Electromagnetic flowmeter work and measurement principle

Working principle

The working principle of electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. In the electromagnetic flowmeter, the conductive medium in the measuring tube is equivalent to the conductive metal rod in the Faraday test, and the two electromagnetic coils at the upper and lower ends generate a constant magnetic field. When a conductive medium flows through it, an induced voltage is generated. Two electrodes inside the pipe measure the induced voltage produced. The measuring pipe is electromagnetically isolated from the fluid and measuring electrodes by a non-conductive lining (rubber, Teflon, etc.).

Measurement principle

According to Faraday’s law of electromagnetic induction, in a uniform magnetic field with a magnetic induction intensity B, a non-magnetic pipe with an inner diameter D is placed perpendicular to the direction of the magnetic field. When the conductive liquid When flowing in a pipe at a velocity v, the conductive fluid cuts the magnetic field lines. If a pair of electrodes are installed on both ends of the pipe cross section perpendicular to the diameter of the magnetic field, it can be proved that as long as the flow velocity distribution in the pipe is axially symmetrical, a pair of electrodes will be generated between the two electrodes. Induced electromotive force: e=KBDv (3-36) In the formula, v is the average flow velocity on the pipe section, and k is the instrument constant. From this, the volume flow rate of the pipeline can be obtained: qv= πeD/4KB (3-37) It can be seen from the above formula that the volume flow rate qv has a linear relationship with the induced electromotive force e and the inner diameter of the measuring tube D, and is inversely proportional to the magnetic induction intensity B of the magnetic field. It has nothing to do with other physical parameters. This is the measurement principle of the electromagnetic flowmeter. It should be noted that in order for equation (3-37) to be strictly established, the measurement conditions of the electromagnetic flowmeter must meet the following assumptions: ① The magnetic field is a uniformly distributed constant magnetic field ; ② The flow velocity of the measured fluid is axially symmetrically distributed; ③ The measured liquid is non-magnetic; ④ The conductivity of the measured liquid is uniform and isotropic.