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

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.

Characterization and field test of an in situ multi‐platform optical sensor

“…5b, using a 500 ms integration time. Turbidity as low as 0.1 NTU can be measured, demonstrating a potential application in monitoring processed drinking water, for which a typical upper limit specified by water authorities is 1 NTU (Ng et al 2012b). Measurement of the same calibration standards after 3 months elapsed time shows that the calibration correlation remained valid.…”

“…A low-cost SEAFLO (Model: SFBP1-G350-01) submersible bilge pump connected to polyethylene (PE) tubing is used to transport sample from the water body into the PORTA LEDIF flowcell manifold. A detailed description of LEDIF technology is available in Ng et al (2012a) and Ng et al (2012b).…”

“…Such observations can help the management to implement effective controls and gauge the effectiveness of the solution for better maintaining the water quality on the test site. The flowrate through the flowcell was measured to be 14.4 cm 3 /s, resulting in a water velocity (V) of 62.7 cm/s in the cell, and a flow retention time of 2.4 s computed based on the flow retention time (t) equation (t = 3V -1.2 ) of LEDIF published by Ng et al (2012b). Although not required, the flushing time of the present work was set to be significantly higher than that of the computed value, at 75 s. LEDIF permits frequency of measurement as high as one per 10 s. A frequency of measurement of one per 7 min, later reduced to one per 23 min, was found to be sufficient to capture temporal trends at this site.…”

“…The instrument, based on a technology named LEDIF (LED-Induced Fluorescence) developed by Ng et al (2012a) was configured for in situ long-term continuous monitoring at a fixed location. The optical performance and sensing capabilities of the instrument were assessed by calibrating against laboratory-prepared and commercial standards, and compared with several commercial sensors and published values.…”

“…Literature on in situ sensing of water chemistry has been reviewed by Ng et al (2012b) and literature cited therein. As such, the review in this article will be brief and will focus on long-term continuous monitoring.…”