In general, there are two ways to verify the measuring instruments: real flow test and dry type test. So, what is the way to calibrate the flow meters? This usually depends on the measurement uncertainty requirements of the measurement system, the type and use of the flowmeter to be tested, the verification conditions and the verification costs, and many other factors.
Next, for you to sort out the flow measurement instrument of several verification methods and differences, I hope to help you.
Selection of verification methods
1.Differences in verification conclusions
Dry calibration of flow meters by means of combined measurement method is based on the measurement results of relevant parameters and their uncertainties. The total uncertainty of flow meters is synthesized by error processing method. The uncertainty range of flow meters is determined indirectly by a certain degree of confidence. It can not give specific errors. Value. It is usually based on abundant test data and standardized technical requirements, maintaining the experimental and consistent characteristics of measurement. For example, the standard orifice throttle device, critical flow Venturi nozzle and other dry verification technologies have been quite mature.
Taking orifice flowmeter as an example, its outflow coefficient formula is based on abundant and sufficient experimental data. The error range of outflow coefficient given in the standard is not more than 0.6%. When the uncertainty of flow measurement of synthetic orifice plate is uncertain, only a certain range of uncertainty can be given with a certain degree of confidence.
Real-flow verification, especially on-line real-flow verification, is most in line with the measurement characteristics of accuracy, consistency, traceability and experimentation. It can realize the real calibration or assignment of flow measurement instruments, and ensure the continuity and closure of value transmission or traceability.
Off-line calibration gives the error value or flowmeter coefficient of flowmeter under calibration conditions, but because the actual operating conditions and installation conditions are different from the calibration conditions, the relevant physical parameters of medium and even the medium itself are different, in fact, this calibration is not a real calibration or assignment. Strictly speaking, the off-line calibration results of flow meters can only explain their measurement characteristics under the verification conditions. Most of the actual conditions of field use, installation and operation of the instruments are quite different from those under the verification conditions. This will bring additional errors to flow meters, and the magnitude of additional errors is always one. Because of the subjective judgment of certain experience, off-line calibration has low requirements for flow measurement results, or even if there are additional errors, it can meet the expected measurement requirements, which is a simple and easy choice.
2.The degree of correction for the influence of physical parameters is different.
Almost all the measurement results of flow meters are affected by the physical parameters of the medium under test, but the degree of influence is different. For the physical parameters which can express their influence on the flow measurement results by explicit function, we can modify them as long as we know the actual values of these parameters, such as the relative density of natural gas, compression factor, isentropic index and so on. But for most flow measurement instruments, the influence of physical parameters on their measurement performance is difficult to express accurately with mathematical formulas. For example, in liquid measurement, volumetric flowmeters and velocity flowmeters are very sensitive to the change of liquid viscosity, especially at low viscosity and the lower limit of instrument measurement range. There is no general formula for viscosity correction.
In the measurement of natural gas flow, the change of natural gas density has obvious influence on the constant velocity flowmeters such as turbine flowmeter and vortex flowmeter. When considering whether the results of flowmeter calibration using air as medium under low pressure can be directly applied to natural gas under high pressure, on-line real-flow calibration can completely eliminate the influence of physical parameters. The only choice is that dry calibration and off-line calibration can not eliminate the influence of physical parameters on the above flowmeter.
3.The degree of correction for operation conditions is different.
The operating conditions or operating conditions of flowmeter directly affect its metering performance. The most direct influence of the change of operating pressure or temperature on flowmeter is the change of its metering chamber. Its indirect influence is the physical properties of the medium under test, such as viscosity and density. The indirect influence can be considered when modifying the influence of physical parameters. Because of the complexity of the structure and shape of the flowmeter and the discreteness of the assembly, it is almost impossible to accurately modify its cavity with the change of operating conditions by using the calculation method, nor to fit the empirical formula to meet the accuracy requirements for all flowmeters according to the test data.
For volumetric flowmeters, the change of measurement chamber will lead to the change of volume as measurement reference, and the change of internal leakage, which will affect the flow measurement results. For the speed flowmeter, the change of operating conditions will cause the change of flow area, which will lead to the change of instrument coefficients. If the operating conditions of flow meters can be the same or close to the actual use, the off-line verification can meet the requirements. However, due to the complexity and variability of the operating conditions, off-line verification can not always restore the actual operating conditions. Only on-line verification can solve the problem of flow measurement with high accuracy, otherwise we should consider it. Additional errors.
Summary
In summary, for flow meters, dry calibration is very limited, and it can only determine the uncertainty range of flow measurement, and can not directly calibrate or assign the flow meters. Off-line verification is suitable for occasions where the accuracy of flow is not high. It reflects the dynamic characteristics of flow traceability, but it is difficult to achieve traceability or closure of the transmission chain. On-line real-flow verification is most in line with the dynamic traceability characteristics of flow value. It fully considers the physical parameters, operating conditions, environmental conditions, installation conditions and many other factors, and ensures that the verification conditions are fully consistent with the actual use conditions.