A classification, according to the exciting current mode of electromagnetic flowmeter
(1), DC excitation
DC excitation electromagnetic flowmeter is used to measure the flow of liquid metal, such as mercury at room temperature and high temperature liquid sodium, lithium, potassium etc.. In a loop of fast neutron breeder reactor, the melt flow rate of 500 degrees C was used to measure the flow rate of degrees Celsius. Dry efficacy flow value, compared with wet (flow) efficacy, precision can reach 3%. Except for by an electromagnet value flow excitation OTC, useful also for magnetic field of permanent magnet small aperture transducer.
This kind of instrument is used in the exploration of the general process industry in our country. In twentieth Century 60 ~ 70, it has been applied successfully to measure the flow rate of the electrolytic tank.
(2), AC excitation
Early electromagnetic flowmeter for 50Hz mains power frequency excitation and sine wave alternating magnetic field; by reason of AC excitation is to avoid a DC excitation electromagnetic surface polarization phenomenon, but due to susceptible to cause the electric lock, with traffic signal with corresponding induction noise superposition, formed the zero drift of, now has been gradually replaced by the low frequency rectangular wave excitation. The induced noise is generated by some of the following reasons.
1) the noise and the zero drift of the electrode pollution caused by the eddy current in the liquid are same as the phase noise;
2) the magnetic circuit iron loss offset the magnetizing current and the magnetic field phase, in the original signal circuit transformer effect of orthogonal (90 degrees) noise phase shift, produce the same noise;
3) the excitation coil and the signal line and the excitation coil and fluid between the electrostatic induction noise.
But AC excitation flow sensor of the magnetic induction intensity usually involves high (about 1mV per 1m / s, the rectangular wave excitation is only 0.2 ~ 0.3mV) and a high signal to noise ratio the advantages of large signal electromotive force. In addition, in the measurement of solid-liquid biphasic slurry fluid, low frequency rectangular wave excitation mode in the solid rub over the electrode surface producing spikes of slurry noise, the output signal wave, frequency of AC excitation electromagnetic flow meter does not exist the shortcomings. So at home and abroad and some instrument factory to provide electromagnetic flowmeter or other power frequency AC excitation.
(3), low frequency rectangular wave excitation
Since 1975 low-frequency rectangular wave excitation published, with its low power consumption, zero point stability, electrode pollution small advantages, quickly developed, so far to become the main exciting mode. "Positive negative" value, "plus zero negative zero" three value low-frequency rectangular wave excitation waveform, as shown right in B and C shown, respectively. Three valued signal excitation with no excitation during the improvement of zero stability. The signal voltage of flow signal is obtained by the interval sampling of the signal voltage. 1/32 to the exciting frequency for electric 1/2, between 1/8 ~ 1/4. Small caliber instruments with high frequency, large diameter meter with a lower frequency, by the manufacturing plant in accordance with the sensor diameter to set. There are also available from the user to set the frequency of the instrument, such as the commercial name of the programmable key storage (ProgrammableKeyedStorage) electromagnetic flowmeter".
Influence of rectangular wave AC excitation polarization voltage variation, though not by the slow, but due to the stable magnetic field and short period of DC by relative to the drastic changes in the polarization voltage, but also produce a sharp change in the output. In addition fluid solid particles impact electrode, the surface of the oxide film is damaged, the surface potential changes in the formation of spike like noise, the output shows a substantial shake. This phenomenon is called the slurry noise, which occurs in the liquid of high concentration of slurry or sand, which affects the application of low frequency rectangular wave electromagnetic flowmeter.
High frequency power frequency excitation than low-frequency rectangular wave excitation anti slurry noise performance is superior to that of reason; also can be illustrated by the slurry noise frequency characteristics. The following picture is a case of pulp size noise spectrum, the low frequency band noise level is high, the high frequency part of the noise is small, that is called 1/? Characteristic Therefore, the high frequency excitation noise level is low, can obtain higher signal than S/N.
(4), dual frequency excitation
Dual frequency excitation is in the low-frequency rectangular wave superimposed on high frequency square wave, such as pictured to the right of using two frequency sampling and low pass, high pass filter, high frequency and low frequency signal and synthesized to traffic signals to overcome the low frequency rectangular wave excitation in the presence of slurry and flow noise noise, and improve the stability of the instrument and response. Fast response can reflect the flow of the piston pump with frequency up to 1Hz.
Two, electromagnetic flowmeter connected according to the output signal and the excitation (or power) connection standard classification
(1), four wire system
The traditional electromagnetic flowmeter output signal line and a power line (or flow sensors and sensor excitation current line) respectively by 2 groups of 2 wire of composition is the main standard.
