PRELIMINARY EVALUATION OF A NEW IMGC-CNR FLOWMETER PROTOTYPE

摘要

In a new IMGC-CNR flowmeter prototype, still based on the constant pressure-variable volume method, volume variations (ΔV) are generated by a piston-bellows coupling. Such mechanical arrangement permits to avoid any oil or O-ring connected with the moving parts inside the measuring volume, that, in the present case, is essentially given by the internal volume of the bellows. The piston-bellows coupling guarantees both the advantages of the accurate ΔV measurements of the moving piston method and leak sealing of the bellows, without introducing complicated measurement procedures; in fact ΔV values are calculated by knowing the piston section and its displacement, but the effective ΔV is generated by the bellows directly connected to the piston itself. The most important part of the work is represented by the search, and consequently the construction, of the best piston related to the chosen bellows, in order to minimize the volume correction due to not avoidable differences between the piston and bellows sections. Several pistons have been constructed and tested to get the best coupling, which is represented by a minimum correction to ΔV given by the piston. The tests consist in the measurement of pressure variations in a suitable chamber surrounding the bellows, when the piston is moving (the optimum is a null variation). In the final configuration, a correction of the order of 10~(-3) must be applied to ΔV given by the piston, and, consequently, a small additional contribution to the uncertainty of the actual varied volume will result. In addition to the system description and characterization, preliminary results of gas flow measurements through two capillaries previously calibrated by the other IMGC-CNR flowmeters are presented.