Currently, the piston-cylinder assembly known as PG39 is used as a primary pressure standard at the National Institute of Standards and Technology (NIST) in the range of 20 kPa to 1 MPa with a standard uncertainty of 3 × 10 − 6 as evaluated in 2006. An approximate model of gas flow through the crevice between the piston and sleeve contributed significantly to this uncertainty. The aim of this work is to revise the previous effective cross sectional area of PG39 and its uncertainty by carrying out more exact calculations that consider the effects of rarefied gas flow. The effective cross sectional area is completely determined by the pressure distribution in the crevice. Once the pressure distribution is known, the elastic deformations of both piston and sleeve are calculated by finite element analysis. Then, the pressure distribution is recalculated iteratively for the new crevice dimension. As a result, a new value of the effective area is obtained with a relative difference of 3 × 10 − 6 from the previous one. Moreover, this approach allows us to reduce significantly the standard uncertainty related to the gas flow model so that the total uncertainty is decreased by a factor of three.

The National Institute of Metrology (NIM) has constructed a dual optical cavity for the optical pressure standard project. The elastic deformation induced by pressure is investigated by finite element analysis (FEA) simulation. The distortion coefficient, fractional length change per unit pressure, is calculated to be (9.936 ± 0.014)×10-12 /Pa for the measurement channel and (1.49 ~ 1.54)×10-11 /Pa for the reference channel. The gravityinduced deformation is also modelled to evaluate the sensitivity to the low-frequency vibration. The cavity is modelled to be supported symmetrically on four points. With optimized support position, the sensitivity to vertical acceleration reduces to 19 kHz/(m/s 2) at 633 nm.