JP3996464B2 - 2-wire electromagnetic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flowmeter that measures the flow rate of a fluid having conductivity in various process systems. In particular, an external voltage is supplied from a pair of signal lines connected to a DC power source, and a pair of signals that supply the external voltage. The present invention relates to a two-wire electromagnetic flow meter that outputs a measurement value by adjusting an output current flowing through a wire.
[0002]
[Prior art]
Conventionally, in this type of two-wire electromagnetic flow meter, a rectangular wave-like excitation current Iex is applied at a predetermined frequency to an excitation coil arranged with the direction of generation of the magnetic field perpendicular to the flow direction of the fluid flowing in the measurement tube. The signal electromotive force (a signal proportional to the flow rate) obtained between the electrodes arranged in the measurement tube perpendicular to the magnetic field generated by the excitation coil is detected, and the CPU calculates based on the detected signal electromotive force. A measured value is obtained as a value of 0 to 100% by processing, and a current (output current) flowing through a pair of signal lines supplying an external voltage to this two-wire electromagnetic flow meter is 4 to 20 mA depending on the obtained measured value. The current range is adjusted.
[0003]
FIG. 6 shows an outline of a conventional two-wire electromagnetic flow meter. In the figure, 100 is a two-wire electromagnetic flow meter, 200 is a DC power source, and the two-wire electromagnetic flow meter 100 is supplied with an external voltage Vs from the DC power source 200 via a pair of signal lines L1 and L2. The For example, a resistor of 250Ω is connected to the signal line L2 as the external load RL. Further, the DC power source 200 is set to 24V DC. In this case, the external voltage Vs supplied to the two-wire electromagnetic flow meter 100 is a value obtained by subtracting the voltage drop at the external load RL from the power supply voltage (DC24V) of the DC power supply 200.
[0004]
In the two-wire electromagnetic flow meter 100, 1 is a measurement tube, 2 is an excitation coil arranged with the direction of the magnetic field generated perpendicular to the flow direction of the fluid flowing in the measurement tube 1, and 3 is a rectangle to the excitation coil 2 Excitation circuits for periodically supplying a wavy excitation current Iex, 4a and 4b are detection electrodes arranged in the measuring tube 1 perpendicular to the magnetic field generated by the excitation coil 2, 5 is a ground electrode, 6 is a detection electrode 4a, A signal electromotive force obtained between 4b is detected, a measured value is obtained based on the detected signal electromotive force, and an output current Iout returned to the DC power source 200 according to the obtained measured value is a current of 4 to 20 mA. It is a flow rate measurement output circuit that adjusts within a range.
[0005]
The excitation circuit 3 has an excitation voltage circuit (constant voltage circuit) 3-1 and a current adjustment circuit (CCS) 3-2. The excitation voltage circuit 3-1 generates a constant voltage of 8.5V as the excitation voltage Vex. The current adjustment circuit 3-2 applies the excitation voltage Vex from the excitation voltage circuit 3-1 to the excitation coil 2 and generates a rectangular wave excitation current Iex whose polarity is alternately switched. The value (crest value) of the excitation current Iex is switched in multiple stages as shown in FIG. 7 according to the measurement value in the flow measurement output circuit 6.
[0006]
The flow rate measurement output circuit 6 includes a signal electromotive force detection circuit 6-1, a sample hold circuit 6-2, an A / D conversion circuit 6-3, a CPU 6-4, a D / A conversion circuit 6-5, It has a current adjustment circuit (CCS) 6-6 and a constant voltage circuit 6-7 for supplying a power supply voltage to these circuits.
[0007]
The signal electromotive force detection circuit 6-1 detects the signal electromotive force obtained between the electrodes 4a and 4b with reference to the potential of the ground electrode 5. The sample hold circuit 6-2 receives the signal electromotive force detected by the signal electromotive force detection circuit 6-1, and samples and holds the value of the signal electromotive force immediately before the polarity is switched. The A / D conversion circuit 6-3 converts the signal electromotive force (analog value) sampled and held by the sample hold circuit 6-2 into a digital value, and sends the digital value to the CPU 6-4.
