The precise, real-time measurement of liquid concentration is important in
fundamental research, chemical analysis, mixing processes, and manufacturin
g, e.g., in the food and semiconductor industries. This work presents a las
er-based, noninvasive technique to measure concentration changes of flowing
liquids in real time. The essential components in the system include a 5 m
W laser diode coupled to a single-mode optical fiber, a triangular optical
cell, and a high-resolution beam position sensor. The instrument provides a
large range of concentration measurement, typically 0%-100% for binary liq
uid mixtures, while providing a resolution on the order of 0.05% concentrat
ion or better. The experimental configuration is small, reliable, and inexp
ensive. Results are presented for NaCl and MgCl2 aqueous solutions with con
centrations ranging from 0% to 25%, with very good agreement found between
measured and true concentrations. (C) 2000 American Institute of Physics. [
S0034-6748(00)00410-X].