Study on the Potential Use of Mg(NO₃)₂, CsCl, MnSO₄, and KNO₃ Salt Solutions as Secondary Standards for Reproducing the Unit of Relative Humidity with Established Traceability to the State Primary Standard GET–151–2020

The widespread use of continuous relative humidity sensors has created a need for on-site transfer of the humidity unit. The use of salt-based hygrostats for this purpose is significantly limited by their insufficient range and the requirement for a reference hygrometer. The authors of this article propose a method to expand the range of applicable salt solutions. Through preliminary analysis, they selected salt solutions based on MgNO₃, LiCl, MgCl₂, CsCl, NaCl, KNO₃, MnSO₄. These solutions were then tested for the reproducibility of their relative humidity values after being subjected to various thermal regimes. Modeling confirmed the possibility of maintaining a temperature distribution not exceeding 0.1 °C within the flask’s working zone. The presented results show that the relative humidity value of all salt solutions remains within ±0.6 % after exposure to both lower and higher temperatures. The reproducible relative humidity values for LiCl, MgCl₂, Mg(NO₃)₂, CsCl, MnSO₄, KNO₃ solutions were experimentally established. Based on a synthesis of the obtained data, the use of thermostated salt cells is proposed for reproducing fixed points of relative humidity. This will eliminate the need for reference hygrometers during the unit transfer process. Furthermore, a draft of an extended verification scheme is presented. The presented results may be useful for developers of relative humidity measurement instruments and specialists in the field of environmental parameter assurance or monitoring.

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