Problems of uniform mixing of liquids during electromagnetic flowmeter measurements

Case introduction

In industrial production, liquid flow meter measurement must be accurate. The requirements for working conditions are different. In most cases, ideal measurement conditions cannot be achieved, such as the selection of liquid phase and ideal measurement conditions. , the liquid medium required to be measured is a pure clean liquid flowmeter. There are three factors in the physical properties of the liquid flowmeter that will cause the output to slosh. They are: the conductivity of the liquid is close to the lower limit; the liquid contains solid particles or bubbles; the two liquids in the two-component liquid have different conductivities and are not evenly mixed, or the chemical reaction in the pipeline has not been completely completed. 1. If the liquid conductivity is close to the lower limit, shaking will occur. Because the lower limit value stipulated in the manufacturer's instrument specification is the lowest value that can be measured under various good conditions of use, and the actual conditions may not be ideal. Sometimes we encounter low-strength distilled water or deionized water whose conductivity is close to the lower limit of 5×10-6S/cm stipulated in the electromagnetic flowmeter specification, but the output oscillates during use. It is generally believed that the lower limit of conductivity that can be stably measured is 1 to 2 orders of magnitude higher. The conductivity of liquids can be consulted in relevant manuals. If there is no ready-made data, samples can be taken and measured with a conductivity meter. However, sometimes there are cases where samples taken from pipelines and measured in the laboratory are considered usable, but the actual electromagnetic flowmeter cannot work. This is because the liquid when measuring conductivity is different from the liquid in the pipeline. For example, the liquid has absorbed CO2 or NOX in the atmosphere to produce carbonic acid or nitric acid, which changes the conductivity. For noisy slurries produced by liquids containing particles or fibers, increasing the excitation frequency can effectively improve output sloshing. 2. The measured liquid contains many solid particles, which will act like bubbles, causing spikes and pulse-like noises in the flow signal, causing the output to shake. If the solid phase is powdery, it usually does not cause output fluctuation. In the fine chemical industry, food industry, pharmaceutical industry and water supply treatment engineering, chemical liquid is often added to the liquid, and the chemical liquid is usually injected by a reciprocating pump or diaphragm pump in proportion to the main liquid flow rate. After the liquid is injected, the main liquid appears in a series of segments separated by liquid segments and segments without liquid. If the two liquids with different conductivities are not mixed evenly, the output of the electromagnetic flowmeter measuring the flow rate downstream will slosh. In this case, the liquid filling point should be moved downstream, or the electromagnetic flowmeter should be moved to the liquid filling point, or the electromagnetic flowmeter should be moved upstream of the liquid filling point; if it is limited by site conditions or the modification project is too large, , you can also install a mixer downstream of the liquid addition point to remedy the problem. However, after installing a static mixer, the liquid flow will produce a rotating flow, which may cause an additional error of 1% or more. However, compared with the fact that the output jitter cannot be measured, this is the lesser of two disadvantages. 3. If the mixed liquid enters the electromagnetic flowmeter for measurement before the chemical reaction in the pipeline is completed, output shaking may also occur. In this case, you can only change the position of the measuring point, making sure that the measuring position is upstream of the mixing point or downstream away from the mixing section. However, the distance between the principle mixing sections needs to be very long.