^4fGAS FLOW MEASUREMENTS CONNECTED WITH THE CONTINUOUS EXPANSION SYSTEM

摘要

^7fThe IMGC-CNR-F3 constant-pressure and variable-volume gas flow-meter has been recently automated and completely characterized. At the reference temperature of 21°C, it can generate and measure pure gas flows from 10{sup}(-3) Pa m{sup}3/s to 10{sup}(-8) Pa m{sup}3/s (corresponding to molar flow ranging from about 10{sup}(-6) mol/s to 10{sup}(-11) mol/s); gas flows are used for both vacuum primary standards and standard-leak calibration. The attention will be focused on the automatic electronic control unit (and related data acquisition system) and the uncertainty evaluation in the whole range. The various input quantities of the combined uncertainty are considered, starting from the linear speed of the piston during the measurements (by the slope of the displacement curve vs. time) and its repeatability, that is practically the repeatability of the flow-meter itself. The temperatures of the various parts of the device affect the value of the measured flow and consequently attention is paid on the evaluation of the influence that thermal effects may have on the system accuracy. The combined standard uncertainty is increasing both at low and at high gas flows which are connected with the limit of each of the two pistons (5 mm and 20 mm diameter respectively). The relative combined standard uncertainty is included between 0,2% and 0,8%.