Electromagnetic flowmeter error analysis

Everyone should be familiar with the electromagnetic flowmeter, but it seems to be a simple thing, one head and one circle. In fact, there are many mysteries inside, and problems often arise! Today we will analyze and analyze how to deal with problems when encountering them based on on-site examples.

1. The liquid to be measured contains bubbles

This is a common phenomenon. It is caused by inhalation from the outside and dissolution of the internal liquid. However, the electromagnetic flowmeter cannot distinguish between liquid and bubbles. Bubbles, so calculating and measuring them together will cause errors.

Solution:

1. As shown in the picture above, it is not easy to install at a high point of the pipeline, so change the installation location.

2. If the installation location is difficult to change, a gas collector can be installed upstream of the flow meter to exhaust air regularly.

2. The tube to be tested is not full of liquid

It can be said that the tube is not full of bubbles. It means that the liquid in the tube is not full and the top contains a large number of bubbles. If the liquid If the electrodes are not submerged, the measurement results will be greatly compromised. This is definitely an engineering design error.

Solution:

Figure 2 mainly shows the installation diagram for the correct installation of the electromagnetic flowmeter. Keep it in mind that it is correct.

3. Short-circuit effect of the conductive deposition layer

Since the conductive material is gradually deposited, the flow signal will be short-circuited. This type of fault usually does not appear during the debugging period, but will appear after a period of operation.

Solution:

Disassemble the flow meter and clean the insulation layer. Most of them have yellow rust, which is caused by the deposition of a large amount of iron oxide in the electrolyte. If it starts to run normally, but as time goes by, the flow rate becomes smaller and smaller, the possibility of such a failure should be analyzed.

4. When encountering crystallized liquids

When selecting a product, we often encounter materials whose medium is easy to crystallize. Although most materials can be measured normally at normal temperatures, and the pipe has heat tracing and insulation effects. However, the sensor measuring tube inside the flow meter does not have this function, so when the fluid frequently flows through the measuring tube, a layer of solid is formed on the inner wall due to the cooling.

Solution:

1. Try to choose a flowmeter with a small measuring tube.

2. Disassembly is complicated and it is not recommended to choose this type of flow meter.

5. The conductivity of the liquid exceeds the allowable range

Since the output impedance of the electrode is determined by the conductivity of the liquid being measured and the size of the electrode, when the conductivity is lower than the lower limit , the instrument cannot work normally, and the indication value appears to be shaking.

Solution: According to the above problems, the tantalum electrode electromagnetic flowmeter works normally after the factory replaces it

1. Choose other low conductivity electromagnetic flowmeters that meet the requirements, such as capacitive electromagnetic flowmeters Meter

2. Use other principle flow meters, such as orifice plates, etc.

6. Space electromagnetic wave interference

Generally speaking, if the interference between the sensor and the converter The cable is long and there is strong electromagnetic interference around it, the cable may introduce interference signals, forming common mode interference, causing display distortion, non-linearity or substantial shaking.

Solution:

1. Try to stay away from strong magnetic fields (such as near large motors, large transformers and power cables).

2. Shorten the cable length as much as possible and use shielding measures.

7. Asymmetric flow of the fluid to be measured

Under normal circumstances, the flow velocity of the fluid in the pipeline is required to be axially symmetrically distributed and the magnetic field is uniform. In fact, non-axisymmetric flow velocity distribution often occurs. When vortex flow occurs, it will affect the output and cause errors.

Solution

1. There is enough straight pipe section upstream to ensure that the flow velocity is distributed in concentric circles.

2. The inner diameter of the flow meter should be the same as the inner diameter of the pipes within a certain range upstream and downstream, otherwise the flow velocity distribution will be uneven.

3. If the upstream straight pipe section is insufficient, a flow regulator can be installed, which can only partially compensate.

8. Vibration problem at the installation point

Electromagnetic flowmeters have strict requirements for the vibration at the installation point and must be installed in a place with low vibration, otherwise errors will definitely occur.