At NMIA, volumetric standards such as Brooks or bell provers are used to calibrate critical flow Venturi nozzles or “sonic nozzles”. These nozzles, which are extremely stable, are used by both NMIA and Australian accredited laboratories to establish continuous flows for the calibration of gas flow meters. For operational reasons, sonic nozzles are generally calibrated using dry air but later used with standard atmospheric air at various humidity levels either drawn or blown through the meterunder-test. Although the accepted theoretical calculations for determining the mass flow through a sonic nozzle incorporate corrections for the resulting change in air density, as laboratories seek to reduce uncertainties the validity of this assumption warrants further examination. In this paper we report on the calibration of sonic nozzles from 0.1 mm to 6.5 mm ID (0.005 to 25 m³/ h) in humidified air, at atmospheric pressure and room temperature, provided by a divided flow generator over the range from dry to 95%RH air. The measured change, of up to 0.12%, somewhat larger than predicted by the theoretical calculations of other workers, is comparable with the typical calibration uncertainty for calibrations using sonic-nozzles.

LNE has ability to calibrate micro gas flow rates using the dilution method in the range from 2 μg/s to 20 μg/s of nitrogen or 0.75 μg/s to 30 μg/s of helium. In addition, a primary constant pressure flowmeter for leak rates measuring from 0.05 μg/s to 35 μg/s is also available. In order to compare these reference facilities and to validate the dilution method below 30 μg/s LNE is developing a micro flow transfer standard (μFTS) with the collaboration of ATEQ France, manufacturer of control equipment for leak test. The flowmeter consists mainly of an array of three stainless steel capillaries designed to cover the ranges from 0.035 μg/s to 0.35 μg/s 0.35 to 3.5 μg/s and 3.5 to 35 μg/s of nitrogen (0.01 ml/h to 100 ml/h). A dynamic model of the mFTS determines the mass flow rate from the input presure, the differential pressure of the capillary, the gas temperature, the gas properties (viscosity and density) and the dimensional parameters of the capillary (length and radius). The comparison of both reference methods was carried out with the MFTS from 0.35 μg/s to 35 μg/s

In NIM, the old 2 m³ and 20 m³ facilities were built in 1986. The temperature stability of the collection tank was easily influenced by the surroundings. So, the new 0.1 m³ and 2 m³ pVTt facilities were built in 2013. The collection tanks were covered in water bath. The uncertainty for the new pVTt facilities were analyzed, and the measurement capablity was verified by comparisons among the new pVTt facility, the old pVTt facility and gas flow facilities in PTB with 14 sonic nozzles.

A primary high pressure air flow measurement standard was constructed at Center for Measurement Standards (CMS) in Taiwan with a capacity of 18000 m3/h and pressure range of 1 to 60 bar. With present calibration setup, the temperature of air flowing through the meter under test (MUT) decreased significantly during a test. To reduce thermal effect during meter calibration, improvements have been made by expanding the upstream air storage tank from 19 m³ to 34 m³, and installing a new compact sonic nozzle array (CSNA) downstream of the MUT as a new working standard (WS). A re-circulating loop at CMS was retrofitted and completed at the end of 2012. The calibration results of an ultrasonic flow meter (USM) between two different facilities and two different CSNA were compared. Results showed that after expanding the capacity of the upstream air storage tank, the change rate of air temperature during calibration could be reduced. With such decreasing rate, the time-delay problem of temperature measurement at the meter has been alleviated. The calibration of a DN 150 SICK USM showed good comparison between the two different facilities and the two different CSNA as working standard.

The paper describes the design, measurement results and uncertainty analyses of the hydraulic driven piston-prover system which is in operation at VSL since 2008. The 12 meter long, 0.6 m bore piston-prover is used for the realization of Reference Values for Gas-Volume at pressures between 1 and 65 bar(a) and ‘any’ type of Gas. 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 230 m³/h. The system is designed to calibrate reference meters. The Calibration and Measurement Capability (CMC) of the system is proven 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’.