The difference between several verification methods of flow measuring instruments

Abstract: The information on the difference between several verification methods of flow measuring instruments is provided by excellent flowmeter and flowmeter manufacturers and quotation manufacturers. The difference between several verification methods of flow measuring instruments The verification of flow measuring instruments usually adopts two methods: real flow verification and dry verification. Which method to use to verify the flow meter depends on the measurement system's requirements for measurement uncertainty and the type of flowmeter being tested. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following are the details of the article on the differences between several verification methods of flowmeters. There are several verification methods of flow measuring instruments to distinguish the verification of flow measuring instruments. Usually, two methods of real flow verification and dry verification are used. Which method to use to verify the flow meter depends on the measurement system's requirements for measurement uncertainty, the type of flowmeter to be inspected, its use, the verification conditions it has, and the cost of verification. Generally, flow instruments used for liquid measurement (such as crude oil and water measurement instruments) basically adopt real flow verification, while flow instruments for gas measurement (such as differential pressure flow instruments for natural gas trade settlement) are mostly dry-type verification. method, only a few use the critical flow nozzle online real-flow verification or off-line verification. 1. Selection of several verification methods The verification of flow measuring instruments usually adopts two methods: real flow verification and dry verification. Which method to use to verify the flow meter depends on the measurement system's requirements for measurement uncertainty, the type of flowmeter to be inspected, its use, the verification conditions it has, and the cost of verification. Generally, flow instruments used for liquid measurement (such as crude oil and water measurement instruments) basically adopt real flow verification, while flow instruments for gas measurement (such as differential pressure flow instruments for natural gas trade settlement) are mostly dry-type verification. method, only a few use the critical flow nozzle online real-flow verification or off-line verification. 2. Differences in several verification methods 1. Differences in verification conclusions The combined measurement method is used to perform dry verification of the flow meter. The flow rate of the meter is synthesized according to the measurement results and uncertainty of each relevant parameter according to the error processing method. The measurement of the total uncertainty is to indirectly determine the uncertainty range of the flow meter with a certain degree of confidence, and it cannot give a specific error value. It is usually premised on a large amount of rich test data and standardized technical requirements, and maintains the characteristics of measurement experimentation and consistency. For example, standard orifice throttling devices, critical flow venturi nozzles, etc. have quite mature dry verification technologies. Taking the orifice flowmeter as an example, its outflow coefficient formula 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%. When synthesizing the uncertainty of orifice flow measurement, only a certain uncertainty range can be given with a certain degree of confidence. Real-flow verification, especially online real-flow verification, is most suitable for measurement features such as accuracy, consistency, traceability, and experimentation. It can realize real flow measurement instrument calibration or assignment, and can ensure the continuity and closure of value transmission or traceability. Offline verification gives the error value or flowmeter coefficient of the flow meter under the verification conditions, but because the actual operating conditions and installation conditions are different from the verification conditions, the relevant physical parameters of the medium and even the medium itself are also different. In fact, this verification is not True calibration or assignment. Strictly speaking, the off-line verification results of the flow meter can only describe its measurement characteristics under the verification conditions. Most of the actual use site environmental conditions, installation conditions and operating conditions of the instrument are very different from the verification conditions. It brings additional error to the flow meter, and the size of the additional error is always subjectively judged by a certain experience, so offline verification does not require high flow measurement results, or even if there is additional error, it can meet the expected measurement requirements. Simple and easy choice. 2. The correction degree of influence on physical parameters is different. The measurement results of almost all flow measuring instruments are affected by the physical parameters of the measured medium, but the degree of influence is different. For the physical parameters that can express their influence on the flow measurement results by an explicit function, as long as the actual values ​​of these parameters are known, they can be corrected, such as the relative density of natural gas, compression factor, isentropic index, etc. influences. However, for most flow measurement instruments, the influence of physical parameters on their measurement performance is difficult to accurately express with mathematical formulas. For example, in liquid measurement, positive displacement flowmeters and velocity flowmeters are very sensitive to changes in liquid viscosity. , especially at low viscosities and at the lower end of the meter's measuring range, there is currently no general formula for viscosity correction. In the measurement of natural gas flow, the change of natural gas density has a significant impact on the velocity type flowmeter such as turbine and vortex.If considering whether the result of the flowmeter using air as a medium under low pressure can be directly used for natural gas under high pressure, online real flow verification becomes the best choice to completely eliminate the influence of physical parameters, because dry verification and offline verification cannot eliminate physical parameters. Effects on the above flow measuring instruments.

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