This paper describes an optical instrument that uses the transmission and scattering properties of water droplets in hydraulic fluids to measure the water fraction in the fluid. The instrument consists of a small optical chamber that can be built into an active hydraulic system. Light is supplied via a near-infrared light-emitting diode and is collected via a simple system of lenses and two wavelength-matched detectors placed at 90° to the beam path and directly in the path of the transmitted light. A novel method of measuring the water contamination is utilized that greatly reduces the signal noise caused by dynamic changes in the system during operation.
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