Paper VT-ThP12
Development of a Static Expansion Vacuum Standard at the National Institute of Standards and Technology

Thursday, October 18, 2007, 5:30 pm, Room 4C

The NIST Pressure and Vacuum Group maintains sub-atmospheric pressure standards that range from 10^-7 to 10⁺⁵ Pa. These consist of orifice flow standards from 10^-7 to 10 Pa and Ultrasonic Interferometric Manometers from 10^-1 to 10⁺⁵ Pa. A new Static Expansion Standard is in development with an operating range of 10^-3 to 10⁺³ Pa. The standard is composed of two spherical stainless-steel vessels with a volume ratio of about 100:1. The two vessels are interconnected with a stepper motor driven valve. An initial pressure of gas in the small volume is measured to high accuracy using a resonant silicon gauge and then is expanded into the evacuated larger volume. This expansion results in a pressure reduction of about 100. Further evacuation and expansion cycles are used to produce lower pressures. The system is temperature controlled to within 0.1C with a thermoelectrically cooled enclosure. This new standard provides many useful benefits: First, the Static Expansion Standard generates pressures that encompass both the orifice flow and manometry standards without relying on gas flowmeters; Second, the standard enables fast and simple calibration of spinning rotor (SRG’s) and capacitance diaphragm gauges (CDG’s). Currently these calibrations require a very skilled technician and are done at very high cost to customers; Third, the system allows for a convenient comparison of manometer and vacuum standards using CDG’s and SRG’s respectively. Work is ongoing to characterize the system, including determining the volume ratio and estimating the uncertainties.