Toshiba LF150 Instruction Manual

— —25６Ｆ８Ａ０９１６7. Principle of OperationThe operating principle of the electromagnetic flowmeter is based on Faraday's Law ofelectromagnetic induction and it is designed to measure the volumetric flow rate of fluid. Aninsulated pipe of diameter D is placed vertically to the direction of a magnetic field with fluxdensity B (see Figure 7.1). When an electrically conductive fluid flows in the pipe, an electrodevoltage E is induced between a pair of electrodes placed at right angles to the direction ofmagnetic field. The electrode voltage E is directly proportional to the average fluid velocity V.The following expression is applicable to the voltage.E = K × B × D × V [V] ......... (Eq. 7.1)Volumetric flow rate Q [m3 /s] is:Q = × V ................(Eq. 7.2)Using Equation 7.1 and 7.2E = K × B × D × × QE = × Q ............(Eq. 7.3)Therefore, volumetric flow rate is directly proportional to the induced voltage.Figure 7.1 Principle of OperationThe LF150 flange-type electromagnetic flowmeter uses square-wave excitation method, whichprovides a long-term stable operation. With square-wave excitation, the LF150 offers reliablemeasurement without being affected by electrostatic or electromagnetic interference, orelectrochemical polarization between the electrode and the fluid to be measured.E = induced electrode voltage [V]K = constantB = magnetic flux density [T]D = meter pipe diameter [m]V = fluid velocity [m/s]π × D²44π × D²4 × K × Bπ × DSquare Wave Excitation Method