[0008]
The CPU 6-4 obtains a measured value (0 to 100%) based on the signal electromotive force from the A / D conversion circuit 6-3, and outputs it to the D / A conversion circuit 6-5. The D / A conversion circuit 6-5 converts the measurement value (digital value) from the CPU 6-4 into an analog value and sends it to the current adjustment circuit 6-6. The current adjusting circuit 6-6 includes a comparator CP1, a transistor Q1, and a resistor R1. By adjusting the base current of the transistor Q1 by the comparator CP1, the current Iccs flowing between the collector and the emitter of the transistor Q1 is converted to D / A. It adjusts according to the measured value from the conversion circuit 6-5. Further, the CPU 6-4 supplies the exciting current Iex to the exciting coil 2 in the relationship shown in FIG. 7 according to the measured value obtained based on the signal electromotive force from the A / D conversion circuit 6-3. A command is given to the current adjustment circuit 3-2 of the excitation circuit 3.
[0009]
[DC excitation circuit method]
In the two-wire electromagnetic flow meter 100, the excitation circuit 3 and the flow measurement output circuit 6 are connected in series between the signal lines L1 and L2, and the current flowing through the excitation circuit 3 is supplied to the flow measurement output circuit 6. The output current Iout flows in and returns to the DC power supply 200. Such a system is called a series excitation circuit system.
[0010]
For example, when the measurement value in the CPU 6-4 is 0%, the value of the excitation current Iex in the excitation circuit 3 is 3.5 mA. In the exciting circuit 3, a current of 0.5 mA is required for generating the exciting voltage Vex in the exciting voltage circuit 3-1, and if the current flowing through the exciting voltage circuit 3-1 is Ia, the current flowing through the exciting circuit 3 I1 becomes I1 = Ia + Iex = 0.5 mA + 3.5 mA = 4 mA.
[0011]
This 4 mA current flows into the flow rate measurement output circuit 6. In the flow measurement output circuit 6, when the current flowing to the constant voltage circuit 6-7 side is Ib, this current Ib needs 3 mA to drive the CPU 6-4 and the like, and the current Iccs flowing through the transistor Q1 side is 1 mA. , The current I2 flowing through the flow measurement output circuit 6 (I2 = Iccs + Ib) is 4 mA, the current I1 flowing through the excitation circuit 3 is equal to the current I2 flowing through the flow measurement output circuit 6, and the DC power supply 200 The output current Iout returned to is 4 mA indicating a measured value of 0%.
[0012]
If the measured value at the CPU 6-4 is 10%, for example, the CPU 6-4 adjusts the current Iccs flowing through the transistor Q1 to 2.6 mA so that the output current Iout becomes Iout = 4 mA + 1.6 mA = 5.6 mA. To do. In this case, in the excitation circuit 3, since the excitation current Iex = 3.5 mA, the current Ia flowing through the excitation voltage circuit 3-1 side is Ia = 2.1 mA.
[0013]
[Reason for changing the excitation current Iex value in multiple steps according to the measured value]
The value of the excitation current Iex is switched in multiple stages according to the measured value by the CPU 6-4 according to the relationship shown in FIG. Such a method of switching the value of the excitation current Iex in multiple stages is called a multi-point excitation switching method. If the multipoint excitation switching method is not used, that is, if the value of the excitation current Iex is fixed to 3.5 mA, the signal electromotive force obtained by the signal electromotive force detection circuit 6-1 is small because the magnetic flux density penetrating the fluid is small. The output fluctuates greatly under the influence of noise superimposed on the electrodes 4a and 4b according to the flow velocity. That is, as the flow rate increases, the ratio of noise included in the signal electromotive force increases, the S / N ratio decreases, and stable flow rate measurement cannot be performed.
[0014]
If the signal electromotive force obtained by the signal electromotive force detection circuit 6-1 is e, the signal electromotive force e is expressed as e = k · B · v · D. In this equation, k is a constant, D is the diameter of the measuring tube 1, v is the average flow velocity, and B is the generated magnetic flux density. Here, B is proportional to the exciting current Iex. If the exciting current Iex is increased, the signal electromotive force e is increased even at the same flow rate. Therefore, in the conventional two-wire electromagnetic flow meter 100 shown in FIG. 6, if the output current (4 to 20 mA) corresponding to the measured value increases, the increased amount is used. The excitation current Iex is switched to a large value.
[0015]
For example, when the measured value reaches 20%, the value of the excitation current Iex is switched to 6.7 mA. That is, the output current Iout corresponding to the measured value 20% is 7.2 mA, and the excitation circuit 3 requires Ia = 0.5 mA. Therefore, the excitation current Iex can flow up to 6.7 mA. When the measured value reaches 40%, the value of the excitation current Iex is switched to 9.9 mA. That is, the output current Iout corresponding to the measured value 40% is 10.4 mA, and the excitation circuit 3 requires Ia = 0.5 mA. Therefore, the excitation current Iex can flow up to 9.9 mA. Thus, by switching the excitation current Iex to a large value according to the measured value, the signal electromotive force e is increased, the S / N ratio is improved, and stable flow rate measurement is possible.
