point B to the next reference clock rising point is Cy, If the clock period is T2, Taa ′ and Tbb ′ can be expressed by (Equation 8) and (Equation 9).

Taa ′ = Cx × T2 (Formula 8)
Tbb ′ = Cy × T2 (Formula 9)
The phase difference detection means 14 obtains the phase difference Δθ between the two zero cross points using (Equation 10), where the period of the reference clock is T1.

Δθ = (Taa′−Tbb ′) / T1 × 360 ° (Formula 10)
The frequency setting means 13 sets the reference clock frequency of the oscillator 10 so that the value of (Equation 10) matches the target value. For example, when the number of zero cross detections by the waveform detection means 8 is 4, the value of (Equation 10) is set to 90 °. In this case, when two received waveforms that differ by T1 / 4 on the time axis are compared, at least one of the four zero cross points indicates a different value. Become. Similarly, every time the waveform changes on the time axis by T1 / 4, the propagation time obtained by the time calculation means 11 becomes a different value, so that the time resolution of 1/4 of the reference clock can be obtained. I can say that.

  When the phase difference Δθ deviates from the target value of 90 ° by a predetermined value or more, the frequency setting unit 13 changes the reference clock frequency of the oscillator 10 and transmits an ultrasonic wave. The phase difference Δθ is acquired. Then, the reference clock frequency is adjusted while repeating the setting operation until the deviation between the target value and Δθ becomes less than a predetermined value.

  As described above, by acquiring the phase difference by the phase difference detection unit 14, the reference clock frequency of the oscillator 10 can be set so that the detected phase difference matches the target value. Not only can adjustment be performed, but it is possible to set a reference clock frequency at which an optimum phase difference can be obtained according to the number of detections of the zero-cross point, and a high measurement resolution can be always obtained.

  Further, during the adjustment operation, that is, when the deviation of Δθ is large, the measurement reliability can be improved by invalidating the propagation time obtained by the time calculation means 11.

(Embodiment 3)
FIG. 5 is a block diagram of a flow measuring apparatus according to Embodiment 3 of the present invention. In FIG. 5, since the main part is the same as FIG. 1 and FIG. 3, detailed description is abbreviate | omitted and only a different part is demonstrated.

  In FIG. 5, the temperature detection means 17 obtains the fluid temperature using the output of the time calculation means 11. The storage means 18 stores the relationship between the fluid temperature acquired when adjusting the reference clock frequency, the reference clock frequency of the oscillator 10, and the phase difference of the reference clock between adjacent zero crosses acquired by the phase difference detection means 14. The learning unit 19 refers to the contents of the storage unit 18 to obtain a reference clock frequency for obtaining a target phase difference according to a change in the fluid temperature.

  First, the measurement principle of the temperature detection means 16 will be described. In (Expression 1) and (Expression 2), the flow velocity v generally takes a value much smaller than the sound velocity C, and therefore can be modified as follows.

t ₁ = (L / C) (1 + V cos θ / C) ⁻¹ ≈ (L / C) (1−V cos θ / C)
(Formula 11)
t ₂ = (L / C) (1−Vcos θ / C) ⁻¹ ≈ (L / C) (1 + V cos θ / C)
(Formula 12)
From (Expression 11) and (Expression 12),
C = 2L / (t ₁ + t ₂ ) (Formula 13)
In general, the fluid temperature T and the sound velocity C can be expressed by a proportional relationship as shown by the approximate expression of (Expression 14).

C = α · T + β (Formula 14)
Here, α and β are constant terms. Therefore, (Expression 15) is established by transforming the expression using (Expression 13) and (Expression 14).

T = α ⁻¹ · {2L / (t ₁ + t ₂ ) −β} (Formula 15)
Therefore, by knowing the round-trip propagation time, it is possible to detect the fluid temperature without providing a specially configured temperature detection means.

  As a storage format of the storage means 18, there can be considered a relationship in which the relationship between the reference clock frequency and the phase difference with respect to the temperature T is expressed by a mathematical formula, a table stored in a table format, or the like.

  The learning unit 19 selects an optimum reference clock frequency corresponding to the temperature detected by the temperature detection unit 17 based on the information in the storage unit 18 and transmits the selected frequency to the frequency setting unit 13.

  The above operation makes it possible to immediately obtain the reference clock frequency for obtaining the target phase difference based on the change in the vibration frequency of the vibrator accompanying the change in the temperature of the fluid. However, the phase relationship between the received waveform and the reference clock can always be kept appropriate.

(Embodiment 4)
FIG. 6 is a block diagram of the flow measuring apparatus according to the fourth embodiment of the present invention. In FIG. 6, since the main part is the same as FIG. 1, detailed description is abbreviate | omitted and only a different part is demonstrated.

  In FIG. 6, the flow rate calculation means 20 obtains the fluid flow rate using the output of the time calculation means 11. The storage unit 18 stores the relationship between the fluid flow rate obtained at the time of adjusting the reference clock frequency, the reference clock frequency of the oscillator 10, and the phase difference of the reference clock between adjacent zero crosses acquired by the phase difference detection unit 14. The learning unit 20 refers to the contents of the storage unit 19 to obtain a reference clock frequency for obtaining a target phase difference in accordance with a change in the fluid flow rate.

  The storage format of the storage means 18 may be a mathematical expression of the relationship between the transmission frequency and the phase difference with respect to the fluid flow rate, or a table stored in a table format.

