Electromagnetic flowmeter conductivity measurement problem introduction

A decrease in fluid conductivity will increase the output impedance of the electrode and cause an error due to the loading efficiency caused by the input impedance of the converter. Therefore, the lower limit of the conductivity of the fluid in the application of the electromagnetic flowmeter is specified in accordance with the principles described below.
The output impedance of the electrode determines the amount of input impedance required by the converter, and the output impedance of the electrode can be considered to be dominated by the conductivity of the fluid and the size of the electrode.
In the theoretical analysis, the electrode is used as the spot electrode, and the size can be neglected. In fact, the electrode has a certain size. When the disk electrode with the diameter d is in contact with the semi-infinitely broadened fluid with the conductivity K, the stretch resistance is 1 /2Kd, therefore, if the pipe diameter D>>d, the output impedance of the electrode is the sum of the two stretch resistances, which is equal to 1/Kd.
Generally, the lower limit of the fluid conductivity measured is 5 μS/cm to 10 μS/cm. Therefore, if the electrode diameter is 1 cm, the output impedance of the electrode is 1/Kd=100 kΩ to 200 kΩ, so that the influence of the output impedance is limited to 0.1. Below %, the input impedance of the converter should be around 200MΩ.