Water contamination in lubrication and hydraulic oils is one of the primary causes of destructive wear and corrosion in machinery, making early detection desirable. Fourier transform infrared spectroscopy can detect water in oil at concentrations as low as 100 ppm, but researchers at the Pacific Northwest National Laboratory in Richland, Wash., have found that photoacoustic spectroscopy has a detection limit up to 10 times lower. Photoacoustic spectroscopy measures the acoustic wave that is generated in a substance returning to the ground state after optical stimulation. The researchers designed a layered-prism cell for their experiments with transmission, hydraulic and engine oils. A pulsed Nd:YAG laser Raman-shifted deuterium to produce the 2.93-µm excitation light source.