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The UHV/XHV calibration apparatus is composed of the XHV system, UHV system, separated flow system and gas supply system. The ultimate pressure of 10-10 Pa was obtained in the XHV calibration chamber by the combination of magnetically levitated turbomolecular pump and non-evaporable getter (NEG) pump. The separated flow method was used to extend the lower limit of vacuum gauge calibration to 10-10 Pa. Because NEG pump has no pumping speed for inert gas, the uncertainty at lower calibration limit is reduced when inert gas is used as test gas. The molecular flow conductance of laser orifice was measured with linear vacuum gauge method, which reduced the measurement uncertainty of the orifice conductance. The calibration apparatus combined the separated flow method, pressure attenuation method and direct measurement method to calibrate vacuum gauge. Its calibration range is from 10-1 to 10-10 Pa, and the combined standard uncertainties are from 0.41% to 3.5%. |