JPH07333031A - Mass flowmeter converter - Google Patents

Abstract

PURPOSE:To measure a time difference signal requiring no special clock at a high sensitivity free from effect of changes in flow rate. CONSTITUTION:A leading pulse (T+DELTAT) is inputted into an input terminal 1 and a delay pulse (T-DELTAT) into an input terminal 2 separately. N each of the above pulses are inputted at the time when contacts 7 and 8 determined by a switch SW1 are turned ON while being inputted into operational amplification circuits 12 and 13 with a time constant R1C1=R2C2 to perform a charging at a voltage proportional to the number of the pulses. Then, contacts 9 and 8 are turned on with switches SW2 and SW3 and connected to a reference power source 11 to discharge a charged voltage during the period when the contacts 7 and 8 are turned off of the switch SW1. A time difference of a zero- crossing voltage detected by the discharging is detected with zero-crossing detection circuits 16 and 17 to measure a mass flow rate proportional to the time difference DELTAT multiplied by 2N by a CPU 18, namely, to (2NDELTAT).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass flowmeter converter, and more specifically, it is suitable for measuring a time difference of a Coriolis flowmeter of a straight pipe type having a small detection sensitivity and capable of measuring a highly sensitive time difference. Mass flowmeter converter.

[0002]

2. Description of the Related Art As is well known, a Coriolis flowmeter measures a measurement pipe in which a fluid to be measured flows at both ends, and the center portion of the supported measurement pipe is alternately driven in a direction perpendicular to a support line. It is a mass flow meter that detects a phase difference signal proportional to the mass flow rate at a symmetrical position between both ends of the pipe and the central part. The phase difference signal is an amount proportional to the mass flow rate, but when the driving frequency is constant, the phase difference signal can be detected as a time difference signal at the observation position of the measuring tube.

If the frequency of the alternating drive of the measuring tube is made equal to the natural frequency of the measuring tube, a constant driving frequency according to the density of the fluid to be measured can be obtained, and it is possible to drive with a small driving energy. Recently, it has become common to drive the measuring tube at a natural frequency, and the phase difference signal is detected as a time difference signal.

FIG. 5 is a diagram for explaining an example of time difference measurement of a conventional Coriolis mass flowmeter. Figure 5 (a)
FIG. 4 is a diagram showing a displacement signal of the measuring tube at the detection position of the measuring tube, which is a trapezoidal wave signal obtained by amplifying and shaping the detected sine wave signal, with the horizontal axis representing time and the vertical axis representing voltage. There is. In the figure, the trapezoids A, B, C, D, ... And the trapezoids A ₁ , B ₁ , C ₁ , D ₁ , ... Are positive and negative (± E) with the peak value of the absolute value of the voltage being equal to the time axis XX. The displacement signals of the measuring tubes having different phases with voltage are shown. Each displacement signal is a continuous trapezoidal waveform of the same shape on the time axis, and the reference time representing the phase difference time is one trapezoidal waveform A, B. ，
For example, the time between the peak value C (+ E) or D (-E) of C and D of C and D and the position O crossing the time axis is T.

In the displacement signals of the trapezoids A, B, C, D and A ₁ , B ₁ , C ₁ , D ₁ having different phases, for example, the phase difference signal is explained at the sides C · D and C ₁ · D ₁ . To do.
The quadrilaterals C, C ₁ , D ₁ and D ₁ are parallelograms and the parallel sides C
The time difference ΔT between D and C ₁ · D ₁ is the phase difference signal, and the side C ·
C ₁ , D · D ₁ have the same length as the time axis O · O _1, and the projection points from the points C ₁ and D ₁ to the time axis are O ₂ , respectively.
Assuming O ₃ , the side O ₂ · O is the time (T−ΔT), and the side O · O ₃
Indicates time (T + ΔT).

