## 3 Results and Discussion  
### Test Results for Density Matching Approach  
The Coriolis mass flowmeter for the test selected here is the type most frequently used in hydrogen refueling stations in Korea. The flowmeter is designed for flowrates from 0.2 kg/min to 10 kg/min. The maximum pressure is up to 1050 bar, and the temperature ranges are from \(-\)50 \({}^{\circ}\)C to 120 \({}^{\circ}\)C. The autoclave 3/8\" MP process connections were used for the installation. In order to ensure reliable zero-flow stability, zero-point adjustments of the Coriolis mass flowmeter were carried out under the operating pressure and temperature conditions in the present study. The measurement characteristics of the Coriolis mass flowmeter for the density-matching approach were assessed using reference sonic nozzles with different throat diameters and working pressure conditions in the High-Pressure Gas Flow Standard System of KRISS. The sonic nozzles, for which the diameters of the throat are 1.4 mm, 2.0 mm, 2.9 mm, and 3.6 mm, used for the reference flowmeter at different working pressures were calibrated with the gravimetric method using a diverter, a weighing tank,  
Figure 5: High-Pressure Water Flow Test Facility of KRISS. and a gyroscopic scale, all of which were traceable to the national SI-unit standard of KRISS, as shown in Figure 4. The four sonic nozzles with different throat diameters as used here are capable of covering a flowrate range of 1.0 kg/min to 4.5 kg/min, depending on the working pressure condition. The measurement performance tests of the Coriolis mass flowmeter at the four different working pressure conditions of 10, 20, 30, and 40 bar were conducted with the four sonic nozzles of the High-Pressure Gas Flow Standard System of KRISS. Figure 6 shows the test results of the Coriolis mass flowmeter at the different working pressures from 10 to 40 bar when using the four sonic nozzles for the flowrate range from 1.0 kg/min to 4.5 kg/min. The pressure conditions of the incoming air from 10 to 40 bar correspond to the different density conditions of 12 kg/min\({}^{3}\) to 46 kg/min\({}^{3}\). At each measuring point, three repeated measurements were carried out in different pressure and flowrate conditions. The error (%) is defined as the relative difference between the measurement results of the Coriolis mass flowmeter and the reference sonic nozzles at each pressure and flowrate condition. The error of the Coriolis mass flowmeter ranged from \(-\)2.0% to 0.5% within the flowrate range of 1.0 kg/min to 4.5 kg/min. Large errors occurred at a low flowrate, below 1.0 kg/min, from 10 to 30 bar, which is typical behavior of Coriolis mass flowmeters. Above 1.0 kg/min, the error was approximately linear, and the average error was within \(\pm\)0.2%. This tendency toward error of the Coriolis mass flowmeter with different pressure conditions is similar to previous research [9]. Figure 7 shows the test results of the Coriolis mass flowmeter at 40 bar and at an ambient temperature (around 20 \({}^{\circ}\)C) with the density set to approximately 46 kg/m\({}^{3}\), equivalent to hydrogen gas at 700 bar and \(-\)40 \({}^{\circ}\)C. The error of the Coriolis mass flowmeter was within \(\pm\)0.30% at flowrates from 1 kg/min to 4.5 kg/min, and the average error was within \(\pm\)0.20%. The repeatability of three measurements at each flowrate was less than \(\pm\)0.02. These results are similar to the typical water calibration results of Coriolis mass flowmeters installed at hydrogen dispensers at stations. The results are from the density-matching approach of the Coriolis mass flowmeters for hydrogen gas refueling conducted with an alternative fluid. However, in the present study the operating temperature for the density-matching approach was limited to an ambient temperature (around 20 \({}^{\circ}\)C).