Empty pipe detection and alarm of electromagnetic flow meter

Abstract: The empty pipe detection and alarm information of electromagnetic flowmeters are provided by excellent flowmeter and flowmeter manufacturers. When discussing the principle of electromagnetic flowmeter, it was mentioned that a measurement condition is that the conductive liquid needs to fill the measuring tube. This is because the area of ​​the dissatisfied pipeline is uncertain, and the correct flow rate actually flowing through the measuring tube cannot be obtained. kind of interference letter. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the details of the empty pipe detection and alarm article of electromagnetic flowmeters. When discussing the principle of electromagnetic flowmeter, it was mentioned that a measurement condition is that the conductive liquid needs to fill the measuring tube. This is because the area of ​​the dissatisfied pipeline is uncertain, and the correct flow rate actually flowing through the measuring tube cannot be obtained. an interfering signal. This kind of unreal measurement will cause trade disputes as measurement, and will cause adjustment and control errors as production process control. Therefore, in the intelligent electromagnetic flow converter, the empty pipe detection circuit is often designed to alarm the sensor to measure the non-full state of the pipe. There are many ways to detect empty pipes. The simplest and most reliable method for empty pipe detection is to use liquid level detection. However, this method requires the simultaneous installation of a liquid level sensor on the measuring tube. Therefore, the cost of the measuring instrument is increased, and the installation is troublesome. Generally, it is rarely used. Even if this method is used, it is only alarmed by the liquid level contact. In the following, the method of measuring the conductivity of the liquid will be used to introduce the detection of empty pipes, which can also indicate the relative conductivity of the liquid to a certain extent. Figure 4-56 shows the principle block diagram of the hollow tube alarm of the domestic L-mag electromagnetic flow converter. Figure. E, is the high-frequency AC power supply (v) supplied to the conductance measurement circuit; R is the voltage divider resistance (Q) used to measure the conductance; Rx is the resistance between the electromagnetic current and the sensor electrode and the fluid (signal ground), usually We call it the internal resistance of the signal. The relationship between the size of Rx and the fluid conductivity d (unit is S/m, S=1/n) is: using the conductivity measurement to detect the empty pipe, there are two points worth noting. First, the frequency of the AC power source should be selected to be different from the flow signal frequency, and no additional polarization voltage should appear on the electrodes. So 2000Hz AC power can be used. Secondly, the use of electrodes to detect the empty pipe state must not conflict with the sampling timing of the flow signal at timing seven. Empty pipe detection typically uses a time that is backwards from the leading edge of the magnetic field waveform, but comes into the flow sampling time. The sampling time of the flow signal of the electromagnetic flowmeter is from the trailing edge of the waveform forward, leaving the integral transition period. zoom in y. Compared with the set high voltage E, the voltage is a monostable mode of operation. Considering that the conductivity of the fluid medium has a wide application range, that is, the voltage v. The range is relatively wide, the dynamic range of the monostable is too large, and the alarm will be unreliable. So the application software sets a“Empty Pipe Span Correction”Correction coefficient is used to adjust the gain of the amplifier, so that the voltage v with different conductivity is detected, and after being amplified by the amplifier I, a voltage value with a relatively close output amplitude is obtained. This is similar to the range adjustment of instrument measurement. due to v. Inversely proportional to conductivity, the detected conductivity ratio and conductivity are non-linear. so,“Empty Pipe Span Correction”The correction factor and the displayed conductance are also non-linear. Thus, in different“Empty range correction”Under the coefficient, there is a phenomenon that the auxiliary and displayed conductivity values ​​vary widely, and the setting range of the threshold value may be smaller than the measured conductivity ratio value range, so it is easy to cause false alarms for empty pipes. (l) Empty pipe range correction“ATC limit correction”The larger the value of , the lower the conductance ratio displayed by the detection;“Empty Pipe Span Correction”The smaller the value of , the higher the conductance ratio displayed by the test. Therefore, for the detection of commercial conductivity, use a small“Empty Pipe Span Correction”value. For low conductivity detection of electromagnetic flowmeter, it is necessary to use high“Empty Pipe Span Correction”value, taking the conductivity of tap water with conductivity higher than about 50 μS/crn as an example,“Empty Pipe Span Correction”The value is set to 1. oooo, the detected conductance ratio can fully meet the threshold value of 999.9%. For 12 higher conductivity, you can reduce“Empty Pipe Span Correction“value or lower the threshold to improve the detection sensitivity i and for fluids with low conductivity f50μS/cm, it is necessary to“Empty Pipe Span Correction”Set the value to a large value, say 3.0000, and set the threshold to 999.9%. At this time, reduce the displayed conductance ratio to make it smaller than the threshold value to prevent false alarms when there is fluid. (2) The actual detection setting needs to be explained. The data in Figure 4-58 was obtained with no transmission cable connected. Practical application due to the existence of distributed capacitance between the signal line and the ground line of the transmission cable, the capacitive reactance of the distributed capacitance is reduced by V1. value. In this way, when a long cable is connected, the displayed conductance ratio will be lower than the actual conductance ratio. Just now, the actual empty pipe detection setting should be adjusted first when the fluid is full of pipes.“Empty Pipe Span Correction”value, so that the actual conductance ratio of silicon is shown as an appropriate value, and then set the threshold according to the value of 3 to 5 times the measured conductance ratio. This results in the best settings for sensitivity and reliability.
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