Knowledge of the dew point of compressed humid gases is needed for many new technical applications, e.g., compressed air energy storage (CAES), humid air turbine (HAT), or zero emission power plants. A new method was developed to measure the dew point, expressed as vapor concentration enhancement factor, by Fourier transform infrared (FTIR) spectroscopy. This method has already been successfully applied to compressed humid air (Koglbauer and Wendland, J. Chem. Eng. Data 2007, 52, 1672-1677). Here, measurements in pure air components were performed at temperatures from (20 to 100) °C and pressures up to 25 MPa for nitrogen and argon and up to 5.5 MPa for carbon dioxide. The estimated combined standard uncertainties (68 % confidence level) of the new experimental data are 0.02 K for temperature, 3.2 kPa for pressure, (0.12 to 1.2) % for the vapor concentration enhancement factor in argon and nitrogen, and (0.5 to 5) % for the vapor concentration enhancement factor in carbon dioxide.

Water activity is an important property of aqueous solutions and humid solids. It can be measured via the relative humidity of a gas which is in equilibrium with the solution or solid. A new method was developed to measure the water activity via the relative humidity by fourier transform infrared (FTIR) spectroscopy. This method is based on earlier work on the measurement of the dew point of compressed humid air (Koglbauer and Wendland, J. Chem. Eng. Data 2007, 52, 1672-1677. The accuracy of the method was tested on salt fixed points which are widely used as humidity fixed points for the calibration of water activity sensors. The estimated uncertainty of the new method and the deviation of the new data from the reference values of the humidity fixed points are within the uncertainty of the reference values.

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