塩類の飽和水溶液による湿度定点の実現方法

抄録

Water vapor pressure of a saturated salt solution is inherently constant at a given temperature and utilized for calibration of hygrometers as a humidity fixed point. In this paper, the humidity fixed points were realized respectively in a glass desiccator and in a box with an air stirrer and examined experimentally.
The state of saturated solution and the change of ambient temperature are important and effective for realizing the humidity fixed point. It is suitable for a rapid attainment of humidity equilibrium that the saturated solution is in a state of slushy mixture with water and salt. Although the equilibrium relative humidity of the saturated solution does not change much with temperature, a transient change of the relative humidity is observed following a sudden change of ambient temperature. The transient change amounted to -1.5%RH/°C at the maximum.
The equilibrium relative humidities by respective saturated solutions of magnesium chloride, sodium bromide, sodium chloride and potassium chloride were accurately measured in a glass desiccator with use of a humidity sensor calibrated against the humidity standard of the National Research Laboratory of Metrology. The humidity values measured agree with those of the salt-humidity table in the literature and accordingly the reliability of the table was confirmed. The humidity fixed point by saturated salt solutions can be realized in a desiccator with the precision of 1%RH, if temperature is well controlled.
To improve utility for practical use, the humidity characteristics of the fixed point realized in a cubic box were also studied under various conditions, such as stirring in the box and changing surface area of saturated solutions. It becomes clear that the stirring is important for uniform distribution and for quick attainment of equilibrium of the humidity, and the time required for the humidity equilibration is decreased to about 30min under stirring condition.