The time (T-ΔT) is shown in FIG.
+ ΔT) is represented by the pulse shown in FIG. 5D, and the time difference ΔT is conventionally 2 times as long as the time pulse having the time width M of one period of the trapezoidal wave shown in FIG. 5B. The average value is obtained by subtracting the added value of the pulse widths of. That is, (4 (T + ΔT) −4 (T−ΔT)) / 8 = ΔT (1)

FIG. 6 shows a block diagram of a mass flowmeter converter for obtaining a conventional time difference ΔT, in which a pulse (T + Δ
The pulse width of T) and (T-ΔT)
As a number of clocks transmitted from each counter 34
The CPU (central processing unit) 35 measures the calculation based on the equation (1), and outputs a digital value proportional to the mass flow rate from the terminal 36.

[0008]

In the above-mentioned conventional mass flowmeter converter, the time proportional to the phase difference of the measuring tube is digitally measured as the number of clocks, but the time with the resolution commensurate with the accuracy is obtained. In order to measure, the number of clocks that is proportional to the time difference counted at all flow rates must be a sufficient number of clocks commensurate with accuracy. In a curved tube Coriolis flowmeter of the type in which the Coriolis force generated when the curved tube is alternately driven in the direction perpendicular to the axis of symmetry is generated around the axis of symmetry, the moment around the axis is measured. By making it large, a highly sensitive time difference signal can be detected, and the time difference signal can be counted by the number of clocks, but like a straight pipe type Coriolis flowmeter, the bending rigidity of the measuring pipe is large, and therefore the natural frequency is If it is large, the time difference signal generated by the Coriolis force is also small, and a clock oscillator that generates a high oscillation frequency (for example, 100 MHz) clock commensurate with accuracy currently has problems in reliability and stability.
It is difficult to obtain a satisfactory clock oscillator at low cost.

It is an object of the present invention to provide a mass flowmeter converter which can measure a time difference simply and highly accurately without using a clock oscillator having a high time resolution.

[0010]

In order to solve the above-mentioned problems, the present invention provides (1) the Coriolis force acting on a measuring tube which is alternately driven around a support point at a constant frequency, and the Coriolis force applied to the measuring tube. In a mass flow rate converter that detects a time difference ΔT between displacement signals which form a pair and is detected at a position, and measures a mass flow rate proportional to the time difference ΔT, the displacement signal has a positive and negative absolute value with respect to a reference time axis. When a pair of trapezoidal wave signals having a peak voltage is used, and T is a time difference between the time when the hypotenuse of one trapezoidal wave signal passes the reference time axis and the time when the peak voltage position of the hypotenuse passes. , The input pulse is a lead pulse and a lag pulse each having a constant peak value with a pulse width equal to the lead time (T + ΔT) and the lag time (T−ΔT) through which the peak value of the hypotenuse of the other trapezoidal wave signal passes. A gate circuit that opens and closes at a predetermined time to capture the input pulse, and when the gate circuit is opened, N (N> 1) leading and lagging pulses are simultaneously input and charged, When the gate circuit is closed, the first and second charging / discharging circuits for discharging the charged electric charges by the reference power source and the zero cross of the discharging voltage discharged after being charged in the first charging / discharging circuit are detected. First
A zero-cross detection circuit, a second zero-cross detection circuit that detects a zero-cross of the voltage discharged after being charged in the second charge / discharge circuit, and each zero detected by the first and second zero-cross detection circuits. A time difference detection circuit for detecting a time difference of the cross time, and outputting a mass flow rate proportional to the detected time difference, and (2) in (1), the first charge / discharge circuit is input. Input pulse switching circuit for switching the leading pulse to the input of the delayed pulse and switching the delayed pulse input to the second charging / discharging circuit to the input of the leading pulse, and inputting the leading pulse to the first charging / discharging circuit, 2 The time difference of the zero crossing time obtained by inputting the delay pulse to the charge / discharge circuit and the delay pulse input to the first charge / discharge circuit and the advance pulse input to the second charge / discharge circuit. Time difference of zero cross time It is characterized in that it has a time course correction means of the charge and discharge circuit for correcting the mass flow rate which is measured in proportion to the time difference deviation.

