[en] This report presents a detailed description of the High Pressure Gas Loop at RNL and its operation for the measurement of pressure drop v flow rate characteristics of test components. The computer analysis of the measured data is described and a typical Euler number v Reynolds number characteristic is included as an example. It is shown that the shape of the curve-fitted characteristic can be determined to better than +- 1% on Euler number; in line with the accuracy required in the setting of fast reactor sub-assembly flows. (author)