CAN A PRESSURE STANDARD BE BASED ON CAPACITANCE MEASUREMENTS

We consider the feasibility of basing a pressure standard on measureme nts of the dielectric constant epsilon and the thermodynamic temperatu re T of helium near 0 degrees C. The pressure p of the helium would be calculated from fundamental constants, quantum mechanics, and statist ical mechanics. At present, the relative standard uncertainty of the p ressure u(r)(p) would exceed 20 x 10(-6), the relative uncertainty of the value of the molar polarizability of helium A(epsilon) calculated ab initio. If the relativistic corrections to A(epsilon) were calculat ed as accurately as the classical value is now known, a capacitance-ba sed pressure standard might attain u(r)(p)< 6 x 10(-6) for pressures n ear 1 MPa, a result of considerable interest for pressure metrology. O ne obtains p by eliminating the density from the virial expansions for p and epsilon - 1. If epsilon - 1 were measured with a very stable, 0 .5 pF toroidal cross capacitor, the small capacitance and the small va lues of epsilon - 1 would require state-of-the-art capacitance measure ments to achieve a useful pressure standard.