JPS6079225A - Electromagnetic flow meter - Google Patents

Abstract

PURPOSE:To enable measurement of super-low electric conductive fluid, for instance, 10<-7>OMEGA/cm and less, by compensating electrodes arranged extendedly along the outer periphery in the direction of pipe axis and providing these compensating electrodes with the specified electrodes in the direction respectively in which they intersect with detecting electrodes through an angle in a section perpendicular to the pipe axis. CONSTITUTION:An angle of 90 deg. is divided into 4 sectors and in each sector is provided with the compensating electrodes arranged by eight pieces in top and bottom positions respectively. On the other hand, an operational amplifier 12 as a preamplifier converting a detected potential into a non-inverted output obtained in a detecting electrode 3, and a divider 13 dividing at inside and outside diameters a, a' and angle of compensating electrode theta, an output voltage of an amplifier 12 into a products of (a/a') cos theta are installed. The voltage obtained in each output point of the divider 13 is applied to a compensating electrode 11 corresponding to each angle. Outside part of the compensating electrode 11 is covered by conventional insulating material and its outside part is covered by metallic shielding pipe and the assembly is grounded. A magnitude of signals similar with that which would have been obtained, by using a pipe of finite thickness with its outer periphery being grounded when an electroinductive pipe of infinite thickness were employed. At the same time, damping of a leader line shield of the detecting electrode by floating capacity can be compensated also.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an electromagnetic flow meter for electromagnetic flow, particularly for ultra-low conductivity variable fluids.

[Prior art] Conventionally, an alternating current magnetic field is generated perpendicular to the flow whose flow rate is to be measured, and the electromotive force generated in the fluid is absorbed by a pair of electrodes installed directly behind the fluid on the inner wall of a pipe through which the fluid flows. The lower limit of the measurable fluid conductivity of an ordinary electromagnetic flowmeter to be taken out is 2 to 5 x 10-'σ/cm. On the other hand, although it is generally called "oil", its horn conductivity is 10-"-i!1-"07cm, and not only this "oil" but also intermediate entities such as acetate/ammonia etc. , it is considered impossible to measure the flow rate using the method described above. however,
On the other hand, since electromagnetic transport is easier to maintain than mechanical methods, there is a strong desire to realize electromagnetic transport that can measure even the ultra-low conductivity variable fluids mentioned above3. [Summary of the invention] The present invention has been made in view of the above circumstances, and
Its purpose is conventionally considered to be T-ability, for example 10-'
An object of the present invention is to provide an electromagnetic flowmeter that enables measurement of ultra-low conductivity variable fluids of U 2 /an or less.

In order to achieve such an object, the present invention interposes a dielectric material between the fluid and the detection electrode, prevents the electrode surface from directly contacting the fluid, and eliminates the electromotive force that may be generated in the fluid by using a custom coupling. The so-called container to be taken out 1. Compensate for the disturbance and attenuation of the internal electric field distribution due to the use of a coupled type electromagnetic flowmeter, in which case the thickness of the dielectric tube is finite and the outer periphery must be shielded and grounded. In order to respectively,
As , , +, a potential that is −cosθ times the potential obtained at the detection electrode was applied. It is something.

In addition, with respect to the minute coupling capacitance between the detection electrode and the fluid, the input capacitance of the operational amplifier whose non-inverting input is the detection potential obtained from each detection electrode provided in the front stage of the differential amplifier is approximately the same or In order to compensate for the drop in output voltage caused by exceeding this level, a circuit is provided that amplifies the output voltage with a constant gain and provides positive feedback via a capacitor.
It is assumed that attenuation due to stray capacitance between the inverting and non-inverting input terminals of the operational amplifier is compensated for by a follower connection.

The present invention will be explained in detail below using examples. First, we will explain the validity of the combined bond and analyze the above-mentioned problems that must be solved in that case. Now, we will move on to an explanation of an embodiment as a means for solving the problem.

〔Example〕

There is an analysis by V. Cushing ("Induction Flowmeter", R.
ev, Sci, Instrum.

