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Dec 13, 2018

# How to realize quantitative control of electromagnetic flowmeter

In order to realize the batch, quantitative control of electromagnetic flowmeter, we must first understand the main content of this function, only in this way can we better realize its function.The so-called batch, quantitative control function, in fact, refers to the flow from 0 accumulated to a set value, the flow meter output a switch signal, and the user can use the signal to complete the specific control function.How do you do this? ## Structure of electromagnetic flowmeter

Eletromagnetic Flowmeters (EMF) are new type of Flowmeters which rapidly develop with the development of electronic technology in the 1950s and 1960s.Electromagnetic flowmeter is made according to Faraday's law of electromagnetic induction, which is used to measure the volume flow of conductive liquid.Due to its unique advantages, electromagnetic flowmeter has been widely used in the industrial process of various conductive liquid flow measurement, such as a variety of acid, alkali, salt and other corrosive media;Electromagnetic flow meter various slurry flow measurement, forming a unique field of application.

Structurally, the electromagnetic flow meter consists of electromagnetic flow sensor and converter.The sensor is installed in the industrial process pipeline. Its function is to convert the liquid volume flow value into the induced potential signal linearly and send the signal to the converter through the transmission line.Located not too far from the sensor, the converter amplifies the flow signal sent by the sensor and converts it into standard electrical signal output proportional to the flow signal for display, accumulation and adjustment control. ## Working principle of electromagnetic flowmeter

Electromagnetic flowmeter is a flowmeter for flow measurement according to Faraday's law of electromagnetic induction.The advantage of electromagnetic flowmeter is that the pressure loss is very small and the flow range can be measured is large.The ratio of maximum flow rate to minimum flow rate is generally more than 20:1, the applicable industrial pipe diameter range is wide, the maximum can reach 3m, the output signal and measured flow rate is linear, the accuracy is high, the electrical conductivity is greater than 5 microns /cm acid, alkali, salt solution, water, sewage, corrosive liquid and mud, pulp, pulp and other fluid flow.But it cannot measure the flow of gas, steam or pure water.

When the conductor cuts the magnetic force line movement in the magnetic field, the induction potential will be generated in the conductor. The magnitude of the induction potential is proportional to the effective length of the conductor in the magnetic field and the speed of the conductor in the magnetic field moving perpendicular to the magnetic field.Similarly, when conducting fluid flows vertically in a magnetic field and cuts magnetic induction lines, it will also generate induction potential on the electrodes on both sides of the pipe.The direction of the induced potential is determined by the right hand rule, and the magnitude of the induced potential is determined by the following formula:

Ex=BDv---------------- -- -formula (1)

Where Ex -- induced potential, V;B -- magnetic induction intensity, TD -- pipe diameter, mv -- average liquid velocity, m/s

However, volume flow qv is equal to the product of the velocity v of the fluid and the product D squared PI over the area of the pipe. Substitute equation (1) into this equation to get:

Qv=(PI /4B)* Ex ---- PI (2).

As can be seen from the above equation, when the pipe diameter D has been determined and the magnetic induction intensity B remains unchanged, the measured volume flow has a linear relationship with the induced potential.If an electrode is inserted on each side of the pipe, an induced potential Ex can be introduced. The magnitude of this potential can be measured and the volume flow can be obtained.

According to Faraday electromagnetic induction principle, with the sounding pipe axis and the magnetic field lines in the vertical tube wall installed on a pair of detecting electrodes, when conducting liquid along the measuring tube axis movement, conductive liquid cutting lines of magnetic force induced electric potential, the induction electric potential by two detection electrodes detection, numerical size and velocity of proportional, its value is:

E = B, V, D, K

Where: e-induced potential;

K- coefficient related to magnetic field distribution and axial length;

B- magnetic induction intensity;

V- average velocity of conductive liquid;

D- electrode spacing;(measuring pipe diameter)

The sensor takes the induced potential E as the flow signal and transmits it to the converter. After signal processing such as amplification, transformation and filtering, the sensor displays the instantaneous flow and cumulative flow with the point-matrix liquid crystal with backlight.Converter has 4~20mA output, alarm output and frequency output, and rs-485 communication interface, and support HART and MODBUS protocol. ## Achieve batch (quantitative) control function

Batch (quantitative) control function refers to the flow from 0 accumulated to a set value, the flow meter output a switch signal.Users use this signal to perform specific control functions.

1. Interface signal of electromagnetic flowmeter

A) we use batch (quantitative) control AH and COM port input signals, when the two terminal short circuit (switch closure), resource control (Σ S) reset, the total response time for 300 ms, that is, the user should be given the control signals of the time width is greater than 300 ms.

B) batch (quantitative) control USES AL end as the output control signal, and the common end of the signal is COM.

2. Setting parameters of electromagnetic flowmeter

When users use batch (quantitative) control function should be set in the parameter Settings, "numerical control total" parameter, numerical control points should be the total value of its maximum value can be set to 59999 "special note, the value is 59999, not represent a certain unit of a single value of 59999 m3 or 59999 l, 59999 valid number but also is to control total Σ S after five, so special attention should be paid to enterprise user Settings, set the value of some reasonable."When the control total amount (Σ S) reaches this value, AL output low level.

3. Operation control of electromagnetic flowmeter

Users first use AH will control the total input control end (Σ S) reset 【 if switch remained closed, the total control (Σ S) reset 】, at this moment, AL is in a state of high level (switch).If AH input control the input high level (switch), control (Σ S) start the accumulative total, after reaching the set value, AL output low level (switch is closed), complete one cycle control.

4. The communication software specification for the electromagnetic flowmeter is l-mag-p-51, which is a special command communication for batch control communication in a single machine.When the total control amount is cleared, the total control time and numerical specifications start to accumulate. When the value set by the total control amount is reached, the total control time and numerical value stop

The upper and lower alarm functions of the instrument have no output function except display.