Accurate gas properties are needed to take full advantage of the low uncertainties provided by NIST’s Gas Flow Calibration Services. If a flowmeter user and NIST use different values for these properties (molecular mass, compressibility, density, viscosity, and critical flow factor), the user’s flow measurements will have errors. Since January 2009, calibrations conducted by NIST’s Fluid Metrology Group use the NIST-supported database REFPROP version 8.1 to reduce calibration data. Prior to 2009, NIST’s calibration data were reduced using REFPROP 7.0. Flowmeter users who recalibrate their meters at NIST and ignore the January 2009 change of database may erroneously conclude that either their meters or NIST’s standards are not stable.
The newer database, REFPROP v. 8.1, calculates properties for mixtures that include up to 2 % water vapor. By measuring the dew point temperature of air used during our calibrations and by incorporating the REFPROP v. 8.1 dynamic link library (REFPROP.dll) into our Labview* and Excel data acquisition and reduction programs, NIST reduced the uncertainty of air flow calibrations from 0.05 % to 0.025 %. NIST now reduces calibration data for critical flow venturis using the "real" critical flow factor generated by REFPROP v. 8.1 instead of approximate "ideal" critical flow factors. This change improved the mutual consistency of critical flow venturi calibrations performed in N₂ and air from > 0.05 % to < 0.012 %. In anticipation of future calibrations at higher pressures, we tabulate estimates of property uncertainties for pressures up to 70 MPa for air, argon, helium, N₂ , CO₂ , H₂ and CH₄ by comparing REFPROP to primary data sources.

More and more chemical analysis laboratories using gas chromatographs need to measure the different flowrates involved during the analysis process (split ratio, column flow). The measurement of the column flow is certainly the most difficult because the flowrate of carrier gas is generally very small. For example, the flowrate of helium which is the most often used carrier gas is typically less than 15 µg/s (5 cm³/min). To calibrate helium flowmeters, the "traced gas method" developed at LNE for nitrogen has been optimized for helium microflow measurements in the range of 0.75 µg/s to 30 µg/s. The traced gas method, the uncertainty estimation of the calibration bench and the calibration results of a laminar flow element type Molbloc and an industrial mass flowmeter are presented in this paper.

Thermal time-of-flight (TOF) technology has been considered to be one of the most effective approaches that could provide an accurate flow measurement at ultra low flow speed. While the technology remains on paper for over half a century without real implementation, the lack of market drive may be one of the major reasons. Current demands in energy management such as city natural gas metering, medical applications for respiratory machines and others have revitalized this technology. Thermal TOF technology in principle can provide accurate flow speed measurements for gases regardless of its gas compositions. However, traditional design of TOF sensors is often very vulnerable to fluidic conditions, in particular, moisture, particles and other contaminations. In this paper, we present a thermal TOF gas flow meter that is equipped with a robust MEMS thermal TOF sensor which can be used in fluid where moisture and particles exist. The design and fabrication of the MEMS TOF sensor are described followed by its circuit scheme. The design of the flow meter and the test results are presented.

Thermal mass flow meters are commonly of full scale accuracy and not considered for applications where custody transfer is required. Recent advancement of MEMS mass flow technology in city natural gas metering has demanded a better calibration and verification procedure so that the custody transfer can be justified with regard to the traditional thermal mass flow meter technology. The thermal mass flow sensor could be impacted by the humidity, gas composition and temperature. Calibrations with one standard may often be not applicable to the others if each of the factors is not fully accounted; even the meter itself has been well designed and immune to the variables. In this paper, we will discuss the calibration procedures and measurement uncertainties of the MEMS mass flow meters. The results indicated that with the MEMS meter designed for processing the variables, the MEMS thermal mass flow meters either with calorimetric and/or energy dispersion principle can be applied for custody transfer with a generally acceptable accuracy of ±1.5% or better.

A simple, accurate, relatively compact and light gas mass flow meter has been developed with diagnostic capabilities. This fundamental mass flow meter concept revises a simple concept originally suggested four decades ago before the computer power existed to make a practical system. This paper revises the concept, discusses the development of a modern meter design and adds new diagnostic capabilities. The mass meters outputs are the volume flow rate, the fluid density and the mass flow rate. The system does not require the fluid density from an external source and can meter gas mass flows and measure gas density at low pressures. The system has significant qualitative diagnostics capable of signaling when the meter is malfunctioning. In cases where the fluid density is known from an external source some quantitative diagnostics are also available.