  The learning unit 19 selects an optimum reference clock frequency corresponding to the temperature detected by the temperature detection unit 17 based on the information in the storage unit 18 and transmits the selected frequency to the frequency setting unit 13.

  With the above operation, it is possible to immediately determine the reference clock frequency for obtaining the target phase difference based on the change in the vibration frequency of the vibrator accompanying the change in the flow rate of the fluid, and the preparation time can be shortened. It becomes.

  The above operation makes it possible to immediately obtain the reference clock frequency for obtaining the target phase difference based on the change in the vibration frequency of the vibrator accompanying the change in the temperature of the fluid. However, the phase relationship between the received waveform and the reference clock can always be kept appropriate. This is particularly effective when a high measurement resolution is required while maintaining low power consumption, such as a household gas meter that is continuously used for 10 years by battery drive.

  The fluid flow measuring device of the present invention can maintain a high measurement resolution even when the received waveform changes due to environmental changes, so it can be applied not only to gas but also to liquid flow measurement. Is possible.

The block diagram of the flow measurement apparatus in Embodiment 1 of this invention Signal characteristics of the device The block diagram of the flow measuring device in Embodiment 2 of this invention Signal characteristics of the device The block diagram of the flow measuring device in Embodiment 3 of this invention The block diagram of the flow measurement apparatus in Embodiment 4 of this invention Block diagram of a conventional flow measurement device Signal characteristics of the device

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fluid line 2 1st vibrator | oscillator 3 2nd vibrator | oscillator 6 Transmission means 8 Waveform detection means 9 Timekeeping means 10 Oscillator 11 Time calculation means 12 Time difference calculation means 13 Frequency setting means 14 Phase difference detection means 17 Temperature detection means 18 Storage means 19 Learning means 20 Flow rate calculating means

Claims (10)

A first vibrator that is provided in a fluid line and transmits an ultrasonic signal; a second vibrator that receives the ultrasonic signal; a transmission unit that outputs an AC signal to the first vibrator; and the second vibration Waveform detection means for detecting a predetermined number of zero-cross points that are generated a plurality of times in succession by an ultrasonic signal received by the child, an oscillator for generating a reference clock having a constant frequency, and the reference clock based on the reference clock A time measuring means for measuring an elapsed time from transmission of the first vibrator to a zero cross point detected by the waveform detecting means; a time calculating means for obtaining a propagation time of the ultrasonic signal based on a measurement result of the time measuring means; A time difference calculating means for obtaining an occurrence time interval of each zero cross point, and a frequency setting means for setting a reference clock frequency of the oscillator,
The fluid flow measuring device, wherein the frequency setting means sets a reference clock frequency so that an output of the time difference calculating means satisfies a predetermined condition. 2. The fluid flow measuring device according to claim 1, wherein the frequency setting means sets the reference clock frequency so that the phase relationship of the reference clock at each zero cross point is different. Phase difference detection means for obtaining the phase difference of the reference clock at two different zero-cross points detected by the waveform detection means instead of the time difference calculation means,
2. The fluid flow measuring device according to claim 1, wherein the frequency setting means sets the reference clock frequency of the oscillator so that the phase difference detected by the phase difference detecting means matches a target value. 4. The frequency setting means sets the reference clock frequency of the oscillator by setting a target value of the phase difference detected by the phase difference detection means to a value obtained by dividing 360 degrees by the number of zero cross detections. Fluid flow measuring device. 5. The frequency setting means adjusts the reference clock frequency by repeating the setting operation until the deviation between the phase difference detected by the phase difference detecting means and the phase difference target value becomes less than a predetermined value. Fluid flow measuring device. 5. The fluid flow measurement according to claim 3, wherein the frequency setting means invalidates the output of the time calculation means when the deviation between the phase difference detected by the phase difference detection means and the phase difference target value is larger than a predetermined value. apparatus. A second oscillator that starts operation at a zero-crossing point and stops operation at a rising point of a reference clock; and a second timing unit that counts the count number of the second oscillator;
7. The phase difference detection unit according to claim 3, wherein the phase difference detection unit obtains a phase difference between the reference clocks at two different zero-cross points detected by the waveform detection unit based on a count number counted by the second timing unit. The fluid flow measuring device according to claim 1. Temperature detection means for detecting fluid temperature, storage means for storing the relationship between the fluid temperature, the reference clock frequency of the oscillator, the phase difference detected by the phase difference detection means, and the storage contents of the storage means. Learning means for selecting a reference clock frequency for the phase difference target value;
The fluid flow measuring device according to any one of claims 3 to 7, wherein the frequency setting means sets the value selected by the learning means to the reference clock frequency. 9. The fluid flow measuring device according to claim 8, wherein the temperature detecting means is constituted by an arithmetic circuit that calculates the fluid temperature using the output result of the time calculating means. A flow rate calculation unit that calculates a fluid flow rate using a measurement result of the time calculation unit, a storage unit that stores a relationship between the fluid flow rate, a reference clock frequency of the oscillator, and a phase difference detected by the phase difference detection unit; Learning means for selecting a reference clock frequency for the phase difference target value based on the storage content of the storage means;
The fluid flow measuring device according to any one of claims 3 to 7, wherein the frequency setting means sets the value selected by the learning means as a reference clock frequency.

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