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 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 (Cuccaro et al 2018 Meas. Sci. Technol. 29 054002). 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.

Trace water is one of the most critical matrix contaminants in ultra-high-purity (UHP) process gases, like argon (Ar), nitrogen (N₂), and many others. Even trace amounts can severely degrade the quality of many products that are reliant on these gases. Despite its importance to advanced technology sectors, notably semiconductor manufacturing, it has proven quite difficult to realize preparative or analytical trace water metrology over the full amount fraction range needed or in the broad spectrum of industrially relevant matrix gases. Within the EU-funded PROMETH2O project consortium, this challenge has been addressed through the development or significant improvement of traceable measurement methods and standards spanning 5 nmol⋅mol⁻¹ to 5 µmol⋅mol⁻¹, tailored for use in UHP process gas production, such as Ar, N₂ and clean dry air (CDA). The measurement ranges were extended and the uncertainties were improved while being consistent with the current best practice at primary humidity standard laboratories. The developed standards provide combined standard uncertainties ranging from approximately 0.4 % to 1.5 % in water vapor amount fraction and from 0.03 °C to 0.07 °C in frost-point temperature, while the comb-assisted CRDS system achieves detection limits in the sub-ppb to ppt range. These capabilities were validated in applications that are relevant to process instrumentation and the gas industry. A distributed metrological infrastructure at various European national metrology institutes and partner sites now provides SI-traceable trace water measurements in UHP gases, strongly supporting and extending the calibration capabilities for the gas and semiconductor industries and the associated stakeholders.

Trace water is one of the most critical matrix contaminants in ultra-high purity (UHP) process gases, like argon (Ar) and nitrogen (N₂), and many others. Even trace amounts can severely degrade the quality of many products reliant on these gases. Despite its importance to advanced technology sectors, notably semiconductor manufacturing, it has proven quite difficult to realize preparative or analytical trace water metrology over the full amount fraction range needed or in the broad spectrum of industrially relevant matrix gases. Within the EU-funded PROMETH2O project consortium, this challenge has been addressed through the development or significant improvement of traceable measurement methods and standards spanning 5 nmol mol⁻¹ to 5 µmol mol⁻¹, tailored for use in UHP process gas production, such as Ar, N2 and hydrogen (H2). The measurement ranges were extended and the uncertainties were improved, while being consistent with current best practice at primary humidity standards laboratories. These capabilities were validated in applications relevant to process instrumentation and the gas industry. A distributed metrological infrastructure at various European National Metrology Institutes and partner sites now provides SI-traceable trace water measurements in various UHP, strongly supporting and extending the calibration capabilities for the gas and semiconductor industries and the associated stakeholders.