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Comprehensive verification of the accuracy of electromagnetic flowmeters to ensure the authenticity of the measurement data
2012-3-14 9:15:20


How to maintain and maintain the electromagnetic flowmeter in a stopped state

在停机即不使用的时候,我们要对电磁流量计采取科学合理的维护和保养方法。 When the electromagnetic flowmeter is not used when it is stopped, we must adopt scientific and reasonable maintenance and maintenance methods for the electromagnetic flowmeter. In providing electromagnetic flowmeter technical services to users, users often encounter inaccurate measurement data after purchasing the electromagnetic flowmeter for a period of use. In fact, this is not a quality problem of the electromagnetic flow meter itself. Because the user does not follow the post-use care and maintenance methods in the manual.

I. Zero check and adjustment of electromagnetic flowmeter maintenance:

Before the electromagnetic flowmeter is put into operation, the zero point must be adjusted after the electromagnetic flow sensor is full of liquid after the power is turned on. After being put into operation, it is necessary to periodically check the zero point according to the conditions of use. Especially for precipitates and easily polluted electrodes, non-cleaning liquids containing solid phases should be checked at the beginning of operation to obtain experience to determine the normal inspection cycle. The AC magnetic field flowmeter and rectangular wave excitation ratio are more likely to produce zero drift, so more attention should be paid to inspection and adjustment. Take two examples of application errors where the deposits have failed. One is that in the oil drilling and cementing project, the total flow of cement slurry is an important process parameter, and high pressure electromagnetic flow meters are often used. Use the meter intermittently. Rinse the sensor measuring tube with clean water after use. The remaining time is empty. Due to incomplete cleaning, the residual cement slurry on the inner wall of the measuring tube has solidified into a thin layer, which has accumulated in the past two months to form an insulating layer, covering the entire electrode surface, resulting in abnormal operation and eventually failure to work. The other is an electrolytic cutting process inspection device, which uses an electromagnetic flowmeter to control the flow of saturated saline solution. After a period of use, the flow signal is gradually weakened, and the signal is zero after 2 months. The reason is that iron oxide deposits on the tube wall during electrolytic cutting, resulting in a short circuit. Clearing the stratum immediately returned to normal.

Second, the periodic inspection of the electrical performance of the electromagnetic flowmeter maintenance sensor:

First, roughly measure the resistance between electrodes. Disconnect the signal connection between the sensor and the converter, the sensor is filled with liquid, and use a multimeter to measure the resistance value of the two electrodes and the ground terminal, whether it is within the range specified by the manufacturer, and the measured two values are substantially the same. Record the resistance value measured for the first time. This value is useful for later determining the cause of the sensor failure (such as whether the deposited layer is conductive or insulating). Secondly, drain the sensor from the liquid and wipe the inner wall. After it is completely dry, use a megohmmeter to measure the resistance between the two electrodes and the ground terminal. Finally, check the insulation resistance of the excitation coil, remove the sensor excitation coil, wire the terminal indirectly to the converter, and measure the insulation resistance of the coil with a megohmmeter


必须是在线连续使用,几乎不可能拆除再运输到国家计量检测中心进行检定。 Because the electromagnetic flowmeter must be used continuously online, it is almost impossible to dismantle and transport it to the National Metrology and Testing Center for verification. Therefore, it is necessary to verify the accuracy of the large-diameter electromagnetic flowmeter used in the field. Accuracy verification of electromagnetic flowmeters It is also very useful for the management of electromagnetic flowmeters to ensure their accuracy and reliability, accumulate original comparison data, and conduct future verification and verification. The accuracy of the electromagnetic flowmeter can be verified by the volume of the clear water tank and the electromagnetic flowmeter. Fully verify the accuracy of electromagnetic flowmeters to determine the accuracy of electromagnetic flowmeters in water plant applications.

