Oceanic Measurements: Fiber Optic Chemical Sensor (FOCS) Brings a New Dimension

Abstract

ABSTRACT The emerging technology, fiber optic chemical sensors (FOCS), holds great potential in environmental and ocean monitoring. The promising features of FOCS include the ability to make sensitive, species or group specific, in-situ, real-time measurements over a prolonged period of time. The FOCS is an integrated continuous monitoring system consisting of an optical fiber, sensing chemistry, a light source, optical assembly, detection electronics, and signal processing and data logging equipments. FOCS have been used to make preliminary measurements of p02 and pC02 in the open ocean. The performances of these sensors will be discussed. INTRODUCTION Great progress has been made in the area of fiber optic sensor technology over the past few years and fiber optic sensors have proven effective for in situ measurement of physical and chemical parameters in a variety of mediums. One of the most promising environmental applications for fiber optic sensors to date is the measurement of specific chemical parameters in water. This has been demonstrated in both the laboratory and the field by researchers in government, academia and in the private sector [1-3]. Fiber Optic Chemical Sensors (FOCS) are ideally suited for remote sensing applications by using remotely located sensors in coastal and ocean environments [4,5]. If properly designed, a FOCS system would result in observational networks that would provide superior spatial and temporal coverage of selected geographic areas of diverse marine environments. Remote sensing techniques are valuable measurement tools that can also provide improved sampling strategies by making near synoptic observations possible. Shore-based FOCS systems could offer many advantages for monitoring the health and safety of these often impacted areas. An observational network of strategically placed FOCS can effectively monitor a coastal area located near a potential source of pollution. Monitoring data could be transmitted to a central data collection and processing facility, which would, in turn, disseminate the information to the appropriate regulatory authorities. The data obtained could also be used for compliance for pollution discharge limits, and for monitoring pollution levels in controlled ecosystems experiments. Systems of this type would be valuable tools in monitoring the environmental condition of enclosed bays and estuaries. Because of the great potential, the National Oceanic and Atmospheric Administration (NOAA) has been engaged in a program to develop, test and evaluate dissolved oxygen and dissolved carbon dioxide FOCS. This work was accomplished through contract and the results and accomplishments are detailed below. FOCS DESIGN The need for high resolution, long active lifetimes and good reproducibility between sensors for a particular chemical species has resulted in the use of the reservoir sensor concept for CO2 and O2. In this design (Figure 1) the sensing reagents are used as aqueous solutions. A membrane is used for selective permeation of the dissolved gases as well as for containing the sensing reagent inside the cell. Using this approach, all of the key elements of the sensor can be very accurately controlled [6].