According to the different types of applied magnetic field, electromagnetic flowmeters mainly have two types: DC type and induction type.
The external constant magnetic field B of the DC electromagnetic flowmeter is perpendicular to the tube axis, and two electronic components are installed at C and D to measure the electromotive force U induced by the fluid when the fluid traverses the magnetic field. The flow rate Q can be obtained by the following formula:
Q=1/k*UA/Bd
In the formula, A is the pipe cross-sectional area; d is the pipe diameter; k is the correction coefficient, which is used to correct the influence of factors that are not taken into account when the formula is derived (such as the fact that the flow velocity in the flowmeter pipe is not uniform). In normal flow meters, k is about 0.8, but for electromagnetic flow meters of specific dimensions and for specific working conditions, the volume method (volume that flows through a certain period of time) is used to calibrate the value of k.
The magnetic field can be generated by a *magnet, which is generally made of aluminum-nickel-diamond alloy. When the flow rate is large, due to the larger pipe diameter, an iron-core excitation winding is used, and a constant direct current is passed through it to generate an approximately uniform magnetic field.
The induction electromagnetic flowmeter can be used in the case that the temperature of the measured fluid is too high or the electronic components are strongly corrosive. A and B are two AC excitation windings with the same number of turns (the cross section is drawn in the figure). The windings are connected in series, but their current directions are opposite. When the fluid is at rest, the resultant magnetic flux passing through the induction coil C is zero, so there is no induced alternating electromotive force in the coil. When the fluid flows, the alternating electromotive force is generated in the induction coil, and its magnitude is proportional to the flow velocity. Based on this principle, there are many modifications, such as using an excitation winding, and placing an induction coil in the opposite direction on both sides of its symmetry and connecting them in series. When the fluid flows, the lines of magnetic force move in the direction of the flow, causing both sides to induce induction. The surprise electromotive force in the coil is not zero, which can indirectly indicate the size of the flow.
There are no other parts in the pipeline of the electromagnetic flowmeter, so in addition to measuring the flow of conductive fluids, it can also be used to measure the flow of non-conductive liquids of various viscosities (in which easily ionizable substances are added). Electromagnetic flowmeters are often used in the nuclear energy industry.