Evaluation of Measurement Uncertainty of Gas Flow Device with Steam Flow Meter Calibration Function

Abstract: The measurement uncertainty evaluation information of the gas flow device with the calibration function of the steam flow meter is provided by the excellent flow meter and flow meter production and quotation manufacturers. Abstract: Taking the critical flow venturi nozzle method gas flow standard device as the representative, this paper briefly expounds the measurement uncertainty evaluation method and steps of the standard table method gas flow standard device with the function of steam flowmeter calibration. 1. Introduction of gas flow standard device. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the details of the measurement uncertainty evaluation article for gas flow devices with steam flowmeter calibration function. Taking the critical flow venturi nozzle method gas flow standard device as the representative, the method and steps of measurement uncertainty evaluation of the standard table method gas flow standard device with the function of steam flowmeter calibration are briefly expounded. 1. Introduction of gas flow standard device The standard meter method flow standard device uses the standard flowmeter as the standard device, so that the fluid continuously passes through the standard flowmeter and the tested flowmeter, and compares the output instantaneous flow value of the two, or compares the cumulative amount over a period of time. flow value to determine the metering performance of the spare flowmeter. As a typical standard table method gas flow standard device, the critical flow sonic nozzle gas flow standard device is mainly composed of gas source, test pipeline, parallel critical flow sonic nozzle group and control equipment. In 2007, the company built a set of gas flow standard device with the critical flow sonic nozzle group as the standard table. The device is used as the flow standard shared by the two medium flowmeters of compressed air and steam. The most scientific and authoritative steam flowmeter is used in the design. The mathematical model of temperature and pressure compensation, through the gas flow device and the national steam flow measurement Yantai steam flow standard device real flow calibration comparison, data fitting processing, to ensure accurate and reliable steam flow meter calibration. 2. Uncertainty evaluation of gas flow standard device 2.1 Mathematical model The device adopts critical flow sonic nozzle as the standard table, the fluid medium is air, and the calculation formula of the actual mass qm and volume flow qv of a single critical flow sonic nozzle is: middle:ρ, p, T are the air density (kg/m3), absolute pressure (Pa), thermodynamic temperature (K) A—Nozzle throat area; (mm) C—Nozzle Outflow Coefficient; C*—Critical flow function; P0—Stagnation pressure of the gas before the nozzle (Pa); T0—The stagnation temperature of the gas before the nozzle (K); R—Universal gas constant, its value is 8314.51J/(kmol.K); M—The mass of air in kilomol, its value is 28.9625kg/(kmol). As a standard meter gas flow standard device, the device utilizes multiple critical flow sonic nozzles (standard flow meters) in parallel to accumulate the gas flow through each critical flow sonic nozzle to expand the gas flow range of the device. Since each critical flow sonic nozzle works independently, the measurement uncertainty of the gas flow of the device after the parallel combination of n critical flow sonic nozzles can be expressed as The parallel combination of tubes will not reduce the measurement uncertainty of the standard device. At this time, the measurement uncertainty of the device must not exceed the maximum value of the gas flow uncertainty of a single critical flow sonic nozzle in the nozzle group. Therefore, according to the mathematical model of formula (1), the uncertainty of gas flow measurement of the device after combining multiple critical flow sonic nozzles can be determined by evaluating the uncertainty of gas flow rate of a single critical flow sonic nozzle. 2.2 Evaluation of the standard uncertainty of the input quantity Since the verification and use of the sonic nozzle are all carried out in the air medium, the values ​​of A, C*, M and R are the same, so the uncertainty introduced by these four items can be ignored. (1) The uncertainty of the outflow coefficient C is in accordance with the requirements of JJG620-2008 'Critical Flow Venturi Nozzle Verification Regulations', with a 95% confidence probability, U=0.2%. Because the number of measurements is 6, t95=2.45 can be obtained, then there are (2) the uncertainty of the stagnant pressure P0 at the inlet of the sonic nozzle. The full scale of the pressure transmitter used is 1000kPa, and the allowable error is 0.1% of the full scale. Therefore, , when P0 takes 800kPa, according to the uniform distribution, there are (3) the uncertainty of the stagnation temperature T0 at the entrance of the sonic nozzle. The full scale of the temperature transmitter used is 100 °C, and the allowable error is 0.1% of the full scale; consider Therefore, when T0 is 300K, according to the uniform distribution and 90% reliability, there are (4) the uncertainty of temperature T and pressure P at the flowmeter under test. The measuring instruments for temperature T and pressure P at the flowmeter under test in the device are the same as those for measuring the stagnant temperature and pressure at the inlet of the sonic nozzle, and the temperature inhomogeneity in the test pipeline is considered to be 0.5K.

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