JP2002310750A - Electromagnetic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an electromagnetic flowmeter by which a flow rate value can be measured easily in a short time, by a method wherein the conductivity of a fluid is measured by using electrodes used to measure an electromotive force, and an error in the flow rate value is corrected on the basis of the conductivity of the fluid. SOLUTION: The electromagnetic flowmeter is constituted in such a way that a magnetic field is generated by an excitation coil installed at the outside of a measuring pipe, and that the flow rate value of the fluid flowing inside the measuring pipe is measured by detecting the electromotive force generated across the electrodes installed inside the measuring pipe. The electromagnetic flowmeter is provided with a means by which a constant current is supplied across the electrodes, by which the conductivity of the fluid is measured and by which the error due to the conductivity in the flow rate value is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electromagnetic flow meter,
In particular, the present invention relates to shortening of the distance between the surfaces of the conduit (shortening of the surfaces).

[0002]

2. Description of the Related Art The measurement principle of an electromagnetic flowmeter is based on JIS B7.
As described in 554 “Electromagnetic flowmeter”, a magnetic field is applied to the measurement tube at right angles, an electromotive force is measured between a pair of electrodes provided on the inner surface of the measurement tube, and the flow rate in the tube is obtained from the measured value. Things. The relationship between the flow rate and the measured value at this time is as follows.

Q = πDE / (4 kB) (1) where, Q: volume flow rate (m ³ / S) π: pi D: inner diameter of measurement tube (m) E: electromotive force (V) k: Constant B: magnetic flux density (T)

FIG. 3 shows an example of a conventional electromagnetic flow meter based on such a principle. The figure is a main part configuration diagram. An excitation coil 3 for generating a magnetic field is attached to an outer peripheral surface of a cylindrical measurement tube 1. This exciting coil 3 is covered by a housing 4. The measurement tube 1 is also provided with a pair of electrodes 2a and 2b for detecting the generated electromotive force.

The flow rate calculating circuit 5 calculates the flow rate based on the above equation (1) based on the electromotive force detected by the electrodes 2a and 2b.

[0006] The electromagnetic flow meter can measure the flow rate of the fluid flowing in the measuring tube 1 in this manner, but the measured value of the electromotive force is affected by the conductivity of the fluid. In order to make the device less susceptible to the influence of the electrical conductivity and maintain high accuracy, a certain inter-plane dimension L (also simply referred to as “plane-to-plane”) is required.

FIG. 4 shows the relationship between the conductivity and the span error of the flow rate value when the distance L between the surfaces of the measuring tube is changed. As is clear from the figure, the measured flow rate is the conductivity (μS / cm).
It can be seen that it is necessary to make the distance L between the surfaces longer than a certain value in order to be hardly affected by the deviation and to maintain high accuracy.

[0008]

As described above, the conventional electromagnetic flowmeter requires a certain distance between the surfaces, and has a problem that the distance between the surfaces cannot be easily shortened to a smaller dimension.

For example, if the conductivity of the fluid flowing through the measuring tube 1 changes within the range of 20 μS / cm to 2000 μS / cm, the distance L between the surfaces must be 1.5D to suppress the span error to within ± 1%. It was necessary and could not be shorter.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and to measure a fluid conductivity using an electrode for measuring an electromotive force, and to correct an error in a flow value from the fluid conductivity. It is another object of the present invention to provide an electromagnetic flowmeter capable of easily shortening the distance between the surfaces.

[0011]

In order to achieve the above object, according to the present invention, a magnetic field is generated by an exciting coil provided outside a measuring tube, and an electrode provided on the measuring tube is provided. In an electromagnetic flowmeter configured to measure a flow rate of a fluid flowing in a measurement tube by detecting an electromotive force generated between the electrodes, a constant current is supplied between the electrodes to measure a fluid conductivity, and the flow rate of the fluid is measured. It is characterized by comprising means for correcting an error due to conductivity.

In the present invention, the fluid conductivity is also measured using an electrode for measuring the electromotive force, and an error in the flow value is corrected from the measured fluid conductivity. Such correction of the flow value makes it possible to easily shorten the distance between the surfaces.

In this case, the fluid conductivity can be measured intermittently or periodically at the time of measuring the flow rate.

In addition, the fluid conductivity is as follows:
The liquid contact resistance between the electrodes is measured using two electrodes, and σ = 2 / (R _ab d) where σ: fluid conductivity R _ab : liquid contact resistance d: electrode diameter It can also be configured.

Alternatively, the fluid conductivity is determined by measuring each liquid contact resistance between the electrode and the ground electrode by using three electrodes, ie, two electrodes and a ground electrode, according to claim 4, and σ _ac = 1 / (R _ac × d) σ _bc = 1 / (R _bc × d) where σ _ac , σ _bc : fluid conductivity between each electrode and the ground electrode R _ac , R _bc : liquid contact resistance between each electrode and the ground electrode d: electrode diameter It can be configured to be obtained based on the following relational expression. In this case, the conductivity can be obtained more accurately even if the distribution of the conductivity differs in the measurement tube, and the span error can be corrected more accurately.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a main part configuration diagram showing one embodiment of an electromagnetic flow meter according to the present invention. In FIG. 1, the same parts as those in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted.

In the figure, _Rab is the liquid contact resistance between the electrodes 2a and 2b. Reference numeral 10 denotes a constant current source. S1 and S
2 is an interlocking ON / OFF switch. Reference numeral 20 denotes a switch drive circuit that generates a signal (conductivity measurement control signal) for driving the switches S1 and S2. Reference numeral 5a denotes a flow rate calculation circuit having a flow rate calculation function similar to the conventional one and a span error correction calculation function unique to the present invention. The illustration of the excitation coil and its drive circuit is omitted.

