Paper VT-MoM4
Uncertainty of a NIST Transfer Standard for Inter-Laboratory Comparison of National Metrology Institutes

Monday, October 28, 2013, 9:20 am, Room 202 C

For over a decade, NIST has been developing portable Transfer Standards Packages (TSP) which can disseminate pressure at significantly lower uncertainties than commercially available transfer standards. Because of this experience, NIST was tasked with piloting an International Inter-laboratory Comparison of seven countries. From April 2012 to June 2013, a NIST designed TSP circled the globe visiting each National Metrology Institute as a reference to provide an International Inter-laboratory comparison from 1 Pa to 10 kPa. The most important aspect of the TSP was to have an uncertainty value ranging from 0.5 % at 1 Pa to 0.01 % at 10 kPa.

Key to reaching the uncertainty goal was using special techniques to improve the uncertainty of commercial gauges. These techniques include temperature stabilization, maintaining level, and improving the calibration stability of the gauges. The crucial advantage of the NIST TSP is the ability to exploit the advantage of the Capacitance Diaphragm Gauge’s (CDG) unbeatable resolution at lower pressures (typically 1 part in 10⁶ of full scale range) and the Resonance Silicon Gauge’s (RSG) drift uncertainties of 0.01% (k=2). Each gauge type individually cannot achieve the necessary uncertainty, with the CDGs subject to up to 0.5% uncertainty (k=2) due to long term stability (drift uncertainty) and RSGs low resolution/inability to measure low pressures. However, the short term stability (< 8 hours) of a CDG is excellent (less than 0.01%) and has been found to drift in a correctable manor. Because of these characteristics, it is possible to calibrate the CDGs using the RSG just before use and wind up with a very low uncertainty measurement. The talk will cover primary gauge stability data on two transfer standard packages and will discuss some of the technical challenges in running an international key comparison.