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Electromagnetic flowmeter inspection contents note introduction

The application of electromagnetic flowmeters in the industry is mainly based on flow control, and the measured fluids are mostly corrosive and abrasive. In practical applications, the failure of the electromagnetic flowmeter is mostly caused by corrosion leakage, insulation degradation, electrode contamination or foreign matter adhesion. The purpose of the daily inspection of the electromagnetic flowmeter is to ensure or prove that the electromagnetic flowmeter is operating under controlled conditions. The daily inspection methods generally include online inspection and offline inspection, mainly to verify whether the flow measurement value of the electromagnetic flowmeter meets and maintains the expected measurement requirements. The main inspection contents are: checking the electromagnetic flowmeter, in addition to the zero point check, the flow sensor, the converter and the connecting cable are separately separated.
First, the connection cable check
The inspection content is to check the conduction and insulation resistance of each core of the signal line and the excitation line, and check whether the grounding of each shielding layer is intact.
Second, the converter check
The inspection consists of using a universal instrument and an analog signal that matches the flowmeter model to provide a flow signal for zeroing and calibration. Calibration includes zero check and adjustment, setpoint check, field current measurement, current/frequency output check, and more. It should be noted that the inspection item should be compared with the last inspection value (or factory value) to analyze whether it has changed or changed to meet the original measurement requirements.
Third, the whole machine zero check
The technical requirements for the zero check of the whole machine are: the flow sensor measuring tube is full of liquid and no flow, which is not suitable for many enterprises and abandons the zero check and adjustment of the whole machine, but can turn to the converter for separate zero check and Adjustment. Technically, this must be meaningful after the sensor has been checked and the insulation resistance of the sensor excitation circuit and signal circuit is normal (both cables are included), otherwise the whole machine will not operate normally. Usually, the zero point of the converter is negative and the value is small. If the absolute value is greater than 5% of the full scale, it needs to be checked first, and then the adjustment is made after confirming the cause. Usually, the zero point of the electromagnetic flowmeter and the individual zero difference of the converter are less than 1%. There are many cases where the zero difference value is greater than 5% due to the user's incorrect zero adjustment when the pipeline valve is closed.
Fourth, flow sensor check
The inspection content is: indirectly evaluate whether the magnetic field strength changes by checking the excitation coil and checking the excitation current measured by the converter; measuring the electrode liquid resistance to evaluate the contamination of the electrode surface and the adhesion layer of the lining; Insulation resistance to determine the degree of component degradation to assess whether interference is introduced. The thickness of the electrode and liner adhesion layer can be observed and measured for the pipeline that can stop the flow of the medium to estimate the change in flow value introduced by the change in flow area before and after cleaning the adhesion layer.
(1) Measuring the excitation coil copper resistance
Measure the coil resistance with a high-precision digital multimeter or Wheatstone bridge, and if necessary, compare the temperature coefficient with the meter file value. Check that the coil is conducting well and there is no inter-turn short circuit.
(2) Check the insulation resistance of the excitation coil
After the excitation coil and its terminals are wetted, the excitation circuit is grounded and the insulation is lowered. It is very likely that the excitation signal is introduced into the flow signal transmission circuit, so that the electrode is added with a large insulation resistance and the signal resistance is divided by the excitation voltage to form a larger Common mode interference signal. When this interference signal exceeds the suppression capability of the converter preamplifier, the converter will drift at zero. When the insulation resistance drops not very severe, this phenomenon is not easy to detect when the instrument is running. Except for the IP68 terminal box, in practice, due to negligence, the terminal box is not sealed into moisture, and the terminal insulation resistance is reduced to 5 to 6 MΩ or less. Drying the terminals can usually be eliminated by a fault.
(3) Check the electrode liquid resistance
The electrode fluid resistance of the flow sensor should be measured immediately after the new instrument is commissioned and recorded. After each maintenance measurement, the analysis and comparison of these data can help determine the cause of the instrument failure.
The electrode-to-liquid contact resistance value depends on the measured liquid conductivity of the contact surface. The resistance values measured by different media are significantly different. The electrode wetted resistor can be used to measure the resistance between each electrode terminal and the ground when the measuring tube is filled with liquid by an analog multimeter. Experience has shown that the difference between the contact resistance values of the two electrodes should be less than 10% to 20%, otherwise it indicates a fault.
If the measured electrode contact resistance is different from the original measured value, the reason is: a, the insulation coverage of the two electrodes is inconsistent or the insulation resistance of one electrode signal circuit is decreased; b, the increase of the resistance value is the surface of the electrode is insulated Layer coverage; c, the reduction of the resistance value is the adhesion of the conductive deposition layer or the electrode assembly (such as the insulating collar) on the lining surface near the electrode. Sometimes there is no failure, but it should be taken as a precursor to failure.
(4) Measuring electrode/liquid polarization voltage
Measuring this voltage will help to determine if the electrode is dirty or covered, which may result in a zero point instability or output sloshing fault.
(5) Check the insulation between the signal circuit insulation and the excitation circuit/signal circuit
The purpose of this inspection is to assess whether interference is introduced due to a drop in insulation. When checking the signal circuit, the signal line should be temporarily disconnected from the electrode. The cause of the insulation drop is that the junction box is not sealed into the moisture, and the cable of the protective sensor is cut and not moisture-proof.
(6) Check the insulation resistance and lining condition of the electrode
The inspection shall be carried out from the pipeline for the small-diameter instrument. For the large-diameter instrument, the effusion may be emptied and then enter the pipeline from the inlet hole. The inner surface of the lining is wiped and the insulation resistance of the two electrodes is tested with a megohmmeter; If there is an adhesion layer, the cleaning cycle shall be removed and the thickness of the laminate shall be determined; if the adhesion layer is not thick and the conductivity is the same as the liquid, the additional error of the area change may be neglected; if the conductivity of the adhesion layer is less than the liquid, a positive additional error will be generated, and vice versa. Then a negative additional error is generated.

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