The Russian National Standard of Gases Humidity and Traceability System of Humidity Measurements

The Russian national humidity standard of gases has been modernized in order to increase the number of reproducible quantities of humidity (relative humidity, dew/frost-point temperature, mole fraction) and to extend the humidity and operating temperature ranges. The basis of the standard comprises two humidity generators with operating temperature ranges from 5 degrees C to 90 degrees C and from -60 degrees C to 15 degrees C. The common working range (from 5 degrees C to 15 degrees C) allows comparison of the generators. The generators use the two-pressure method to generate humid gas defined in terms of the relative humidity (from 5 % rh to 98 % rh at temperatures from 90 degrees C to -60 degrees C) and the one-pressure (or phase equilibrium) method to generate humid gas defined in terms of the vapor mole fraction (from 0.6 ppm to 700 x 10(3) ppm) and dew/frost-point temperature (from -79 degrees C to 90 degrees C). The expanded uncertainty in the relative humidity is no more than 0.2 % rh, no more than 1.2 % in the vapor mole fraction, and no more than 0.12 degrees C in the dew/frost-point temperature. The ordinary hygrometers are traceable to the national primary standard in accordance with the state hierarchical chain for measuring means of gas humidity. The state hierarchical chain consists of three branches for means of measurements: (a) mole fraction, (b) dew/frost-point temperature, and (c) relative humidity with each branch represented as the scheme: primary standard-secondary standard-working standard-ordinary hygrometer. Calibration and verification of working standards and ordinary hygrometers, and their traceability to the primary standard use methods of (i) direct measurements, (ii) direct comparison, or (iii) comparison with a comparator.