Our state-of-the-art property measurements require extensive calibration facilities of equal quality. Regular calibrations are essential for realistic uncertainty estimates. Temperature and pressure are the most important independent variables for most of our measurements, and we concentrate our resources on those quantities. We also maintain mass, resistance, and density standards. We do most of the calibrations of our working instruments ourselves for reasons of economy, timing, and most importantly, accuracy. Calibrations at the level of accuracy that we require are very expensive and often involve long turn-around times; sending a working temperature or pressure instrument to an outside calibration lab could result in lengthy down-times. The highest levels of accuracy often require an on-site calibration because sensors can shift when subjected to the shock and vibration of shipping.
We also have a standard platinum resistance thermometer (SPRT) that was calibrated by the Temperature Group of NIST-Gaithersburg that is used with a resistance bridge and standard resistor in a "comparison block" calibration furnace for temperatures up to 933 K. The uncertainty for this type of calibration is 0.01 K.
In experiments requiring the utmost accuracy in pressure we directly connect a piston gauge to the fluid property instrument. This approach limits the experiment to inert gases compatible with the piston gage (nitrogen, helium, neon, or argon); it has been used in calibrations with the two-sinker densimeter.
Davis, R.S. (1992). Equation for the determination of the density of moist air (1981/91). Metrologia 29: 67-70.
Harris, G.L. and Torres, J.A. (2003). Selected laboratory and measurement practices and procedures, to support basic mass calibrations. National Institute of Standards and Technology, NISTIR 6969.
Related Programs and Projects:
Intimately related to the issues of calibration is the expression of measurement uncertainty. This is approached in a consistent way at NIST, following Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results: