WO2020141426A1

WO2020141426A1 - Electromagnetic flow meter with self-correction capability, increasing the measurement accuracy, reduced startup and magnetic field protection

Info

Publication number: WO2020141426A1
Application number: PCT/IB2019/061419
Authority: WO
Inventors: Amirhossein GHADAMOSSOLTANI
Original assignee: Ghadamossoltani Amirhossein
Priority date: 2018-12-30
Filing date: 2019-12-28
Publication date: 2020-07-09

Abstract

A method for self-correction of a fluid flow measured by an electromagnetic flow meter is utilized by applying an excitation signal to a coil, where the coil is configured to apply a magnetic field to a fluid that passes through a tube, generating a first signal by a processor, where a first signal frequency is different from an excitation signal frequency, applying the first signal to a first electrode of the electromagnetic flow meter in the intervals between two consecutive excitation signals, detecting a second signal by a second electrode of the electromagnetic flow meter, determining a flow rate factor based on comparing the second signal with a data stored in a storage unit, and updating the fluid flow according to the flow rate factor.

Description

Electromagnetic flow meter with self-correction capability, Increasing the Measurement Accuracy, Reduced Startup and Magnetic Field Protection

This application claims priority from grant IR Patent Application, Application Number 139750140003008498, filed on December 30, 2018, entitled “Improved Electromagnetic Flowmeter with Self-correction Capability Against the Fluid Hardness Parameter, Increasing the Measurement Accuracy, Reduced Startup and Magnetic Field Protection ”, which is incorporated by reference herein in its entirety.

This application is related to an electromechanical apparatus, particularly related to the electromagnetic flow meter, and more particularly related to a self-correction method for the electromagnetic flow meter.

Electromagnetic flow meters operate based on Faraday’s law of induction. If an excitation signal is connected to a coil, a magnetic field is created around the coil. If a conductive liquid, such as water, passes through this magnetic field, the magnetic field is disturbed, and a weak signal proportional to the speed of the liquid is produced. This voltage is detected by two electrodes. Finally, the velocity of the liquid is determined according to the Faraday’s law. Since the tube diameter is known, the flow rate is determined based on the velocity of the liquid.

A major drawback of these types of flow meters is the extreme dependence of their performance accuracy on the fluid characteristics such as hardness and density and the requirement for their local calibration. Another problem with all flow meters is the minimum detectable flow rate below which flow rates cannot be measured. This problem is due to the weak voltage created by the passing current and the level of thermal noise and other types of noise in the system.

Hence, there is a need for developing a new method and related apparatus to detect the low flow rate of fluids. In addition, developing a method to determine the flow rate of fluids with different hardness would be of great benefit.

Summary of Application

This summary is intended to provide an overview of the subject matter of this patent, and is not intended to identify essential elements or key elements of the subject matter, nor is it intended to be used to determine the scope of the claimed implementations. The proper scope of this patent may be ascertained from the claims set forth below in view of the detailed description below and the drawings.

In one general aspect, the present disclosure is directed to an exemplary operation method of an electromagnetic flow meter. The method may include applying an excitation signal to a coil, wherein the coil connected to a current source is configured to apply a magnetic field to a fluid that passes through a tube, detecting a first signal by a first electrode of the electromagnetic flow meter and a second signal by a second electrode of the electromagnetic flow meter, generating a first digital signal by entering the first signal and the second signal to a first path. The first path includes a first amplifier, a first analog to digital convertor, and at least one filter, indicating a fluid flow as a function of the first digital signal by a processor, generating a third signal by the processor, wherein a third signal frequency may be different from an excitation signal frequency, applying the third signal to the first electrode in the intervals between two consecutive excitation signals, detecting a fourth signal by the second electrode, generating a second digital signal by entering the fourth signal to a second path, wherein the second path including a second amplifier, a second analog to digital convertor, and at least one filter, determining a fluid characteristic based on the second digital signal, determining a flow rate factor based on comparing the fluid characteristic with a data stored in a storage unit, and correcting the fluid flow according to the flow rate factor.

