Since 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 large-diameter electromagnetic flowmeters used in the field. The accuracy verification of the electromagnetic flowmeter is also very useful for the management of the electromagnetic flowmeter, ensuring its accuracy and reliability, accumulating the original comparison data, and doing future verification and verification. The accuracy verification of the electromagnetic flowmeter can use the clean water volume and the electromagnetic flowmeter calibration equipment.
Comprehensively verify the accuracy of the electromagnetic flowmeter to determine the accuracy of the electromagnetic flowmeter in the water plant application process to ensure that the measurement data is true and reliable or whether to replace the electromagnetic flowmeter.
1. Use visual method and instrument method, use GS8 to check whether the resistance of the excitation coil of the sensor, the insulation resistance between the signal lines, the grounding resistance and other items meet the pre-factory standards, and whether the zero point and output current of the electromagnetic flowmeter converter meet Accuracy requirements. The specific detection method is:
(1) Measure the resistance of the excitation coil to determine whether there is a short-circuit phenomenon between the turns of the excitation coil (the resistance value between the measuring line number "7" and "8"), 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 indirectly assesses that the magnetic field strength of the electromagnetic flowmeter sensor has not changed.
(2) Measure the insulation resistance of the excitation coil to the ground (wire number "1" and "7" or "8") to determine whether the sensor is damp, the resistance value should be greater than 20 megohms.
(3) Measure the resistance value of the contact between the electrode and the liquid (line numbers "1" and "2" and "1" and "3"), and indirectly evaluate the general condition of the surface of the electrode and lining layer. Such as whether the electrode surface and the lining layer are attached to the deposited layer, whether the deposited layer has conductivity or insulation. The resistance value between them should be between 1 kiloohm and 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 of signal cable and excitation cable, and check whether the shielding layer is intact.
(6) Use GS8 to calibrate the instrument and test the output current of the converter. When given zero flow, the output current should be: 4.00mA; when given 100% flow, 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 converter terminals "7" and "8"), the positive and negative values ​​of the excitation current should be within the specified range, roughly 137 (5%) mA.
Evaluate the influence of the external environment of the electromagnetic flowmeter on it, such as the laying of the excitation line and the signal line in the same pipeline, the excitation line and the signal line parallel to the high-voltage cable, and the surrounding large transformer or motor. This assessment mainly uses a visual method to observe whether the electromagnetic flowmeter in operation is abrupt or fluctuating, and roughly judge whether the electromagnetic flowmeter is interfered by electromagnetic waves or other stray waves or whether there are 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 common tools.
2. Verification by the clear water volume method:
The verification of the measurement accuracy of the electromagnetic flowmeter of the factory water factory adopts the clear water volume method, which is one of the methods often used by water supply enterprises.
Under the condition of measuring the geometric dimensions of the clear water pool accurately and reducing various operating errors, a higher comparison reference effect can be obtained. The principle of the clear water volume method is to measure the actual space plane size of the clear water tank and the suction well with a high-precision steel ruler, and accurately calculate the actual flat area of ​​the clear water tank and the suction well. First adjust the water level of the clear water tank to a higher water level, and close all outlet valves.
After the water level of the clear water tank is stable, the water level transmitter of the clear water tank and a high-precision steel ruler are used to manually and accurately measure the water level of the clear water tank and the suction well. In order to correct the error caused by the leakage of the clear water tank and other valves, the water level of the clear water tank and the suction well is measured again after a certain time, and the amount of water leakage per unit time is calculated in order to correct the water metering and reduce the error. The cumulative flow of the electromagnetic flowmeter to be verified is recorded, and the purpose of manually measuring the liquid level of the clear water tank and the suction well is to verify the accuracy of the liquid level transmitter. Then turn on the water pump, turn on the water outlet valve, and after a certain time, close the water outlet valve to stop the water pump.
When the water level of the cleaning tank is stable, use the liquid level transmitter of the cleaning tank again and manually measure the water level of the cleaning tank and the suction well with high precision steel ruler, record the water level of the cleaning tank and the suction well again, and record the cumulative flow of the electromagnetic flowmeter to be verified . Finally, the water level height difference ⊿h of the clear water tank and the suction well is calculated, so that the actual water volume of the clear water tank and the suction well is calculated. The actual water volume 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, and subtract the cumulative amount of the electromagnetic flowmeter from the actual water volume of the clear water pool to obtain the error between them, thereby verifying the accuracy of the measurement system of the factory-made water electromagnetic flowmeter. The measurement accuracy of the factory water electromagnetic flowmeter using the clear water volume method must be verified when the state of the clear water tank is completely static, so that the data obtained is more accurate. Calculated as follows:
E = (Q standard—Q instrument) / Q standard × 100%
In the formula: E is the error value between the two;
Q is the volume calculated by the difference in the drop height of the clear water tank;
Q meter is the flow value accumulated by the flowmeter during verification.
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