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Humidity Fixed Points of Binary Saturated Aqueous Solutions

Humidity Fixed Points of Binary Saturated Aqueous Solutions,Lewis Greenspan

Humidity Fixed Points of Binary Saturated Aqueous Solutions   (Citations: 523)
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An evaluated compilation of equilibrium relative humidities in air versus temperature from pure phase to approximately 105 pascal (1 atm) in pressure is presented for 28 binary saturated aqueous solutions. The relative humidities of the solutions range from about 3 to 98 percent. Using a data base from 21 separate investigations comprising 1106 individual measurements, fits were made by the method of least squares to regular polynomial equations with two through four coefficients. Equations and tables are presented along with the estimated uncertainties in the correlated results. Research, hygrometer calibration, testing and material conditioning often require the accurate control of humidity in a working space. The common methods of controlling the humidity accurately use either a humidity generator (1A)1 or the equilibration of a closed space with a chemical system (IB) which produces the desired equilibrium vapor pressure. Humidity generators tend to be expensive and complex whereas equilibration with chemical systems that provide fixed points is a relatively inexpensive and simple method of humidity control. Among the chemical systems used for this purpose are aqueous sulphuric acid solutions, glycerine and water solutions and single and binary salt solutions. Each such solution offers a degree of humidity adjustment that can be achieved by changing its concentration. On the other hand, special problems are associated with the use of solu- tions because their concentrations must be measured and controlled. Not only must the concentration of the solution be determined initially but the presence of any humidity sources or sinks in the controlled space and even the initial equilibra- tion process of the space can alter the solution concentration. An especially useful method of humidity control by chemi- cal system involves the use of binary saturated aqueous solutions (primarily of single salts) in which the solute is highly non-volatile. At any temperature, the concentration of a saturated solu- tion is fixed and does not have to be determined. By provid- ing excess solute, the solution will remain saturated even in the presence of modest sources or sinks. Where the solute is a solid in the pure phase, it is easy to determine that there is indeed saturation. Due to the ease of its use, this is a popular method of humidity control. Since a given saturated salt solution provides only one relative humidity (RH) at any desired temperature, a different relative humidity must be achieved by selecting another appropriate salt. Though much data on saturated salt solu-
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