Development of an underwater in-situ spectrophotometric sensor for seawater pH

A pH sensor based upon spectrophotometric techniques has been developed for in-situ analysis of surface seawater. This sensor utilizes a spectrophotometric pH indicator (Thymol Blue) which has been calibrated for use in seawater as a function of temperature and salinity. Shipboard spectrophotometric pH analyses routinely demonstrate a precision on the order of +/- 0.0004 pH units. in-situ analysis of seawater pH has demonstrated a precision on the order of +/- 0.001 and an accuracy, using shipboard measurements as a standard, on the order of +/- 0.01. The sensor is a self contained system which pumps seawater, meters in indicator, spectrophotometrically determines indicator absorbance and stores data with a 1Hz acquisition frequency. The sensor employs two absorbance cells, each with three wavelength channels, to obtain the spectrophotometric absorbance. The sensor system, rated for depths up to 500m, provides pH, conductivity, temperature and can be operated via computer or in a standalone mode with internal data storage. The sensor utilizes less than 12 watts of power and is packaged in a 29 '' long x 4.5 '' diameter aluminum housing.