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Electromagnetic Flow Meter, Five Steps To Solve The Zero Instability Phenomenon Analysis

Dec 10, 2018

Electromagnetic flowmeter, five steps to solve the zero instability phenomenon analysis

Intelligent electromagnetic flowmeter,five steps to solve the zero instability phenomenon analysis

First, let's see the structure of electromagnetic flowmeter:

The structure of electromagnetic flowmeter is mainly composed of magnetic circuit system, measuring pipe, electrode, shell, lining and converter.

Electromagnetic flow meter, five steps to solve the zero instability phenomenon analysis

Magnetic circuit system: its function is to produce a uniform DC or AC magnetic field.Dc magnetic circuit with magnets, its advantage is simple structure, small influence of the AC magnetic field, but it is easy to make by measuring the electrolyte liquid polarization in the ducts, the positive electrode is surrounded by negative ion, negative electrode is surrounded by positive ions, the electrode polarization, and lead to increased internal resistance between two electrodes, thus seriously affect the normal work of the instrument.When the pipe diameter is large, magnet is also large, bulky and uneconomic, so the electromagnetic flowmeter generally USES alternating magnetic field, and is generated by 50HZ power frequency power supply.

Measuring catheter: the function of the measuring catheter is to let the conductive liquid pass through.In order to make the magnetic field line be shunt or short-circuit the magnetic flux when passing through the measurement catheter, the measurement catheter must be made of non-magnetic conductivity, low conductivity, low thermal conductivity and material with a certain mechanical strength, and non-magnetic stainless steel, glass fiber reinforced plastic, high-strength plastic, aluminum and so on.

Electrode: the function of the electrode is to elicit an induced potential signal proportional to the measured one.The electrodes are generally made of non-magnetic stainless steel and are required to be flush with the lining so that the fluid can pass through unimpeded.It should be installed in the vertical direction of the pipe to prevent sediment accumulation on it and affect the measurement accuracy.

Enclosure: made of ferromagnetic material, it is the outer enclosure of the excitation coil of the distribution system and isolates the interference of external magnetic field.

Lining: a complete electrical insulating lining is provided on the inside of the measuring pipe and on the flange sealing surface.It directly touches the liquid under test, and its function is to increase the corrosion resistance of the measuring pipe and prevent the inductive potential from being short-circuiting by the metal measuring pipe wall.Lining materials for corrosion resistance, high temperature resistance, wear resistance of polytetrafluoroethylene plastics, ceramics and so on.

Converter: the induction potential signal generated by liquid flow is very weak, which is greatly affected by various interference factors. The role of the converter is to amplify the induction potential signal and convert it into a unified standard signal and suppress the main interference signal.Its task is to amplify the induction potential signal Ex detected by the electrode and convert it into a unified standard dc signal.

Next, we use five steps to solve the zero-point instability phenomenon. When analyzing industrial production, we often encounter the zero-point instability of electromagnetic flowmeter.There are many reasons for this problem. According to the experience of Huaheng instrument technicians for many years. There are mainly the following factors:

1. The pipe is not filled with liquid or the bubbles in the liquid objectively assume that there is no activity in the flow sensor, but in practice there is micro activity. 

2, The use of the valve for a long time or liquid pollution makes the valve airtight incomplete will also lead to zero point instability. 

3, Liquid conductivity change or uneven, at rest will make zero point change, make the output shaking when moving. 

4. Due to the impossibility of complete symmetry between the scale formation on the inner wall surface and the electrode contamination level, the equilibrium of the initial zero setting is destroyed. 

5. The change of the state of power equipment near the left of the flow sensor (such as the increase of leakage current) constitutes the change of grounding potential, which also causes the zero point change of the intelligent electromagnetic flowmeter.Through the analysis of the problem, we can find the corresponding solution according to the corresponding phenomenon.

ⅰ.Firstly, check the unstable flow of zero-point intelligent electromagnetic flowmeter. Firstly, make a preliminary investigation and judgment according to the overall consideration of the flow, and then carefully check and try to troubleshoot each item.

(1) it can be understood by observation or inquiry that the operation is not required to be large before, that is, easy before difficult; 

(2) according to the past on-site maintenance experience, the one with higher occurrence frequency and higher possible occurrence probability in the future is the first; 

(3) check the sequence of requirements required by the inspection itself. If the reasons for the faults are confirmed after preliminary investigation, detailed inspection can be carried out in advance.

ⅱ.Second, imperfect intelligent electromagnetic flowmeter sensor according to the ground by the stray current interference, such as pipeline stray current interference effects such as main electromagnetic flowmeter good grounding protection, usually requires grounding resistance less than 1000, don't and he Shared grounding electric machines and appliances. Sometimes better environmental conditions, intelligent electromagnetic flowmeter earthing can not work normally, but once the good environment does not exist, the instrument will fail, then checked again will bring much trouble flow sensor near wall power equipment state changes (such as leakage current increase) formed by the ground potential changes, electricity will cause the intelligent electromagnetic flowmeter zero change.

ⅲ. Reasons for the liquid aspect of the intelligent electromagnetic flowmeter (such as the uniformity of liquid conductivity, electrode pollution, etc.);In static liquid conductivity changes or uneven make zero changes, shook the output flow. So the flowmeter position should be far away from the injection solution point or pipe section downstream of the chemical reaction, good flow sensor is installed in the upstream of the sites. If the liquid containing solid phase, or the sediment measurement tube inner surface, or on the inner wall of the measuring tube fouling, or electrode by grease filth and so on, all have zero changes may occur. Because the lining surface scale and degree of filth and electrode can't completely the same and symmetrical, destroy the balance of plant initial die set. Positive steps foot remove filth and scale deposit layer;If the zero gang changes little, you can also try to reset the zero.

ⅳ. electromagnetic flowmeter signal circuit insulation. Insulation fall will form the zero signal circuit. Soil to reason for the declining signal circuit insulation of foot, package parts drop caused by the insulation, but also can't rule out the signal cable and terminal decline or insulation damage. Because the environment is very harsh, sometimes slightly one careless negligence meter cover/wire joint sealing, filled with moisture acid mist or powder dust intrusion meter connector or cable protection layer, insulating bligh. Check respectively according to the signal circuit insulation resistanceTwo posts on the cable side and one on the flow sensor side shall be tested with a megohm meter. As the signal cable is easy to do first, the flow sensor shall be tested in two parts: liquid filled velider surface contact resistance on the electrode and one pipe after measurement,U electrode insulation resistance.

Electromagnetic flow meter, five steps to solve the zero instability phenomenon analysis

ⅴ.Check electrode contact resistance and electrode insulation resistance in two steps.

(1) the liquid contact resistance flow sensor on the surface of the liquid-filled measurement electrode discharges the signal cable wiring, and USES a multimeter to measure the resistance between each electrode and the connection point, respectively. The left of the value of the two electrodes to the geoelectric major shall be 10%-20%. The "measurement of one/electrode contact resistance" in section 9 will be further explained.

(2) the atc measuring electrode insulation vent pipe, wipe with dry cloth in the inner surface, after being completely dry, use H500VDC megohmmeter measure between the electrode and ground resistance, resistance must be more than 100 m Ω. In general, through the above five steps of the inspection and operation, basic can solve zero unstable situation.

Know more about the flow meter!