[0011]

The negative (or positive) leading pulse of the peak value (-E) having the pulse width of the time (T + ΔT) and the pulse width of the time (T-ΔT) at the peak value equal to the leading pulse. N (N> 1) negative (or positive) delayed pulses are charged into a charge / discharge circuit having the same time constant. The charged electric charges of the charge / discharge circuit are simultaneously discharged by the same positive (or negative) reference power source. At the time of charging, N leading and lagging pulses are charged with a gradient according to the magnitude of the pulse width and have different charging voltages, but at the time of discharging, they are discharged with parallel discharge straight lines with a same time constant. , The zero crossing causes the difference in the zero crossing position of the discharge line to be the time difference ΔT due to the Coriolis force.
2N times larger.

[0012]

【Example】

Embodiment 1 (corresponding to claim 1) FIG. 1 is a circuit block for explaining Embodiment 1 of a mass flowmeter converter according to the present invention, in which 1 is a pulse (T + ΔT) input terminal and 2 Is an input terminal of pulse (T-ΔT), 3 is an input terminal of switch SW ₁ , 4 is an input terminal of switch SW ₂ , 5 is an input terminal of switch SW ₃ , 6 is an input terminal of switch SW ₄ , 7 and 8 , 9, 10, 14,
Reference numeral 15 is a contact, 11 is a positive reference power source, 12 and 13 are operational amplifier circuits, 16 and 17 are zero cross detection circuits, and 18 is a CPU.
(Central processing circuit).

A negative leading pulse (T + ΔT) having a pulse width of time (T + ΔT) and a voltage (-E) having a constant voltage value.
The input terminal 1 for inputting (hereinafter simply referred to as (T + ΔT) pulse) is driven by the switch SW ₁ and simultaneously turned on.
Via a contact 7 that is (closed) and turned off (open), it is connected to an integrator composed of an input resistor R ₁ , a feedback capacitor C ₁ and an operational amplifier circuit 12 to form a charge / discharge circuit.

At the same time, a negative delay pulse (T) having a pulse width of time (T-ΔT) and a constant voltage value (-E) is used.
The input terminal 2 for inputting −ΔT (hereinafter, simply referred to as (T−ΔT) pulse) is connected to the input resistance R via the contact 8.
₂ , the feedback capacitor C ₂ and the operational amplifier circuit 13 are connected to an integrator to form a charge / discharge circuit.

Contacts 14 and 15 driven by switches SW ₄ and SW ₅ are connected in parallel to the feedback capacitors C ₁ and C ₂ of the operational amplifier circuits 12 and 13, respectively, and are turned on immediately before the charging operation. The charges of the capacitors C ₁ and C ₂ are discharged. Zero-cross detection circuits 16 and 17 are connected to the operational amplifier circuits 12 and 13, respectively, and the detected zero-cross signals are input to the CPU 18. Here,
The resistance value and capacitance are selected as R ₁ = R ₂ and C ₁ = C ₂ .

On the other hand, a contact 9 driven by the switch SW ₂ and a contact 10 driven by the switch SW ₃ are connected in series between the contact 7 and the resistance R ₁ and between the contact 8 and the resistance R ₂ . A reference power source 11 having a positive reference voltage Es is connected between the contacts 9 and 10. Next, the operation of the mass flowmeter converter configured as described above will be described based on FIG.

FIG. 2 is a time chart for explaining the mass flowmeter converter shown in FIG. Here, for the sake of explanation, it is assumed that the measurement start time is after time t ₁ . First, the switch SW _4, SW ₅ ON the contacts 14 and 15 by the driving of (FIG. 2 (i), (h) ) and After Death charged charges the feedback capacitor C _1, C ₂ and, High switch SW ₁ The contacts 7 and 8 are simultaneously turned on by the level signal (FIG. 2C) during the period of time t ₁ to t ₂ , and N (T + ΔT) pulses (FIG.
(A)) is input to the resistor R ₁ , and at the same time, N (T−Δ
The (T) pulse (FIG. 2B) is input to the resistor R ₂ . During this time, the switches SW ₂ and SW ₃ are low level signals (see FIG. 2).
As shown in (d) and (e), the contacts 9 and 8 are turned off.

