NEAR-INFRARED MEASUREMENT OF RELATIVE AND ABSOLUTE-HUMIDITY THROUGH DETECTION OF WATER ADSORBED ON A SILICA-GEL LAYER

Near-IR spectroscopy is especially well suited to moisture determinati on because of the relatively high absorptivity of water compared to mo st other substances. In the present work, near-IR diffuse-transmittanc e spectroscopy is applied to the measurement of humidity via observati on of adsorbed water on a high-performance thin-layer chromatography s ilica gel plate. The adsorbed water is detected through both ordinary absorption of radiation by water molecules and the increased scatterin g of near-IR radiation by the silica gel as it adsorbs more water. Thi s technique shows promise as a highly accurate and sensitive humidity sensor with a time constant of less than 1 min. The addition of inorga nic salts to the silica gel layer is shown to increase the response to water vapor. However, it also increases the time constant of the sens or. A miniature humidity sensor using a commercially available near-IR transmissive switch is also demonstrated. Possible interferences and hysteresis effects are investigated.