1. Use visual inspection method and meter method, and use GS8 to check whether the sensor's excitation coil resistance, insulation resistance between signal lines, ground resistance and other items meet the pre-factory standards, and whether the zero point and output current of the electromagnetic flowmeter converter meet Precision requirements. The specific detection methods are:

(1) Measure the resistance of the excitation coil to determine whether there is a turn-to-turn short-circuit phenomenon in the excitation coil (resistance value between line number "7" and "8"), and the resistance value should be between 30 ohms and 170 ohms. If the resistance is the same as the factory record, the coil is considered good, and the magnetic field strength of the electromagnetic flowmeter sensor is indirectly assessed to have not changed. (2) Measure the insulation resistance of the excitation coil to ground (line number "1" and "7" or "8") to determine whether the sensor is wet, and the resistance value should be greater than 20 megohms. (3) Measure the contact resistance between the electrode and the liquid (measurement line numbers "1" and "2" and "1" and "3"), and indirectly evaluate the general condition of the surface of the electrode and the backing layer. Such as whether the electrode surface and the lining layer are attached to the deposited layer, whether the deposited layer is conductive or insulating. The resistance value between them should be between 1 kohm to 1 megohm, and the resistance values of the wire numbers "1" and "2" and "1" and "3" should be roughly symmetrical. (4) Close the valve on the pipeline, and check the zero point of the whole machine when the electromagnetic flowmeter is full of liquid and no liquid flows. Make appropriate adjustments as appropriate. (5) Check the insulation resistance of each core wire of the signal cable and the excitation cable, and check whether the shielding layer is intact. (6) Use the GS8 calibration instrument to test the output current of the converter. When zero flow is given, the output current should be: 4.00mA; when 100% flow is given, the output current should be: 20.00mA. The error of the output current value should be better than 1.5%. (7) Test the excitation current value (between the converter terminals "7" and "8"). The positive and negative values of the excitation current should be within the specified range, which is approximately 137 (5%) mA.

Evaluate the impact of the external environment of the electromagnetic flowmeter, such as laying the same line as the excitation line and signal line, paralleling the excitation line and signal line with high-voltage cables, and surrounding large transformers or motors. This evaluation mainly uses the visual inspection method to observe whether the electromagnetic flowmeter is abrupt or fluctuating during operation to roughly judge whether the electromagnetic flowmeter is interfered by electromagnetic waves or other stray waves or whether there are air bubbles in the pipeline. The instruments and tools required for the verification of the electromagnetic flowmeter itself: one GS8, one 4-1 / 2 multimeter, one 500V megohmmeter, one pointer multimeter and commonly used tools.

2. Clear water volume method verification:

计量精度的验证采用清水池容积法,是供水企业经常采用的方法之一。 The verification of the measurement accuracy of the electromagnetic water flowmeter manufactured by the water factory adopts the clear tank volume method, which is one of the methods often used by water supply enterprises.

Under the condition that the geometric size of the clear water tank is measured accurately and the operation errors are reduced, a higher comparison reference effect can be obtained. The principle of the clear water tank volume method is: use high-precision steel ruler to measure the actual space plane size of the clear water tank and the suction well, and accurately calculate the actual planar area of the clear water tank and the suction well. First set the clear water level to a higher level and close all water outlet valves. After the water level of the clear water tank is stable, use the clear water level transmitter and manually measure the water level of the clear water tank and suction well with high precision steel ruler. In order to correct the error caused by the leakage of valves such as the clear water tank, the water level of the clear water tank and the suction well is measured again after a certain period of time, and the water leakage per unit time is calculated in order to correct the water measurement and reduce the error. Record the accumulated flow of the electromagnetic flowmeter to be verified. The purpose of manually measuring the level of the clear water tank and the suction well is to verify the accuracy of the level transmitter. Then turn on the water pump and the water outlet valve. After a certain period of time, close the water outlet valve to stop the water pump. After the water level of the clear water tank is stable, the clear water tank and suction wells are manually measured with a high-precision steel ruler, and the water level of the clear water pool and the suction well is recorded again, and the cumulative flow of the electromagnetic flowmeter to be verified is recorded. . Finally, the water level difference ⊿h between the clear water tank and the suction well is calculated, so as to calculate the actual water volume of the clear water tank and the suction well. The actual water amount is equal to the height difference ⊿h times the plane area and the corrected water volume. Then calculate the water volume of the electromagnetic flowmeter to be verified, subtract the accumulated volume of the electromagnetic flowmeter from the actual water volume of the clear water tank, and get the error between them, so as to verify the accuracy of the metering system of the factory water electromagnetic flowmeter. The clear water volume method is used to verify the measurement accuracy of the electromagnetic water flowmeter of the factory water and the factory water when the state of the water tank is completely static, so the data obtained is more accurate. Calculated as follows:

E = (Q standard—Q instrument) / Q standard × 100%

Where: E is the error value between the two;
Q is the volume calculated for the difference in the height of the clear water tank;
Q meter is the flow value accumulated by the flowmeter during verification.