Therefore, the voltage on the straight line A / B (FIG. 2 (f)) rising in proportion to the number of input pulses from the integrator of R ₁ / C ₁ by the input of N (T + ΔT) pulses. Is output. At the same time, by inputting N (T-ΔT) pulses, the voltage shown by the line A · D (FIG. 2 (f)) is output from the R ₂ C ₂ integrator. Clearly the (T + ΔT) pulse is
Since the pulse width is larger than the (T-ΔT) pulse, the voltage at point B is larger than the voltage at point D.

Next, during the period from time t _{2 to} t ₃ , the switch S
The contacts 7 and 8 are turned off by the Low level signal of W ₁ (FIG. 2C), and the HIGH level signals (FIGS. 2D and 2E) are output from the switches SW ₂ and SW ₃ by the OFF signal, Reference power source 1 with positive reference voltage Es when contacts 9 and 10 are turned on
3 are respectively input to the R ₁ C ₁ integrator and the R ₂ C ₂ integrator,
The electric charge charged during each period t _{1 to} t ₂ is discharged. Here, R ₁ = R ₂ and C ₁ = C ₂ are selected, the R ₁ C ₁ integrator is discharged according to the straight line B · C, and the R ₂ C ₂ integrator is straight line D.
-Discharge according to E (Fig. 2 (f)), and each discharge straight line crosses zero with the voltage zero volt line at the time position of CE.

Since the time constant R ₁ C ₁ = R ₂ C ₂ and the negative reference voltage Es are discharged by the reference power source 13, the straight line B · C
And D and E are parallel, and the straight line B and D and the straight line E and C are proportional. The straight line E · C indicates the time axis, the zero-cross position C is detected by the zero-cross detector 16 as a zero-cross time signal g ₁ , and the zero-cross position E is detected by the zero-cross detector 17. is detected as g _2, is output as a pulse having a pulse width g ₁ g ₂ (Fig. 2 (g)).

The period between the time signals g ₁ and g ₂ is a time difference signal obtained by multiplying the time difference ΔT corresponding to N (T + ΔT) −N (T−ΔT) = 2NΔT (2) by 2N.
The zero crossing time signals g ₁ and g ₂ are converted into digital signals by the clock of the CPU 18 and digitally processed to output a mass flow rate signal proportional to the time ΔT multiplied by 2N.

Next, the switches SW ₂ , SW ₃ , SW ₄ , S
The operation of W ₅ will be described. The High level signal (FIG. 2 (d)) of the switch SW ₂ which turned on the contact 10 at time t ₂ is the zero cross signal g ₂ (FIG. 2 (g)) of the discharge line D · E at time g ₂ .
The signal is switched to the low level signal. At the same time, the zero crossing signal g ₂ is turned ON raised contact 15 up the switch SW ₅ to the High level signal (Fig. 2 (h)), (T- △ T)
The charge on the capacitor C ₂ of the integrator on the pulse side is discharged. The High level signal of the switch SW ₅ is lowered to the Low level signal (h) by the High level signal of the switch SW ₁ (FIG. 2C) that determines the time t ₃ when the next measurement cycle is started.

Similarly, the High level signal (FIG. 2 (e)) of the switch SW ₃ whose contact 9 is turned on at time t ₂ is raised by the zero cross signal g ₁ (FIG. 2 (g)) of the discharge straight line B / C. It is lowered and becomes a Low level signal. At the same time, the zero-cross signal g
₁ raises the switch SW ₄ to a high level signal (FIG. 2 (i)), turns on the contact 14 (T + ΔT) and discharges the capacitor c ₁ of the pulse side integrator. Switch S
High level signal W ₄ is lowered to the High level signal of the switch SW ₁ defining a time t ₃ when the next measurement cycle is started (FIG. 2 (c)) by a Low level signal (FIG. 2 (i)) .

