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

Abstract

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.