How to verify the electromagnetic flowmeter of electronic measuring instrument

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 calibration. Therefore, it is necessary to verify the accuracy of large-caliber electromagnetic flowmeters used on site. The accuracy verification of electromagnetic flowmeters is also very useful for the management of electromagnetic flowmeters, ensuring their accuracy and reliability, accumulating original comparison data, and doing future verification and checking. The accuracy verification of electromagnetic flowmeters can use the clear water tank volume and electromagnetic flowmeter calibration equipment .

Comprehensively verify the accuracy of the electromagnetic flowmeter to determine the accuracy of the electromagnetic flowmeter during its application in the water plant, ensure that the metering data is authentic and reliable, or determine whether to replace the electromagnetic flowmeter.

1. Use visual inspection and GS8 to check whether the excitation coil resistance, insulation resistance between signal lines, grounding resistance and other items meet the standards before leaving the factory, and whether the zero point and output current of the electromagnetic flowmeter converter meet the accuracy requirements. The specific detection methods are:

(1) Measure the resistance of the excitation coil to determine whether there is a short circuit between the turns of the excitation coil (measure the resistance between line numbers 7 and 8). The resistance should be between 30 ohms and 170 ohms. If the resistance is the same as the factory record, the coil is considered to be good, and the magnetic field strength of the electromagnetic flowmeter sensor has not changed.

(2) Measure the insulation resistance of the excitation coil to ground (measurement line 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 contact resistance between the electrode and the liquid (measurement line numbers 1 and 2 and 1 and 3) to indirectly evaluate the general surface conditions of the electrode and lining layer. For example, whether there is a sedimentary layer attached to the electrode surface and the lining layer, and whether the sedimentary layer is conductive or insulating. The resistance value between them should be between 1 kilo-ohm and 1 megohm, and the resistance values ​​of line numbers 1 and 2 and 1 and 3 should be roughly symmetrical.

(4) Close the pipeline and check the zero point of the electromagnetic flowmeter when it is full of liquid and there is no liquid flowing. Make appropriate adjustments as needed.

(5) Check the insulation resistance of each core wire of the signal cable and excitation cable, and check whether the shielding layer is intact.

(6) Use GS8 to calibrate and 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 converter terminals 7 and 8). The positive and negative values ​​of the excitation current should be within the specified range, approximately 137 (5%) mA.

Evaluate the impact of the external environment on the electromagnetic flowmeter, such as the excitation line and signal line are laid in the same pipeline, the excitation line and signal line are parallel to the high-voltage cable, and there are large transformers or motors around. This evaluation mainly uses visual inspection to observe whether the electromagnetic flowmeter in operation has sudden changes or fluctuations, 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 are: one GS8, one 4-1/2, one 500V megohmmeter, one pointer multimeter and common tools.

2. Clear water tank volumetric method verification:

The verification of the metering accuracy of the electromagnetic flowmeter for water leaving the water plant adopts the clear water tank volume method, which is one of the methods frequently used by water supply companies.

Under the condition of accurate measurement of the geometric dimensions of the clean water tank and reduction of various operating errors, a higher comparison reference function can be obtained. The principle of the clean water tank volume method is: use a high-precision steel ruler to measure the actual spatial plane dimensions of the clean water tank and the water absorption well, and accurately calculate the actual plane area of ​​the clean water tank and the water absorption well. First, adjust the water level of the clean water tank to a higher water level and close all water outlets.

After the water level in the clear water tank is stable, the water level in the clear water tank and the water well is measured manually and accurately with a high-precision steel ruler using the clear water tank level transmitter. In order to correct the error caused by the leakage of the valves such as the clear water tank, the water level of the clear water tank and the water well is measured again after a certain interval, and the leakage per unit time is calculated to correct the water output measurement and reduce the error. The purpose of recording the accumulated flow of the electromagnetic flowmeter to be verified and manually measuring the liquid level of the clear water tank and the water well is to verify the accuracy of the level transmitter. Then turn on the water pump, open the water outlet valve, and after a certain period of time, close the water outlet valve to stop the water pump.

When the water level in the clean water tank is stable, use the clean water tank level transmitter again and use a high-precision steel ruler to accurately measure the water levels of the clean water tank and the water absorption well, record the water levels of the clean water tank and the water absorption well again, and record the accumulated flow of the electromagnetic flowmeter to be verified. Finally, calculate the water level height difference ⊿h between the clean water tank and the water absorption well, and then calculate the actual water volume of the clean water tank and the water absorption well. The actual water volume is equal to the height difference ⊿h multiplied by the plane area and the corrected water volume.

Then calculate the water volume of the electromagnetic flowmeter to be verified, and subtract the accumulated volume of the electromagnetic flowmeter from the actual water volume in the clean water tank to obtain the error between them, so as to verify the metering system accuracy of the electromagnetic flowmeter for factory water. The verification of the metering accuracy of the electromagnetic flowmeter for factory water using the clean water tank volume method needs to be carried out when the clean water tank is completely static, so that the data obtained is more accurate. The calculation formula is as follows:

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

Where: E is the error value between the two;

Q is the volume calculated by the height difference of the clear water tank;

Q meter is the flow value accumulated by the flow meter during the verification period.