Measuring volume ratios of vacuum vessels using non-evaporable getters

Several methods have been used in vacuum metrology for volume determination. Volume ratios can be determined by different gas expansion methods, which were primarily developed for the precise determination of expansion ratios in static expansion systems. Measurements of the volume ratios using the gas expansion methods below 10−2 Pa are influenced by outgassing from chamber walls. To reduce outgassing in the expansion chambers during measurements at low pressures, we have installed non-evaporable getter (NEG) pumps, which pump hydrogen and other active gases but have a negligible pumping speed for inert gases. The volume ratio of two chambers of an experimental static expansion system has thus been measured using the inert gases argon, helium and krypton. The obtained results were compared with the measurements without the use of an NEG pump. Measurements of the pressure ratios were performed with a spinning rotor gauge (SRG).

► Use of nonevaporable getter (NEG) pumps in vacuum metrology.
► Use of a spinning rotor gauge for volume ratio measurements at a very low pressure.
► The selective pumping of NEG reduces errors due to outgassing when inert gas is used for calibration.
► The NEG pump assures the amount of calibrating gas is the same before and after expansion.
► It is also important to use an NEG pump to purify the gas before expansion.