Limitations and shortcomings of electromagnetic flowmeters

1) Working temperature and working pressure

The maximum working temperature of the electromagnetic flowmeter depends on the temperature at which the pipe and lining materials expand, deform and change, which varies depending on the specific instrument and is generally lower than 120'C. The maximum working pressure depends on the strength of the pipeline, the sealing of the electrode part and the specifications of the flange, which is generally 1.6X105-2.5X105Pa. Since too thick a pipe wall will increase eddy current loss, the measuring conduit is made thinner.

2) Conductivity of the measured fluid

The measured medium must have a certain conductivity. The conductivity is generally required to be 10-4 - 10-1s/cm, and the minimum is not less than 50 5S/cm. Therefore, the intelligent electromagnetic flowmeter cannot measure the flow of non-conductive fluids such as gas, steam and petroleum products. For conductive media, theoretically, any flow relative to the magnetic field will generate an induced potential. In fact, the increase in internal resistance between electrodes is limited by the distributed capacitance of the transmission line, the input impedance of the amplifier and the measurement accuracy.

3) Flow rate and flow rate distribution

Electromagnetic flowmeter is also a velocity instrument. The induced potential is proportional to the average flow rate. And this average flow rate is calculated under the condition that the flow rate at each point is symmetrical to the center of the pipe. Therefore, when the fluid flows in the pipe, the flow rate distribution at each point on the cross section has a great influence on the instrument value. For the transmitter of circular pipe point electrode commonly used in general industry, if the symmetrical distribution of flow rate relative to the central axis of the pipe is destroyed, the electromagnetic flowmeter cannot work properly. Therefore, there must be enough straight pipe sections before and after the electromagnetic flowmeter to eliminate the influence of various local resistances on the symmetry of flow rate distribution.