Electromagnetic flowmeter (EMF) is made by using Faraday's law of electromagnetic induction of a conductive liquid volume flow measurement instrument. Early 50 s EMF realized industrial application, in recent years the world EMF production accounted for about 5% ~ 6.5% of the industrial flow meter number. Have appeared since 70 s of the keying low-frequency rectangular wave excitation mode, gradually replace the early application of power frequency ac excitation mode, has made great improve instrument performance, get more extensive application.
1. The principle and mechanism
The EMF is the basic principle of Faraday's law of electromagnetic induction, when cutting lines of magnetic force in the conductor in magnetic field movement on both ends of the induced electromotive force. As shown in figure 1, the electrical conductivity of liquid flow in the perpendicular to the magnetic field of magnetic measurement tube, in a direction perpendicular to the direction of flow and flow rate is proportional to the induced potential, the direction of the electromotive force according to the "Ming right hand rule", its value in the following try E -- -- -- -- -- induction electromotive force, namely traffic signals, V. K -- -- -- -- -- coefficient; B -- -- -- -- -- the magnetic induction intensity, T; D - tube inner diameter measurement, m; - average flow velocity, m/s. Sets the volume flow of liquid to be, then in the type for instrument constant K, K = 4 KB/D of PI. The EMF by flow sensor and converter composed of two parts. Sensor signal as shown in figure 2, a typical structure measurement tube with excitation coil, excitation current to produce magnetic field through the measuring tube, a pair of electrodes in measuring tube wall in contact with the liquid phase, which leads to the induction electric potential, to the converter. Exciting current is provided by the converter.
2. The advantages
EMF measurement channels is a shine a smooth straight pipe flow detection, because of not easy to block suitable for measuring the liquid containing solid particles or fibers solid two phase fluid, such as pulp, coal water slurry, pulp, mud and water, etc. EMF is not formed by the testing flow pressure loss, the resistance of the meter is just the same along the length of pipe resistance and energy-saving effect is remarkable, for low resistance loss of large diameter water pipe is most suitable. EMF measured by volume flow, in fact is not affected by fluid density, viscosity, temperature, pressure and conductivity changes (as long as the above a certain threshold) apparent effect. Compared with most other flowmeters, lower front straight pipe requirements. EMF measurement range of large, usually those days ~ 50:1, optional wide flow rate range. Full value of the liquid flow rate can be selected within 0.5 ~ 10 m/s. Some models of instrument can be according to the need to expand and to reduce the flow at the scene (for example, has 4 digits potentiometer setting instrument constants) don't have to take low offline real flow calibration. EMF diameter range than other varieties of flow meters wide, from a few millimeters to 3 m. Measurable positive and negative two-way flow, also measurable pulsating flow, as long as the pulse frequency is below a lot of excitation frequency. The instrument output is linear in nature. Easy to contact with the fluid are selected material varieties, can be used in corrosive fluid.
3. The shortcomings
EMF cannot measure the low conductivity of liquid, such as oil products and organic solvents etc. Not measuring gas, steam and liquid containing more bigger bubbles. Universal EMF because of lining materials and electrical insulation materials, can not be used in high temperature liquid; Some models of instrument used for below room temperature of the liquid, due to the measuring tube outer roller (or cream) and damage the insulation.