Two-stage flow dividing system has been developed for the calibration of ionization vacuum gauges (IG) and residual gas analyzers (RGA) from 10-7 to 10-2 Pa for N2, Ar, He and H2.
This system consists of three chambers; an initial chamber V0, a flow divider VFD and a calibration chamber V1, connected each other with capillaries (capillary 0 between V0 and VFD, and capillary 1 between VFD and V1). The calibration chamber V1 is evacuated by a main turbo molecular pump (550 l/s) thorough an orifice (Φ30 mm). The flow divider VFD is also evacuated by a subsidiary turbo molecular pump (220 l/s) to reduce the gas flow into the calibration chamber V1.
The pressure in the initial chamber, P0, is kept constant using a pressure controller from 102 to 105 Pa. The pressure in the flow divider, PFD, is determined from P0, the conductance of the capillary 0, C0, and the effective pumping speed of the subsidiary turbo molecular pump, Ssub. C0 and Ssub are designed to generate PFD from 10-4 to 10 Pa by changing P0 from 102 to 105 Pa.
The pressure in the calibration chamber, P1, is determined by PFD, the conductance of the capillary 1, C1, and the effective pumping speed of the main turbo molecular pump through the orifice, C2. C1 and C2 are designed to generate P1 from 10-7 to 10-2 Pa by changing PFD from 10-4 to 10 Pa.
At the molecular flow condition C1 and C2 are constant, and P1 is proportional to PFD. Thus, P1 is defined quantitatively from PFD and the constant pressure ratio of P1 to PFD. The ratio was measured using two spinning rotor gauges at PFD from 0.1 to 10 Pa corresponding to P1 from 10-4 to 10-2 Pa prior to the calibration. The ratio for N2, Ar, He and H2 is 1.16±0.01 x10-3, 1.12±0.01 x10-3, 1.41±0.02 x10-3 and 1.63±0.02 x10-3, respectively. The difference of these ratios is explained by the mass dependence of C1 and C2.
The uncertainty of the generated pressure P1 from 5x10-7 to 8x10-3 Pa is from 6.2% to 0.9% (k=1). This system is useful for the calibration of IG and RGA by easy handling. |