Looking at the above measurement time, as shown in FIG. 2 (j), during the period from time t _{1 to} t ₂ , N (T +
△ T) is a pulse and (T-△ T) charging period for charging the pulse, a period of time t ₂ ~t ₃ is simultaneously discharges the electric charge charged (T + △ T) pulses and (T-△
T) Determine the zero-cross time g ₁ and g ₂ of the pulse and set 2N · ΔT
Is measured, and the charge of the integrating capacitors c ₁ and c ₂ is discharged in the period of time g _{2 to} t ₃ to prepare for the next measurement cycle. Therefore, (T + ΔT) pulse,
After 2N (T-ΔT) pulses are input and charged and then discharged, the capacitor of the integrator is discharged to prepare for the next measurement. The period from t ₁ to t ₃ is one measurement cycle. This measurement cycle is repeated in the same manner thereafter.

According to the mass flowmeter converter to which the first embodiment shown in FIG. 1 is applied, the time t ₁ to t _{2 is set} by the switch SW _1.
Since N (T + ΔT) pulses and (T-ΔT) pulses are simultaneously taken in during the period of time t and the flow rate of the fluid to be measured fluctuates during the period of time t _{1 to} t ₂ , the driving frequency is the same, so Equal number of (T + ΔT) pulses and (T−ΔT)
The pulse is arithmetically processed, and a highly sensitive and accurate mass flow rate signal is obtained without being affected by flow rate fluctuation.

In the mass flow converter according to the first embodiment described above, although the time constant R ₁ C ₁ = R ₂ C ₂ is selected, the resistance R
Also, the capacitance C is affected by temperature and changes over time for a long period of time, and accordingly, the time constant changes, which may cause an error in the time difference measurement result. The second embodiment relates to a detection circuit that detects a time difference deviation caused by changes in time constants R ₁ C ₁ and R ₂ C _2, and corrects a mass flow rate calculated based on the detected time difference deviation. It is intended.

Embodiment 2 (corresponding to claim 2) FIG. 3 is a circuit block for explaining a mass flow converter according to Embodiment 2, in which 20 is an input terminal of a switch SW ₆ , 21, 22. , 23, 24 are contacts, 25, 26
Is an inverter. The same reference numerals as those in FIG. 1 are attached to the portions having the same functions as those in FIG.

The circuit block of FIG. 3 differs from the circuit block of FIG. 1 in that the contact point 23 is applied between the contact point 7 of the line to which the (T + ΔT) pulse is applied and the resistor R _1, and the (T-ΔT) pulse is applied. A contact 24 is connected between the contact 8 of the line and the resistor R _2, and the contacts 23 and 24 are driven by the High level signal of the switch SW ₆ connected to the input terminal 20.

Further, the input terminal 20 has an inverter 25
Is connected to turn ON the contacts 21 and 22 by the Low level signal of the switch SW ₆ , and the inverter 26 is further connected to the inverter 25 so that the contact 24 is substantially switched.
It is driven by a high level signal of ₅ .

One side of the contact 21 is connected between the contacts 7 and 23 to which the (T + ΔT) pulse is input, and the other side is connected between the resistance R ₂ of the (T-ΔT) pulse line and the contact 24. Connected to. Similarly, one side of the contact 22 is (T-Δ
T) is connected between the contacts 8 and 24 to which the pulse is input, and the other side is connected between the resistor R ₁ of the (T + ΔT) pulse line and the contact 23.

In the circuit block diagram shown in FIG. 3, the contacts 23 and 24 are turned on by the high level signal of the switch SW ₆ during the normal time difference measurement period, and the same operation as the mass conversion circuit shown in FIG. 1 is performed. A time difference signal g ₂ g ₁ proportional to the mass flow rate as shown in FIG. 2G is detected.