The above general aspect may have one or more of the following features. In an exemplary implementation, the method may further include separating the first path from the second path by a divider and two respective filters with different frequency. In an exemplary implementation, the method may further include separating the first path from the second path by a switch. In an exemplary implementation, the method may further include comparing the second digital signal with a threshold, wherein the tube is empty and the second digital signal is larger than the threshold. In an exemplary implementation, the first amplifier may include a differential amplifier. In an exemplary implementation, the first amplifier may include a low noise differential amplifier. In an exemplary implementation, the first path may also include a third amplifier. In an exemplary implementation, the second path may further include a fourth amplifier. In addition, in an exemplary implementation, the fluid characteristic may be indicated according to a difference of the third signal and the second digital signal. In an exemplary implementation, the fluid characteristic may be indicated according to a phase shift between the third signal and the second digital signal. In an exemplary implementation, the method may further include amplifying the third signal by a variable-gain amplifier to generate an amplified signal and applying the amplified signal to the first electrode. In an exemplary implementation, the method may further include amplifying the third signal by a variable-gain amplifier to generate an amplified signal and applying more than one amplified signal to the first electrode in the intervals between two consecutive excitation signals. In an exemplary implementation, the excitation signal frequency may be between 0.5 to 40 Hz. In an exemplary implementation, the third signal frequency may be between 100 to 5000 proportions larger than the excitation signal frequency. In an exemplary implementation, and the filter may include a finite impulse response filter. In an exemplary implementation, the current source may include an AC current source.

In another general aspect, the present disclosure is directed to a method for self-correction of a fluid flow measured by an electromagnetic flow meter. In an exemplary implementation, the self-correction method may include applying an excitation signal to a coil, wherein the coil is configured to apply a magnetic field to a fluid that passes through a tube, generating a first signal by a processor, wherein a first signal frequency is different from an excitation signal frequency, applying the first signal to a first electrode of the electromagnetic flow meter in the intervals between two consecutive excitation signals, detecting a second signal by a second electrode of the electromagnetic flow meter, determining a flow rate factor based on comparing the second signal with a data stored in a storage unit, and, finally, correcting the fluid flow according to the flow rate factor.

The above general aspect may have one or more of the following features. In an exemplary implementation, the method may further include comparing the second signal with a threshold, wherein the tube is empty and the second digital signal is larger than the threshold. In an exemplary implementation, the excitation signal frequency may be between 0.5 to 40 Hz. In an exemplary implementation, the first signal frequency may be between 100 to 5000 proportions larger than the excitation signal frequency. In an exemplary implementation, the flow rate factor may be indicated according to a difference of the first signal and the second signal. In an exemplary implementation, the flow rate factor may indicate according to a phase shift between the first signal and the second signal. In an exemplary implementation, the first signal may include more than one signal. In an exemplary implementation, the first signal may include more than one signal with different amplitude.

In another general aspect, the present disclosure is directed to an electromagnetic flow meter. In an exemplary implementation, the electromagnetic flow meter may include a coil, wherein the coil may be placed around a tube, a processor, and the processor may be configured to apply an excitation signal to the coil and a first signal to a first electrode of the electromagnetic flow meter in the intervals between the two consecutive excitation signals. A first signal frequency may be different from an excitation signal frequency, a first path, wherein the first path may include a first amplifier, at least one filter, and a first analog to digital convertor, wherein a second signal induced by the excitation signal in the first electrode and a second electrode of the electromagnetic flow meter may be configured to pass through the first path, and a second path, wherein the second path may include a second amplifier, at least one filter, and a second analog to digital convertor, wherein a third signal induced by the first signal in the second electrode may be configured to pass through the second path, an storage unit, wherein a data stored in the storage unit may compare with a second path signal to indicate a flow rate factor, the processor may indicate a correct fluid flow according to the flow rate factor.

The above general aspect may have one or more of the following features. In an exemplary implementation, may further include a low noise divider and two respective filters with different frequency, wherein the low noise divider and two respective filters with different frequency may configure to separate the first path from the second path. In an exemplary implementation, may further include a switch, wherein the switch may configure to separate the first path from the second path. In an exemplary implementation, the first amplifier may include a differential amplifier. In an exemplary implementation, the first signal frequency may be between 100 to 5000 proportions larger than the excitation signal frequency. In an exemplary implementation, the flow rate factor may indicate according to a difference of the first signal and the third signal. In an exemplary implementation, the flow rate factor may be indicated according to a phase shift between the first signal and the third signal. In an exemplary implementation, the first signal may include more than one signal. In an exemplary implementation, the first signal may include more than one signal with different amplitude.