[0016]
[Problems to be solved by the invention]
However, since the conventional two-wire electromagnetic flow meter 100 described above has a series excitation circuit method, the external voltage Vs supplied to the two-wire electromagnetic flow meter 100, that is, the power supply voltage DC24V of the DC power supply 200 is used as an external load. The voltage Vs obtained by subtracting the voltage drop Iout × RL in RL is divided into the excitation circuit 3 and the flow rate measurement output circuit 6. For this reason, the excitation voltage Vex generated by the excitation voltage circuit 3-1 is as small as 8.5V, and the rectangular wave excitation current Iex supplied to the excitation coil 2 has a rise time up to a required value as the value increases. become longer.
[0017]
FIG. 8 is a rising waveform when the value of the excitation current Iex is switched to Iex = 3.5 mA, 6.7 mA, 9.9 mA, and 12 mA. When the value of the excitation current Iex is as small as 3.5 mA, It rises immediately like the waveform I shown in the figure. However, since the voltage does not change due to the excitation generated by the excitation voltage circuit 3-1, when the value of the excitation current Iex increases to 6.7 mA, 9.9 mA, and 12 mA, the waveforms II, III, and IV shown in FIG. As described above, the rising time becomes longer, and the steady region (flat waveform portion after Iex reaches the required value) ta just before the polarity is switched becomes shorter.
[0018]
The sample and hold circuit 6-2 samples and holds the value of the signal electromotive force e immediately before the polarity is switched. For example, the signal electromotive force e is sampled for 5 ms immediately before the polarity of the signal electromotive force e is switched, and the average value is held. When the value of the excitation current Iex is 12 mA, the steady region ta immediately before the polarity of the excitation current Iex is switched is about 5 ms, and the sampled electromotive force e is a value obtained by the barely stable excitation current Iex.
[0019]
However, if the value of the excitation current Iex is approximately 12 mA, the signal electromotive force e is sampled while the excitation current Iex is changing. Due to the eddy current generated in the electrodes 4a and 4b, An error is included in the measured value of the flow rate. For this reason, in the conventional two-wire electromagnetic flow meter 100 of the multipoint excitation switching method, the limit value of the excitation current Iex set in multiple stages according to the measured value is about 12 mA, and the excitation current Iex is more than this. The value of could not be increased.
[0020]
The present invention has been made to solve such a problem, and the object of the present invention is a two-wire system that can further increase the limit value of the excitation current set in multiple stages according to the measured value. It is to provide an electromagnetic flow meter.
[0021]
[Means for Solving the Problems]
In order to achieve such an object, according to the present invention, in the above-described two-wire electromagnetic flow meter, when a measurement value in the flow measurement output circuit exceeds a predetermined value, the two-wire electromagnetic flow meter is connected in series between a pair of signal lines. Switching means to switch the connection between the excitation circuit and the flow measurement output circuit in parallel and switch the excitation voltage to the excitation coil in the excitation circuit to an excitation voltage higher than the previous excitation voltage within the range of the external voltage. It is a thing.
According to the present invention, when the measured value in the flow measurement output circuit exceeds, for example, 20%, the excitation circuit and the flow measurement output circuit that have been connected in series between the pair of signal lines until then are connected in parallel. At the same time, the excitation voltage to the excitation coil in the excitation circuit is switched to an excitation voltage (within an external voltage range) higher than the excitation voltage so far. That is, what was previously divided into the excitation circuit and the flow measurement output circuit is applied to both the excitation circuit and the flow measurement output circuit, and the excitation voltage generated in the excitation circuit. Becomes larger than the previous excitation voltage. As a result, the rise time of the excitation current can be shortened , and the steady region immediately before the polarity is switched can be lengthened. Even if the excitation current is set to 12 mA or more, for example, 5 ms or more is secured as the steady region immediately before the polarity is switched. Thus, the limit value of the excitation current set in multiple stages according to the measured value can be set to 12 mA or more.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an outline of an embodiment of a two-wire electromagnetic flow meter according to the present invention. In FIG. 6, the same reference numerals as those in FIG. 6 denote the same or equivalent components as those described with reference to FIG.
