###### Abstract  
Hydrogen fuel cell electric vehicles are emerging as a means of transportation using renewable and carbon-free energy due to global warming and air pollution. Hydrogen fuel cell electric vehicles are typically refueled at a wide range of temperatures (\(-40\)\({}^{\circ}\)C to 85\({}^{\circ}\)C) in hydrogen refueling stations in accordance with globally accepted standards. Currently, there is no traceable method by which to verify and calibrate the Coriolis mass flowmeters used at hydrogen refueling stations, except for a water calibration process as a conventional method for mass flowrate calibration. To verify the hydrogen flow metering to a suitable level of accuracy under the challenging condition of high pressures and a wide range of temperatures, necessary methodologies and calibration facilities are developed in the present study. A flow measurement characteristic test of the hydrogen mass flowmeter under identical density conditions of the refueled hydrogen was conducted using the high-pressure gas flow standard system of the Korea Research Institute of Standards and Science to assess the effects on the medium and pressure of the mass flowmeter in a density-matching approach. To investigate the pressure dependence of the mass flowmeter at a hydrogen refueling station, a high-pressure water flow test was conducted in the pressure range of 2 bar to 700 bar, which is a pressure-matching approach. Finally, the KRISS Hydrogen Field Test Standard based on the gravimetric principle was developed to verify the measurement accuracy of the mass flowmeter to be used at hydrogen refueling stations for the first time in Korea. hydrogen refueling station; hydrogen flow metering; Coriolis mass flowmeter +
Footnote †: journal: Accepted: 7 December 2022  
station is shown in Figure 1. Hydrogen refueling stations typically dispense high-pressure hydrogen gas of around 5 kg into the tanks of hydrogen vehicles, with wide span ranges of pressure and temperature [2,3]. A pre-cooling process to lower the temperature to \(-40\) degC is done to prevent any substantial increase in the temperature of the compressed hydrogen gas during the fueling process, given that the pressure is as high as 70 MPa in the tank of the hydrogen vehicle. This process of fueling a hydrogen vehicle at a hydrogen fueling station is in accordance with the worldwide accepted fueling protocol SAE J2601 [4]. To measure the amount of hydrogen gas dispensed into the hydrogen vehicle for customer billing, a Coriolis mass flowmeter is typically installed inside the dispenser at a hydrogen refueling station. Because the hydrogen fueling process operates under a condition in which the flowrate, pressure, and temperature vary widely, a Coriolis mass flowmeter, which can directly measure the mass flow, is used to determine the hydrogen filling amount at a hydrogen refueling station. In order to ascertain the measurement reliability of hydrogen gas transferred to the hydrogen vehicle for customer billing, the international recommendation OIML R 139-1 requires certain accuracy levels for the flowmeters used at hydrogen fueling stations [5]. Table 1 shows the two classes of the maximum permissible error (MPE), which are defined as class 2 and class 4 to represent corresponding accuracy rate limits of 1.5% and 2% for these flowmeters. The MPE values are 2% to 5% for the two accuracy classes to ensure a complete measuring system during the type evaluation, verification, and in-service inspection processes that must take place when the stations are operating. Several studies have investigated how to inspect and verify such accuracies of flow measurements under hydrogen-filling operating conditions at hydrogen refueling stations. It should also be noted that several portable gravimetric standards in national metrology institutes and the manufacturing companies of hydrogen dispensers in several countries have been developed for field verification according to the international recommendation for hydrogen refueling stations [6,7].