Paper VT-MoM4
Analysis of a Quantum Based Refractometer to Replace Mercury Manometers as the Primary Standard for the United States

Monday, November 7, 2016, 9:20 am, Room 104C

NIST has developed a technique to measure pressure using the gas refractivity of nitrogen for pressures in the range of 1 Pa to 360 kPa. This range is critical to many application including altimetry, weather, process control, etc.; all of which require high accuracy calibration of vacuum gauges. Currently the highest claimed accuracy of a primary standard is the NIST mercury Ultrasonic Interferometer Manometers (UIMs) operating at an uncertainty of U(P_UIM)=[(6 mPa)² +(5.2*10^-6*P)²]^1/2. NIST proposes replacement of these standards with an optical gas refractometer with an uncertainty of U(P_OGR) = [(2.0 mPa)² + (8.8*10^-6*P)²]^1/2.

The optical refractometer has many benefits over the current UIMs, however we also need to show the feasibility of the fixed length refractometer as a primary standard. The two key requirements to define a primary standard are traceability of the standard back to the International System of Units (SI) and the ability to transfer the measurement/uncertainty to a high accuracy gauge or a transfer standard. The traceability and associated uncertainty will be discussed along with the derivation of the above stated uncertainty for the optical gas refractometer. Additionally, results of a calibration using the refractometer will be compared to that obtained using the NIST UIM. The capability and limitations of both the refractometer and UIM will be discussed and will show that the refractometer outperforms the UIM and will be slated to replace the mercury standards in the near future.