[0023]
In the two-wire electromagnetic flow meter 100A, the first switch SW1 is provided in the connection line LA between the excitation circuit 3 and the flow rate measurement output circuit 6, and is on the connection line between the excitation voltage circuit 3-1 and the switch SW1. A second switch SW2 is provided between the point P1, the point P2 on the connection line between the resistor R1 and the external load RL (in the connection line LB), and the signal line L1 and the excitation voltage circuit 3-1 are connected. A third switch SW3 is provided between the point P3 on the line and the point P4 on the connection line between the switch SW1 and the flow rate measurement output circuit 6 (in the connection line LC).
[0024]
Further, a function for controlling on / off of the switches SW1 to SW3 is added to the CPU 6-4 in the flow rate measurement output circuit 6, and the measured value in the CPU 6-4 output from the A / D conversion circuit 6-3 is 0 to 0. In the range of 20%, the switch SW1 is turned on (switches SW2 and SW3 are turned off). When the measured value by the CPU 6-4 exceeds 20%, the switches SW2 and SW3 are turned on (the switch SW1 is turned off). I am doing so.
[0025]
Further, according to the measured value obtained based on the signal electromotive force from the A / D conversion circuit 6-3, the current of the value determined by the relationship shown in FIG. 2 is determined as the excitation current instead of the relationship shown in FIG. Thus, a command is given from the CPU 6-4 to the current adjustment circuit 3-2 of the excitation circuit 3. If the measured value at the CPU 6-4 exceeds 20%, the CPU 6-4 gives a command to the excitation voltage circuit 3-1, and the value of the excitation voltage Vex is set to 8.5V in accordance with the switching of the switches SW1 to SW3. To a constant voltage of 14V.
[0026]
[Series excitation circuit method: 0% to 20%]
The CPU 6-4 obtains a measured value based on the signal electromotive force from the A / D conversion circuit 6-3, and when the measured value is in the range of 0 to 20%, the switch SW1 is turned on and the switches SW2 and SW3 are turned off. To do. As a result, the excitation circuit 3 and the flow measurement output circuit 6 are connected in series between the signal lines L1 and L2, and the two-wire electromagnetic flow meter 100A operates in the same manner as the conventional two-wire electromagnetic flow meter 100. Do.
[0027]
FIG. 3 shows a simplified circuit connection in the two-wire electromagnetic flow meter 100A in this case. In this case, the excitation voltage Vex generated by the excitation voltage circuit 3-1 in the excitation circuit 3 is 8.5 V, and the value of the excitation current Iex supplied to the excitation coil 2 by the current adjustment circuit 3-2 is 3.5 mA. The current Ia flowing through the exciting voltage circuit 3-1 side is 0.5 to 3.7 mA corresponding to the measured value of 0% to 20%. In the flow measurement output circuit 6, the current Ib to the constant voltage circuit 6-7 side is 3 mA, and the current Iccs to the transistor Q1 side is 1 to 4.2 mA corresponding to the measured value 0% to 20%. Is done.
[0028]
[Parallel excitation circuit method: 20% to 100%]
When the measured value obtained based on the signal electromotive force from the A / D conversion circuit 6-3 exceeds 20%, that is, the output current Iout returned to the DC power supply 200 exceeds 7.2 mA, the CPU 6-4 The switch SW1 is turned off and the switches SW2 and SW3 are turned on. Thereby, the excitation circuit 3 and the flow rate measurement output circuit 6 that have been connected in series between the signal lines L1 and L2 until then are connected in parallel between the signal lines L1 and L2.
[0029]
FIG. 4 shows a simplified circuit connection in the two-wire electromagnetic flow meter 100A in this case. In this case, the input current Iin = 7.2 mA from the DC power supply 200 is divided into a current I1 (I1 = Iex + Ia) to the excitation circuit 3 and a current I2 (I2 = Ib + Iccs) to the flow rate measurement output circuit 6. In this example, the value of the excitation current Iex to the excitation coil 2 in the excitation circuit 3 is 3.5 mA, and a current of Ia = 0.5 mA is required to generate the excitation voltage Vex in the excitation voltage circuit 3-1. The flow rate measurement output circuit 6 requires 3 mA as the current Ib to the constant voltage circuit 6-7 side. When these currents Iex, Ia, and Ib are added together, 3.5 + 0.5 + 3 = 7 mA, which is almost the same as the input current Iin = 7.2 mA from the DC power supply 200. The difference 0.2 mA between 7.2 mA and 7 mA is the current Iccs flowing through the transistor Q1 side in the flow measurement output circuit 6.
