Investigation of a method for measurement of water vapor coverage on technical surfaces

• We describe a new method for measuring the water vapor coverage on technical surfaces.
• Unlike previous methods, this method accounts for practically all of the water desorbed from the sample during the evacuation.
• Adsorption and desorption on the surface of measurement chamber is compensated by integrating the pressure pulse over sufficiently long time period.
• The surface coverage on samples of Ni foil varied from 1.8 to 3 monolayers when the foil was exposed to water vapor at humidity from 20% to 90%.
• The surface coverage increased significantly (multilayer adsorption) only at relative humidity above 95%.
• The measured surface coverage has an uncertainty less than 10%.

Gas coverage on a surface can be determined by a desorption method, where the amount of gas is calculated by integrating the released flux. This measurement is straightforward in cases where desorbed gases do not re-adsorb on the surface of the measurement chamber. In that case, mass conservation laws are applicable, and the desorbed flux can be determined by the measurement of a pressure drop across an orifice of known conductance. In the case that gases re-absorbed on the surface, such as H2O on a metal surface, the flux through the orifice is not equal to the flux coming from the sample. To determine the surface coverage of H2O on the technical surface of different materials, we have developed a method where adsorption and desorption on the surface of the measurement chamber can be compensated during measurement.The sample is exposed to pure water vapor in a sample chamber at pressures between 6 mbar and 25 mbar. Measurements start with an isothermal expansion of water vapor from the sample chamber into the continuously pumped measurement chamber which has more than 100 times larger volume than the sample chamber. The pressure of the gas pulse during expansion and subsequent pump down is precisely measured with a capacitance diaphragm gauge, initially, and, at later times, with spinning rotor gauge. To detect water vapor which is adsorbed during expansion on the surfaces of the measurement chamber, the measurement and integration of pressure pulse is extended over several hours, until the pressure in measurement chamber drops to 1 × 10−7 mbar, which is the same pressure as before expansion. The product of effective pumping speed and the time integral of the measured pressure pulse is equal to the initial gas quantity in the sample chamber. This gas quantity is a sum of the fraction of the water vapor in the sample chamber volume and water adsorbed on the surfaces within the sample chamber. The latter is determined in a background measurement without the sample.We tested the method with samples of Ni foil having surface areas of 860 cm2 and 1720 cm2. The measured amount of adsorbed H2O was proportional to the area. Measured surface coverage was from 1.8 ML ± 0.2 ML at exposure to 20% of saturation pressure to 4.5 ML ± 0.4 ML at exposure to 95% of saturation pressure of H2O at temperature 23 °C.