A multi-platform optical sensor for in situ sensing of water chemistry

Chee-Loon Ng, Schuyler Senft-Grupp, Harold F. Hemond

Limnol. Oceanogr. Methods 10:978-990 (2012) | DOI: 10.4319/lom.2012.10.978

ABSTRACT: A compact field-deployable optical instrument using fluorescence, absorbance, and scattering to identify and quantify contaminants and natural substances in water bodies is described. The instrument is capable of deployment on autonomous underwater and surface vehicles, manned vehicles, fixed platforms such as buoys, or access points in water supply or drainage networks. The instrument comprises (1) a flowcell, (2) multiple optical systems, (3) a data logger, (4) a power control board and computer, and (5) a battery. The instrument has been packaged in a cylindrical pressure case of 200 mm diameter and 300 mm length for electrically and mechanically seamless insertion as a STARFISH AUV payload module. The same module can be fitted with watertight end caps for use aboard other platforms, or simpler packaging can be employed for use in less demanding environments. For spectrofluorometry, the system uses six (expandable to twelve) electronically switchable excitation sources, allowing the construction of fluorescence excitation-emission matrices (EEMs). A deuterium-tungsten light source (185 to 1100 nm) is used in making UV-VIS absorbance measurements. Turbidity can be measured by nephelometry, using observations of light scattering at each excitation wavelength. The absorbance and turbidity capabilities provide useful water quality information and can also be used for correction of inner shielding effects. Validation of the instrument includes (1) comparison with a commercial luminescence spectrometer in measuring both standards and field samples, (2) comparisons of observed spectra with published optical characteristics for several chemicals, and (3) field demonstration aboard an AUV.