[0030]
Further, when the measured value exceeds 20%, the CPU 6-4 gives a command to the excitation voltage circuit 3-1, and switches the value of the excitation voltage Vex from 8.5V to 14V. The switching of the excitation voltage Vex from 8.5V to 14V is realized by connecting the excitation circuit 3 and the flow rate measurement output circuit 6 in parallel between the signal lines L1 and L2 by switching the switches SW1 to SW3. It is possible.
[0031]
That is, before the measured value exceeds 20%, the excitation circuit 3 and the flow rate measurement output circuit 6 are connected in series between the signal lines L1 and L2. In this case, the external voltage Vs of the two-wire electromagnetic flow meter 100A is divided into the excitation circuit 3 and the flow rate measurement output circuit 6, and the supply voltage to the excitation circuit 3 is small. For this reason, the value of the excitation voltage Vex can only be about 8.5V. On the other hand, when the measured value exceeds 20%, the excitation circuit 3 and the flow rate measurement output circuit 6 are connected in parallel between the signal lines L1 and L2. In this case, the external voltage Vs of the two-wire electromagnetic flow meter 100A is applied to each of the excitation circuit 3 and the flow rate measurement output circuit 6, and the supply voltage to the excitation circuit 3 increases. As a result, the excitation voltage Vex generated by the excitation voltage circuit 3-1 can be increased to about 14V.
[0032]
When the excitation voltage Vex generated by the excitation voltage circuit 3-1 is 14V, the rise time of the excitation current Iex is earlier than the excitation voltage Vex = 8.5V so far, and the steady region immediately before the polarity is switched become longer. There is no significant difference when the value of the excitation current Iex is about 3.5 mA, but the difference increases as the value of the excitation current Iex increases as the flow rate signal (input voltage) increases. When the current Iex is 16.5 mA that allows the maximum flow, as shown in FIG. 5, it is possible to secure about 6 ms as the steady region ta. That is, in the past, the limit value of the excitation current Iex could only be about 12 mA, but the limit value of the excitation current Iex is maximized by switching from the series excitation circuit method to the parallel excitation circuit method with the measured value of 20% as a boundary. The current can be increased up to 16.5 mA.
[0033]
When the measured value exceeds 40%, the CPU 6-4 switches the value of the excitation current Iex to 6.9 mA. That is, the output current Iout corresponding to the measured value 40% is 10.4 mA, the excitation circuit 3 requires Ia = 0.5 mA, and the flow rate measurement output circuit 6 requires 3 mA. If the current Iccs to the transistor Q1 side is set to 0 mA, 6.9 mA can be passed as the exciting current Iex.
[0034]
The CPU 6-4 sets the value of the excitation current Iex to 6.9 mA until the measured value exceeds 60%. The surplus current during this period flows as current Iccs to the transistor Q1 side. When the measured value exceeds 60%, the value of the excitation current Iex is switched to 10.1 mA. That is, the output current Iout corresponding to the measured value of 60% is 13.6 mA, the excitation circuit 3 requires Ia = 0.5 mA, and the flow rate measurement output circuit 6 requires 3 mA. If the current Iccs to the transistor Q1 side is set to 0 mA, 10.1 mA can be passed as the exciting current Iex.
[0035]
Similarly, the CPU 6-4 sets the value of the excitation current Iex to 13.3 mA when the measured value exceeds 80%, and sets the value of the excitation current Iex to 16.5 mA when the measured value reaches 100%. Even if the excitation current Iex exceeds 12 mA, the excitation voltage Vex is increased to 14 V as a parallel excitation circuit system in this embodiment, so that the rising time of the excitation current Iex is fast and the steady region immediately before the polarity is switched Since it becomes longer, it becomes possible to sample the signal electromotive force e obtained by the stable excitation current Iex and obtain an accurate measurement value.
[0036]
In the embodiment described above, when the measured value exceeds 20%, the series excitation circuit method is switched to the parallel excitation circuit method. However, 20% is not necessarily switched from the series excitation circuit method to the parallel excitation circuit method. It does not have to be a point. For example, the measured value 50% may be used as a switching point from the serial excitation circuit method to the parallel excitation circuit method. The relationship between the measured value and the excitation current Iex shown in FIG. 2 is merely an example, and various modifications such as changing the switching point of the excitation current Iex and changing the value of the excitation current Iex are possible. It is.
