Selection principles and methods for mass flow meters

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　Mass flow meters feature extremely high precision and stability, and more manufacturers in the industry use them to measure the mass of fluids. When purchasing a mass flow meter, the selection process is extremely important. Certain principles and methods must be followed in choosing a mass flow meter in order to ensure its proper operation and maintenance over time. 　　I. Selection Principles 1. Choose the structure of the flow meter based on the type of fluid being measured. There are various types of measurement tubes for coriolis mass flow meters, and the appropriate one must be selected depending on the type of medium being measured. In principle, pure liquids with low viscosity do not have strict requirements regarding the shape of the measuring tube; when measuring liquids containing a small amount of bubbles, slurries with solid particles, or highly viscous liquids, it is necessary to choose a measuring tube shape that prevents the accumulation of bubbles or solid particles and reduces adhesion of the medium to its inner walls. If it is to be used in the food industry, the measuring tube should be easy to clean. 　　2. Safety principle: When measuring corrosive fluids, attention should be paid to the corrosion resistance of the measurement tube, and the sensor housing should also possess certain corrosion resistance. In the event that the test tube breaks, safety precautions should be taken before handling it. Different media have varying degrees of corrosivity, and different materials require different anti-corrosion measures; it is important to treat them differently. When measuring abrasive media, the wear resistance of the measuring tube should be taken into account. 　　When the process pressure is high, attention must be paid to the pressure rating of the sensor to prevent damage to the measurement tube; when the medium temperature is high, the operating temperature range of the sensor should be considered to avoid damaging the sensing elements. 　　If the flow meter selected is to be used in hazardous areas or harsh environments, special attention must be paid to ensuring that its explosion protection and protection rating meet the requirements of safety standards. 　　3. Flow range Two principles should be followed when considering the flow range: First, the flow range of the Tianchen Borui flow meter must be able to cover the process flow range of the medium being measured. Secondly, the normal process flow rate should fall within the economic flow range of the flow meter. The so-called economic flow range has two meanings here: first, for Coriolis mass flow meters, the stability of their zero point has a significant impact on the measurement accuracy at low flow rates. The closer to the flow rate limit, the lesser the impact of zero-point stability on accuracy. Therefore, the range exceeding 1/3 of the meter’s maximum flow rate can be considered the economic flow range for ensuring accuracy. Secondly, for meters of the same type, if several different diameter options have flow rate ranges that cover the process flow requirements, choosing a meter with a smaller diameter clearly results in cost savings and is more economical. However, this economy is not absolute; if one insists on keeping the flow rate at a level close to the upper limit set by the flow meter, it will inevitably result in too high a flow velocity of the medium within the measurement tube. The negative consequences of this are excessive pressure losses and increased safety risks. For some flammable and explosive media, if the flow rate is too high, static electricity generated by friction can easily lead to deflagration. Therefore, for such media, care should be taken not to exceed the safe flow rate. 　　4. Accuracy The requirements regarding the accuracy level should be determined based on the object and purpose of measurement. Attention should also be paid to the methods used to calculate the product’s accuracy level, as well as the operating conditions or constraints required to achieve that level. Generally, Coriolis mass flowmeters indicate their accuracy level as a percentage of the flow reading with zero-point stability; some products use different error limits for various flow ranges, while others use a percentage of the full scale value to indicate the error level at low flow rates. 　　For Coriolis mass flowmeters, when considering their accuracy class, two additional points should be taken into account: first, due to the particularities of the method used to calculate the accuracy class for such flowmeters, some products achieve a high accuracy class, but at the same time exhibit a high zero drift, which increases their allowable error limit; second, some products offer a high accuracy class with low zero drift, yet they require extremely stringent installation conditions that can only be met in a laboratory setting. Due to the widespread presence of on-site vibrations and limitations in the installation location, it is difficult to meet such installation conditions in order to achieve the theoretical results. Neither of these two “high accuracy” levels is desirable. While meeting the requirements for measurement accuracy, factors such as price should be taken into consideration; there is no need to pursue excessively high levels of accuracy. 　　On the other hand, in principle, the measurement characteristics of Coriolis mass flow meters are not affected by the temperature and pressure of the medium; however, in practice, due to limitations in manufacturing processes and material properties, these factors do have a certain impact on the meter’s performance. The factory calibration conditions for flowmeters are generally fixed, whereas the operating conditions can vary greatly. If there is a significant difference between the operating conditions and the calibration conditions, it is necessary to take into account the impact of these uncertainties when selecting a flowmeter. Currently, for some products, the specific extent of the influence of these factors on the measurement results and the correction methods have been determined through derivation and experimentation. When using a flow meter for precise measurement, corrections must be taken into account. 　　