Establishment of Thailand's national primary vacuum standard by a static expansion method

A novel primary standard system for a vacuum pressure calibration has been established at the National Institute of Metrology, Thailand (NIMT). The system, which cover the pressure range from 1.3 • 10^-1 Pa up to 1.3 • 10³ Pa is based on the static expansion method. By the series of re-expansion, the range could be extended to lower calibration pressures. The facility consists of two small and one large vessels, which provide an expansion ratio by a factor of approximately 10² to 10³ for a single expansion and more for a multiple one. Six platinum resistance thermometers are attached to the vessel to ensure the correction of the non-isothermal effect during an expansion. The system could deliver the expanded (k = 2) relative uncertainties under 0.6% of reading at the lowest operating pressure. The measurement results of the system were verified by comparing with the calibration results of the capacitance diaphragm gauge (CDG) from Physikalisch-Technische Budensanstalt (PTB). The E_n ratios represent an excellent agreement between NIMT and PTB calibration results. (paper)