However, during the time difference inspection for inspecting whether or not the time constants R ₁ C ₁ and R ₂ C ₂ are changing with time, the signal of SW ₆ is set to a low level signal to turn off the contacts 23 and 24, and at the same time, the contact 21. , 22 are turned on, the N (T + ΔT) pulses applied to the input terminal 1 are converted into the time constant R.
₂ C ₂ integrator and N (T-ΔT) pulses applied to the input terminal 2 are input to the integrator with time constant R ₁ C ₁ to check the time difference signal during normal flow rate measurement. The time difference signal at time is compared.

FIG. 4 is an output voltage characteristic for explaining the operation of the mass flowmeter arithmetic circuit shown in FIG.
4A shows an output voltage characteristic according to a time difference signal at the time of normal measurement of the time difference signal, and FIG. 4B shows an output voltage characteristic at the time of inspection.

FIG. 4 (a) is a diagram showing a time difference signal in the normal flow measurement, the N input to the input terminal 1 (T + ΔT) pulses of constant R ₁ C ₁ when the period t ₁ It is input to the integrator and charged in proportion to the number N of input pulses according to the straight line A ₁ · B ₁ until the period t ₂ . On the other hand, N (T-ΔT) pulses input to the input terminal 2 at the same time are also charged according to the straight line A ₁ D ₁ with the time constant R ₂ C ₂ in the same period, and the switches SW ₁ , SW ₂ , SW ₃ are switched. by being discharged in accordance with each linear _{B 1 · C 1, D 1} · E 1 by the reference power source 11, measurement time difference _{E 1 · C 1 = 2N ·} ΔT = ΔT 1 is measured.

Next, the time difference signal ΔT ₂ at the time of inspection shown in FIG. 4 (b) is the N (T + Δ) input to the input terminal 1.
T) The pulse is input to the integrator with the time constant R ₂ C ₂ and the straight line A ₂
・ The charging voltage according to D ₂ is output, and at the same time, the N (T-ΔT) pulses input to the input terminal 2 are input to the integrator with the time constant R ₁ C ₁ , and the straight line A ₂・ B ₂ is input. The corresponding charging voltage is output. During the next period of t _{2 to} g ₃ , the reference power supply 11 discharges along the parallel straight lines D ₂ · E ₂ and B ₂ · C, and the measurement time difference E ₂ C ₂ = 2NΔT = ΔT ₂ is measured.

If the time constant R ₁ C ₁ = R ₂ C ₂ , the measurement time difference ΔT ₁ = ΔT ₂ and the time constant R ₁ C ₁ and the time constant R
It is judged that there is no change in ₂ C ₂ with time. However, the time constant R
_{If 1} C ₁ −R ₂ C ₂ = ΔTε, the measurement time difference ΔTε is detected as twice as large as | ΔT ₁ −ΔT ₂ | = 2 | ΔTε | (3), and the time difference deviates and the mass flow rate changes. An error occurs.
From this, ΔTε is calculated in the CPU 18 and the mass flow rate is corrected.

As described above, according to the mass flowmeter conversion circuit of the second embodiment, in addition to the effect of the first embodiment, an error due to a temperature change or a temporal change of the time constant, which is a main part of the time difference measurement, is measured. In addition, since the correction is added, it is possible to measure the mass flow rate with high accuracy and high reliability.

[0038]

As is apparent from the above description, the mass flowmeter converter of the present invention has the following effects. (1) Effects corresponding to claim 1: N (T + Δ)
(T) pulse and (T-ΔT) pulse are taken in at the same time, charged into the integrator with the same time constant, and then discharged simultaneously by the reference power source to measure the time difference of the zero-cross voltage. Therefore, the time difference ΔT expanded by 2N times is obtained. A signal is detected, a highly sensitive time difference measurement is measured without using a special clock pulse, and an accurate and highly sensitive mass flow rate is measured without being changed by the flow rate even if there is a slight change in the flow rate. (2) Effect corresponding to claim 2: N (T + Δ)
The (T) pulse and the (T-ΔT) pulse are input to an integrator that has an equal time constant for charging and discharging, and the (T + ΔT) pulse and the (T-ΔT) pulse are different from each other in the normal measurement and the inspection. It is switched to input to the side integrator.
In addition to the effect of claim 1, highly reliable over a long period of time, since it is detected whether or not the time difference deviation of 2NΔT obtained by the switching causes a time difference deviation of ΔTε, and if the deviation occurs, correction is performed. The mass flow rate measurement signal of can be output.