The drawing figures depict one or more implementations in accordance with the present teaching, by way of example only, not by way of limitation. In the figures, like reference numerals refer to the same or similar elements.

Fig.1

illustrates a schematic of an electromagnetic flow meter, consistent with one or more exemplary embodiments of the present disclosure.

Fig.2

illustrates an isometric view of the electromagnetic flow meter assembly, consistent with one or more exemplary embodiments of the present disclosure.

Fig.3

illustrates an exploded view of the electromagnetic flow meter, consistent with one or more exemplary embodiments of the present disclosure.

Fig.4

illustrates a block circuit diagram of one exemplary embodiments of the present disclosure.

Fig.5

illustrates timing chart of a measurement mode and a fluid characteristic detection mode, consistent with one or more exemplary embodiments of the present disclosure.

Detail Description

In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent that the present teachings may be practiced without such details. In other instances, well-known methods, procedures, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present application is defined only by the appended claims.

Further, terms used herein are only for the purpose of describing particular embodiments and are not intended to limit to the present disclosure. The singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. It should be further understood that the terms “include,” “including,” “have,” and/or “having” specify the presence of stated features, integers, steps, operations, elements, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It should also be understood that, although the terms “first”, “second”, “third’, “fourth”, etc. may be used herein to describe various elements, the elements are not limited by the terms, and the terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element without departing from the scope of the present disclosure. The term “and/or” includes combinations of one or all of a plurality of associated listed items.

Electromagnetic flow meters operate based on Faraday’s law of induction.

Equation1

Equation2

Equation1 displays the relationship between the inductive voltage received from the electrodes and the velocity of the passing fluid according to Faraday’s law which states that the inductive voltage is the product of magnetic field intensity, the distance between the two electrodes, and the fluid velocity. The flow rate is the product of cross-sectional area and the fluid flow rate. Here, Ue is the inductive voltage, B is the magnetic field intensity, L is the distance between the electrodes, V is the flow velocity, Q is the flow rate, and A is the cross-sectional area of the spool.

If an excitation signal is applied to a coil, a magnetic field is created around the coil. If a conductive liquid passes through this magnetic field, the magnetic field is disturbed, and a weak signal proportional to the speed of the liquid is produced. This voltage is detected by two electrodes. Finally, the velocity of the liquid is determined according to Faraday’s law. Since the tube diameter is known, the flow rate is determined based on the velocity of the liquid.

The objective of exemplary embodiments of the present discloser is to provide a self-correction method of a fluid flow measured by an electromagnetic flow meter against changes in the characteristics of the fluid such as hardness and density. Also, it’s an apparatus to determine the low flow rate of fluids.

In an exemplary embodiment, in order to self-correct the fluid flow, a self-correction signal with a frequency much higher than the excitation signal frequency may be applied in the interval between the transmissions of two consecutive excitation signals in the measurement mode to the first electrode and received from the second electrode. In an exemplary embodiment, the self-correction signal frequency may be between 100 to 5000 proportions larger than the excitation signal frequency.

In an exemplary embodiment, the reason for the difference in the frequency of the excitation signal and self-correction signal is to enable a more effective filtration in order to separate the self-correction signal from the signal transmitted in the measurement mode.

In an exemplary embodiment, the fluid characteristics can be computed according to, the potential difference or phase shift between applied and received signals. Using a storage unit such as a look-up table and the fluid characteristics, a suitable factor is produced and substituted in the formula for flow measurement, and the flow measurement relationships are updated.

In an exemplary embodiment, when the impedance between the two electrodes in the self-correction mode is more than threshold this may indicate that the tube is empty.

In an exemplary embodiment, a low-pass filter may be used to reduce the bandwidth and noise and, as a result, increase the signal-to-noise ratio (SNR). This filter is an analog one and is located before the analog to digital (A/D) unit. Also, a digital FIR filter can limit the bandwidth to the operating bandwidth of the system due to its sharpness and reduce the noise level and increase the accuracy and reduce the start-up flow rate of the system.

