How to deal with the phenomenon of bubbles in the liquid measured by electromagnetic flowmeter

Causes: The formation of bubbling gas in liquid has two ways: inhalation from the outside and conversion of dissolved gas (air) in liquid into free bubbles. If the liquid contains large bubbles, it can cover the entire electrode when rubbing against the electrode, causing the flow signal input circuit to be open instantly, resulting in the output signal to fluctuate.

Judgment method: The simplest judgment method is to cut off the excitation circuit current of the magnetic field when encountering shaking. If the instrument still has a display and is unstable at this time, it means that it is mostly caused by the influence of bubbles. If the electrode resistance is measured with a pointer multimeter at this time, the loop resistance of the electrode can be measured to be higher than normal, but this test requires the long-term accumulated test experience and data of professionals.

Solution: Regarding the situation where the measured medium contains air, if it is judged to be caused by the installation position, such as the electromagnetic flowmeter is installed at the high point of the pipe system and retains gas or the air is inhaled from the outside to cause the flowmeter to shake, changing the installation position is the most thorough solution, as shown in Figure 1, at the lowest point of the pipeline or using a U-shaped tube installation. However, in many applications, the diameter is large or the installation position is not easy to change. It is recommended to install an air collection bag and an exhaust valve upstream of the flow meter, as shown in Figure 2. For a DN2200 electromagnetic flowmeter, the fluctuation of the display due to bubbles can reach 20-50%. After the exhaust device is installed, the measurement returns to normal.

Figure 1 Changing the installation position

Figure 2 Electromagnetic flowmeter with exhaust device installed