Principle and design of electromagnetic flowmeter with fluid impedance measurement

Contents

The present invention relates to an electromagnetic flowmeter with fluid impedance measurement, and more particularly to a dual-excitation electromagnetic flowmeter that uses a controllable excitation source based on a photocell to generate an additional AC excitation potential, performs fluid flow velocity measurement under magnetic excitation, and performs electrode impedance measurement under electrical excitation. The present invention particularly relates to an electromagnetic flowmeter that implements fluid conductivity measurement, detects electrode insulation attachment, and determines whether a pipe is empty based on the measured electrode impedance value.

Background of the Invention

The electromagnetic flowmeter is based on a measurement equation formed by the fluid cutting the magnetic lines of force under the state of magnetic field excitation to generate an induced electromotive force proportional to the flow rate, and thus forms a measurement system in which the induced electromotive force output by the two electrodes of the sensor and the signal amplification by the measurement amplifier are formed. However, this measurement system is based on the condition that the internal resistance ratio between the equivalent fluid internal resistance of the fluid conductivity and the input internal resistance of the signal amplifier can be ignored. Once the fluid conductivity is low enough, its equivalent internal resistance cannot be ignored, and the measurement accuracy of the above measurement system cannot be guaranteed.

A dual-excitation electromagnetic flowmeter has been developed that uses a new excitation in addition to the magnetic field excitation to measure the fluid impedance. It is mainly used to monitor the electrode contamination and the empty pipe status of the measuring pipeline while ensuring the original fluid flow measurement. Patents include CN1409093A, CN100344940C, CN100491928C, CN1731106A, CN1760643A and CN101718565A.

Generally, there are two ways to add new excitation: one is to connect the new excitation in parallel to the measuring electrode, such as patents CN1409093A, CN100491928C and CN1760643A. In principle, any circuit connected in parallel to the measuring electrode with high internal resistance will cause the induced potential signal under magnetic field excitation to be lost; the other is to connect the new excitation in series in the connection loop between the two measuring electrode ends of the electromagnetic flow sensor and the two input ends of the measuring amplifier, such as patents CN100344940C, CN1731106A and CN101718565A. In principle, the induced potential signal under magnetic field excitation will not be lost, but this requires the use of electrical isolation from the measuring electrode circuit to achieve the generation and control of the additional new excitation.

Summary of the invention

The purpose of the present invention is to address the defects of the existing technology and provide an electromagnetic flowmeter with fluid impedance measurement, which can accurately measure the electrode impedance, thereby being able to identify the type of fluid, detect electrode contamination and the adhesion of the inner lining of the pipe wall, and determine whether the measuring pipe is empty.

The present invention is based on patent CN101718565A. The signal terminals a and b formed by the series connection of the fluid induction potential E0 generated by the sensor (1) and the excitation potential E1 generated by the controllable excitation potential (2) are connected to the two input terminals of the impedance signal amplifier (4) through capacitors Ca and Cb coupling, and resistors Ra and Rb connected to the signal reference ground are respectively connected to the two input terminals of the impedance signal amplifier (4). When the excitation magnetic field B=0, the impedance signal amplifier (4) has an output signal U4 to estimate the fluid impedance Z0, that is, information such as the change in fluid conductivity, electrode contamination, and empty measuring tube can be obtained through the signal U4.

According to the above invention concept, the present invention adopts the following technical scheme: an electromagnetic flowmeter with fluid impedance measurement, comprising: an electromagnetic flow sensor with a pipe diameter D to allow the measured fluid Q to pass through, an excitation magnetic field B in the electromagnetic flow sensor applied to the measured fluid Q, and an induced potential E0=K0×D×B×V with an amplitude proportional to the flow velocity V is generated between two measuring electrodes P1 and P2 in the electromagnetic flow sensor corresponding to the flow velocity V of the fluid Q, wherein K0 is a coefficient; a controllable excitation potential generates an excitation potential E1 with an internal resistance r1 and an alternating frequency f; the induced potential E0 and the excitation potential E1 are connected in series to form two signal terminals a and b. In the above electromagnetic flowmeter with fluid impedance measurement, the resistors Ra and Rb can use digitally controlled potentiometers, which can be adjusted to the required resistance value according to actual needs, thereby improving the measurement accuracy.

Compared with the prior art, the present invention has the following obvious outstanding substantial features and remarkable advantages: based on the electric field excitation of photoelectric coupling in series in the flow measurement loop, it is easy to control and monitor, and will not reduce the input impedance of the flow measurement signal amplifier, thereby not causing the problem of loss of the induced potential signal under magnetic field excitation.

Secondly, the impedance signal amplifier is connected to the two signal output terminals a and b in the flow rate measurement loop through coupling capacitors, so that the fluid flow rate measurement and the electrode impedance measurement do not interfere with each other, and the measurement is performed in time, frequency, and hardware. In this way, a dual-excitation electromagnetic flowmeter can be provided that can maintain high-performance flow measurement characteristics under magnetic field excitation and monitor the fluid impedance between the two ends of the electrode under additional potential excitation. This enables the traditional electromagnetic flowmeter to have new functions such as monitoring electrode contamination, empty measuring tubes, and changes in fluid impedance.