JPH0228091B2 - DENJIRYURYOKEI - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD OF THE INVENTION The present invention relates to an electromagnetic flowmeter, and particularly to an electromagnetic flowmeter for ultra-low conductivity 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 extracted by a pair of electrodes installed on the inner wall of a pipe through which the fluid flows, in direct contact with the fluid. In a normal electromagnetic flowmeter, the measurable fluid conductivity is 2 to 5×
The lower limit is 10 ^-6 ≠/cm. On the other hand, the electrical conductivity of what is generally called "oil" is 10 ^-11 ~
10 ^-17 ≠/cm, and it is considered impossible to measure the flow rate of this "oil" as well as intermediate substances such as acetone and ammonia using the method described above. However, on the other hand, because electromagnetic flowmeters are easier to maintain than mechanical ones, there is a strong desire to realize an electromagnetic flowmeter that can measure even the above-mentioned ultra-low conductivity fluid. [Summary of the Invention] The present invention was made in view of the above circumstances, and its purpose is to make it possible to measure ultra-low conductivity fluids of, for example, 10 ^-7 ≠/cm or less, which was previously considered impossible. Our goal is to provide electromagnetic flowmeters that meet the needs of our customers. In order to achieve such an objective, the present invention
A so-called capacitively coupled electromagnetic flowmeter is used, in which a dielectric material is interposed between the fluid and the detection electrode, and the electromotive force generated in the fluid is extracted through capacitive coupling while the electrode surface is not in direct contact with the fluid. In order to compensate for the fact that the internal electric field distribution is disturbed and attenuated due to the fact that the thickness of the dielectric tube is finite and the outer circumference must be shielded and grounded, a A compensation electrode extending along the tube is provided, and the detection electrode is provided with a compensation electrode extending along the tube axis in a direction that makes an angle θ with the detection electrode in a cross section perpendicular to the tube axis direction, with the inner and outer radii of the tube being a and a', respectively. A potential that is a'/acosθ times the potential that is applied is applied. Furthermore, in order to cope with the minute coupling capacitance between the detection electrode and the fluid, the input capacitance of the operational amplifier, which uses the detection potential obtained from each detection electrode provided at the front stage of the differential amplifier as a non-inverting input, is equivalent to or greater than that. In order to compensate for the drop in output voltage caused by this, 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 follower connection. The present invention will be explained in detail below using examples. First, the validity of capacitive coupling will be explained, and the above-mentioned problems that must be solved in that case will be analyzed. Now, we will move on to an explanation of an embodiment as a means for solving the problem. [Example] Regarding the flow rate detection potential difference due to the Faraday effect in ultra-low conductivity fluids, there is an analysis by V. Cushing (“Induction Flowmeter”, Rev. Scl.
Instrum., 29, pp692−697, 1958). According to this, when a tube 1 made of a dielectric material has a finite thickness and its outer periphery is shielded and grounded by a metal shield tube 2, as shown in FIG. The potential difference ΔV obtained between the electrodes 3 is expressed by the following equations (1) and (2). Note that 4 indicates a fluid. ΔV=K・B・D・U……(1) K=σ ₁ A+jε ₀ (ε〓 ₁ −1)ω/(σ ₁ +j
ε ₀ ε〓 ₁ ω)+(σ ₂ +jε ₀ ε〓 ₂ ω)1+(a/b) ²
/1-(a/b) ² ...(2) However, B: Magnetic flux density D: Diameter (inner diameter) of the tube (=2a) U: Average flow velocity σ ₁ : Electrical conductivity of the fluid σ ₂ : Electrical conductivity of the tube ε ₀ : Permittivity of vacuum ε ₁ : Relative permittivity of fluid ε ₂ : Relative permittivity of tube ω : Angular frequency of magnetic field a : Inner radius of tube b : Outer radius of tube This is generally the case for electromagnetic flowmeters. This is the basic equation that gives the output signal, and in a normal electromagnetic flowmeter that is not intended for ultra-low conductivity fluids, σ ₁ is sufficiently large, so it can be safely assumed that K=1. Now, since σ ₂ can be made sufficiently small in practice (for example, 10 ^-18 ≠/cm for Teflon), if we ignore it, equation (2) can be expressed as equation (3) below. K=σ ₁ +jε ₀ (ε〓 ₁ −1)ω／σ ₁ +jε ₀ (
ε〓 ₁ +1+(a/b) ² /1−(a/b) ² ε〓 ₂ )ω…
...(3) If we examine the practical limit from equation (3), we will find that in reality, the thickness of the tube cannot be made zero or infinite, and the range of a/b is is 0<a/b
<1, but a/b _nio = 0.5 a/b _nax = 0.8 is considered to be the limit. Here, to summarize the numbers, a/b = 0.65. Next, ε〓 ₂ 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 Teflon is used, and in that case ε〓 ₂ = 2 Therefore, equation (3) becomes the following equation (4). K=σ ₁ +jε ₀ (ε〓 ₁ −1) ω/σ ₁ +jε ₀ (ε〓 ₁
+5) ω……(4) ε〓 Regarding ₁ , we will consider two types of fluids. That is, one is petroleum-based, ε〓 ₁ = 2, and the other is pure water, ε〓 ₁ = 80. Alcohols are located in between these. Next, the frequency characteristics of the absolute value of K obtained from equation (4) using these values are shown in FIGS. 2 and 3.