The switch S1 is connected between the constant current source 10 and the electrode 2a, and the switch S2 is connected between the electrode 2b and the common line. The constant current source 10 has a negative side connected to the common line and a positive side connected to the switch S1.

In such a configuration, both switches S1 and S2 are turned off when measuring the flow rate of the fluid, and both switches S1 and S2 are turned on when measuring the fluid conductivity. At the time of measuring the fluid conductivity, the current from the constant current source 10 is switched by the switch S1, the electrode 2a, and the liquid contact resistance R as indicated by an arrow in the figure.
_The current flows to the common line via _ab , the electrode 2b, and the switch S2. The flow rate calculation circuit 5 determines the electrodes 2a, 2
The voltage generated between b is measured.

The liquid contact resistance _Rab is determined from the voltage generated between the electrodes 2a and 2b and the current (known) of the constant current source 10,
The electrical conductivity has the following relationship. σ = 2 / (R _ab d) (2) where σ: fluid conductivity d: electrode diameter (both electrodes 2a and 2b have the same diameter)

The flow rate calculation circuit 5 calculates the fluid conductivity by the calculation of the equation (2), and corrects the span error with respect to the flow rate value measured separately from the relationship shown in FIG. Thereby, the span error of the flow rate value can be corrected according to the distance L between the surfaces. The measurement of the fluid conductivity is interrupted at the time of measuring the flow rate, and the measuring operation is performed at regular intervals.

As described above, even if the distance L between the surfaces is reduced to 1.5 D or less, the flow rate value can be easily obtained with high accuracy.

FIG. 2 shows another embodiment of the present invention. FIG.
The difference from the first embodiment is that the conductivity is measured at two points of the measuring tube 1. The switch S21 is a three-contact switch. The constant current source 10 is connected to the electrode 2a or the electrode 2b when measuring the conductivity, and is not connected to any electrode when measuring the flow rate. Also, switch S
22 is an ON connected between the ground electrode 2c and the ground.
This is an / OFF switch, which is ON when measuring the conductivity and OFF when measuring the flow rate. These switches S21, S22
Are driven by the switch drive circuit 20.

In such a configuration, when measuring the fluid conductivity, the switches S21 and S22 are driven to measure the conductivity at two points between the electrode and the ground. When measuring the conductivity σ _ac between the electrodes 2a and 2c, the switch S21 is connected to the a side, and when measuring the conductivity σ _bc between the electrodes 2b and 2c, the switch S21 is connected to the b side.

The following relationship exists between the conductivity σ _ac and the wetted resistance R _ac and between the conductivity σ _bc and the wetted resistance R _bc . σ _ac = 1 / (R _ac × d) σ _bc = 1 / (R _bc × d) where d: electrode diameter (electrodes 2a, 2b, 2c have the same diameter)

As a result, the electric conductivities σ _ac and σ _bc are measured at two points of the measuring tube, and the electric conductivity can be obtained more accurately even if the distribution of electric conductivity is different in the measuring tube. Correction is also performed more accurately. Thus, the flow rate value can be obtained with higher accuracy than in FIG.

The present invention is not limited to the above-described embodiment, but includes many more changes and modifications without departing from the essence thereof.

[0028]

As described above, according to the present invention, the fluid conductivity is measured by using the electrodes for measuring the flow rate, and the error of the flow rate value from the fluid conductivity can be corrected. In addition, there is an effect that it is possible to realize a reduction in the length between the surfaces.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of an electromagnetic flow meter according to the present invention.

FIG. 2 is another embodiment of the present invention.

FIG. 3 is a configuration diagram illustrating an example of a conventional electromagnetic flow meter.

FIG. 4 is a diagram showing a relationship between a flow rate value span error and conductivity.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Measurement tube 2a, 2b, 2c Electrode 5a Flow rate calculation circuit 10 Constant current source 20 Switch drive circuit S1, S2, S21 Switch

Claims (4)

[Claims] A magnetic field is generated by an excitation coil provided outside a measuring tube, and an electromotive force generated between electrodes provided in the measuring tube is detected to measure a flow rate of a fluid flowing in the measuring tube. The electromagnetic flowmeter according to any one of claims 1 to 3, further comprising means for supplying a constant current between the electrodes to measure a fluid conductivity, and correcting an error caused by the conductivity of the flow rate. 2. The electromagnetic flowmeter according to claim 1, wherein the measurement of the fluid conductivity is performed intermittently or at a constant cycle when measuring the flow rate. 3. The fluid conductivity is obtained by measuring a liquid contact resistance between two electrodes by using two electrodes. Σ = 2 / (R _ab d) where σ: fluid conductivity R _ab : liquid contact resistance d: The electromagnetic flowmeter according to claim 1, wherein the electromagnetic flowmeter is configured to be obtained based on a relational expression of an electrode diameter. 4. The fluid conductivity is determined by measuring each liquid contact resistance between an electrode and an earth electrode by using three electrodes of two electrodes and an earth electrode, and σ _ac = 1 / (R _ac × d) σ _bc = 1 / ( _Rbc × d) where σ _ac , σ _bc : fluid conductivity between each electrode and the ground electrode R _ac , R _bc : liquid contact resistance between each electrode and the ground electrode d: electrode diameter The electromagnetic flowmeter according to claim 1, wherein the electromagnetic flowmeter is configured to be determined based on an equation.