What is electromagnetic flow meter?

Historical Development of Electromagnetic Flow Meters

Faraday discovered the possibility of induced electromotive force in a magnetic field for a moving fluid in 1832, but it was not until 1930 that Williams reported the first component similar to a flow meter. The earliest true development of electromagnetic flow meters came from the medical field, and many of the ideas proposed by Colin (1936, 1941) have now become standard practices. In 1941, Thürlemann first provided a general proof, later in this chapter (see Thürlemann, 1955 and Shercliff, 1962 for more foundations)
- In the 1950s, the development of electromagnetic flowmeters in industry was accompanied by the following points:
- In 1953, in the Netherlands, Tobiflux flow meters (Tobi, 1953) were used to measure adhesive synthetic silk, sand, water, and acid slurry;
-  Foxboro was granted patent rights in 1952;
- The first commercial instrument appeared in 1954 (Balls and Brown, 1959);
- Application in nuclear reactors;
- The related work that led to the publication of an important book by J.A. Shercliff (1962).

Principle of electromagnetic flow meter

We start with the simple induction of a conductor moving in a magnetic field. As shown in Figure 1, a copper wire cutting a permanent magnet generates magnetic induction lines. The copper wire moves at a speed V in a direction perpendicular to the magnetic induction line and the length of the copper wire, thereby generating an electromotive force of magnitude BlV at both ends of the copper wire, where l is the length of the copper wire and B is the magnetic induction intensity.


Figure 2 shows the basic characteristics of an electromagnetic flowmeter. The fluid passes through a circular cross-section pipeline, usually with a magnetic field generated vertically by alternating current passing through the pipeline. To ensure that the magnetic field can pass through the pipeline, the pipeline itself must be made of non-magnetic material. From the figure, we can imagine that the liquid wire spanning the two electrodes of the pipeline is like a copper wire cutting magnetic induction lines at different speeds, thereby generating electromotive force at both ends. To prevent short circuits, the pipeline is lined with insulation material. The voltage in the liquid is measured through electrodes mounted on the tube wall.


As shown in Figure 3, assuming that more than one wire is moving within a magnetic field. The P wire moves at a velocity V in the strong magnetic field region B, the Q wire moves at a velocity V/2 in the same region, and the R wire moves at a velocity V in the relatively weak magnetic field region B/4. The magnetic induction intensity they generate is different from each other:

Q: ΔUq =BIV/2

R: Δ UR = BIV/4

If these wires are connected together, a current will be generated, but due to the difference in potential, Δ will decrease due to Ohmic losses. Bringing the same discussion back to Figure 2, we will obtain a complex potential diagram and cyclic current. Despite this obvious complexity, the practical application equation of flow meters is still very simple in various situations. The induced voltage between the electrodes is:

Among them, B is the magnetic induction intensity, unit T; D is the pipeline diameter, unit m;  Vm
is the average flow velocity in the pipeline, unit m/s. The basic equation of this flowmeter is used under the following conditions:
·The magnetic field is uniform;
·The velocity distribution of fluids is axisymmetric.
In modern design, it is difficult to meet the above conditions. Therefore, the design has evolved to minimize the impact of fluid velocity distribution on the output signal as much as possible while being far away from a uniform strong magnetic field.

Electromagnetic flow meter contractions

Figure 4 shows the main components of an industrial point electrode electromagnetic flowmeter. Industrial flow meters consist of two parts: Sensor Unit (primary component) and Converter Unit (secondary component).

The sensing part includes an insulated measuring tube, a flange joint, a coil that generates a magnetic field, and electrodes. In addition, means are usually provided to generate reference signals proportional to the magnetic field. For AC power supply systems, current transformers or detection coils are usually used; For DC systems, a constant induced current is usually measured.

The conversion part is usually referred to as a converter, which functions to:
1. Amplify and process fluid signals;
2. Eliminate stray electromotive force;
3. Ensure that the converter is insensitive to voltage fluctuations and  electromagnetic interference;
4. Meet the safety level;
5. Efficiently communicate with the user's control computer.

Electromagnetic flow meter Sensor Unit (primary component)

To ensure the magnetic field passes through, the measuring tube (see Figure 5) is usually non-magnetic. Its diameter may be 2~3000mm,normal request is 2" size, 3 " size flow meter, 4 inch flow meter, 8 inches flow meters and so on. The flow range it can pass through can be 0~28500m³/h or higher. The inner surface of the tube is insulated to prevent signal short circuits. The electrodes are set at both ends of the diameter, and their connection is perpendicular to the magnetic induction line. Electrodes are usually very small (with a diameter of 5~20mm) and are sometimes referred to as point electrodes or button electrodes. In some designs, there are also large electrodes that may have a 90 °arc and a length close to the diameter.

Table 1 presents some typical insulation materials, which are often formed around pipe segments and combined with flange joints. Excessive flow velocity (greater than 4m/s) may cause wear, and lining protection can be used (Ginesi and Annarummo, 1994). In the past, grounding rings were used as reference points for measuring electrodes in lined or non-conductive pipelines. In some designs, the reference board or electrode may not be grounded. Due to the current carried by the fluid, it will flow into the ground through grounding and cause current damage. In this case, the instrument potential can be made to float, and the safety of the instrument can be ensured through an isolation converter.