In an exemplary embodiment, a real or an I/Q signal may be used, and one can eliminate the negative phase from the input signal via the quadrature algorithm. This helps increase the SNR and accuracy and decrease the start-up flow rate. Furthermore, since the flow variation is proportional to changes in the amplitude of the potential difference between the two electrodes, one can use an improved method of detecting and estimating the direct sequence spread spectrum signals combined with the first-order second-moment method, averaging the non-overlapping windows of the input signal, and finally estimating the noise variance in each window to make use of event-based threshold processing in order to separate signal from noise.

In an exemplary embodiment, signals as low as 6 dB under the noise level can be detected, resulting in a twofold increase in measurement accuracy, and for example, the minimum detected flow rate in a 3-inch flow meter is 10 times less than the conventional minimum detectable flow rate. This, in turn, considerably increases the performance of the system.

In an exemplary embodiment, after the placement of hardware inside the flow meter body in this application, a shell includes one or more layers of silica or related alloys, which may be inserted around the coils to immunize the system against adverse effects of being subjected to strong magnetic fields, preventing vandalism using strong magnets.

In an exemplary embodiment, this application method may utilize time division includes the time intervals between two excitation cycles that are used to measure the flow rate and the time for self-correction mode.

Hereinafter, embodiments according to the present disclosure will be described in detail with reference to the accompanying drawings. Fig. 1 and Fig. 2 are an isometric view of the electromagnetic flow meter according to one embodiment of the present disclosure.

Fig. 2 displays an isometric view of the electromagnetic flow meter assembly where a plate 201 is connecting the electronic boards including the transmitter, and a cylinder 202 is containing the cables connecting the electrodes to the electronic boards. A couple of

coils

203a and 203b placed in front of each other and the first electrode 204a are also shown in Fig.2.

Fig. 3 displays an exploded view of the electromagnetic flow meter, consistent with one or more exemplary embodiments of the present disclosure. In an exemplary embodiment, two

flanges

301a and 301b, which connect to the flow meter body,

support

302a and 302b. A liner 303 is placed between two

flanges

301a and 301b. Two

rings

304a and 304b hold the internal components of the flow meter and connect to a spool 305. Two

plates

306a and 306b hold the

coils

203a and 203b and two

electromagnetic cores

307a and 307b. The

electrodes

204a and 204b may be insulated by two

coatings

308a and 308b, and two

insulators

309a and 309b, are placed between the electrodes and the spool. Two

gaskets

310a and 310b hold the insulators. The shell 311 may insulate the magnetic wave and a cover 312 holds all the components.

Fig. 4 illustrates the block diagram of the electromagnetic flow meter body 401 and the circuit board 402, consistent with one or more exemplary embodiments of the present disclosure. The electromagnetic flow meter body may include a coil 403, wherein the coil is placed around a spool 404, and two

electrodes

204a and 204b. The circuit board may include a coil excitation unit 405, an amplifier 406, a low noise divider 407, two

DC blocker

408 and 409, a measurement path, a self-correction path, a processor 425, and a control unit 424. A low noise divider 410 and two low-

pass filters

411 and 413 before the measurement path and a band-pass filter 412 before the self-correction path separate the measurement path from the self-correction path. In an exemplary implementation, the low noise divider 410 may be replaced by a switch.

The measurement path may include a low noise differential amplifier 419, a sample and hold unit 420, an amplifier 421, a first analog to digital convertor 422, and a finite impulse response filter 423. A self-correction path may include a second amplifier 414, a sample and hold unit 415, an amplifier 416, a second analog to digital convertor 417, and a sharp finite impulse response filter 418. The processor 425 is based on the processing algorithms and especially processing an algorithm for detecting signals, even with negative SNR.

Furthermore, the control unit 424 may include a timing unit 424a and a control timing unit 424b, a timing unit 424a that measures the signals transmitted to and received from the electrodes for the self-correction mode and also a control timing unit 424b that controls the timing of the excitation signal production. Two low pass filters 411 and 413 are tasked with reducing the bandwidth and the noise and, therefore, increasing the SNR. It is worth noting that this signal is before the A/D converter, and a special analog filter has been used. The sharp FIR

digital filters

418 and 423 have been used so as to restrict the input bandwidth to the operational bandwidth of the system and to considerably reduce noise levels and, therefore, increase the accuracy of the system and decrease the start-up flow rate.

