## 3 Results and Discussion
### Matching Approaches and Hydrogen Field Test Standard  
Figure 11 shows the comparison of the calibration test results of the Coriolis mass flowmeters by the density-matching approach and the pressure-matching approach under the vehicle hydrogen refueling condition at hydrogen refueling stations. The two measurements were taken with air at a pressure of 40 bar with the high-pressure Gas Flow Standard System of KRISS and with water at a pressure of 2 bar to 700 bar using the high-pressure water flow test facility. As shown in Figure 11, the test results between the density-matching approach and the pressure-matching approach were within the corresponding measurement uncertainty rates. The difference in the errors between the results of the two approaches is due to the speed of sound being different in air and water. Although the medium was not hydrogen gas but was instead a substitute fluid, i.e., compressed air or water, and the pressure conditions differed from those when refueling at a hydrogen station, this is the first study in Korea to investigate the measurement performance capabilities of the Coriolis flowmeters for use at hydrogen stations with alternative calibration by a density-matching approach and a pressure-matching approach. In Korea, the Coriolis mass flowmeters in the hydrogen refueling station are installed near the upstream of the  
Figure 10: Measurement results of the Coriolis mass flowmeter at the inlet pressure condition from 2 bar to 700 bar.  
pre-cooler; this means that the operating condition of the flowmeter is not as cold as the condition after the pre-cooler (\(-40\)\({}^{\circ}\)C). Nevertheless, the temperature dependency on the Coriolis mass flowmeter should be addressed in the density and pressure-matching approaches. If temperature dependency for the operating condition of the Coriolis mass flowmeters is addressed in the future, with the development of new facilities for temperature control in the operating conditions for flowmeters, the results of these approaches can be presented as the best means of accurately calibrating Coriolis mass flowmeters for hydrogen refueling stations where there is no facility to calibrate them with hydrogen gas at 700 bar and \(-40\)\({}^{\circ}\)C.  
According to the newly enforced regulation of the High-Pressure Gas Safety Control Act, KRISS is developing a new Hydrogen Field Test Standard, as shown in Figure 12. This new facility utilizes a trailer and is designed to maintain safe and strict explosion-proof rules. Field experiments with KRISS HFTs are planned at hydrogen refueling stations for the first time in Korea in the near future. The Coriolis mass flowmeter used in the density-matching approach and pressure-matching approach will be installed at the KRISS HFTs during field experiments at the hydrogen refueling stations. This will allow a comparison between the test results from hydrogen gas and alternative fluids such as compressed air or water with the density- and pressure-matching approaches.  
Figure 11: Comparison between the test results of Coriolis mass flowmeters for the density-matching approach and the pressure-matching approach.