Electromagnetic flow meters are one of the most successful types of flow meters. According to Faraday’s law of electromagnetic induction, the basic concept that a conductive fluid generates an electric potential proportional to the flow velocity when passing through a magnetic field has been known since the beginning of the last century. However, it was not until the 1950s that industrial applications became a reality. Some of the earliest applications involved measuring blood flow velocity, Kolin (1960). Kolin, A. (1960) Circular system; Methods; Blood flow determination by the electrical method In: Glasser, O. (Ed) Medical Physics, 3:141 Chicago I11: Year Book Medical Publishers
In 1952, KROHNE (Cologne) produced its first electromagnetic flowmeter for industrial applications. (From the official website of Cologne). Over the past few decades, electromagnetic flowmeter technology has evolved from the earliest AC excitation to the later pulse square wave DC excitation, and then to Yokogawa’s innovative hybrid excitation; Early electromagnetic flow meters required a strong magnetic field to generate a strong enough signal, resulting in a huge power consumption of -300 W; Up to now, it can operate at a power of 10 W to 15 W; Even instruments powered by a two wire loop can now have a larger diameter of 8 inches, indicating technological progress in the field of electromagnetic flow meters.
The basic principle of electromagnetic flow meters:
Faraday’s law of electromagnetic induction: E=K * B * D * V
Among them, E: induced electromotive force K: instrument constant B: magnetic induction intensity
D: Measurement tube diameter V: Average flow velocity
Through the principle, it can be seen that:
1. Electromagnetic flow meters can only measure conductive media, and generally require conductivity б≥ 1-5us/cm (water>20us/cm)
2. The actual measured value of the electromagnetic flowmeter is the fluid velocity
3. Magnetic field stability is the key to measuring stability
The structure of electromagnetic flow meters:
Measurement catheter
The conduit must be made of non magnetic materials (ensuring that the magnetic field passes through the conduit), such as stainless steel, ceramics, plastics, etc. Conduits are the means of medium flow; The conduit bears the rated working pressure.
electrode
Electrodes are used to extract signals from the medium, and due to direct contact with the medium, their corrosion resistance characteristics should be matched with the medium.
In terms of electrode configuration, many manufacturers offer differentiated options, such as scraper electrodes, replaceable electrodes, and electrode cleaning.
lining
When the lining comes into direct contact with the medium, it is necessary to choose a lining material that is suitable for the medium. A reasonable selection should be made by considering the temperature, pressure, corrosiveness, and abrasiveness of the medium comprehensively.
Returning to Faraday’s law of electromagnetic induction, magnetic field stability is the key to measuring stability; In the early stages of the development of electromagnetic flow meters, an AC power supply system (AC sine wave excitation) was directly used to supply power to the excitation coil. The advantage of this technology is that the excitation magnetic field of the mains is strong (high magnetic flux “B”), but the disadvantage is particularly obvious: there is zero drift when the phase changes, and manual zero adjustment is also required when the medium is stationary.
Non sine wave AC excitation is a method of excitation that uses square or triangular waves below the industrial frequency. It can be considered to generate a constant DC and periodically change polarity. Due to the stability of this excitation power source, there is no need to perform calculations to remove changes in magnetic field strength.
The main problem with the AC excitation method is severe induced noise.
The direct current excitation method becomes an important obstacle due to the polarization potential on the electrode. Therefore, a certain value of DC excitation method is only applicable to the measurement of non electrolyte (such as liquid metal) liquids.
When the excitation frequency is reduced, the zero point stability can be improved, but the instrument’s ability to resist low-frequency interference is weakened and the response speed is slow. If the excitation frequency is high, the ability to resist low-frequency interference is enhanced, but the zero point stability of the instrument is reduced. By the 1970s, low-frequency rectangular waves (1/2 to 1/32 of 50Hz) had been developed to address the long-standing power frequency interference that plagued electromagnetic flow meters, improving zero point stability and measurement accuracy; In the 1980s, ternary low-frequency rectangular wave excitation technology emerged again (with 1/8 of 50Hz as the cycle and sinusoidal excitation current), which has better zero point stability and solves the influence of interference potential, but reduces the response speed. In addition, when measuring solid particles and fiber containing fluids such as mud and pulp, as well as low conductivity fluids, electrical noise is generated (due to fluid friction with the electrode, the oxide film on the electrode surface peels off and forms again), causing unstable output signal oscillation; In the late 1980s, a dual frequency rectangular wave excitation method was introduced to address these issues.
