Measurements of liquid film thickness, concentration and temperature of aqueous NaCl solution by NIR absorption spectroscopy

摘要

A multi-wavelength diode laser absorption sensor was investigated for real-time monitoring of the thickness, solute concentration and temperature of thin films of liquid solutions, and the concept was demonstrated with aqueous NaCl solutions. The sensor monitors the transmittance of laser intensity of four near-infrared diode lasers through a thin liquid film deposited on a quartz plate. The variation of the absorption spectrum of the solution with temperature and solute concentration was used to select wavelengths for determining film thickness, NaCl concentration and liquid temperature from ratios of the transmission measurements. The spectral database needed to select laser wavelengths was measured by a Fourier transform infrared spectrometer in the near infrared (5500-8000 cm(-1)) for NaCl solutions between 5 and 26 wt% and temperatures between 298 and 338 K. A prototype sensor was constructed, and the sensor concept was validated with measurements using a calibration cell consisting of two parallel quartz plates providing liquid layers of variable thickness (200-1500 mu m), NaCl concentration (5-15 wt%) and temperature (293-318 K). The sensor was demonstrated for real-time constant temperature evaporation of an evaporating liquid film deposited on a heated quartz window.