Detection of trace levels of water in oil by photoacoustic spectroscopy

Abstract

We have demonstrated the feasibility of non-destructively analyzing trace levels of water in petroleum-based transmission and hydraulic oils and in a synthetic polyol ester engine-lubricating oil using photoacoustic spectroscopy (PAS) at a wavelength of 2.93 μm. Using the method of standard additions, detection limits of 45, 60, and 515 ppm were obtained for water in the transmission, hydraulic, and synthetic engine oils, respectively. Lower sensitivity for water in the synthetic polyol ester oil was due to a 0.16 μm shift in the water absorption maximum from the wavelength used for the other oils. A detection limit of 62 ppm was also observed for analysis of water in standard reference material (SRM) 8507 from the National Institute of Standards and Technology (NIST), which is a petroleum oil having a nominal water content of 77 ppm. The results for the petroleum-based oils exceed the current best detection limits of approximately 100 ppm obtained using Fourier transform infrared spectroscopy (FTIR) and compare favorably with the desired detection limits for water in oil of 10–50 ppm. Factors involved in implementing this technology in on-line sensors are discussed.