[0037]
【The invention's effect】
As is apparent from the above description, according to the present invention, when the measurement value in the flow measurement output circuit exceeds a predetermined value, the excitation circuit and the flow measurement output circuit connected in series between the pair of signal lines. And switching means to switch the excitation voltage to the excitation coil in the excitation circuit within the range of the external voltage to an excitation voltage higher than the excitation voltage so far, so that the measured value is a predetermined value what external voltage has been pressed excitation circuit and flow measurement output circuit and a secondary minutes to over is made to that measurement value is applied to each of the the excitation circuit and the flow measurement output circuit exceeds a predetermined value, also excitation voltage generated in the excitation circuit is increased within the range of the external voltage than the excitation voltage so far, and faster rise time of the exciting current, the constant region immediately before the polarity switches Comb, so that the limit value of the exciting current to be set in multiple stages may be further increased in accordance with the measured value.
[Brief description of the drawings]
FIG. 1 is a diagram showing an outline of an embodiment of a two-wire electromagnetic flow meter according to the present invention.
FIG. 2 is a diagram showing a relationship between a measured value and an indicated value of an exciting current Iex in the two-wire electromagnetic flow meter.
FIG. 3 is a diagram showing a simplified circuit connection (series excitation circuit system) before the measured value in this two-wire electromagnetic flow meter exceeds 20%.
FIG. 4 is a diagram showing a simplified circuit connection (parallel excitation circuit method) when a measured value in this two-wire electromagnetic flow meter exceeds 20%.
FIG. 5 is a diagram illustrating a rising waveform when the value of the excitation current Iex is 16.5 mA when switching to the parallel excitation circuit method;
FIG. 6 is a diagram showing an outline of a conventional two-wire electromagnetic flow meter.
FIG. 7 is a diagram showing a relationship between a measured value and an indicated value of an excitation current Iex in a conventional two-wire electromagnetic flow meter.
FIG. 8 is a diagram illustrating a rising waveform when the value of the excitation current Iex is switched to Iex = 3.5 mA, 6.7 mA, 9.9 mA, and 12 mA.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Measuring tube, 2 ... Excitation coil, 3 ... Excitation circuit, 3-1 ... Excitation voltage circuit, 3-2 ... Current adjustment circuit, 4a, 4b ... Electrode, 5 ... Ground electrode, 6 ... Flow measurement output circuit, 6 DESCRIPTION OF SYMBOLS -1 ... Signal electromotive force detection circuit, 6-2 ... Sample hold circuit, 6-3 ... A / D conversion circuit, 6-4 ... CPU, 6-5 ... D / A conversion circuit, 6-6 ... Current adjustment circuit , CP1 ... comparator, Q1 ... transistor, R1 ... resistor, 6-7 ... constant voltage circuit, SW1-SW3 ... switch, LA, LB, LC ... connection line, L1, L2 ... signal line, RL ... external load, 100A ... 2-wire electromagnetic flow meter, 200 ... DC power supply.

Claims (1)

An excitation coil arranged with the direction of generation of the magnetic field perpendicular to the flow direction of the fluid flowing in the measurement tube;
An excitation circuit for generating an excitation voltage to the excitation coil and supplying an excitation current whose polarity changes periodically;
Based on the detected signal electromotive force, a signal electromotive force obtained between the electrodes arranged in the measurement tube is detected orthogonal to the flow direction of the fluid flowing in the measurement tube and the direction of the magnetic field generated by the excitation coil. The measured value is obtained, and the output current flowing through the pair of signal lines for supplying the external voltage is adjusted in a predetermined current range according to the obtained measured value, while the excitation circuit from the excitation circuit is adjusted according to the obtained measured value. A flow measurement output circuit that switches the value of the excitation current to the excitation coil in multiple stages,
In the two-wire electromagnetic flow meter in which the excitation circuit and the flow measurement output circuit are connected in series between the pair of signal lines,
When the measurement value in the flow measurement output circuit exceeds a predetermined value, the connection between the excitation circuit and the flow measurement output circuit connected in series between the pair of signal lines is switched in parallel, and A two-wire electromagnetic flow meter comprising switching means for switching an excitation voltage to the excitation coil in the excitation circuit to an excitation voltage higher than the excitation voltage within the range of the external voltage .

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