Leakage flow rate is one of the most important contributions to the measurement uncertainty when measuring small gas flow rates with a clearance-sealed piston prover. Our previous study has shown ...that the systematic effects related to the change of the gas viscosity can be successfully corrected, whereas the effects related to the reproducibility of the leakage flow rate are poorly understood. This paper focuses on the interpretation and correction of the reproducibility related effects by analysing experimentally identified variations of the leakage flow rate and the gas pressure in the inclined piston prover. Small changes of the leakage flow in the inclined piston prover indicate that the position of the piston relative to the cylinder remains approximately the same. The obtained relationship between the leakage flow rate and the gas pressure below the piston led to the pressure-based correction of the leakage flow rate.
•The leakage flow rate represents important uncertainty contributions at low gas flows.•The study tries to explain the effects related to the reproducibility of the leakage flow rate.•The flow cell inclination is not a significant uncertainty contribution to the leakage flow rate.•The variations of the leakage flow rate can be linked to the pressure difference acting on the piston.•The pressure correction reduces the uncertainty of the leakage flow related to its reproducibility.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
This paper deals with an uncertainty analysis of gas flow measurements using a compact, high-speed, clearance-sealed realization of a piston prover. A detailed methodology for the uncertainty ...analysis, covering the components due to the gas density, dimensional and time measurements, the leakage flow, the density correction factor and the repeatability, is presented. The paper also deals with the selection of the isothermal and adiabatic measurement models, the treatment of the leakage flow and discusses the need for averaging multiple consecutive readings of the piston prover. The analysis is prepared for the flow range (50 000:1) covered by the three interchangeable flow cells. The results show that using the adiabatic measurement model and averaging the multiple readings, the estimated expanded measurement uncertainty of the gas mass flow rate is less than 0.15% in the flow range above 0.012 g min−1, whereas it increases for lower mass flow rates due to the leakage flow related effects. At the upper end of the measuring range, using the adiabatic instead of the isothermal measurement model, as well as averaging multiple readings, proves important.
•The piston provers are in general susceptible to non-stable ambient temperature.•The temperature inhomogeneity in the piston prover is related to the mass flow errors.•The magnitude of heat transfer ...effects decreases with the gas flow rate.•Covered flow cell shows systematic flow errors at stable temperature conditions.
This paper deals with the quantification of ambient temperature effects on the measured mass flow rate of a gas using a clearance-sealed piston prover. The variations of temperature were simulated by placing the piston prover in a climate chamber. The tests were made for flow rates of air between 13 mg/min and 15 g/min for three different flow cells. The temperature in the chamber and the piston prover, and the deviations of the piston prover’s readings from the flow source, were recorded. The results confirm that the temperature inhomogeneity in the piston prover can be related to the observed mass flow errors, which increase with the magnitude and the rate of temperature change and decrease with the gas flow rate. Placing the measurement cylinder under a protective cover reduces the sensitivity to variable ambient conditions, but introduces a systematic mass flow error, even in the case of a stable temperature.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
The leakage flow rate represents an important input quantity in the measurement model for the gas flow rate in a clearance-sealed piston prover, especially when measuring small flow rates. This study ...deals with the experimental identification of leakage flow effects. The following influences were examined: the flow rate supplied to the piston prover, the inclination of the flow cell, the type of gas and the temperature of the gas. The leakage flow rate was measured during the typical operation of the piston prover using the dynamic summation method. The tests were carried out in a climate chamber to ensure stable and homogenous temperature conditions. It is demonstrated that the leakage flow rate is significantly affected by the piston prover's inclination and the viscosity of the gas, which depends mainly on the type and the temperature of the gas. Based on the experimental findings that confirm the leakage flow rate is inversely proportional to the gas viscosity, a correction model was proposed. The uncertainty analysis shows that reproducibility is the most important component in the combined uncertainty of the leakage flow rate.
This paper presents a computational model written in Modelica for the high pressure piston prover (HPPP) used as the national primary standard for high pressure natural gas flow metering in Germany. ...With a piston prover the gas flow rate is determined by measuring the time a piston needs to displace a certain volume of gas in a cylinder. Fluctuating piston velocity during measurement can be a significant source of uncertainty if not considered in an appropriate way. The model was built to investigate measures for the reduction of this uncertainty. Validation shows a good compliance of the piston velocity in the model with measured data for certain volume flow rates. Reduction of the piston weight, variation of the start valve switching time and integration of a flow straightener were found to reduce the piston velocity fluctuations in the model significantly. The fast and cost effective generation of those results shows the strength of the used modelling approach.
