First, the selection of several test methods
Flow measurement instrument verification, usually using real-flow test and dry test in two ways. Which way to check the flow meter depends on the measurement system requirements of measurement uncertainty, was seized flow meter type, use, with the test conditions, the cost of testing and many other factors. Generally, the flow meters used for liquid metering (such as crude oil and water meters) basically use the real flow test, while the gas flow meters (such as differential pressure flow meters used for settlement of natural gas trade) mostly adopt dry verification Way, only a very small number of critical flow nozzle online real flow test or offline test.
Second, the difference between several test methods
1. Different conclusions on the test
The combination of measurement methods for dry flow meter verification, is based on the measurement results of the relevant parameters and their uncertainty, in accordance with the error processing method of synthesis of the instrument flow measurement total uncertainty, is indirectly determined with a certain degree of confidence Flow measurement instrument uncertainty range, it can not give a specific error value. It is usually based on a large number of rich experimental data and standardized technical requirements as a precondition to maintain the characteristics of the experimental and consistent measurement. For example, the standard orifice throttling device, the critical flow Venturi nozzle has a very mature dry test technology. Taking the orifice flowmeter as an example, the outflow coefficient formula is based on extremely rich and sufficient experimental data. The error range of the outflow coefficient given by the standard is not more than 0.6%. In the synthesis of orifice flow measurement uncertainty, it can only give a certain degree of confidence in a certain range of uncertainty.
Real-flow verification, especially on-line flow verification, is the most accurate, consistent, traceable, and experimental metrology feature that enables true flow meter calibration or assignments to ensure continuity and containment of volume delivery or traceability. Off-line calibration gives the flow meter under test conditions, the error value or flow meter coefficient, but because of its actual operating conditions and installation conditions are different from the test conditions, the media of the physical parameters and even the media itself is different, in fact, this test is not True calibration or assignment. Strictly speaking, the flowmeter off-line verification results only indicate its measurement characteristics under test conditions, most of the actual use of the site environmental conditions, installation conditions and operating conditions of the instrument and the test conditions are very different, so will The additional error to the flow meter, and the additional error size is always a subjective experience to judge, so off-line verification for the flow measurement results are not high, or even with additional error can meet the expected measurement requirements, after all, a Easy to choose.
2. The correction of the influence of physical parameters is different
The measurement results of almost all flow measuring instruments are affected by the relevant physical parameters of the measured medium, but the influence degree is not the same. For physical parameters that can express their influence on flow measurement by explicit function, they can be corrected by knowing the actual value of these parameters, such as the relative density of natural gas, compression factor, isentropic index, etc. influences. However, for most flowmeters, the influence of physical parameters on theirmeasuring performance can not be accurately expressed by mathematical formulas. For example, in liquidmeters, positive displacement flowmeters and velocity flowmeters are sensitive to changes in liquid viscosity , Especially in the low viscosity and the lower limit of the instrument measuring range, there is no universal viscosity correction formula. In the natural gas flow measurement, natural gas density changes on the turbo, vortex and other speed flowmeter has a significant impact, if you consider the flow meter under low pressure with air as a medium test results can be directly used in high pressure natural gas, online Actual flow verification becomes the only option to completely eliminate the influence of physical parameters because dry verification and off-line verification can not eliminate the influence of physical parameters on the above-mentioned flow measurement instruments.
3. The impact of the operating conditions to amend the degree of difference
Flow measuring instruments operating conditions or operating conditions directly affect the measurement performance, operating pressure or temperature changes on the flow meter is the most direct impact on the changes in its metering chamber, the indirect impact of the measured medium viscosity, density and other physical, indirect impact Can be considered when modifying 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 apply a calculation method to precisely correct its cavity with the change of operating conditions. It is also impossible to accurately fit all the flowmeters according to the experimental data Degree of experience required formula.
For positive displacement meters, changes in the metering chamber will result in a change in volume as a basis for measurement, which will cause changes in the internal leakage to affect the flow measurement. For the speed flowmeter, changes in operating conditions will cause changes in its flow area, resulting in changes in instrument coefficients. However, due to the complexity and variability of the operating conditions, off-line verification often fails to reproduce the actual operating conditions, and only online verification can be performed Solve the problem of high-accuracy flow measurement, or consider additional errors.
In summary, the flow meter, dry test for the instrument is very limited, and it can only determine the uncertainty of the flow measurement range can not directly calibrate the flow meter or assignment. Offline verification is suitable for occasions where the accuracy of traffic is not critical. It reflects the dynamic characteristics of traffic tracing, but it is difficult to trace the source or the closure of the transmission chain. Online real-time verification is most suitable for the dynamic traceability characteristics of the flow rate value. It takes into full account the physical parameters, operating conditions, environmental conditions, installation conditions and many other effects to ensure that the test conditions and actual conditions of use is fully consistent, for trade settlement flow measurement Instrumentation, both buyers and sellers hope to use online flow verification, in order to achieve a real measurement of justice.