The high-pressure piston prover is at the top of the calibration chain for flow measuring devices in the gas transport networks of Germany and Europe. It is implemented in pigsar™, a calibration facility for gas flow measuring devices, which is operated as a grid-connected test bench. Due to econonical and technical reasons, pigsar™ will be extended by a new, significantly larger test stand part, which will be designed as a closed-loop system. The Physikalisch-Technische-Bundesanstalt (PTB) is therefore building a new primary standard, which significantly larger and new, untested technology, the so-called Flow Comparator. Due to the new concept and the closed-loop system, the dynamic behaviour and interactions of the two systems have to be determined.