Piston prover has been widely used as a gas flow standard for its advantages of high accuracy in standard volume, flow stability and repeatability. It has also been employed as the primary gas flow ...standard in many countries to calibrate meters. However, it is difficult to ensure the uniformity of the inside dimension of the piston, thus the application of conventional piston provers are limited by the maximum calibration flow generated by the piston cylinder volume. In this paper, an improved piston gas prover that mainly consists of two uniform plungers was proposed. Their external diameter constitutes the flow standard. The plungers are driven by servo motor, and the high speed fieldbus EtherCAT has been introduced as the control unit. Hence the two pistons could work collaboratively and operate in three modes: single-piston mode, double-pistons parallel mode, and double-pistons reciprocating mode. Besides generating steady-flow rate, the double-plunger prover can even produce an unsteady-flow rate which could be used to research the dynamic characteristics of flow meters. The structure and working principle of the three modes were carefully introduced. Then experiments for calibrating critical nozzles were carried out, and the results show that the repeatability of the discharge coefficient could be better than 0.06%, and the pressure fluctuation during the process was less than 50 Pa.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This paper deals with heat exchange effects in a compact, high-speed, clearance-sealed version of a piston prover for gas flow measurements that has the temperature measurements limited to the ...time-averaged temperature of the gas flow. A lumped-element mathematical model is used to study the physical background of the heat exchange effects. Experimental testing is performed to validate the theoretical results, estimate the required temperature homogeneity in the piston prover and propose a modified measurement model that considers the heat exchange effects. These effects are almost linearly related to the temperature difference between the gas flow into the piston prover and the cylinder wall, with the sensitivity coefficient being dependent on the measured flow rate. The piston-prover configuration with the gas temperature sensor in the mixed inlet /outlet flow is found to be advantageous in comparison to a measurement of the inlet temperature.
The paper describes the design, measurement results and uncertainty analyses of the hydraulic driven piston-prover system which has been in operation at VSL since 2008. The 12-meter long, 0.6m bore ...piston-prover is used for the realization of Reference Values for Gas-Volume at pressures between 1 and 65bar(a) at several gases. The principle is based on the displacement of a piston acting as a Gas–Oil separator. The standard has a flow-rate range from 5 to 230m3/h. The system is designed to calibrate reference meters. The Calibration and Measurement Capability (CMC) of the system is proven to be smaller than 0.1% (k=2). The paper also explains the coherence between the Gas–Oil piston-prover and other traceability generators and ‘flow rate bootstrapper systems’.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
We describe a flow standard for gas flows in the range from 0.01sccm to 100sccm with a relative standard uncertainty (68% confidence) of 0.03% at 1sccm (1sccm≡1cm3/min of an ideal gas at 101325Pa and ...0°C ≈ 0.74358μmol/s). The flow standard calibrates a secondary meter by withdrawing a piston from a cylinder held at constant pressure P while gas flows from the secondary meter into the cylinder. The flow standard can operate anywhere in the range 10kPa<P<300kPa, and it can act as a flow source as well as a flow receiver. The flow standard incorporated features that improved its convenience and lowered its cost without sacrificing accuracy, specifically (1) dry sliding seals made with commercially available, easily replaced, o-rings, (2) a compact design based on a commercially available, hollow piston, and (3) a linear encoder with a small Abbe error.
•Primary standard for gas flows in the range from 0.01 to 100cm3/min.•It can act as a flow source or a flow meter at pressures from 10kPa to 300kPa.•Its standard uncertainty at 1cm3/min is 0.03%.•Innovations improve convenience and lower cost without sacrificing accuracy.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
The dynamic pressure effects and their corrections in a high-speed, clearance-sealed realization of a piston prover for gas flow measurements are discussed. The experimental results show the ...deterministic, rather than stochastic, nature of the dynamic pressure conditions and, consequently, the repeatable nature of their influence on the flow measurements. The experimental validation proves the advantage of the polytropic/adiabatic pressure correction model, which was proposed by the authors, as compared with the isothermal pressure correction model. The paper ends with an estimation of the measurement uncertainty related to the pressure corrections using either the adiabatic or isothermal model.
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