Figure 2 shows the petroleum system, and A, B, C, and D in the diagram show the cases where σ ₁ is 10 ^-15 , 10 ^-13 , 10 ^-11 , 10 ^-9 ≠/cm, respectively, and Figure 3 shows the pure water system. In the figure, σ ₁ is
The cases of 10 ^-7 and 10 ^-5 ≠/cm are shown. In addition, C in FIG. 3 is shown next to theoretically pure water having the minimum σ ₁ 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 in direct contact with the fluid, but in process control systems, which are the main fields in which electromagnetic flowmeters are used, a response of at least 1 to 2 seconds is required. Because it will be done,
The lower limit of ω must be considered to be about 20 (3 Hz in frequency). 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, the lower limit of conductivity that can be measured is 10 ^-10 ≠/cm. Furthermore, 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), σ ₁ /ω
→ Setting it to 0 corresponds to capacitive coupling. limK limK σ ₁ /ω→0=ε〓 ₁ −1/ε〓 ₁ +1+(a/b) ² /1
-(a/b) ² ...(5) Here, if a/b=1, 1imK=0, so as shown in Figure 1, a tube 1 made of a dielectric material whose outer periphery is shielded and grounded. When the thickness 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 present invention, a/b=0, that is, infinite thickness, was achieved equivalently by the following method. Now, as b → ∞, a point p (θ ) is V(θ), and the potential at point q(θ) with radius a′ opposite to point p is V′(θ). Since there is no electric field in the axial direction of the tube, V′( θ) = a/a′V(θ) This means that if we place an electrode at point q and
This shows that if a potential of V'(θ) is given, it becomes equivalent to n=∞ even if nothing exists outside the radius a'. Since a and a′, that is, the outer radius of the actual tube, are known, if V(θ) is known,
V′(θ) is determined. V(θ) at θ=0 is the potential V of the detection electrode, and since it can be set that V(θ)=Vcosθ, V'(θ)=a/a'Vcosθ. In other words, the inner radius a and the outer radius are made of dielectric material.
If a compensation electrode extending in the tube axis direction is provided on the outer periphery of the tube a', and a potential distribution given by the above equation is given to it, then
Signal attenuation due to the finite thickness of the tube can be compensated for. Approximately, this can be achieved by providing a band-shaped compensation electrode 11 divided into a plurality of parts in the axial direction, as shown in FIG. can. FIG. 5 is a block diagram showing an embodiment (part) of the present invention, in which a 90° angle 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).
A voltage divider 13 is provided which divides the output voltage of the operational amplifier 12 by a/a′ cos θ (θ is the angle at which each compensation electrode 11 is arranged), and the voltage obtained at each output point of the voltage divider 13 is The voltage is applied to the compensation electrodes 11 at corresponding angles. 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 provided offset from the detection electrode 3 (θ≠0°), but this is for convenience of drawing out, and it may be arranged so that θ=0°. Needless to say. The outside of the compensation electrode 11 is covered with a normal insulating material,
Cover the outside with a metal tube for shielding and ground it. For example, this can be easily realized by using something like a flexible printed board as the compensation electrode 11, pasting it on the outside of the tube 1 made of, for example, Teflon, and grounding the outside as a shield. For the tube 1, for example, Teflon is used, and a structure is used in which a detection electrode is embedded near the inner wall. However, for the part that comes into direct contact with the fluid, ceramic, which has higher wear resistance, is used mainly from the viewpoint of wear resistance. It's okay. In that case, for example, a detection electrode may be attached to the outer periphery of the ceramic tube, and the outside thereof 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 inputs the signal from the detection electrode, becomes a problem. That is, although the signal taken out by the detection electrode through capacitive coupling is originally very small, the input capacitance becomes identical to or more than the coupling capacitance and significantly attenuates the signal. That is, in FIG. 6, 21 is the coupling capacitance,
With 22 as a bias resistor, the capacitance value C of the coupling capacitor 21 is determined by the design of the capacitance meter body. That is, the area of the detection electrode 3 is S, the gap between this detection electrode and the fluid is t, and the relative permittivity of the dielectric between them is ε〓 ₂
C is expressed as C=S/tε ₀ ε〓 ₂ , but in actual design, for example, t=0.15 (cm) S=2.5×11.5 (cm ² ) ε〓 ₂ = 2 (in case of Teflon) Therefore, C=4.43×10 ^-12 (F) is considered to be the structural limit. On the other hand, the input capacitance 23 (C _IN ) is 3~
It is about 4 pF, which is almost the same as the coupling capacitance mentioned above, that is, the signal source series capacitance. Therefore, by using an operational amplifier with a FET input with a small bias current and connecting a high bias resistor of 3000 MΩ, the voltage division ratio of the CR coupling part can be adjusted.

