NIR sensor for aqueous urea solution film thickness and concentration measurement using a broadband light source

We demonstrate a multi-wavelength near-infrared (NIR) broadband absorption sensor for the simultaneous monitoring of layer thickness and urea concentration of aqueous urea solutions. Samples were prepared in thin-layer quartz transmission cells. Film thickness and urea mass fraction (at constant temperature) were determined from measured transmittance ratios in characteristic wavelength bands selected by narrowband filters in front of the detector and converted to absorbance ratios. Suitable emission bands were selected depending on the sensitivity of the NIR absorption spectrum of the solution with respect to temperature and solute concentration. For this purpose, Fourier transform IR spectra of aqueous urea solutions were recorded in the 1250-2500 nm wavelength range for urea concentrations between 0 and 40 wt.% and temperatures between 298 K and 338 K. A prototype sensor was designed using a continuous-wave fiber-coupled incoherent tungsten lamp, subsequent intensity modulation, and lock-in detection of the transmitted radiation. The sensor concept was validated with measurements using a calibration cell providing liquid layers of variable thicknesses (7-1000 μm).