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Principles and methods for the selection of mass flow meters - focus on microfluidic control technology

Mass flow meters have high precision and stability. More manufacturers in the industry use mass flow meters to measure the quality of fluids. When purchasing a mass flow meter, the problem of the selection problem is very critical. The quality flow meter selection must pay attention to certain principles and methods to ensure normal use and maintenance in the future.
First, the principle of selection
1. Select the structure of the flowmeter according to the type of fluid to be tested.
The measuring tube of the Coriolis mass flowmeter has many forms and needs to be selected according to the type of the medium to be tested. In principle, a pure liquid with a low viscosity is not required for the shape of the measuring tube; when measuring a liquid containing a small amount of bubbles, a slurry containing solid particles, and a highly viscous liquid, the measuring tube should not be used to accumulate bubbles or solid particles or inner walls. It is not easy to adhere to the shape of the medium; if used in the food industry, the measuring tube should be easy to clean.
2, the principle of safety
When measuring corrosive fluids, care should be taken to measure the corrosion resistance of the tubes and the sensor housing should also have some corrosion resistance. In case the test tube is broken, there should be safety precautions before handling. The corrosiveness of different media is different. Different anti-corrosion objects of different materials should be treated differently. To measure abrasive media, the wear resistance of the measuring tube should be considered.
When the process pressure is high, pay attention to the pressure level of the sensor to avoid damage to the measuring tube. When the medium temperature is high, the temperature range of the sensor should be considered to avoid damage to the detecting component.
If the selected flowmeter is used in a hazardous area or in a harsh environment, special attention should be paid to the explosion-proof of the flowmeter and the degree of protection should meet the requirements of safety standards.
3, the flow range
Considering the flow range should follow two principles: First, the flow range of the Tianchen Borui flowmeter should cover the process flow range of the measured medium. Second, the common process flow should fall within the economic flow range of the flow meter. The so-called economic flow range has two meanings here: First, due to the Coriolis force mass flowmeter, its zero stability has a great influence on the measurement accuracy of the lower limit flow. The closer to the upper flow limit, the smaller the effect of zero stability on accuracy. Therefore, the range of more than 1/3 of the upper flow rate of the flowmeter can be called the economic flow range of accuracy; the second is the same type of flow meter, if There are several measurable flow ranges that cover the process flow range. Selecting a smaller diameter flow meter can obviously save money and be more economical. However, this kind of economy is not absolute. If the pursuit of using the flow rate is close to the upper flow rate of the flowmeter, it will inevitably cause the medium flow rate in the measuring tube to be too high, and the negative effect will be excessive pressure loss and unsafeness. increase. Some flammable and explosive media, if the flow rate is too high, it is easy to cause static electricity due to friction and cause deflagration. Therefore, care should be taken not to exceed safe flow rates for such media.
4, accuracy
The accuracy level requirements should be determined according to the measurement object and purpose, and attention should be paid to the calculation method of the product accuracy level and the use conditions or constraints to achieve the level. In general, Coriolis mass flow meters represent the accuracy level as a percentage of the flow reading with zero stability. Other products are represented by different flow segments corresponding to different error limits, and some products are in the low flow rate segment. The percentage of full scale is used to indicate the error level.
For Coriolis mass flowmeters, when considering the accuracy level, the following two points should also be noted: First, due to the particularity of the accuracy calculation method of such flowmeters, some products have given high accuracy. Degree, while the zero drift value is also high, which increases its allowable error limit; second, some products give high accuracy levels under low zero drift, however, the installation conditions required are almost Harsh, can only be done in the laboratory. Due to the ubiquity of on-site vibration and the limitations of the installation location, it is difficult to meet such installation conditions and achieve theoretical results. Both of these "high accuracy" levels are not sufficient. In the case of meeting the measurement accuracy requirements, factors such as price should be considered, and it is not necessary to blindly pursue an excessively high level of accuracy.
On the other hand, in principle, the metering characteristics of the Coriolis mass flowmeter are not affected by the temperature and pressure of the medium. In fact, due to the limitations of the manufacturing process and material properties, the above factors are still characteristic of the flowmeter. Have a certain impact. The factory calibration conditions of the flowmeter are generally fixed, and the conditions of use are ever-changing. If the conditions of use differ greatly from the verification conditions, the influence of these uncertain factors must be considered in the selection. At present, some products have been deduced and experimentally obtained the specific degree of influence of these factors on the measurement results and the correction method. When using a flow meter for meticulous metering, consider making corrections.
5, pressure loss
In the field measurement, there is always a certain requirement for the pressure loss of the flow meter. When the density, viscosity and flow rate of the medium are determined, the pressure loss of the flowmeter depends on its structure. The Coriolis mass flowmeter depends on the caliber, flow area and shape of the measuring tube; when the structural form of the sensor is determined, The larger the flow rate, the larger the pressure loss. When calculating the pressure loss in the selection, the following points should be fully considered: the flow rate in the process pipeline and the allowable pressure loss; whether the sensor meets the measurement accuracy requirement under the pressure loss condition; the change of the viscosity and density of the process fluid The effect of pressure loss; in the application should avoid the vaporization of the liquid due to excessive pressure loss. When the other conditions are the same, a flow meter with a small pressure loss should be selected.
6. Consideration of other performance factors
First, consider the additional measurement performance of the flowmeter, such as the accuracy of the Coriolis mass flowmeter to measure the temperature and density of the medium; the second is to consider the factors affecting the installation, such as the volume and weight of the mass flowmeter and the limitations of the special cable.
7, performance and price ratio is a basic principle of selection. In the specific application, in addition to considering good performance and low price, there is often a preference for functional diversity, and the more functions, the better. In fact, as a mass flow meter, its main purpose is to detect fluid mass flow. Although the use of microcomputer technology and (super) large-scale integrated circuits for transmitters can achieve more diverse functions, it is bound to increase manufacturing costs, and some functions may not be necessary and practical in specific applications. On the other hand, from the perspective of reliability theory, the more complex the performance and structure, the lower the reliability. Therefore, it is the correct principle to consider the performance price ratio of the flowmeter from the actual need.
Consider the price-performance ratio, which is what we usually say is worth the money. As a mass flow meter, this "value" should be reflected in the sales and service of the product, in addition to the function and performance. With the advancement of technology and continuous efforts of its own, the manufacturing level gap between manufacturers is shrinking, and the unique show has become a complement to each other. According to the above selection principle, sometimes a measurement point can first select several brands of available products. In this case, the powerful flowmeter of Guangjia should be selected. Investigating the strength of the manufacturer depends not only on its manufacturing level, production capacity, but also on its service level. This service not only represents the so-called after-sales service, but also includes pre-sales and in-sale services. In addition to introducing common practices such as product performance, quotation, and commitment service, the user is pre-trained to master the installation and debugging and some common troubleshooting methods, until the installation and assistance are debugged. The length of the delivery period, the specific guarantee measures for product quality, the time of free service, the response speed in the event of failure, the level of treatment, the protection of spare parts, and even the service attitude of maintenance personnel, etc., represent the service level of the manufacturer.
The above selection principles are sometimes mutually constrained. In the actual selection work, everything must be based on the actual needs of the site and considered comprehensively. According to the different characteristics of different occasions, we can focus on it. As long as the selection work is done, the instrument can be used well. It can be said that the correct selection is the prerequisite for satisfactory use.

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