###### Abstract
In a cooperative project for hydrogen vehicles by several European national metrology institutes, mobile gravimetric standards for verifying the measurements by the flowmeters inside the hydrogen dispensers were developed [8]. Several hydrogen refueling stations were tested in Europe using the gravimetric standard  
Figure 1: Sample layout for a hydrogen refueling station and the Coriolis mass flowmeter in the dispenser. developed during this project for the purpose of research related to the MPE acceptance criteria as defined in OIML R 139-1, which is the international recommendation [6; 7; 8]. The Korea Research Institute of Standards and Science (KRISS) also developed a Hydrogen Field Test Standard (HFTs) that can be used for the field verification and calibration of the measurement accuracy of the mass flowmeters used at hydrogen refueling stations, as shown in Figure 2. The KRISS HFTs consists of three 52 L pressure cylinder tanks (type IV), a 300 kg weighing scale with a 0.5 g resolution, the Coriolis mass flowmeter, needle valves, temperature and pressure sensors, and a receptacle. The testing method is based on the gravimetric principle. The hydrogen delivered from the receptacle is passed through the Coriolis mass flowmeter, and the amount is determined by the weighing of the hydrogen gas collected in the pressure tank on the weighing scale. However, only hydrogen vehicles are allowed access to hydrogen gas at hydrogen refueling stations in Korea under the Korean High-Pressure Gas Safety Control Act for all high-pressure gas, including hydrogen gas. A field test at a hydrogen refueling station using the KRISS HFTs could not be carried out due to legal and safety restrictions in Korea. Recently, regulations in the High-Pressure Gas Safety Control Act have been revised to enable filling processes for verification and research purposes at hydrogen refueling stations. Because administrative evaluations and certification are still required, actual field experiments with the KRISS HFTs have not been conducted thus far. Currently, there is no traceable method by which to verify and calibrate the Coriolis mass flowmeters to be used at hydrogen refueling stations, except for a water calibration process as a conventional method for mass flowrate calibration. Typically, the Coriolis mass flowmeters in the hydrogen dispenser in Korea are calibrated with water by their manufacturers. In order to address these measurement challenges, several approaches to calibrate mass flowmeters for hydrogen refueling stations have been devised using alternative fluids \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Accuracy Class**} & \multirow{2}{*}{\begin{tabular}{c} **MPE for the Completer Measuring System** \\ **(in \% of the Measured** \\ **Quantity Value)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **MPE for the Completer Measuring System** \\ **(in \% of the Measured** \\ **Quantity Value)** \\ \end{tabular} } \\ \cline{3-4}  & \begin{tabular}{c} **(in \% of the Measured** \\ **Quantity Value)** \\ \end{tabular} & \
\begin{tabular}{c} **Type Evaluation** \\ **Initial or Subsequent** \\ **Verification** \\ \end{tabular} &
\begin{tabular}{c} **In-Service Inspection** \\ **Under Rated Operating** \\ **Condition** \\ \end{tabular} \\ \hline
2 & 1.5 & 2 & 3 \\ \hline
4 & 2 & 4 & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: MPE value for the hydrogen meter and measuring system in OIML R 139-1 [5]. Figure 2: KRISS Hydrogen Field Test Standard (primary gravimetric system for flowmeters at hydrogen refueling stations). and matched operating conditions, such as the pressures and temperatures [9; 10]. The performance and the measuring behavior of Coriolis mass flowmeters designed for use in hydrogen refueling stations were investigated with different fluids, including air and water with different pressure and temperature range conditions to simulate the fueling conditions of hydrogen gas when it is transferred to a hydrogen vehicle.