Fig. 5 illustrates timing chart of the measurement mode 502 and the self-correction mode 503. In the measurement mode, an excitation signal, a clock signal 501 with a cycle time T=1/f, wherein f is the measurement signal frequency (T=T_n+T_p),may be applied to the coil. In the time interval between two excitation cycles, the self-correction signal is transmitted to one electrode and received and read from the other.

Examples

This application can be practically implemented by adding the circuit to the electromagnetic flow meter body. The design of the circuit may be according to Fig. 4 so as to be able to transmit a signal via a processor 425 to a second electrode 204b in the self-correction mode and receive this signal using suitable filtering, as described in this disclosure.

In an exemplary embodiment, the self-correction mode is separated from the measurement mode via two separate paths. The desired parameters are measured by receiving via the second electrode the signal transmitted to the first electrode.

Example 1 is an operation method of the electromagnetic flow meter consistent with the teachings of the exemplary embodiments of the present disclosure. The coil excitation unit 405 applies an excitation signal to the coil 403, wherein the coil 403 is configured to apply a magnetic field to a fluid that passes through the tube. The control timing unit 424b controls the timing of the excitation signal production. First and

second electrodes

204a and 204b detect a first signal. The first signal enters the measurement path to generate the first digital signal. The processor 425 indicates a fluid flow as a function of the first digital signal and generates a third signal, wherein a third signal frequency is different from an excitation signal frequency. The timing unit 424a applies the third signal to the first electrode 204a in the intervals between two consecutive excitation signals. In an exemplary embodiment, a variable-gain amplifier generates multiple amplified signals and applies more than one amplified signals to the first electrode 204a with different amplitudes. The second electrode 204b detects a fourth signal. The fourth signal enters the self-correction path to generate a second digital signal. The processor 425 determines a fluid characteristic based on the second digital signal, determines a flow rate factor based on comparing the fluid characteristic with a data stored in a storage unit, and corrects the fluid flow according to the flow rate factor.

In an exemplary embodiment, the divider 410 and two

respective filters

412 and 413 with different frequency or a switch that is controlled by the processor may be used in the receiving path so as to separate a measurement mode path and a self-correction mode path and prevent their interference.

In an exemplary embodiment, after entering the self-correction mode path, the signal is amplified in several stages before the processor.

In an exemplary embodiment, the analog signal is converted to a digital by the analog to digital (A/D) unit and then sent to the processor in order to determine the characteristics of the fluid such as conductivity, density and hardness. According to the potential difference and phase shift between the third signal and the second digital signal, using a storage unit such as a look-up table, a suitable factor is produced and substituted in the formula for flow measurement, and the flow measurement relationships are updated.

In an exemplary embodiment, to detect empty tube, the impedance between the two electrodes in the self-correction mode compared with the threshold when the impedance is more than the threshold indicating that the tube is empty.

