Selection principle and method of mass flowmeter (2)

Selection principles and methods of mass flowmeters (2) 4. Accuracy The requirements for the accuracy level should be determined according to the measurement object and purpose. At the same time, attention should be paid to the calculation method of the product accuracy level and the conditions of use or Constraints. Generally speaking, the Coriolis force mass flowmeter expresses the accuracy grade by the percentage of the flow reading with zero stability, and some products are expressed by the method that different flow segments correspond to different error limits, and some products are in the low flow rate segment. The error level is expressed as a percentage of full scale. For the Coriolis force mass flowmeter, when considering its accuracy level, the following two points should also be noted: First, due to the particularity of the calculation method of the accuracy level of this type of flowmeter, some products give a high accuracy level. Accuracy level, and the zero drift value given at the same time is also very high, thus increasing its allowable error limit; second, some products give a high accuracy level under low zero drift value, but the installation conditions it requires are close to Harsh and can only be done in a laboratory. Due to the ubiquity of on-site vibration and the limitation of the installation site, it is difficult to meet such installation conditions and achieve the theoretical effect. Neither of these 'high accuracy' levels is desirable. 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 accuracy level. On the other hand, in principle, the measurement characteristics of the Coriolis force 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 still affect the characteristics of the flowmeter. have a certain impact. The factory verification conditions of the flowmeter are generally fixed, but the use conditions are ever-changing. If the difference between the use conditions and the verification conditions is too large, the influence of these uncertain factors needs to be considered when selecting the type. At present, some products have obtained the specific degree and correction method of the influence of these factors on the measurement results through derivation and experiments. When the flowmeter is selected for dense measurement, correction should be considered. 5. In the on-site measurement of pressure loss, there are always certain requirements for the pressure loss of the flowmeter. When the density, viscosity and flow rate of the medium are determined, the pressure loss of the flowmeter depends on its structure. For the Coriolis force mass flowmeter, it depends on the caliber, flow area and shape of the measuring tube; when the structure of the sensor is determined, The greater the flow rate, the greater 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 requirements under the allowable pressure loss condition; The influence of pressure loss; in the application, the vaporization of the liquid due to excessive pressure loss should be avoided. When other conditions are the same, the flowmeter with smaller pressure loss should be selected. 6. Consideration of other performance factors One is to consider the additional measurement performance of the flowmeter, such as the accuracy of the Coriolis mass flowmeter in measuring medium temperature and density; the other is to consider the volume, weight and limitations of special cables of the mass flowmeter, etc. Factors affecting installation. 7. Performance-price ratio is a basic principle of model selection. In the specific application, in addition to considering good performance and low price, there is often a preference for the diversity of functions, and the more functions the better. In fact, as a mass flow meter, its main purpose is to detect fluid mass flow. Although the transmitter generally adopts microcomputer technology and (ultra) large-scale integrated circuit, more diversified functions can be realized, but this will inevitably increase the manufacturing cost, and some functions may not be required and practical in specific applications. On the other hand, in terms 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 performance-price ratio, which is what we usually call value for money. As a mass flow meter, this 'value' should not only be reflected in the number of functions and performance, but also in the sales service of the product. With the advancement of technology and continuous efforts, the manufacturing level gap among various manufacturers is narrowing day by day, and the uniqueness has become mutual learning from each other. According to the above selection principles, sometimes a measuring point can first select the available products of several brands, and at this time, the flowmeter of a stronger manufacturer should be selected. Investigating the strength of a manufacturer not only depends on its manufacturing level and production capacity, but also its service level. This service not only represents the so-called after-sales service, but also includes pre-sale and in-sale services. In addition to common practices such as introducing product performance, making quotations, and promising services, pre-training is provided to users so that they can master installation and debugging and how to deal with some common faults, until they guide installation and help debugging. The length of the delivery cycle, the specific guarantee measures for product quality, the time of free service, the response speed in case of failure, the processing level, the guarantee of spare parts, and even the service attitude of maintenance personnel, etc., all represent the service level of the manufacturer. The above selection principles sometimes restrict each other. In the actual selection work, everything should be considered comprehensively based on the actual needs of the site. It can also be emphasized according to the different characteristics of different occasions. As long as the selection work is done well, the instrument can be used well. It can be said that the correct selection is the prerequisite for satisfactory use.

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