PRESSURES IN THE HIGH AND MEDIUM VACUUM RANGE GENERATED BY A SERIES EXPANSION STANDARD

Pressures in the range 10-7 to 10 mbar which are required to calibrate vacuum gauges as, e.g., the capacitance diaphragm gauge, the spinning rotor gauge, and the ionization gauge, are conveniently generated by the series expansion method. In this method, the test gas is initially filled into a vessel of small volume and then isothermally expanded into an evacuated vessel of large volume. The resulting pressure is calculated from the initial high pressure and from the volume ratio by using Boyle's law. The performance of such a series expansion apparatus is investigated in detail. The uncertainties of various gauges for measuring the initial pressure are discussed. Different methods for determining the volume ratio are explored and are found to give inconsistent results. Furthermore, disturbing effects due to the nonideal gas behaviour, temperature changes and gas adsorption at walls are studied. For inert gases, the 1σ-uncertainty of the generated pressure amounts to 0.05% at a pressure of 10 mbar and increases to 0.2% at 10-6 mbar and stems mainly from the uncertainty of the volume ratios. This uncertainty is smaller than the stability of currently available transfer gauges in the corresponding medium and high vacuum range. Thus the apparatus is well suited for calibrations of these gauges. © 1990.