We have built and characterized a refractometer that utilizes two Fabry-Perot cavities formed on a dimensionally stable spacer. In the typical mode of operation, one cavity is held at vacuum, and the other cavity is filled with nitrogen gas. The differential change in length between the cavities is measured as the difference in frequency between two helium-neon lasers, one locked to the resonance of each cavity. This differential change in optical length is a measure of the gas refractivity. Using the known values for the molar refractivity and virial coefficients of nitrogen, and accounting for cavity length distortions, the device can be used as a high-resolution, multi-decade pressure sensor. We define a reference value for nitrogen refractivity as n-1=(26485.28±0.3)×10(-8) at p=100.0000  kPa, T=302.9190  K, and λ(vac)=632.9908  nm. We compare pressure determinations via the refractometer and the reference value to a mercury manometer.

The vapor pressure of hexagonal (Ih) water ice was measured over the temperature range 175K to 253.4K and referenced to the value at the triple point of water. This experiment combined a highly accurate humidity generation system containing an ice-coated saturator (millikelvin-level temperature control) to provide humidified streams of nitrogen to a cavity-enhanced laser absorption spectrometer. The measured ice vapor pressures had relative standard uncertainties ranging from 0.4% to 0.7% over the entire temperature range. We demonstrate that these measurements validate thermodynamic correlations for ice vapor pressure based on integration of the Clapeyron equation. Moreover, they also indicate that some commonly used vapor pressure correlations are inaccurate and should be avoided.

The National Institute of Standards and Technology (NIST) provides low-humidity calibration and testing services for hygrometers and humidity generators over a humidity range between 3 nanomole per...