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Flow measurement instrument several verification methods difference description

First, the selection of several verification methods
The verification of flow measuring instruments usually adopts two methods: real flow verification and dry verification. The method used to verify the flow meter depends on the measurement system's requirements for measurement uncertainty, the type of flowmeter to be inspected, the use, the verification conditions, and the cost of verification. Generally, flow meters for liquid metering (such as crude oil and water meters) basically use solid flow verification, while gas metered flow meters (such as differential pressure flow meters for natural gas trade settlement) mostly use dry verification. In the mode, only a few of the critical flow nozzles are used for online real-flow or offline verification.
Second, the difference between several verification methods
1. Difference in conclusion
The dry measurement of the flow meter is carried out by the combined measurement method. According to the measurement results of the relevant parameters and the uncertainty, the total uncertainty of the flow measurement of the meter is synthesized according to the error processing method, which is determined indirectly by a certain confidence. For the uncertainty range of the flow measurement instrument, it cannot give a specific error value. It is usually based on a large number of rich experimental data and standardized technical requirements, maintaining the testability and consistency of measurement. For example, standard orifice plate throttling devices, critical flow venturi nozzles, etc. have been fairly mature dry calibration techniques. Taking the orifice flowmeter as an example, the formula of the outflow coefficient is based on extremely rich and sufficient test data. The error range of the outflow coefficient given in the standard is not more than 0.6%. In the uncertainty of the flow measurement of the orifice plate, a certain degree of uncertainty can only be given with a certain degree of confidence.
The actual flow verification, especially the online actual flow verification, is the most accurate, consistent, traceable and experimental, and can achieve real flow measurement instrument calibration or assignment, which can ensure the continuity and closure of the value transmission or traceability. The offline test gives the error value or flowmeter coefficient of the flow meter under the verification condition, but because the actual operating conditions and installation conditions are different from the verification conditions, the physical property parameters of the medium and even the medium itself are different. In fact, this verification is not True calibration or assignment. Strictly speaking, the offline verification results of flow meters can only indicate the measurement characteristics under the verification conditions. Most of the actual use of the site environmental conditions, instrument installation conditions and operating conditions are very different from the verification conditions, so It brings additional error to the flow meter, and the additional error is always subjectively judged by certain experience. Therefore, the offline test does not require high flow measurement results, or it can meet the expected measurement requirements even if there are additional errors. Simple and easy to choose.
2. Different degrees of correction for the influence of physical parameters
The measurement results of almost all flow measuring instruments are affected by the physical properties of the measured medium, but the degree of influence is different. For physical parameters that can express their influence on flow measurement results by explicit function, as long as the actual values of these parameters are known, they can be corrected, such as natural gas relative density, compression factor, isentropic index, etc. influences. However, for most flow measuring instruments, the influence of physical parameters on their metering performance is difficult to accurately express by mathematical formula. For example, in liquid metering, volumetric flowmeters and velocity flowmeters are sensitive to changes in liquid viscosity. Especially at the low viscosity and the lower limit of the meter measurement range, there is currently no general viscosity correction formula. In the measurement of natural gas flow, the change of natural gas density has obvious influence on the velocity flowmeter such as turbine and vortex. If the flowmeter is used as the result of medium verification at low pressure, whether it can be directly used for natural gas under high pressure, online The real-flow verification becomes the only option to completely eliminate the influence of physical parameters, because dry verification and off-line verification cannot eliminate the influence of physical parameters on the above flow measurement instruments.
3. Different degrees of correction for the effects of operating conditions
The operating conditions or operating conditions of the flow measuring instrument directly affect its metering performance. The most direct influence of the operating pressure or temperature change on the flow meter is the change of the measuring chamber. The indirect effect is the physical properties such as the viscosity and density of the measured medium, and the indirect effect. Can be considered when correcting the influence of physical parameters. Due to the complexity of the structure and shape of the flow meter and the discreteness of the assembly, it is almost impossible to use the calculation method to accurately correct the change of the cavity with the operating conditions, and it is impossible to fit the accuracy of all the flowmeters according to the test data. The empirical formula required for degree.
For volumetric flowmeters, changes in the metering chamber can result in changes in the volume that is the basis of the measurement, which can cause changes in the amount of internal leakage, thereby affecting the flow measurement. For a velocity flow meter, changes in operating conditions will cause a change in its flow area, resulting in a change in meter factor. If the operating conditions of the flow meter can be the same or close to the actual use, the offline test can meet the requirements. However, due to the complexity and variability of the operating conditions, offline verification often cannot reproduce the actual operating conditions. Only online verification can Solve high-accuracy flow measurement problems, otherwise consider additional errors.
In summary, for flow measurement instruments, the instrument for dry verification is very limited, and it can only determine the uncertainty range of the flow measurement can not directly calibrate or assign the flow meter. Off-line verification is suitable for occasions where traffic accuracy is not high. It embodies the dynamic characteristics of traffic traceability, but it is difficult to trace the source or the transmission chain. On-line flow verification is the most suitable dynamic traceability feature of flow volume. It fully considers the physical parameters, operating conditions, environmental conditions, installation conditions and other effects, ensuring that the verification conditions are fully consistent with the actual conditions of use, and for the flow measurement of trade settlement. Instrumentation, both buyers and sellers hope to use online real-flow verification to achieve true measurement justice.

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