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The electromagnetic flowmeter is subject to process conditions when used in the field. It is impossible to have the same ideal conditions as in the laboratory, and the errors caused are inevitable. Some of the influencing factors (such as the physical properties and properties of the fluid) can be reduced or eliminated through correction, while some are impossible. For example, in the face of the complex flow fields caused by thousands of resistance components and their combinations, they are difficult to reproduce through laboratory simulations and cannot be corrected. However, they are the biggest obstacle to improving the accuracy of flow instruments (especially the classic throttling devices that once accounted for most of the flow instrument market). Excluding the internal reasons such as the principle and structure of the flow meter, the external factors that affect its accuracy are mainly attributed to the following three aspects: 1. Fluid properties The fluids passing through the flowmeter in industrial sites cannot be as clean as the fluids used in laboratories. They may have sediments and are corrosive. After being used for a period of time, scale, wear, and corrosion will also occur on the pipe edge detection parts. Precipitates on the pipe wall will change the wall thickness and roughness of the pipe. For standard orifice plates, the β value will be changed, causing an error of ±3~10%. ； Causes wear and corrosion to the moving parts of the turbine, rotor, and volumetric flow meter, which may cause errors at least, or make it impossible to work at worst. ； Contamination of the electrodes of the electromagnetic flowmeter, the transducer of the ultrasonic flowmeter, and the thermal resistor of the thermal flowmeter will reduce its sensitivity and increase the error. ； Causes obstruction to the pressure tapping hole of the differential pressure flow meter, etc. Of course, this process is slow, but it must not be taken lightly. Generally speaking, as long as you pay attention to regular maintenance, its impact can be reduced (or eliminated). 2. Physical properties of fluids. The commonly used media in tests are water, air and oil. In field applications, tens of thousands of various fluids will be faced, and their physical properties (such as density, viscosity, electrical conductor, thermal conductivity, sound speed, composition, etc.) are different from the media commonly used in tests, which will more or less affect the accuracy of the flow meter. However, the physical properties of these fluids can be checked through some engineering manuals, and corrections can be made to mitigate their effects. This is also the reason why the flow meter is intelligent. 3. Flow characteristics In the laboratory, the flow meter should be in an ideal flow state. Generally, there are the following types:: 1. Newtonian fluid: In the process industry, except for the food industry, most fluids are Newtonian fluids. 2. Steady flow: A flow state in which the flow rate in the measurement pipe section does not change with time, but slow changes are allowed. In the industry, the so-called pulsating flow (a flow state in which the flow rate changes rapidly with time) is unsteady flow. In industrial sites, pulsating flow will be generated due to the oscillation of pumps, compressors, blowers, certain regulators, and valves, which will bring large errors to flow meters. Head raised this issue as early as 1956 and proposed the concept of pulsation coefficient Ip to define the impact of pulsating flow on flow measurement, and believed that when Ip is less than 0.03, it can be regarded as a steady flow. ； If it is greater than 0.03, it should be taken seriously. Pulsating flow will bring errors to the flow meter, and this effect is particularly serious for differential pressure flow meters. The pulsating flow of the electromagnetic flowmeter will cause changes in the rotor speed. ； For vortex flowmeters, if the pulsation frequency is close to the vortex frequency, the so-called "synchronization phenomenon" will occur and a large error will occur. For fluids, the compressibility of gas is better than that of liquid. The pulsating flow will be quickly attenuated during the flow, and the impact on the flow meter will be less than that of liquid. People have been paying attention to the impact of pulsating flow on flow meters for nearly 60 years. Although many studies have been carried out in an attempt to correct it, there is still a lack of sufficient data. The current common approach is to use filters in the pipeline to eliminate (or mitigate) its effects. 3. Single-phase fluid: The measurement results of flow meters on single-phase and multi-phase fluids are very different. Here we only discuss the measurement of single-phase flow meters. Flow meters are only calibrated in single-phase flow laboratories. In field applications, problems with multi-phase fluids will inevitably be encountered. In process engineering, as the fluid flows through various resistance parts, friction and separation will inevitably occur, and due to changes in cross-section and pressure drop, the gas phase dissolved in the liquid phase will separate and generate cavities. For the throttling device, due to the drastic flow changes in the orifice plate, the probability of cavities will be 8 times that of a Venturi tube and three times that of a Doyle tube. If the generation of cavities can be limited within a certain range, the resulting measurement error can be as high as 20%. If the out-of-control development is too large, the instrument may be damaged. For gas-solid and liquid-solid two-phase flows, the measurement of flow meters will also bring large errors and hazards. The way to avoid it is to install the flow meter above the vertical pipe as much as possible to avoid solid phase deposition. As we all know, the flow velocity distribution in the pipeline affects the accuracy of flow meters based on most principles (except Coriolis and volume). Therefore, ISOTC30 stipulates that flow meters must be installed in fully developed turbulent flow to maintain high accuracy. Of course, the flow laboratory should also have fully developed turbulent flow, so that the calibrated flow coefficient is meaningful. Generally speaking, as long as there is a straight pipe length of 30D (D is the inner diameter of the pipe), turbulent flow can be fully developed. However, with the trend of modernization and large-scale projects, the diameter of the pipes in the project is increasing day by day. The process design has never considered the straight pipe length necessary for the electromagnetic flowmeter to maintain high accuracy in order to save space. Moreover, there are thousands of types of resistance parts on site and various combinations. The flow velocity distribution in the pipeline is very complex. Due to the difference in flow field, the flow coefficient verified in the laboratory cannot be accurately transmitted to the flow meter on site, so it is difficult to obtain higher accuracy. For more information, please visit the company’s official website http://www.yb1518.com/. Please keep this link when reprinting!