Abstract
A low frost-point generator (INRIM 03) operating at sub-atmospheric pressure has been designed and constructed at the Istituto Nazionale di Ricerca Metrologica (INRIM) as part of a calibration facility for upper-air sounding instruments. This new humidity generator covers the frost-point temperature range between −99 °C and −20 °C and works at any controlled pressure between 200 hPa and 1100 hPa, achieving a complete saturation of the carrier gas (nitrogen) in a single passage through a stainless steel isothermal saturator. The generated humid gas contains a water vapour amount fraction between 14 × 10−9 mol mol−1 and 5 × 10−3 mol mol−1. In this work the design of the generator is reported together with characterisation and performance evaluation tests. A preliminary validation of the INRIM 03 against one of the INRIM humidity standards in the common region is also included. Based on experimental test results, an initial uncertainty evaluation of the generated frost-point temperature, Tfp, and water vapour amount fraction, xw, in the limited range down to −75 °C at atmospheric pressure is reported. For the frost-point temperature, the uncertainty budget yields a total expanded uncertainty (k = 2) of less than 0.028 °C, while for the mole fraction the budget yields a total expanded uncertainty of less than 10−6 mol mol−1.
Highlights
The measurement of the amount of water present in a mat erial substance represents a critical aspect in several social and economics fields
A standard platinum resistance thermometer (SPRT) is used to measure the satur ated gas temperature, Tsat, which corresponds to the generated frost-point temperature
The amounts of water vapour measured by the two analysers are shown as a function of the frost-point temper ature Tfp of the humid gas generated by the INRIM 03
Summary
The measurement of the amount of water present in a mat erial substance represents a critical aspect in several social and economics fields. In the 90s, at the National Institute of Standards and Technology (NIST), a two-temperature gen erator designed for the generation of precise humidity levels in the frost-point range, including between −100 °C and −5 °C, was developed [18, 19] This generator was a saturator based system, where the passing gas was moisturised with the evapo rated and saturated water vapour rising from the ice surface. Since radiosondes need SI-traceable calibration in conditions similar to those encountered in atmosphere during sounding operations in terms both of pressure and water vapour mole fraction, a new low-frost point primary generator able to operate at sub-atmos pheric pressure has been designed and constructed at INRIM. Experimental tests include measurements of temperature gra dients, pressure stability and saturator efficiency under var ious operating conditions, as well as a comparison between the humidity generated by the INRIM 03 and that one gener ated by the existing primary humidity standard INRIM 02, in the temperature and pressure overlapped region
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