## Atomic Standard for Pressure
Above 300 kPa, NIST’s pressure standards are commercially manufactured piston-cylinder sets. In operation, both the piston and the cylinder deform significantly with pressure and the piston rotates continuously to insure gas lubrication. Because of these complications, piston-cylinder sets are calibrated using NIST’s primary-standard mercury manometer below 300 kPa and the calibration is extrapolated to higher pressures using numerical models of the coupled gas flow and elastic distortions. The data used for extrapolation exhibit poorly understood dependencies on the gas used and its flow rate; therefore, the extrapolation is not fully trusted. The extrapolation cannot be checked with existing technologies; however, the atomic standard of pressure will reduce extrapolation uncertainties. Major Accomplishments:- Initially, we built cross capacitors to measure the dielectric constant
*ε*of helium as the basis of a pressure standard*p*(*ε*,*T*). [**1**] The dielectric constant measurements were limited by the linearity and resolution of commercially-manufactured capacitance bridges. This led us to replace dielectric constant measurements with higher-resolution refractive index measurements obtained from the microwave resonance frequencies of helium-filled, quasi-spherical cavities invented at NIST. [**2**] - We measured the fractional difference (
*p*piston/*p*atomic − 1) = (1.8±9.1)×10−6 between the pressure determined from NIST’s piston-cylinder standards and the atomic standard [*n*He(*p*,*T*)] in the pressure range 0.14 MPa to 7 MPa at the temperature of the triple point of water,*T*W = 273.16 K. [**3**] - We measured the ratios [
*n*Ar(*p*,*T*) − 1]/[*n*He(*p*,*T*) − 1] with a fractional uncertainty of 2.5×10−6 at the triple points of mercury (*T*Hg = 234.3156 K), water (*T*W = 273.16 K), and gallium (*T*Ga = 302.9146 K). [**4**]
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