Abstract
A calibration system has been designed and built at the NPL for the calibration of ultrahigh vacuum gauges in a well distributed maxwellian gas. The system has a base pressure of approximately 2 nPa, and calculable gas pressures can be generated using the orifice flow technique. The apparatus uses a cryopump in a novel way to condense the gas molecules passing through the primary orifice. A full description is given of the metrology of the apparatus and an error analysis is also included. It is concluded that known pressure increments can be produced in the range 3 nPa to 0.1 mPa to within +or-7%.

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