[Brief description of drawings]

FIG. 1 is a circuit block for explaining a first embodiment of a mass flowmeter converter according to the present invention.

FIG. 2 is a time chart for explaining the mass flowmeter converter shown in FIG.

FIG. 3 is a circuit block for explaining a mass flow converter according to a second embodiment.

FIG. 4 is an output voltage characteristic for explaining the operation of the mass flowmeter arithmetic circuit shown in FIG.

FIG. 5 is a diagram for explaining an example of time difference measurement of a conventional Coriolis mass flowmeter.

FIG. 6 shows a block diagram of a mass flowmeter converter for obtaining a conventional time difference ΔT.

[Explanation of symbols]

1 ... Pulse (T + ΔT) input terminal, 2 ... Pulse (T-
ΔT) input terminal, 3 ... switch SW ₁ input terminal, 4
... switch SW ₂ input terminal, 5 switch SW ₃ input terminal, 6 switch SW ₄ input terminal, 7, 8, 9, 1
0, 14, 15 ... Contact, 11 ... Reference power supply, 12, 13 ...
Operational amplifier circuit 16, 17, ... Zero cross detection circuit, 18 ...
CPU (central processing unit), 20 ... Input terminals of switch 6, 21, 22, 23, 24 ... Contact, 25, 26 ... Inverter.

Claims (2)

[Claims] 1. A Coriolis force acting on a measuring tube which is alternately driven around a support point at a constant frequency is detected as a time difference ΔT between displacement signals which form a pair and which are detected at symmetrical positions of the measuring tube. In a mass flow rate converter that measures a mass flow rate proportional to a time difference ΔT, the displacement signal is a pair of trapezoidal wave signals having positive and negative peak voltages whose absolute values are equal to a reference time axis, and a hypotenuse of one of the trapezoidal wave signals. When the time difference until the peak voltage position of the hypotenuse passes with respect to the time when the traverse of the reference time axis is T, the lead time (T + Δ) at which the peak value of the hypotenuse of the other trapezoidal wave signal passes
T) and a delay time (T-ΔT), and a gate circuit that opens and closes at a predetermined time to take in the input pulse, each of which is a leading pulse or a lag pulse having a constant peak value. When the gate circuit is opened, N (N> 1) pieces of the lead pulse and the lag pulse are simultaneously input and charged, and when the gate circuit is closed, the charged charge is discharged by the reference power source. First and second charge / discharge circuits, a first zero-cross detection circuit for detecting a zero-cross of the discharge voltage discharged after being charged in the first charge / discharge circuit, and after being discharged in the second charge / discharge circuit. And a time difference detection circuit for detecting a time difference between the respective zero cross times detected by the first and second zero cross detection circuits. Due to the time difference Mass flowmeter converter and outputs a mass flow rate that example. 2. An input pulse switching circuit for switching a lead pulse input to the first charge / discharge circuit to a delay pulse input and a delay pulse input to the second charge / discharge circuit to a lead pulse input, The leading pulse is input to the first charging / discharging circuit and the delay pulse is input to the second charging / discharging circuit, and the delay pulse is input to the first charging / discharging circuit. 2 The present invention is characterized by having a time-dependent change correcting means of the charge / discharge circuit for correcting the mass flow rate measured in proportion to a time difference deviation from the time difference of the zero cross time obtained by inputting a pulse to the charge / discharge circuit. The mass flowmeter converter according to claim 1.