Together with tighter quality control expansion, demand on accurate measurement of the leakage rates increases. Consequently, demand on improvement of the calibration of the standard leaks by a primary technique increases, too. There are several techniques available for the calibration of leaks (for review see C. D. Ehrlich and J. A. Basford, J Vac Sci Technol, A10(1), 1 (1992)). However, direct comparison of a gas leakage rate with the gas flow rate from a primary flow rate standard, such as a constant pressure flowmeter, has become preferable technique for primary leak calibration, recently. The comparison is performed by alternately and separately flowing gas from the leak and from the flowmeter into a vacuum chamber equipped with a proper pressure gauge .Since the flow rates relevant at these measurements range from 10-6 down to 10-12 mol/s, the pressure in the vacuum chamber where the comparison is performed extends up to uhv range. Residual gas analyzer (RGA) and ionisation gauge (IV) are the only gauges available for such measurements.
A new primary flow rate standard that was designed for Czech Metrological Institute and finished last year exhibits some advantageous properties (high stability of the pressure in flowmeter, low detection limit of the flowmeter volume changes and fully automatic operation due to a computer controlled driving system) which allow improving the procedure of the comparison.
An analysis of the comparison procedure is presented that demonstrates not only decreasing of the uncertainty of the comparison but also reduction of the impact of the instabilities of the gauge (RGA or IV) that indicates the pressure when the leak and the flowmeter, respectively, are opened to the vacuum chamber. Experimental data that were obtained at international comparisons are presented to confirm efficiency of the method. |