Time-Correction Model Based on Diverter Speed for a pVTt Gas Flow Primary Standard

The volumetric primary standard with a constant volume (pVTt) determines the flow rate from the rate of change of gas density in the defined measuring volume. The key element of the measuring system that uses the flying start-stop method is a diverter. This paper presents a time correction model that adapts the correction according to the diverting speed implemented in our pVTt system developed for flow rates smaller than 12mgmin-1. A detailed study of the diverter's effects on the effective collection time and two methods for determining the time correction are presented. One is based on the ISO 4185 standard and the other on measurements of the dynamic pressure upstream of the diverter. A set of correction time measurements were made at different diversion speeds to define the correction model at a flow rate of 9mgmin-1. The results show very good agreement between the measurements with both methods and the defined correction model. Additional measurements were made at smaller flow rates and the results indicate the independence of the time correction from the measured flow rate.