Assessment of the INRIM trace water generator and analysis of the uncertainty components down to -100 °C frost-point temperature

A low frost-point generator (INRIM 03) able to operate at sub-atmospheric pressure has been recently designed, constructed, and assessed at the Istituto Nazionale di Ricerca Metrologica (INRiM) with the aim of providing the metrological traceability both to instruments developed for the measurement of humidity in atmosphere and to sensors and analysers used in industry for controlling and measuring the amount of water vapour in manufacturing processes. The humidity generator operates in a single temperature single pressure mode, letting the carrier gas (nitrogen) achieve saturation in a single passage through an isothermal saturator. Its working range encompasses a frost-point temperature range from -100 °C to -20 °C, in a pressure range between 200 hPa and 1100 hPa, corresponding to an amount of water fraction range from 13·10-9 mol·mol-1 to 6.2·10-3 mol·mol-1. In a previous work its performance was assessed in the frost-point temperature range from -75 °C to -20 °C [2]. In this work, a comprehensive set of tests for its characterisation and performance evaluation between -75 °C and -100 °C is presented. A detailed uncertainty analysis in the above temperature range is reported, taking into account all the sources of uncertainty that affect the humid gas generation. An expanded uncertainty (k=2) of 0.07 °C was found for frost-point temperature measurements between -75 °C and -95 °C, while an expanded uncertainty of 0.26 °C resulted at a frost-point temperature of -100 °C. The relative expanded uncertainty (k=2) associated with water vapour amount fraction measurements was estimated equal to or better than 1.2 % between 35·10-9 mol·mol-1 and 6.1·10-3 mol·mol-1, increasing up to 6.5 % at 13∙10-9 mol·mol-1.