Influence of Hysteresis Phenomenon on Various Mass Flow Meters

Effect of hysteresis on various mass flowmeters Response speed is a key performance indicator of mass flowmeters, so what impact does response speed have on flow measurement or control accuracy? Below we will discuss the hysteresis phenomenon of general mass flow meters. Hysteresis will cause a mass flowmeter to respond differently to the same input value, resulting in deviations in flow measurement accuracy. What is hysteresis? To put it simply, the hysteresis phenomenon is that after a system passes through a certain input path, even if the same input value is changed back to the original state, the state cannot return to its initial state. For example, usually the chocolate we eat at room temperature comes in the form of a bar, block or ball. Yummy Yummy…! Sometimes chocolate is used to make a cake or bake a cookie by yourself. At this time, the chocolate bar needs to be heated and melted into chocolate sauce. Have you ever noticed that even after cooling and returning to room temperature, melted chocolate can no longer return to its original shape. A piece of normal chocolate and heated and cooled chocolate are in two different states (solid and liquid) under the same input path (ie temperature), which is hysteresis. Hysteresis will prevent things from getting reproducible results. Hysteresis in Coriolis mass flowmeters Now that we understand what hysteresis is, let's explore the differences in the hysteresis of mass flowmeters with different measurement principles. A Coriolis mass flow meter measures the flow of a fluid by vibrating a U-shaped tube. Increasing the flow rate will increase the mass of the medium in the system and produce a secondary vibration directly related to the density and mass of the measured medium, which is called Coriolis effect stress. Just as the mass of the medium passing through the system will increase when the flow rate increases rapidly, the secondary vibration will increase, and vice versa, the secondary vibration will weaken when the flow rate decreases. However, hysteresis occurs when the flow rate drops sharply from one point to a much lower point because it takes time for the secondary vibration to recover and vibrate in response to the new, lower flow value. Actually, on the pendulum of the wall clock in your home, you can“see”Hysteresis in a Coriolis mass flow meter. You might as well try tapping the pendulum lightly from the vertical direction. At this time, the second-order vibration has been introduced into the swing of the pendulum. Although it is only touched once, the influence of the external force on the pendulum will be completely eliminated after a while. If there is another tap before the pendulum stops swinging, the external force vibration is introduced again, and the vibration effect produced by the external force this time will be somewhat different from the previous one. Hysteresis in thermal mass flowmeters Thermal mass flowmeters measure mass flow in an indirect way based on the thermal properties of the fluid. Different from the direct detection of the number of molecules of the measured fluid flowing through the pipeline, the thermal principle detects the heat taken away by the passing fluid, that is, the principle of heat dispersion. There are several types of thermal mass flowmeters. The typical measurement method is to separate a small part of the gas and pass it through a heated capillary. There are multiple temperature sensors in the capillary. When the flow rate increases, the gas molecules pass through the capillary. After the pipe is heated, it carries the heat all the way to the end of the pipe until the temperature returns to that before it was heated. Larger flows carry more heat to the back end of the capillary, while smaller flows carry less heat. How much heat is carried away also depends on the thermodynamic properties of the gas molecules, the pressure, and the temperature before being heated. If the measured medium suddenly drops from a high flow rate to a low flow rate, the heat carried by the large flow rate through the capillary will take a certain time to recover. To be able to measure low flow rates, the capillary tubes used in thermal mass flow meters have excellent thermal conductivity and are better at maintaining elevated temperatures as flow rates drop.

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