29, pp692-697.1958). Accordingly, as shown in Figure 1, if the tube 1 made of dielectric material has a finite size and its outer periphery is shielded and grounded compared to the metal shield tube 2, it is possible to place the tube between the fluid. The potential system Δ■ obtained between a pair of detection electrodes 30 is as follows 11), t
2) It is shown by the formula. Note that 4 indicates one piece.

1゜ Δv=ni-n-p-u l　---(1)□ 1...1(2) □ −゛゛′″″″″″.

D = Diameter (inner diameter) of the tube (-21L) U: Average flow velocity (σ,: Electrical conductivity of the fluid σ2° Electrical conductivity of the tube ε.: Permittivity of vacuum ε7 □: Relative permittivity of the fluid εt-2' Tube Relative permittivity ω: Angular frequency of the magnetic field a: Inner radius of the tube b: Outer radius of the tube This is generally the basic equation that gives the output signal of an electromagnetic flowmeter, and is used for ordinary, that is, ultra-low conductivity fluids. In an electromagnetic flowmeter that does not have σ1, σ1 is sufficiently large, so it can be safely assumed that σ1 is equal to 1.

Now, since σ2 can be made sufficiently small in practice (for example, 1t for Teflon) -18U 7cm), equation (2) can be expressed as the following equation (3).

Examining the practical limit from Equation (3), first of all, the actual thickness of the upper layer cannot be made zero or infinite, and the range of a/b ij Q < a/ b <
1, but n/bm4n=0.5 a/b=0.8 is considered to be the limit. Here, in order to summarize the numbers, aA = 0.65.

Next, εr2 is determined by the lining material, that is, the inner wall of the tube, the dielectric between the detection electrode and the fluid, which requires particularly high insulation, so for example (I'J' Teflon etc. are used). , the equation εr2-2 Therefore, equation (3) becomes the following equation (4).

Regarding ε7, two types of fluids will be considered. That is, one of them is petroleum-based, ε7, -2, and the other is pure water, εr1=80. Alcohols are located in between these.

Next, the frequency characteristics of the absolute value of K calculated from equation (4) using these values are shown in FIGS. 2 and 3.

Figure 2 shows petroleum systems, and σl in (a), (b), (c), and ni) are 10-15110-11" and 10-", respectively.
, 10-' U/C1n, and FIG. 3 shows the pure water system, and (A) and ←) in the figure show the cases where σl is 10-7 and IQ-JJ/c1n, respectively. Further, (c) in FIG. 3 shows theoretical pure water having the minimum σ1 of 5.55×10 −8 . In each curve, the flat part in the low range of ω corresponds to the case where the detection electrode is brought into direct contact with the fluid. Since responsiveness of about 2 seconds is required, the lower limit of ω is 2
0 (3 Hz' in frequency) must be considered as i degrees.

Then, as is clear from the figure, in pure water systems it is possible to measure everything up to theoretically pure water, but in petroleum systems it is possible to measure up to 1O
-10U 71m is the lower limit of conductivity that can be measured. Moreover, since this is a theoretical limit of a completely ideal measurement circuit, it is actually impossible to measure oil using the method described above, and capacitive coupling must be used.

Therefore, in equation (3), if σ□su → 0 as shown in equation (5) below, 1゜εrl-' corresponds to capacitive coupling.Here, if a/b = 1, then 11mK=Ohi. As shown in FIG. 1, if the thickness of the tube 1 made of a dielectric material whose outer periphery is shielded and grounded is reduced, the output signal obtained becomes smaller.

Therefore, it is necessary to use a tube as thick as possible, but there are restrictions due to design and cost considerations (for example, Teflon is expensive). Therefore, in the unexploded OJJ, a/b・0, that is, infinite thickness, was achieved isometrically using the following method.

Now, as b → ω, a point p (0 ) is the potential at V(θ), and the potential at point q( of radius a' opposite to point p is v'(). Since there is no electric field in the axial direction of the tube, v'(θ) --!17v()Conversely, if you put one electrode at point q and give it the potential of v'(, b-ω even if there is nothing outside the radius a') Since a and a′, that is, the outer radius of the actual pipe, are known, V
If you know (, you can determine V'(.V() at θ-〇
θ) is the potential V′″C of the detection electrode, and ■(-v co
Since it can be set as sθ, v'(θ)-'=V
cos θ.

