Extension of Humidity Standards to $$-105\,^{\circ }\hbox {C}$$ Frost Point.

The KRISS low frost-point humidity generator which has been operated by the two-temperature method in the frost-point range from $$-95\,^{\circ }\hbox {C}$$ to $$-40\,^{\circ }\hbox {C}$$ since 2006 is reformed to a two-temperature, two-pressure type, in order to extend the calibration capability to a frost point of $$-105\,^{\circ }\hbox {C}$$ . The temperature and pressure of the saturator were controlled to $$-95\,^{\circ }\hbox {C}$$ and 1 MPa, respectively. The water-vapor mole ratio generated by the upgraded humidity generator reached $$4\,\hbox {nmol}{\cdot }\hbox {mol}^{-1}$$ . The uncertainty of the generator was estimated by calculations as well as a series of experiments including the stability of the generated frost point, the saturation efficiency with a varied gas flow rate, and the change of water-vapor mole ratio in the tubing line. The standard uncertainty of the generator is less than $$0.09\,^{\circ }\hbox {C}$$ at the frost point of $$-95\,^{\circ }\hbox {C}$$ and is increased to $$0.19\,^{\circ }\hbox {C}$$ at the frost point of $$-105\,^{\circ }\hbox {C}$$ . The increase in uncertainty is mainly due to the water adsorption/desorption on the internal surface of tubing from the saturator to the hygrometer. [ABSTRACT FROM AUTHOR]