Claims

An operation method of an electromagnetic flow meter, comprising: applying an excitation signal to a coil, wherein the coil connected to a current source configured to apply a magnetic field to a fluid that passes through a tube; detecting a first signal by a first electrode of the electromagnetic flow meter and a second signal by a second electrode of the electromagnetic flow meter; generating a first digital signal by entering the first signal and the second signal to a first path, wherein the first path including a first amplifier, a first analog to digital convertor, and at least one filter; indicating a fluid flow as a function of the first digital signal by a processor; generating a third signal by the processor, wherein a third signal frequency is different from an excitation signal frequency; applying the third signal to the first electrode in the intervals between two consecutive excitation signals; detecting a fourth signal by the second electrode; generating a second digital signal by entering the fourth signal to a second path, wherein the second path including a second amplifier, a second analog to digital convertor, and at least one filter; determining a fluid characteristic based on the second digital signal; determining a flow rate factor based on comparing the fluid characteristic with a data stored in a storage unit; and correcting the fluid flow according to the flow rate factor.
The method of claim 1, further comprising separating the first path from the second path by a divider and two respective filters with different frequency.
The method of claim 1, further comprising separating the first path from the second path by a switch.
The method of claim 1, further comprising comparing the second digital signal with a threshold, wherein the tube is empty the second digital signal is larger than the threshold.
The method of claim 1, wherein the first amplifier includes a differential amplifier.
The method of claim 1, wherein the first amplifier includes a low noise differential amplifier.
The method of claim 1, wherein the first path further comprising a third amplifier.
The method of claim 1, wherein the second path further comprising a fourth amplifier.
The method of claim 1, wherein the fluid characteristic indicates according to a difference of the third signal and the second digital signal.
The method of claim 1, wherein the fluid characteristic indicates according to a phase shift between the third signal and the second digital signal.
The method of claim 1, further comprising amplifying the third signal by a variable-gain amplifier to generate an amplified signal and applying the amplified signal to the first electrode.
The method of claim 1, further comprising amplifying the third signal by a variable-gain amplifier to generate an amplified signal and applying more than one amplified signal to the first electrode in the intervals between two consecutive excitation signals.
The method of claim 1, wherein the excitation signal frequency is between 0.5 to 40 Hz.
The method of claim 1, wherein the third signal frequency is between 100 to 5000 proportions larger than the excitation signal frequency.
The method of claim 1, wherein the filter includes a finite impulse response filter.
The method of claim 1, wherein the current source is an AC current source.
A method for self-correction of a fluid flow measured by an electromagnetic flow meter, comprising: applying an excitation signal to a coil, wherein the coil configured to apply a magnetic field to a fluid that passes through a tube; generating a first signal by a processor, wherein a first signal frequency is different from an excitation signal frequency; applying the first signal to a first electrode of the electromagnetic flow meter in the intervals between two consecutive excitation signals; detecting a second signal by a second electrode of the electromagnetic flow meter; determining a flow rate factor based on comparing the second signal with a data stored in a storage unit; and correcting the fluid flow according to the flow rate factor.
The method of claim 17, further comprising comparing the second signal with a threshold, wherein the tube is empty the second digital signal is larger than the threshold.
The method of claim 17, wherein the excitation signal frequency is between 0.5 to 40 Hz.
The method of claim 17, wherein the first signal frequency is between 100 to 5000 proportions larger than the excitation signal frequency.
The method of claim 17, wherein the flow rate factor indicates according to a difference of the first signal and the second signal.
The method of claim 17, wherein the flow rate factor indicates according to a phase shift between the first signal and the second signal.
The method of claim 17, wherein the first signal includes more than one signal.
The method of claim 17, wherein the first signal includes more than one signal with different amplitude.
An electromagnetic flow meter, comprising: a coil, wherein the coil placed around a tube; a processor, wherein the processor configured to apply an excitation signal to the coil and a first signal to a first electrode of the electromagnetic flow meter in the intervals between the two consecutive excitation signals, wherein a first signal frequency is different from an excitation signal frequency; a first path, wherein the first path includes a first amplifier, at least one filter, and a first analog to digital convertor, wherein a second signal that induced by the excitation signal in the first electrode and a second electrode of the electromagnetic flow meter configured to pass through the first path; a second path, wherein the second path includes a second amplifier, at least one filter, and a second analog to digital convertor, wherein a third signal that induced by the first signal in the second electrode configured to pass through the second path; and an storage unit, wherein a data stored in the storage unit compared with a second path signal to indicate a flow rate factor, the processor indicates a correct fluid flow according to the flow rate factor.
The electromagnetic flow meter of claim 25, further comprising a low noise divider and two respective filters with different frequency, wherein the low noise divider and two respective filters with different frequency configured to separate the first path from the second path.
The electromagnetic flow meter of claim 25, further comprising a switch, wherein the switch configured to separate the first path from the second path.
The electromagnetic flow meter of claim 25, wherein the first amplifier includes a differential amplifier.
The electromagnetic flow meter of claim 25, wherein the first signal frequency is between 100 to 5000 proportions larger than the excitation signal frequency.
The electromagnetic flow meter of claim 25, wherein the flow rate factor indicates according to a difference of the first signal and the third signal.
The electromagnetic flow meter of claim 25, wherein the flow rate factor indicates according to a phase shift between the first signal and the third signal.
The electromagnetic flow meter of claim 25, wherein the first signal includes more than one signal.
The electromagnetic flow meter of claim 25, wherein the first signal includes more than one signal with different amplitude.