That is, if a compensating electrode extending in the axial direction of the tube is provided on the outer periphery of a tube with an inner radius a1 and an outer radius a' that is applied from a dielectric material, and a potential distribution given by the above equation is applied to the tube, the thickness of the tube is finite. The attenuation of the signal due to this can be compensated for. Approximately, this can be realized, for example, by providing a band-shaped compensation electrode 11 divided into a plurality of parts in the axial direction, as shown in FIG. 5, and applying a potential of -v cos θ to each of them corresponding to θ.

FIG. 5 is a block diagram showing an embodiment (part) of the present invention, in which 90mm is divided into four parts, and eight compensation electrodes are provided in each of the upper and lower parts. On the other hand, in this embodiment, an operational amplifier 12 as a preamplifier which uses the detection potential obtained at the detection electrode 3 as a non-inverting input is provided before the differential amplifier (not shown), and the output voltage of this operational amplifier 12 is One! 7C
A voltage divider 13 that divides the voltage by O3θ (θ is the angle at which each of the compensation electrodes 11 is arranged) is provided, and the voltage obtained at each output point of the voltage divider 13 is applied to the compensation electrode 1 at the corresponding angle.
1 is applied. Although omitted in the figure, the lower detection electrode also has a circuit with a similar configuration. In the illustrated example, the compensation electrode 11 is shifted from the detection electrode 3 (θ\・
0), but due to the drawer, θ-0
Needless to say, they may be arranged as appropriate.

The outside of the compensation electrode 11 is covered with a normal insulating material, and the outside is covered with a metal tube for shielding and grounded.

- For example, if a flexible printed circuit board or the like is used as the compensation electrode 11, and this is attached to the outside of the tube 1 made of Teflon, for example, and the outside is grounded as a shield, this can be easily realized. Regarding 1, for example, Teflon is used, and a structure in which the detection electrode is embedded near the inner wall is used, but ceramics with higher abrasion resistance and strength are used mainly for the parts that are in direct contact with the fluid, mainly from the viewpoint of wear resistance. Good too. In that case, for example, a detection electrode may be pasted on the outer periphery of the ceramic tube, and the outside may be molded with Teflon.

With the above configuration, while using a tube with a finite thickness whose outer periphery is shielded and grounded, it is possible to obtain the same signal amount as using a dielectric tube with an infinite thickness. It is also possible to compensate for the attenuation due to

Here, the input capacitance of the operational amplifier 12, which is the first-stage amplifier that receives the signal from the detection electrode, becomes a problem. That is, although the signal taken out by the detection electrode at each end coupling is originally very small, the input capacitance becomes equal to or greater than the coupling capacitance, and the signal is significantly attenuated.

That is, in FIG. 6, 21 is a coupling capacitor, 22 is a bias resistor, and the capacitance C of the coupling capacitance 21 is determined by the setting N1 of the capacitor body. That is, if the area of the detection electrode 30 is 81, and the gap between the sensing electrode and the fluid is tl, and the relative dielectric constant of the dielectric between them is εr2, then C is expressed as c=, ε0εr2, but in actual design, for example, t = 0.1
5 <cn+) S = 2.5 x 1.5 (Cm2)ε, 2=2
(In the case of Teflon) c = 4.43X10-'' (F) is considered to be the structural limit.

In contrast, the input capacitance 23 (CIN) is 3 to 49F.
Since the coupling capacitance is approximately the same as the signal source series capacitance, an operational amplifier with a FET input with a small bias current is used to reduce the differential mode floating between the non-inverting and inverting input terminals with a high voltage of 3000 MΩ. Capacity 24
(CD) can be easily compensated by connecting the operational amplifier 12 as a follower, but the input capacitance (CIN
) is not compensated for by this, and since it exists inside the operational amplifier unit, it is not possible to take a compensation signal as in the case of the stray capacitance of the shield of the detection electrode lead line.

Therefore, in the present invention, a compensation circuit consisting of a capacitor 31, resistors 32 and 33, and an operational amplifier 34 is provided, as shown in FIG. 6, for example.

As is clear from the figure, the compensation circuit for and is the operational amplifier 12
This is a stable condition because a positive feedback loop is formed for .

