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The additional components of uncertainty of measurement result of unsteady flow by means of pressure differential flowmeter are investigated in the article. In particular, the authors considered in detail the technique for estimating the additional component of uncertainty, caused by the nonlinearity of the flowrate dependence on the pressure drop on the standard primary device, as well as the additional component, which is related to the absence of the inertial term in the root expression of the quasi-steady flow equation. On the example of four differential pressure signals that reproduce the pulsating and unsteady flow modes of the fluid, the estimating the named components is carried out and the dependence of these components of uncertainty on the basic characteristics of the differential pressure signal (root mean square amplitude of pulsations and the maximum relative deviation) is demonstrated. It is shown that the increase of these signal characteristics leads to an increase in the corresponding components of the uncertainty and, consequently, in the resulting additional uncertainty of the measured value of flowrate. Particularly, for the investigated pressure drop signals for which the root mean square amplitude does not exceed 20% of the mean pressure drop, the resulting additional uncertainty of the measured flowrate reaches 0.5%. Since the additional uncertainty caused by unsteady flow is systematic and has a positive sign, it is possible to determine the correction coefficient based on this additional uncertainty and to correct the measured value of the flowrate by considering the correction factor in the flow equation. However, the authors propose to correct the measured value of the flowrate only if it is impossible to identify the causes of unsteady flow and to eliminate their impact on the flowrate.

This work deals with the investigation of influence of flow pulsation on the accuracy of gas flowrate and volume measurement by means of the differential pressure flowmeters. Experimental studies of the step response curves of an impulse line with a pressure transducer (PT) were carried out with application of a high-frequency analog-to-digital converter of the pressure signal in the PT chamber. Mathematical model of an impulse line with a PT was built. This model provides simulation and investigation of the transient processes with high accuracy. The influence of the impulse line design (length, diameter) on the step response curves and on the frequency response curves as well as on the resonant frequency of the oscillating system (an impulse line with a PT chamber) was studied. The systematic errors of flowrate and volume measurement due to the resonance in the impulse lines which was caused by flow pulsation were simulated and studied for an acting differential pressure natural gas metering system. On the basis of the calculation results, it was defined that amplification of the amplitude of the oscillating signal of the differential pressure due to the resonance may lead to a negative systematic error of gas volume measurement. The systematic errors of gas flowrate and volume measurement caused by the flow pulsation can be reduced by choosing the correct design of the impulse lines in order to avoid the resonance.

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