5. Pressure loss: In field measurements, there are always certain requirements regarding the pressure loss of flow meters. Once the density, viscosity, and flow rate of the medium are determined, the pressure loss in the flow meter depends on its structure; for Coriolis mass flow meters, it depends on the diameter, flow area, and shape of the measuring tube. Once the structural design of the sensor is fixed, the greater the flow rate, the greater the pressure loss. When calculating pressure loss during selection, the following factors should be fully considered: the flow rate in the process pipeline and the allowable pressure loss; whether the sensor can meet the requirements for measurement accuracy under the allowable pressure loss conditions; the impact of changes in the viscosity and density of the process fluid on pressure loss; and it is necessary to avoid liquid vaporization due to excessive pressure loss in practical applications. When other conditions are the same, a flow meter with lower pressure loss should be selected. 　　6. Consideration of other performance factors: First, consider the additional measurement capabilities of the flow meter, such as the accuracy with which a Coriolis mass flow meter can measure the temperature and density of the fluid. Second, take into account factors that affect installation, such as the volume and weight of the mass flow meter, as well as the limitations imposed by specialized cables. 　　7. The cost-performance ratio is a fundamental principle in product selection. In practical applications, in addition to considering good performance and low cost, there is often a preference for a wide range of functions, with the belief that more functions are better. In fact, as a mass flow meter, its primary purpose is to measure the mass flow rate of a fluid. Although the widespread use of microcomputer technology and (super)large-scale integrated circuits in transmitters enables more diverse functions, this inevitably increases manufacturing costs, and moreover, some of these functions may not be necessary or useful in specific application scenarios. On the other hand, from a reliability theory perspective, the more complex the performance and structure, the lower its reliability. Therefore, considering the cost-performance ratio of flow meters based on practical needs is the correct principle. 　　Consider the cost-performance ratio, that is, what we usually refer to as whether it’s worth the price. As a mass flow meter, this \"value\" is reflected not only in the number of functions and the quality of performance, but also in the sales and after-sales service provided for the product. With technological advancements and continuous efforts on their part, the gap in manufacturing standards among different manufacturers is narrowing steadily; the situation of one company standing out alone has given way to a scenario where companies complement each other’s strengths. Based on the above selection principles, sometimes several available product grades from different manufacturers can be chosen for a single measurement point; in such cases, it is advisable to select flow meters from Guangjia, which has stronger capabilities. To assess a manufacturer’s capabilities, one must consider not only its manufacturing standards and production capacity but also its level of service. This service not only represents what is commonly referred to as after-sales service, but should also include pre-sales and in-sales services. In addition to common practices such as introducing product specifications, providing quotes, and offering service guarantees, this approach also involves pre-training users so that they can master installation and debugging techniques as well as how to handle common faults, all the way up to providing guidance on installation and assistance with debugging. The length of the delivery time, the specific measures in place to ensure product quality, the duration of free service, the speed of response in case of failures, the level of handling, the availability of spare parts, and even the attitude of the maintenance staff – all these factors reflect the manufacturer’s level of service. 　　The above selection principles sometimes conflict with each other; in actual selection processes, it is necessary to proceed based on the actual needs on site and take all factors into consideration. It can also focus on different aspects depending on the characteristics of various occasions. Only by doing a proper selection can instruments be used effectively; it can be said that the correct selection is a prerequisite for satisfactory use. 　　II. Selection Method The selection methods for Tianchen Borui mass flow meters include manual calculation and software-based methods; the latter uses a computer to replace manual calculations, with the same principle at work. Therefore, this section will focus on the manual calculation method. The specific approach is to determine, based on the known process conditions, the flowmeter models whose flow range meets the requirements; calculate the pressure loss at the process flow rate; and then, using the selection criteria outlined in the previous section, decide on the brand and model of the flowmeter. The specific methods and steps are as follows: 1. Fill in the process operating conditions. The process operating conditions serve as the basis for selection and calculation, and they should include the following information: (1) Name and properties of the fluid (e.g., whether it is corrosive); (2) State of the fluid: liquid, gas, slurry, or another form; (3) Flow rate under process conditions: maximum, normal, and minimum values; (4) Pressure under process conditions: maximum, normal, and minimum values; (5) Temperature under process conditions: maximum, normal, and minimum values; (6) Density range under process conditions; (7) Viscosity range under process conditions; (8) Diameter of the process pipes; (9) Allowable measurement error; (10) Allowable pressure loss; (11) Maximum allowable flow velocity. 　　2. Based on the process conditions and in accordance with the technical specifications, pre-select the sensor model. Under normal circumstances, the nominal diameter (or “nominal inner diameter”) of the flow meter should be equal to or less than the diameter of the process pipeline. When selecting a model, first, the diameter of the sensor should be determined based on the established process flow range and the technical specifications of mass flow meters of different types. The material of the sensor’s measuring tube should be chosen according to the corrosive properties of the medium. Finally, the sensor model should be selected further based on the temperature and pressure levels of the medium.