However, C' is the capacity W of the capacitor 31, and the value R1 is the resistance 32
The resistance value R2 is the resistance value of the resistor 33, and the signal source capacitance value C in this case is the series capacitance value of the internal fluid capacitance and the electrode capacitance.

Therefore, it is necessary to set the setting so that oscillation does not occur in an empty state (no fluid at all), but the capacitance value for the free space on one side of a flat plate electrode of area S is approximately times ε.

Using the numerical example S −3,75L:m described above, it becomes 0.43 pF. As a result, becomes the stability condition.

The operation of this compensation circuit can be analyzed as follows.

That is, as shown in FIG. 7, the circuit of FIG. 6 is a circuit in which the input of the operational amplifier 12 is positively fed back via the amplifier 35 by a factor of A.
="jt", then ω = 314 C = 4.4 pF 'R = 30 (H) MΩ C, N = 3.5 pF and the relationship between v7E and C' is shown in Fig. 8. It becomes as shown in .

In Fig. 8, one point is C''-CIhh, that is, the state where only the input capacitance is compensated, and one point is the bias resistance R
C' at this time is approximately 3.63 pF, with the effect due to the current flowing through the capacitor being compensated for. The stability of water retained in C' is about to, 6%, assuming that t/true deformation is allowed at 105%1, and can be achieved without any problem by using, for example, an air trimmer capacitor.

For the setting of C', if CIN is known, one point adjustment is completed by setting C' to 01N.
Actual measurement of OS gate capacity is considered dangerous.

The input resistance (CtN) includes the stray capacitance of the bias resistor, the stray capacitance of the Teflon bottle in the insulated wiring, etc.

3. From this point of view, paste metal foil on the inner surface of the tube of the transmitter body in line with the position of the detection electrode, apply a known voltage E, and It is thought that the best method is to measure C' and adjust C' so that V===E.

Note that in order to compensate for the input capacitance using such a compensation circuit, it is a prerequisite that the input capacitance value CTN is stable, but sufficient stability can be obtained with a MOS FET input.

It is desirable that such a first-stage amplifier unit be housed in a container that is electromagnetically shielded and filled with dry gas (for example, NZ). The adjustment screws of the capacitor 31 must be connected by an airtight seal shaft. The part connected to the detection electrode must be insulated with a Teflon bottle and provided with a leakage guard. Botting this part with resin must be avoided from the two viewpoints of increasing stray capacitance and generating stray leaks.

From the above explanation, the first stage amplifier unit is configured as shown in FIG. 9, for example. In the figure, 41 is an N-channel MO
The 8FET input operational amplifier 42 is a general-purpose operational amplifier similar to the operational amplifier 34, and both of these amplifiers correspond to the operational amplifier 12 described above.

Further, 43 is a resistor for leakage guarding the capacitor 31, which includes the operational amplifier 41, the resistor 43, and the capacitor 3.
Circuit elements other than 1 can be printed wiring. 44 is a lead wire shield for the detection electrode. The figure shows only one electrode, but the circuit configuration is similar for the other detection electrode, and the difference between the outputs OUT of both operational amplifiers 42 is amplified by a differential amplifier (not shown) to obtain the final detection output. is obtained. The whole thing is sealed in a N2 gas-filled shield case = -745. Note that the output of the operational amplifier 42 is fed back to the lead wire shield 44 of the detection electrode to compensate for attenuation due to stray capacitance between the lead wire (core wire) and the shield.

〔Effect of the invention〕

As explained above, according to the present invention, a compensating electrode is provided on the outer periphery of the dielectric tube and extends along the tube axis direction using a capacitive coupling method, so that the potential obtained at the detection electrode is By giving a potential distribution that is partially θ times as large as θ, it is possible to compensate for signal attenuation caused by the fact that the dielectric tube has a finite thickness and its outer periphery is grounded as a shield. The above configuration can be approximately realized by applying the output obtained by dividing the output of a follower-connected pre-operational amplifier with a non-inverting input using a voltage divider to the compensation electrode, and furthermore, the output of the above-mentioned operational amplifier is kept constant. By providing a circuit that amplifies with a gain of can be compensated. Thereby, the present invention is different from the conventional electromagnetic flow rate.

