Industry new

Introduction to the principle of electromagnetic flowmeter

An electromagnetic flowmeter (hereinafter referred to as EMF) is a meter that measures the volumetric flow rate of a conductive liquid by using Faraday's law of electromagnetic induction. In the early 1950s, EMF realized industrial application. In recent years, the worldwide EMF production accounted for 5% to 6.5% of the industrial flow meter.

Since the 1970s, there has been a key-controlled low-frequency rectangular wave excitation method, which has gradually replaced the power frequency AC excitation method of the early application. The performance of the instrument has been greatly improved and it has been widely used.

2. Principles and institutions

The basic principle of EMF is Faraday's law of electromagnetic induction, that is, when a conductor cuts a magnetic line in a magnetic field, it generates an induced electromotive force at both ends. As shown in Fig. 1, the conductive liquid flows in a non-magnetic measuring tube perpendicular to the magnetic field, and generates an induced potential proportional to the flow rate in a direction perpendicular to the flow direction. The direction of the electromotive force is in accordance with the "Fleming right-hand rule", and its value As follows

Where E-----induced electromotive force, that is, flow signal, V;
k-----coefficient;
B-----magnetic induction intensity, T;
D----measuring tube inner diameter, m;
--- Average flow rate, m/s.
Let the volume flow rate of the liquid be
, then
Where K is the meter constant, K = 4 KB/πD.

The EMF consists of two parts, a flow sensor and a converter. The typical structure of the sensor is shown in Figure 2. The measuring tube is equipped with an excitation coil on the upper and lower sides. After the excitation current is passed, a magnetic field is generated to pass through the measuring tube. A pair of electrodes are placed on the inner wall of the measuring tube to contact the liquid, and the induced potential is extracted and sent to the converter. The excitation current is provided by the converter.

3, advantages

The EMF measuring channel is a smooth straight tube with an unobstructed flow detection. It is not easy to block and is suitable for measuring liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, coal water slurry, slurry, mud and sewage.

EMF does not produce pressure loss due to the detection of flow. The resistance of the instrument is only the resistance of the pipeline of the same length, and the energy saving effect is remarkable. It is most suitable for large diameter water supply pipelines requiring low resistance loss.

The volumetric flow measured by EMF is virtually unaffected by changes in fluid density, viscosity, temperature, pressure, and electrical conductivity (as long as it is above a certain threshold).

Compared to most other flow meters, the front straight pipe section is less demanding.
The EMF measurement range is large, usually 20:1 to 50:1, and the optional flow range is wide. The full-scale liquid flow rate can be selected within 0.5 to 10 m/s. Some models can expand and reduce the flow rate as needed in the field (for example, with a 4-digit potentiometer to set the meter constant) without having to take off offline real-flow calibration.

The EMF's caliber range is wider than other types of flow meters, from a few millimeters to 3 meters. The forward and reverse bidirectional flow can be measured, and the pulsating flow can also be measured as long as the pulsation frequency is lower than the excitation frequency. The meter output is linear in nature.

It is easy to select the material type of the fluid contact and can be applied to corrosive fluids.

4, missing points

EMF cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. It is not possible to measure gases, vapors and liquids containing more large bubbles.

General-purpose EMF cannot be used for higher temperature liquids due to lining materials and electrical insulation materials; some models are used for liquids that are below room temperature, and the insulation is destroyed by condensation (or frost) outside the measuring tube.

5, classification

General-purpose products and special-type instruments on the market can be classified from different angles.

According to the excitation current method, there are DC excitation, AC (power frequency or other frequency) excitation, low frequency rectangular wave excitation and dual-frequency rectangular wave excitation. The waveforms of several excitation methods are shown in Figure 3.

According to the system of output signal wiring and excitation (or power) connection, there are four-wire and two-wire systems.
Classified by converter and sensor assembly, there are separate and integrated models.

Classified by flow sensor and pipe connection methods, with flange connection, flange clamp connection, sanitary connection and threaded connection.

According to whether the flow sensor electrode is in contact with the liquid to be tested, there are contact type and non-contact type. Classified by flow sensor structure, there are short tube type and insert type.

Classified by use, there are general-purpose, explosion-proof, sanitary, anti-invasive and submersible types.

PREVIOUS：What are the factors that affect the accuracyNEXT：5 vortex flowmeter failure reasons analysis d