(2), second line system
Current temperature, pressure / differential pressure, flow and material level and other parameters such as the field instrument tends to the output signal and power sharing leads to the second tier system development. Two-wire meter without power, and electromagnetic flowmeter is often fitted with in remote places without city electricity supply, the second line of provinces and cities saving electrical wiring engineering cost.
Second line system of electromagnetic flowmeter power supply design ideas are also divided into zero signal output current (4mA) supply, greater than zero signal output supply and battery (or solar cell) supply.
Battery powered electromagnetic flowmeter and the electromagnetic meter adapted to the location of the city from the city with a pool or a sewage treatment after the discharge point, etc.. Some types of meter battery life of a year, some 8 to 10 years.
1993 Japan's revised measurement method provides DN350mm the following trade shows that the use of a variety of water meters from the traditional model of the rotor meter to expand the meter. In the future as a water meter electromagnetic flowmeter to comply with the requirements of standard water meter, measuring accuracy only 2% ~ 5%, but electromagnetic flowmeter body must to display and roommates with integrating external power supply. For example, SU series electromagnetic meter motor power meter, 2mW, built-in battery available for 8 years.
Three, electromagnetic flowmeter by sensor and converter assembly method
(1), of the type
Split type electromagnetic flowmeter is the most commonly used form, as shown in the illustration, sensor access pipeline, transducer mounted on the most easy close to the vicinity of the sensor of the meter room or people apart tens to hundreds of meters. In order to prevent the outside noise intrusion, the signal cable is usually used double layer shielding. When the electric conductivity is lower than 30m, the signal is attenuated, and the inner layer is also required to be shielded from the low impedance of the core wire. Separation type converter can be far from the harsh environment, electronic components inspection, adjustment and parameter setting is more convenient.
(2), a body
The sensor and the converter are assembled together in a direct output DC current (or frequency) standard signal, in fact, become an electromagnetic flow transmitter. The size of a relationship length of connection between the two signal lines and the exciting line, and the external, hidden inside the instrument, thereby reducing signal attenuation and electromagnetic wave noise invasion (see chart 1). The same measurement circuit and the sub body can measure the low conductivity of the liquid. Cancel the signal line and the exciting line of wiring, simplified electrical connections, meter price and installation costs are relatively cheap, more use of tubular diameter instrument. With the development of the second tier system, a meter will have a rapid development. But if due to pipe layout restrictions, the installation is not easy to close to the site, it is difficult to maintain. In addition, the electronic components of the converter are arranged on the pipeline, which will be limited by the fluid temperature and the vibration of the tube.
Four, electromagnetic flowmeter by flow sensor and pipe connection mode
(1) flange connection
The traditional connection mode of the flange connection, the two ends of the sensor are connected with the flange, and the flange of the pipe is fixed with bolts, which can be installed in one way. Large diameter sensors are used in this connection. The volume and weight are more than that of the clip.
DN15 ~ 600mm electromagnetic flow meter of the two flange length, the international standardization organization has been developed standards (ISO13359; 1998) to be unified.
(2) clip connection
In recent years the development of the connection mode, the sensor itself has no flange, with a longer bolt clamp in the pipeline between the two pipe line. This method is small and light weight, and is suitable for different pressure and standard pipe flanges, but only suitable for small tube diameter (200mm), which is lower than the working pressure of liquid.
(3) sanitary connection
The Chinese health type connecting a case, and the International Dairy Federation IDF blessing is connected, to achieve the rapid mounting and dismounting, easy to frequent daily cleaning.
(4) screw connection
Below the Chinese style small caliber meter thread connection. This kind of instrument is more applied in medicine, food and other industrial and pharmaceutical proportion of injection and other places. There are more application in the screw connection of oil, geological exploration, etc. 16 ~ 25MPa above high pressure water injection or cement slurry flow measurement, the shape of the screw is trapezoidal.
Five, the electromagnetic flow meter according to the flow sensor electrode and the measured liquid contact
(1) contact electrode
The contact with the liquid is the traditional structure of EMF, usually a pair of electrodes, large diameter instrument industry useful two pairs of electrodes. Non full pipe type EMF is also useful for 3 pairs of motors or strip electrodes.
(2) non contact electrode
A large area electrode is attached to the lining (or insulated measuring tube), and the discharge signal is detected by the capacitance coupling method. The electromagnetic flowmeter, which can measure the capacitance detection method of the contact electrode, is referred to as the capacitance type electromagnetic flowmeter. A sensor pre amplifier by electrode, excitation frequency is usually high EMF, 50/2Hz, there are more than 100Hz. This kind of instrument does not produce electrode passivation oxidation and catalysis, etc. the effect of electrode surface noise, also there is no opportunity to flow noise