The 1°

[Brief explanation of the drawing]

Figure 1 is a diagram to explain the potential difference obtained on the electromagnetic flow rate side, Figure 2 is a diagram showing the ω dependence of the correction term in oil-based fluids as a function of the electrical conductivity of the fluid.
Fig. 3 is a similar diagram for a pure water system, Fig. 4 is a diagram explaining the principle of compensating for signal attenuation based on the finite thickness of the tube, and Fig. 5 is a diagram for an embodiment of the present invention. 6 is a diagram showing an example of the configuration of a compensation electrode, FIG. 6 is a diagram showing an example of the configuration of a circuit that compensates for signal attenuation based on the input capacitance of a pre-operational amplifier, FIG. FIG. 8 is a diagram for explaining the compensation effect, and FIG. 9 is a diagram showing the configuration of the preamplifier portion in one embodiment of the present invention. 1... Tube made of dielectric material, 2... Shield tube,
3...11 detection poles, 4...11 M bodies, 11...
... Compensation electrical connection, 12... Pre-operational amplifier, 13...
... Voltage divider, 21 ... Coupling capacitance, 23 ... Input capacitance, 31 ... Condenser, 34 ... Feedback operational amplifier, 41.42 ... Pre-operational amplifier 12 Amplifiers that make up the. ! 10 representatives of Yamani-no-Newel Co., Ltd.
Masaki Yamakawa (and 1 other person) Figure 5 1? Figure 6] Il. Figure 7 V Figure 8 C' (ρF)

Claims (1)

[Claims] (11) The pipes are arranged so as to sandwich the fluid flowing in the pipes, which are made of a dielectric material whose outer periphery is shielded and grounded, and are in contact with the fluid through the dielectric material. In a capacitor-coupled electromagnetic flowmeter that is equipped with a pair of detection electrodes that are taken out and a differential amplifier that amplifies the potential difference obtained between these detection electrodes, a A compensation electrode is provided, and this compensation electrode is used to detect FfK [
It is characterized by providing a potential distribution in which the potential in the direction forming the angle between i and θ is −cosθ (a is the inner radius of the tube, a′ is the outer radius) times the potential a′ obtained at the detection electrode. Electromagnetic flowmeter. (2) A pair of detection devices arranged to sandwich the fluid flowing in a pipe made of a dielectric material whose outer periphery is shielded and grounded, and to be arranged so as to be in contact with the fluid through the dielectric material, and to extract the electromotive force generated in the fluid through capacitive coupling. In a capacitively coupled electromagnetic flowmeter equipped with electrodes and a differential amplifier that amplifies the potential difference obtained between these detection electrodes, a compensation electrode is provided on the outer periphery of the tube and extends in the tube axis direction. In the preceding stage of the dynamic amplifier, there is an operational amplifier with a 7-oror connection that uses the detection potential obtained from each detection electrode as a non-inverting input. ) is provided, and the voltage obtained at each output point of this voltage divider is expressed as i! ' An electromagnetic flowmeter whose percentage sign is the voltage applied to the compensation electrode in a direction forming an angle θ with the detection current with respect to the center point within a cross section perpendicular to the +tll+ direction. (3) 610 bodies flowing in a pipe consisting of a dielectric body whose outer periphery is shielded and grounded are placed so as to sandwich and face each other, and each is in contact with the fluid through a dielectric body, and the electromotive force generated in the fluid is capacitively coupled. a pair of detection electrodes taken out by the
In a capacitively coupled electromagnetic flowmeter equipped with a differential amplifier that amplifies the potential difference obtained between these detection electrodes, a compensation electrode extending in the tube axis direction is provided on the outer periphery of the tube, and a differential At the front stage of the amplifier, there is a follower-connected operational amplifier that uses the detection potential obtained from each detection electrode as a non-inverting input, and the output voltage of this operational amplifier is multiplied by -17 cos θ (a is the inner radius of the tube, a' is the outer radius). The voltage obtained at each output point of this voltage divider is detected by detecting the voltage obtained at each output point of the voltage divider with respect to the center point in a cross section perpendicular to the tube axis direction. 1. An electromagnetic flowmeter characterized by being provided with a circuit that applies a voltage to the operational amplifier, amplifies the output of the operational amplifier at a constant gain, and provides positive feedback to the input side via a capacitor.