Pulse DC excitation (PDC)
This is a widely used excitation method that is relatively inexpensive and suitable for most operating conditions, such as:
Uniform medium, pulp, slurry, cement slurry (containing fine particles)
High precision and long-term stability need to be maintained at low traffic levels
When the zero calibration conditions are not available on site
When the power supply is subject to certain limitations
When the pulse flow pulsation frequency is less than 2Hz
When the canning cycle>3 seconds
Pulse AC excitation (PAC)
The excitation frequency of the AC excitation method is faster than that of the DC pulse excitation method, so low-frequency noise signals are easily eliminated.
Due to its fast response speed, it is suitable for fast canning
Suitable for measuring liquids containing large amounts of solid particles
Suitable for liquids that generate noise due to physical or electrochemical reactions, such as two-phase flow (slurry, mud), saline water (conductivity 0.1-20uS/cm)
Suitable for mixtures of multiple liquids
Suitable for pulse flow
The excitation waveform consists of a combination of low-frequency (6.25Hz) rectangular waves and high-frequency (75Hz) rectangular waves. The corresponding flow signals are sampled separately to obtain two types of signals with low-frequency and high-frequency characteristics. After processing, the actual flow signal values can be reproduced. Therefore, this technology has both the excellent zero point stability of low-frequency rectangular wave excitation technology and the strong suppression ability of high-frequency rectangular wave excitation technology on fluid noise.
Two wire electromagnetic flowmeter
The basic concept of two wire technology is that the device using it is powered by the same two wires in the loop and communicates with them.
The first advantage of the two wire system technology is that it reduces the required wiring, conduits, and labor costs; Secondly, it can reduce energy and operating costs. Many four wire Coriolis mass flow meters and electromagnetic flow meters have a rated power consumption of up to 15 W, while two wire transmitters only require 0.1 W. Compared with a four wire flow meter, using a two wire flow meter can reduce energy consumption by 96.4%. The low power consumption of the two wire instrument also makes it easier to apply in intrinsically safe environments. but the maximum diameter can only reach DN200.
Electromagnetic flowmeter supporting PoE
PoE (Power over Ethernet), also known as Ethernet powered. Simply put, it can provide both power and data transmission to PoE supported powered devices (PD) through a single Ethernet cable.
Ethernet technology can maximize the data potential of intelligent instruments and significantly improve communication speed. For example, ABB claims to have launched electromagnetic devices that support PoE, with a communication speed of up to 100 Mbits/sec.
There are significant differences in electrode arrangement between capacitive electromagnetic flow meters and traditional electromagnetic flow meters. For example, Yokogawa’s related products place electrodes outside ceramic measuring tubes and measure electromotive force by measuring the electrostatic capacitance generated between the fluid and the electrodes.
This non-contact electrode structure does not have problems such as electrode corrosion, sticking, wear or leakage, and is suitable for fluids with strong corrosion or containing a large amount of solid particles, as well as measuring fluids with adhesion. Can measure ultra-low conductivity fluids with conductivity as low as 0.01 µ s/cm.
However, the excitation circuit of this type of flowmeter has become more complex, and there are few brands of commercialized electromagnetic flowmeter products, and the caliber range is generally not large.
However, this electrode structure does provide more possibilities for miniaturization of flow meters. Currently, there are many exquisite electromagnetic flow meters on the market based on this structure – fully through, without movable components, and without electrodes;
Battery powered electromagnetic flow meter/electromagnetic water meter
This type of electromagnetic flowmeter can be powered by batteries, with a lifespan of up to 15 years (according to the manufacturer’s promotional manual), or by mains or renewable energy sources (solar/wind); Optional communication method for GSM/SMS. Very suitable for use by municipal pipeline networks, urban water companies, and other water company users.
Post time: 18-12-23