This chapter describes the results of tests on an optical wave guide sensor for measuring refractive index of sea water and hence salinity and density. The optical sensor used to obtain these results is very sensitive to salinity it produces a 10 V signal for a range of salinity of 0–50 ppt. The resolution of the sensor is 0 005 ppt with an excellent stability. However, the primary disadvantage of the sensor is its sensitivity to contamination of its surface with the sea. This is a problem that has plagued sensors which rely on internal reflection to measure refractive index of a fluid.
The use of the optical sensor as a device for measuring salinity in expendable probes is discussed. Expendable probes require that the sensor pass through the surface only once, thus avoiding most of the surface slick contamination problem. This optical sensor promises to be cheap and easy to manufacture, essential features for an expendable probe. We present the experimental results obtained to date in testing the sensor for use in expendable probes.
The sensor consists of a light source, a 2cm long optical wave guide, and a light detector Light is sent along the optical wave guide and is partially lost, depending on the refractive index of the fluid in which the optical wave guide is immersed. More light is lost as the refractive index of the fluid approaches the refractive index of the optical wave guide; less light is lost is there is more of a mismatch. The amplitude of the detected light is therefore directly related to the refractive index of the fluid. The refractive index of sea water is directly related to its salinity and density.
The determination of salinity and density via the measurement of refractive index has several inherent advantages over the traditional measurement of conductivity to determine salinity refractive index is less sensitive to temperature than is conductivity; refractive index increases linearly with salinity to 50 ppt, whereas conductivity does not (sensitivity at saturation is one-sixth sensitivity at low salinities for conductivity); refractive index is hardly affected by changes in the ratios of salts whereas conductivity measurement relies on calibration with a standard salt mixture and is inaccurate when the salt composition changes as in estuaries or ice melt conditions, finally, unlike conductivity, the equation of state for calculating density from refractive index shows that 95% of density change is due to a linear relation with refractive index. Accuracies of 5 X 10−5 cm3 g−1 are attainable easily from the linear relationship. The equation for conductivity is non-linear and temperature dependent (Seaver, 1987).
The main disadvantage of measuring refractive index is that it is more sensitive to pressure than is conductivity. The dependence of refractive index on oceanographic parameters, considering the range of each parameter that a probe would encounter is as follows : temperature -0002/30°C, salinity + 0 009/43 ppt and pressure + 0 003/200 kg/cm2 (Austin and Halikas, 1976).