The next-generation pressure standards will be realized via gas density and the equation of state. One way to access the density is through a measurement of gas refractivity, underpinned by the theoretical calculations that predict the relationship between density and refractivity. At present, calculations with sufficient accuracy that link refractivity to density are only available for helium. To measure helium refractivity we employ interferometry to make ultraprecise measurements of the optical length of gas-filled cells and cavities. The refractivity of helium at atmospheric pressure is about 3.2 × 10~(-5): to measure this to 10~(-6) fractional uncertainty - our goal for a pascal realization - would require the measurement of a 15 cm optical length with 4.5 pm accuracy.
展开▼
