The influence of liquid on electromagnetic flowmeter (I)

   The zero point stability of low-frequency rectangular wave excitation instruments is good, while that of AC excitation instruments is poor. The reasons are:

   1. Noise caused by eddy current in liquid;

   2. Orthogonal (90-degree) noise induced by the signal line, electrode and liquid forming a loop;

   3. When the industrial frequency AC is excited, there is also in-phase noise introduced by the signal line.

   If the magnetic field, measuring tube diameter, tube axis and fluid resistance are completely symmetrical to the electrode axis, no noise will appear on the electrode even if eddy current exists. However, in actual process, the electrode is easily contaminated, which destroys the symmetry of the electrode itself and causes coaxial orthogonal noise.

   These are the measured values ​​of zero point deviation of two battery-powered electromagnetic flowmeters in aluminum hydroxide slurry, AC excitation and low-frequency rectangular wave excitation, over 165 days. The zero point stability of low-frequency excitation is significantly better than that of AC excitation. In principle, orthogonal noise does not directly affect the flow signal, but the iron loss caused by it will cause the signal voltage to shift phase and form in-phase noise.

   The author believes that although these unfavorable factors have been improved to varying degrees in the actual design of AC excitation electromagnetic flowmeters, and AC excitation is no longer the current mainstream, it seems that there is no need to mind them. However, some foreign manufacturers still use AC excitation as the dominant instrument for measuring slurry. Therefore, it is still necessary to be familiar with these characteristics when making analysis and comparison.

   Effect of electrode adhesion layer

   In order to verify the change of differential noise caused by the electrode adhesion layer, a phenolic resin equal to half of the electrode was coated for a simulation test.

   The deviation of industrial frequency AC excitation is as high as 9%, while that of low frequency rectangular wave excitation is less than 0.2%. This experiment provides us with a quantitative concept of the effect of electrode adhesion.

   Flow noise

   People have noticed that open channel flowmeters and electromagnetic flowmeters are affected by fluid noise. Flow noise is the charge generated by the friction between the fluid and the lining surface. These charges form noise when the fluid moves. Flow noise depends on the conductivity, dielectric constant, viscosity and flow rate of the fluid parameters. The relationship between them is shown in the following formula. This set of figures allows us to understand the relationship between them and also have a numerical concept.

   Dielectric constant

   Flow noise ∝───── × (flow velocity) 

   Conductivity × viscosity

   Flow noise is a noise unique to low conductivity and low viscosity liquids such as alcohol and pure water.