〔Effect of the invention〕

As explained above, according to the present invention, a capacitive coupling method is used, and a compensating electrode is provided on the outer periphery of the dielectric tube and extends along the tube axis direction. By providing a potential distribution of /acosθ times, 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 shielded and grounded. Furthermore, the above configuration can be approximately realized by providing the compensating electrode with an output obtained by dividing the output of a follower-connected pre-operational amplifier that uses the detection potential as a non-inverting input using a voltage divider. By providing a circuit that amplifies the output of the amplifier with a constant gain and provides positive feedback via a capacitor, the input capacitance of the operational amplifier is equal to or greater than the minute coupling capacitance of the signal generation source. Signal attenuation can be compensated for. As a result, the present invention makes it possible to measure the flow rate of ultra-low conductivity fluids, which has been impossible with conventional electromagnetic flowmeters.

[Brief explanation of drawings]

Figure 1 is a diagram for explaining the potential difference obtained in an electromagnetic flowmeter, Figure 2 is a diagram showing the ω dependence of the correction term K in petroleum-based fluids using the conductivity of the fluid as a parameter, and Figure 3 is a diagram for explaining the potential difference obtained in an electromagnetic flowmeter. A similar diagram for a water system; FIG. 4 is a diagram for explaining the principle of compensating for signal attenuation based on the finite thickness of the pipe; FIG. 5 is an example of the configuration of a compensation electrode in an embodiment of the present invention. 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. 7 is a diagram for explaining the operation, 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... Pipe made of dielectric material, 2... Shield tube, 3...
Detection electrode, 4...Fluid, 11...Compensation electrode, 12...Pre-operational amplifier, 13...Voltage divider, 21...Coupling capacitor,
23... Input capacitance, 31... Capacitor, 34... Feedback operational amplifier, 41, 42... Pre-operational amplifier 12
Amplifiers that make up the.

Claims (1)

[Scope of Claims] 1. 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, and extract the electromotive force generated in the fluid by capacitive coupling. In a capacitively coupled electromagnetic flowmeter equipped with a pair of detection 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. The compensating electrode is given the potential a′/acosθ (a is the potential of the tube in a direction that makes an angle θ with the detection electrode with respect to the center point in a cross section perpendicular to the tube axis direction). An electromagnetic flowmeter characterized by providing a potential distribution that doubles the inner radius (a' is the outer radius). 2. A pair of detection electrodes which are arranged to sandwich the fluid flowing in the pipe made of a dielectric material whose outer periphery is shielded and grounded, and are arranged so as to be in contact with the fluid through the dielectric material, and to extract the electromotive force generated in the fluid by capacitive coupling. 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 amplifier is provided. At the front stage of the , there is a follower-connected operational amplifier whose non-inverting input is the detection potential obtained from each detection electrode, and the output voltage of this operational amplifier is expressed as a'/acosθ (a is the inner radius of the tube, a' is the outer radius) A voltage divider is provided to divide the voltage twice, and the voltage obtained at each output point of this voltage divider is applied to the compensation electrode in a direction forming an angle θ with the detected current with respect to the center point in a cross section perpendicular to the tube axis direction. An electromagnetic flowmeter characterized in that an electric current is applied to the electromagnetic flowmeter. 3 A pair of detection electrodes that are arranged to sandwich the fluid flowing in the pipe made of a dielectric material whose outer periphery is shielded and grounded, and are arranged so as to be in contact with the fluid through the dielectric material, and to extract the electromotive force generated in the fluid by capacitive coupling. In a capacitively coupled electromagnetic flowmeter equipped with 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, and a differential In 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 expressed as a/a' cos θ (a is the inner radius of the tube, a' is the outer radius). A voltage divider that doubles the pressure (radius) is installed,
The voltage obtained at each output point of this voltage divider is measured between the detection electrode and the θ
An electromagnetic flowmeter comprising a circuit that applies voltage to the compensation electrode in a direction forming an angle of , amplifies the output of the operational amplifier at a constant gain, and provides positive feedback to the input side via a capacitor.